Somatic Sensations pdf - The Nervous System

of the previous chapter; their special characteristics are the following.

tors, but many more similar to these also exist. Some were shown in Figure 46–1

There are at least six entirely different types of tactile recep-

Tactile Receptors.

signals, but some of the same types of receptors as those for touch and pressure

tissues; and (3) vibration sensation results from rapidly repetitive sensory

skin; (2) pressure sensation generally results from deformation of deeper

principal differences among them: (1) touch sensation generally results from

they are all detected by the same types of receptors. There are three

touch, pressure, and vibration are frequently classified as separate sensations,

Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration.

Tactile Sensations

Detection and Transmission of

muscles, and bone. These include mainly “deep” pressure, pain, and vibration.

are those that come from deep tissues, such as from fasciae,

one usually refers specifically to sensations from the internal organs.

are those from the viscera of the body; in using this term,

Visceral sensations

often considered a “special” sensation rather than a somatic sensation).

from the bottom of the feet, and even the sensation of equilibrium (which is

including position sensations, tendon and muscle sensations, pressure sensations

are those having to do with the physical state of the body,

are those from the surface of the body.

grouped together in other classes, as follows.

senses.

rate of movement

senses, and the position senses

tickle

, and

touch

Chapter 48 discusses the thermoreceptive and pain senses. The tactile senses

This chapter deals with the mechanoreceptive tactile and position senses.

, which is activated by any factor that damages the tissues.

, which detect heat and cold; and (3) the

, which include both

mechanoreceptive somatic senses

The somatic senses can be classified into three physiologic types: (1) the

CLASSIFICATION OF SOMATIC SENSES

smell, taste, and equilibrium.

, which mean specifically vision, hearing,

These senses are in contradistinction to the

collect sensory information from all over the body.

The

Position Senses

Organization, the Tactile and

Somatic Sensations: I. General

585

somatic senses are the nervous mechanisms that

special

senses

tactile and position sensa-

tions that are stimulated by mechanical displacement of some tissue of the body;
(2) the thermoreceptive senses

pain

sense

include

, pressure, vibration

include static position and

Other Classifications of Somatic Sensations.

Somatic sensations are also often

Exteroreceptive sensations

Proprio-

ceptive sensations

Deep sensations

Although

stimulation of tactile receptors in the skin or in tissues immediately beneath the

are used.

Meissner’s corpuscles, Iggo dome receptors, hair

Almost all specialized sensory receptors, such as

Transmission of Tactile Signals in Peripheral Nerve Fibers.

mechanical state of the tissues.

second. Therefore, they are particularly important for

skin and deep in the fascial tissues of the body. They

detail in Chapter 46, lie both immediately beneath the

Sixth, pacinian corpuscles, which were discussed in

heavy prolonged touch and pressure signals. They are

continuous states of deformation of the tissues, such as

very slowly and, therefore, are important for signaling

endings, as shown in Figure 46–1. These endings adapt

, which are multibranched, encapsulated

Ruffini’s

Fifth, located in the deeper layers of the skin and

initial contact with the body.

and, like Meissner’s corpuscles, detects mainly (a)

are also a touch receptor. This receptor adapts readily

hair and its basal nerve fiber, called the

stimulates a nerve fiber entwining its base. Thus, each

Fourth, slight movement of any hair on the body

along with the Meissner’s corpuscles discussed earlier,

). These receptors,

myelinated nerve fiber (type A

group of Merkel’s discs is innervated by a single large

extremely sensitive receptor. Also note that the entire

outward, thus creating a dome and constituting an

lium of the skin, as also shown in Figure 47–1. This

, which

Merkel’s discs are often grouped together in a

only slowly. Therefore, they are responsible for giving

receptors differ from Meissner’s corpuscles in that

they have almost no Meissner’s corpuscles. These

ate numbers of expanded tip receptors, even though

47–1. The hairy parts of the skin also contain moder-

shown in Figure

Merkel’s discs,

large numbers of Meissner’s corpuscles usually also

Third, the fingertips and other areas that contain

frequency vibration.

fraction of a second after they are stimulated, which

highly developed. Meissner’s corpuscles adapt in a

tips, lips, and other areas of the skin where one’s ability

These corpuscles are present in the nonhairy parts of

lation are many branching terminal nerve filaments.

) myelinated sensory nerve fiber. Inside the capsu-

(illustrated in Figure 46–1), an

Meissner’s corpuscle

Second, a touch receptor with great sensitivity is the

can nevertheless elicit touch and pressure sensations.

other type of nerve ending besides free nerve endings,

contact with the cornea of the eye, which contains no

detect touch and pressure. For instance, even light

everywhere in the skin and in many other tissues, can

, which are found

First, some

The Nervous System: A. General Principles and Sensory Physiology

586

Unit IX

free nerve endings

elongated encapsulated nerve ending of a large (type
A

the skin and are particularly abundant in the finger-

to discern spatial locations of touch sensations is

means that they are particularly sensitive to movement
of objects over the surface of the skin as well as to low-

contain large numbers of expanded tip tactile receptors,
one type of which is

they transmit an initially strong but partially adapting
signal and then a continuing weaker signal that adapts

steady-state signals that allow one to determine con-
tinuous touch of objects against the skin.

receptor organ called the Iggo dome receptor
projects upward against the underside of the epithe-

causes the epithelium at this point to protrude

play extremely important roles in localizing touch
sensations to specific surface areas of the body and in
determining the texture of what is felt.

hair end-organ,

movement of objects on the surface of the body or (b)

also in still deeper internal tissues are many
end-organs

also found in joint capsules and help to signal the
degree of joint rotation.

are stimulated only by rapid local compression of the
tissues because they adapt in a few hundredths of a

detecting tissue vibration or other rapid changes in the

cle in hairy skin. J Physiol 200:

AR: The structure and function of

epithelium. (From Iggo A, Muir

tightly the undersurface of the

multiple numbers of Merkel’s

Iggo dome receptor. Note the

Figure 47–1

discs connecting to a single large
myelinated fiber and abutting

a slowly adapting touch corpus-

763, 1969.)

systems.

With this differentiation in mind, we can now

mechanoreceptive sensations.

The dorsal system is limited to discrete types of

pain, warmth, cold, and crude tactile sensations;

that the dorsal system does not have: the ability to

The anterolateral system has a special capability

column–medial lemniscal system; that which does not

two systems. That is, sensory information that must

system has much less spatial orientation. These

with respect to their origin, while the anterolateral

40 m/sec.

signals to the brain at velocities of 30 to 110 m/sec,

posed of large, myelinated nerve fibers that transmit

The dorsal column–medial lemniscal system is com-

thalamus.

and lateral white columns of the cord. They terminate

horns of the spinal gray matter, then cross to the oppo-

dorsal spinal nerve roots, synapse in the dorsal

Conversely, signals in the anterolateral system,

site side in the medulla, they continue upward through

Then, after the signals synapse and cross to the oppo-

name implies, carries signals upward to the medulla of

The dorsal column–medial lemniscal system, as its

of the thalamus.

. These

native sensory pathways: (1) the

the entry point into the cord and then to the brain, the

dorsal roots of the spinal nerves. However, from

Nervous System

Transmitting Somatic

Sensory Pathways for

in the cord by lateral inhibition, as described in

or if the scratch is strong enough to elicit pain. The

maneuvers that rid the host of the irritant. Itch can

crawling on the skin or a fly about to bite, and the

The purpose of the itch sensation is presumably to

nated fibers similar to those that transmit the aching,

sations are transmitted by very small type C, unmyeli-

and itch sensations usually can be elicited. These sen-

skin, which is also the only tissue from which the tickle

and itch sensations. Furthermore, these endings are

tence of very sensitive, rapidly adapting mechano-

TICKLE AND ITCH

than pacinian corpuscles.

Meissner’s corpuscles, which are less rapidly adapting

in contrast, stimulate other tactile receptors, especially

frequency vibrations from 2 up to 80 cycles per second,

transmit as many as 1000 impulses per second. Low-

nerve fibers, which can

mit their signals over type A

rapid deformations of the tissues, and they also trans-

signal vibrations from 30 to 800 cycles per second

quencies of vibration. Pacinian corpuscles can detect

tion, although different receptors detect different fre-

Detection of Vibration

nerve bundle than the fast fibers.

tickle, are transmitted by way of much slower, very

crude pressure, poorly localized touch, and especially

fibers. Conversely, the cruder types of signals, such as

the skin, minute gradations of intensity, or rapid

Thus, the more critical types of sensory signals—

serving mainly the sensation of tickle.

the spinal cord and lower brain stem, probably sub-

tion of a meter up to 2 m/sec; these send signals into

30 m/sec.

mit signals mainly by way of the small type A

Conversely, free nerve ending tactile receptors trans-

transmission velocities ranging from 30 to 70 m/sec.

transmit their signals in type A

receptors, pacinian corpuscles, and Ruffini’s endings,

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

587

b nerve fibers that have

d myeli-

nated fibers that conduct at velocities of only 5 to

Some tactile free nerve endings transmit by way of

type C unmyelinated fibers at velocities from a frac-

those that help to determine precise localization on

changes in sensory signal intensity—are all transmit-
ted in more rapidly conducting types of sensory nerve

small nerve fibers that require much less space in the

All tactile receptors are involved in detection of vibra-

because they respond extremely rapidly to minute and

Neurophysiologic studies have demonstrated the exis-

receptive free nerve endings that elicit only the tickle

found almost exclusively in superficial layers of the

slow type of pain.

call attention to mild surface stimuli such as a flea

elicited signals then activate the scratch reflex or other

be relieved by scratching if this removes the irritant

pain signals are believed to suppress the itch signals

Chapter 48.

Signals into the Central

Almost all sensory information from the somatic seg-
ments of the body enters the spinal cord through the

sensory signals are carried through one of two alter-

dorsal column–medial

lemniscal system or (2) the anterolateral system
two systems come back together partially at the level

the brain mainly in the dorsal columns of the cord.

the brain stem to the thalamus by way of the medial
lemniscus.

immediately after entering the spinal cord from the

site side of the cord and ascend through the anterior

at all levels of the lower brain stem and in the

whereas the anterolateral system is composed of
smaller myelinated fibers that transmit signals at
velocities ranging from a few meters per second up to

Another difference between the two systems is

that the dorsal column–medial lemniscal system has a
high degree of spatial orientation of the nerve fibers

differences immediately characterize the types of
sensory information that can be transmitted by the

be transmitted rapidly and with temporal and
spatial fidelity is transmitted mainly in the dorsal

need to be transmitted rapidly or with great spatial
fidelity is transmitted mainly in the anterolateral
system.

transmit a broad spectrum of sensory modalities—

most of these are discussed in detail in Chapter 48.

list the types of sensations transmitted in the two

second-order neurons

). From there,

pass uninterrupted up to the dorsal medulla, where

47–3 that nerve fibers entering the dorsal columns

Note in Figure

The Dorsal Column–Medial Lemniscal Pathway.

Others give rise to the spinocerebellar tracts, which we

cord reflexes, which are discussed in Chapter 54. (3)

local neurons in turn serve three functions: (1) A major

and anterior portions of the cord gray matter. These

gray matter, then divides many times to provide termi-

The lateral branch enters the dorsal horn of the cord

then upward in the dorsal column, proceeding by way

Figure 47–2. The medial branch turns medially first and

, shown by

lateral branch

medial branch

dorsal roots, the large myelinated fibers from the spe-

Anatomy of the Dorsal

Transmission in the

5. Sexual sensations

4. Tickle and itch sensations

3. Crude touch and pressure sensations capable only

2. Thermal sensations, including both warmth and

1. Pain

6. Pressure sensations having to do with fine degrees

5. Position sensations from the joints

4. Sensations that signal movement against the skin

3. Phasic sensations, such as vibratory sensations

2. Touch sensations requiring transmission of fine

1. Touch sensations requiring a high degree of

The Nervous System: A. General Principles and Sensory Physiology

588

Unit IX

Dorsal Column–Medial
Lemniscal System

localization of the stimulus

gradations of intensity

of judgment of pressure intensity

Anterolateral System

cold sensations

of crude localizing ability on the surface of the
body

Dorsal Column–Medial
Lemniscal System

Column–Medial Lemniscal System

On entering the spinal cord through the spinal nerve

cialized mechanoreceptors divide almost immediately
to form a

and a

the right-hand fiber entering through the spinal root in

of the dorsal column pathway all the way to the brain.

nals that synapse with local neurons in the intermediate

share of them give off fibers that enter the dorsal
columns of the cord and then travel upward to the brain.
(2) Many of the fibers are very short and terminate
locally in the spinal cord gray matter to elicit local spinal

will discuss in Chapter 56 in relation to the function of
the cerebellum.

they synapse in the dorsal column nuclei (the cuneate
and gracile nuclei

VII

III

VIII

Dorsal
column

Anterolateral

spinothalamic

pathway

Lamina marginalis

Substantia gelatinosa

Spinal nerve

Tract of

Lissauer

Spinocervical

tract

Dorsal

spinocerebellar

tract

Ventral

spinocerebellar

tract

of the spinal cord.

gray matter and of ascending sensory tracts in the white columns

Cross section of the spinal cord, showing the anatomy of the cord

Figure 47–2

Medulla oblongata

Cortex

Spinal cord

Dorsal root and spinal
ganglion

Medial lemniscus

Dorsal column nuclei

Ascending branches of
dorsal root fibers

Spinocervical tract

Ventrobasal

complex

of thalamus

Internal

capsule

ical types of tactile signals. (Modified from Ranson SW, Clark SL:

Figure 47–3

The dorsal column–medial lemniscal pathway for transmitting crit-

Anatomy of the Nervous System. Philadelphia: WB Saunders Co,
1959.)

contractions and body movements. A major share of

Conversely, that portion of the cerebral cortex ante-

in the temporal lobe.

terminate in the occipital lobe, and

Visual signals

. But the posterior

generally, the anterior half of the parietal lobe is

immediately posterior to the central fissure. And,

the brain. In general, sensory signals from all modali-

based on histological structural differences. This map

Brodmann’s areas

of the human cerebral cortex, showing that it is divided

the various areas of the cortex. Figure 47–5 is a map

somatic sensation, we need to give an orientation to

the thalamus.

mus, and the right side of the body in the left side of

of the medial lemnisci in the medulla, the left side of

medial areas of the complex. Because of the crossing

maintained, with the tail end of the body represented

In the thalamus, distinct spatial orientation is still

laterally.

cord, whereas those that enter the cord at progres-

in the dorsal columns of the spinal cord, the fibers from

the body that is maintained throughout. For instance,

Spatial Orientation of the Nerve Fibers in the

(as shown in Figure 47–6,

, which is

project, as shown in Figure 47–4, mainly to

third-order

From the ventrobasal complex,

in the thalamic sensory relay area, called the

In the thalamus, the medial lemniscal fibers terminate

that the dorsal column fibers subserve for the body.

; these

to the thalamus. In this pathway through the brain stem,

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

589

decussate immediately to the opposite side of the brain
stem and continue upward through the medial lemnisci

each medial lemniscus is joined by additional fibers
from the sensory nuclei of the trigeminal nerve
fibers subserve the same sensory functions for the head

ventrobasal

complex.
nerve fibers
the postcentral gyrus of the cerebral cortex
called somatic sensory area I
these fibers also project to a smaller area in the lateral
parietal cortex called somatic sensory area II).

Dorsal Column-Medial Lemniscal System

One of the distinguishing features of the dorsal
column–medial lemniscal system is a distinct spatial
orientation of nerve fibers from the individual parts of

the lower parts of the body lie toward the center of the

sively higher segmental levels form successive layers

by the most lateral portions of the ventrobasal
complex and the head and face represented by the

the body is represented in the right side of the thala-

Somatosensory Cortex

Before discussing the role of the cerebral cortex in

into about 50 distinct areas called

is important because virtually all neurophysiologists
and neurologists use it to refer by number to many of
the different functional areas of the human cortex.

Note in the figure the large central fissure (also

called central sulcus) that extends horizontally across

ties of sensation terminate in the cerebral cortex

concerned almost entirely with reception and inter-
pretation of somatosensory signals
half of the parietal lobe provides still higher levels of
interpretation.

auditory signals

rior to the central fissure and constituting the poste-
rior half of the frontal lobe is called the motor cortex
and is devoted almost entirely to control of muscle

this motor control is in response to somatosensory

Lower extremity

Upper
extremity

Ventrobasal complex of thalamus

POSTCENTRAL GYRUS

MESENCEPHALON

Spinothalamic tract

Medial lemniscus

Trunk

Face

University Press.)

York: Oxford University Press, 1969, by permission of Oxford

A: Neurological Anatomy in Relation to Clinical Medicine. New

the thalamus to the somatosensory cortex. (Modified from Brodal

Projection of the dorsal column–medial lemniscal system through

Figure 47–4

Central fissure

Lateral fissure

3 2

somatosensory association area

, and areas 5 and 7, which con-

primary somatosensory area I

cerebral cortex. Note specifically areas 1, 2, and 3, which consti-

Structurally distinct areas, called Brodmann’s areas, of the human

Figure 47–5

tute
stitute the

in other layers. Some of these functions are:

Figure 47–8. As would be expected, the neurons in

extending progressively deeper to layer VI, shown in

layers of neurons,

The cerebral cortex contains

Layers of the Somatosensory Cortex and

part of the body is represented medially.

lateral portion of somatosensory area I, and the lower

endings are found in the lips and thumb, whereas only

body. For instance, a great number of specialized nerve

tively small areas. The sizes of these areas are directly

Some areas of the body are represented by large

site side of the body.

however, that each lateral side of the cortex receives

separate regions of somatosensory area I. Note,

at the level of the postcentral gyrus, demonstrating

Figure 47–7 shows a cross section through the brain

Brodmann’s areas 3, 1, and 2).

immediately behind the central fissure, located in the

in Somatosensory Area I.

somatosensory area I. Thus, much of what we know

However, removal of parts of somatosensory area II

auditory areas. Projections from somatosensory area I

sensory areas of the brain, even from the visual and

the body. In addition, many signals come secondarily

the brain stem, transmitted upward from both sides of

area II. It is known that signals enter this area from

the arms centrally, and the legs posteriorly.

II, although roughly, the face is represented anteriorly,

By contrast, localization is poor in somatosensory area

names of virtually all parts of the body in Figure 47–6.

tion of the different parts of the body, as shown by the

“somatosensory cortex” almost always means area I.

somatosensory area II that in popular usage, the term

areas. However, somatosensory area I is so much

. The

Figure 47–6 shows two sep-

Somatosensory Areas I and II.

ent body parts.

cortex, which keep the motor cortex informed at each

The Nervous System: A. General Principles and Sensory Physiology

590

Unit IX

signals received from the sensory portions of the

instant about the positions and motions of the differ-

arate sensory areas in the anterior parietal lobe called
somatosensory area I and somatosensory area II
reason for this division into two areas is that a distinct
and separate spatial orientation of the different
parts of the body is found in each of these two

more extensive and so much more important than

Somatosensory area I has a high degree of localiza-

Little is known about the function of somatosensory

from somatosensory area I as well as from other

are required for function of somatosensory area II.

has no apparent effect on the response of neurons in

about somatic sensation appears to be explained by
the functions of somatosensory area I.

Spatial Orientation of Signals from Different Parts of the Body

Somatosensory area I lies

postcentral gyrus of the human cerebral cortex (in

representations of the different parts of the body in

sensory information almost exclusively from the oppo-

areas in the somatic cortex—the lips the greatest of all,
followed by the face and thumb—whereas the trunk
and lower part of the body are represented by rela-

proportional to the number of specialized sensory
receptors in each respective peripheral area of the

a few are present in the skin of the body trunk.

Note also that the head is represented in the most

Their Function

six

beginning with layer I next to the brain surface and

each layer perform functions different from those

Somatosensory
area I

Somatosensory
area II

Thigh

Thorax

Neck

Shoulder

Hand

Fingers

Tongue

Leg

Arm

Face

Abdomen

Two somatosensory cortical areas, somatosensory areas I and II.

Figure 47–6

Intra-abdominal

Pharynx

Tongue

Teeth, gums, and jaw

Lower lip

Lips

Upper lip

Face

Nose

Eye

Thumb

Index finger

Middle finger

Ring finger

Little finger

Hand

Wrist

For

earm

Elbow

Shoulder

Head Neck

Trunk

Hip

Leg

Foot

Toes

Genital

New York: Hafner, 1968.)

bral Cortex of Man: A Clinical Study of Localization of Function.

sory area I of the cortex. (From Penfield W, Rasmussen T: Cere-

Representation of the different areas of the body in somatosen-

Figure 47–7

objects. This is called

4. The person is unable to judge shapes or forms of

objects.

3. The person is unable to judge the weights of

pressure against the body.

2. The person is unable to judge critical degrees of

legs. Thus, it is clear that the brain stem, thalamus,

a major level of the body trunk, or to one of the

sensations crudely, such as to a particular hand, to

body. However, he or she can localize these

1. The person is unable to localize discretely the

Widespread bilateral excision of somatosensory area I

Functions of Somatosensory Area I

discuss subsequently.

, as we

from somatosensory area I into the parietal cortex, an

tation of sensory signals; the process becomes even

direction. Thus, this is a still higher order of interpre-

I, about 6 per cent of the vertical columns respond only

sensitive to deep pressure.

farther posteriorly, greater numbers of the columns are

slowly adapting cutaneous receptors, and then still

central fissure. These signals play a major role in

sensory columns then spread anteriorly, directly to

stretch receptors. Many of the signals from these

vertical columns respond to muscle, tendon, and joint

Brodmann’s area 3a, an especially large share of the

central gyrus, located deep in the central fissure in

initiate analysis of the meanings of the sensory signals.

At other levels of the columns, interactions occur that

function almost entirely separately from one another.

signals first enter the cortex, the columns of neurons

and so forth. At layer IV, where the input sensory

around joints, some to stimulation of tactile hairs,

ity, some columns responding to stretch receptors

taining perhaps 10,000 neuronal cell bodies. Each of

through the six layers of the cortex, each column

Functionally, the neurons of the somatosensory cortex

Specific Sensory Modality

Different Sensory Spot on the Body with a

Columns of Neurons; Each Column Detects a

The Sensory Cortex Is Organized in Vertical

thalamus.

numbers of axons extend to the thalamus,

transmission. From layer VI, especially large

areas, such as to the basal ganglia, brain stem,

deeper parts of the nervous system. Those in layer

4. The neurons in layers V and VI send axons to the

3. The neurons in layers II and III send axons to

described in Chapter 57. This input mainly

specific regions of the cortex; this system is

2. Layers I and II receive diffuse, nonspecific input

layers.

layer IV first; then the signal spreads toward the

1. The incoming sensory signal excites neuronal

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

591

surface of the cortex and also toward deeper

signals from lower brain centers that facilitate

controls the overall level of excitability of the
respective regions stimulated.

related portions of the cerebral cortex on the
opposite side of the brain through the corpus
callosum.

V are generally larger and project to more distant

and spinal cord where they control signal

providing signals from the cerebral cortex that
interact with and help to control the excitatory
levels of incoming sensory signals entering the

are arranged in vertical columns extending all the way

having a diameter of 0.3 to 0.5 millimeter and con-

these columns serves a single specific sensory modal-

others to discrete localized pressure points on the skin,

In the most anterior 5 to 10 millimeters of the post-

the motor cortex located immediately forward of the

controlling the effluent motor signals that activate
sequences of muscle contraction.

As one moves posteriorly in somatosensory area I,

more and more of the vertical columns respond to

In the most posterior portion of somatosensory area

when a stimulus moves across the skin in a particular

more complex as the signals spread farther backward

area called the somatosensory association area

causes loss of the following types of sensory judgment:

different sensations in the different parts of the

or parts of the cerebral cortex not normally
considered to be concerned with somatic
sensations can perform some degree of
localization.

astereognosis.

VIb

VIa

III

Brodmann]: Anatomy of the Nervous System. Philadelphia: WB

fusiform or polymorphic cells. (From Ranson SW, Clark SL [after

nal granular layer; V, large pyramidal cell layer; and VI, layer of

external granular layer; III, layer of small pyramidal cells; IV, inter-

Structure of the cerebral cortex, showing I, molecular layer; II,

Figure 47–8

Saunders, 1959.)

1 to 2 millimeters. However, on the person’s back,

the fingers, a person can distinguish two separate

points of stimulus are felt or one point. On the tips of

the same time, and the person determines whether two

test, two needles are pressed lightly against the skin at

so-called “two-point” discriminatory ability. In this

test tactile discrimination is to determine a person’s

Two-Point Discrimination.

center.

fire, but those in the center discharge at a considerably

fire. A stronger stimulus causes still more neurons to

cortical “field” for each respective receptor. Thus, a

synaptic stage, divergence occurs. The upper curves of

dorsal column pathway, demonstrating that at each

The lower part of Figure 47–9 shows the basic

in the Dorsal Column

Transmission and Analysis

even exists. This complex sensory deficit is called

feeling objects, the person tends to recognize only

side for motor functions as well. Likewise, when

Therefore, he or she also often forgets to use the other

side of the body—that is, forgets that it is there.

In fact, the person is mainly oblivious to the opposite

or her own body or body parts on the opposite side.

tion, he or she loses most of the sense of form of his

forms felt on the opposite side of the body. In addi-

area is removed on one side of the brain, the person

When the somatosensory association

Effect of Removing the Somatosensory Association Area—

thalamus, (4) the visual cortex, and (5) the auditory

trobasal nuclei of the thalamus, (3) other areas of the

signals from (1) somatosensory area I, (2) the ven-

decipher its meaning. This also fits with the anatomi-

Therefore, it seems clear that the somatosensory asso-

the “feeling” of an object such as a knife or a ball.

experience a complex body sensation, sometimes even

in the somatosensory areas. Therefore, these areas are

area I (see Figure 47–5), play important roles in deci-

Brodmann’s areas 5 and 7 of the cerebral cortex,

somatosensory area I to localize the source.

intensity. But the sensations are poorly localized, indi-

of only somatosensory area I, appreciation of these

of pain and temperature sense. In specific absence

5. The person is unable to judge texture of materials

The Nervous System: A. General Principles and Sensory Physiology

592

Unit IX

because this type of judgment depends on highly
critical sensations caused by movement of the
fingers over the surface to be judged.

Note that in the list nothing has been said about loss

sensory modalities is still preserved both in quality and

cating that pain and temperature localization depend
greatly on the topographical map of the body in

Somatosensory Association Areas

located in the parietal cortex behind somatosensory

phering deeper meanings of the sensory information

called somatosensory association areas.

Electrical stimulation in a somatosensory associa-

tion area can occasionally cause an awake person to

ciation area combines information arriving from mul-
tiple points in the primary somatosensory area to

cal arrangement of the neuronal tracts that enter the
somatosensory association area because it receives

cortex.

Amorphosynthesis.

loses ability to recognize complex objects and complex

one side of the object and forgets that the other side

amorphosynthesis.

Overall Characteristics of Signal

–Medial

Lemniscal System

Basic Neuronal Circuit in the Dorsal Column–Medial Lemniscal
System.

organization of the neuronal circuit of the spinal cord

the figure show that the cortical neurons that discharge
to the greatest extent are those in a central part of the

weak stimulus causes only the centralmost neurons to

more rapid rate than do those farther away from the

A method frequently used to

points even when the needles are as close together as

the needles must usually be as far apart as 30 to 70

Discharges per second

Cortex

Thalamus

Dorsal column nuclei

Single-point stimulus on skin

Strong stimulus

Moderate

stimulus

Weak

stimulus

Transmission of a pinpoint stimulus signal to the cerebral cortex.

Figure 47–9

As a partial explanation of these effects, Figure 46–4

pressure differences of 10,000 to 100,000 times.

as much as a half million times; and the skin can detect

two experiences can vary more than 10 billion times; the

sive sound, even though the sound intensities of these

ences of tremendously varying intensities? For instance,

The first question that comes to mind is, how is it pos-

roundings. Therefore, it is important that we discuss

The ultimate goal of most sensory stimulation is to

Interpretation of Sensory

tional integrity of the dorsal columns.

reason, application of vibration (e.g., from a “tuning

mitted only in the dorsal column pathway. For this

Meissner’s corpuscles as well. These signals are trans-

skin and deeper tissues, but lower-frequency signals

cycles per second. The higher-frequency vibratory

Vibratory signals are rapidly

Vibratory Sensation.

potentials, this system can recognize changing stimuli

peripheral conditions. Based on recorded action

The dorsal column system also is of particular impor-

Transmission of Rapidly Changing and Repetitive Sensations.

two red curves in Figure 47–10, showing complete

lation is blocked. This effect is demonstrated by the

stand out, and much of the surrounding diffuse stimu-

excitatory signal. As a result, the peaks of excitation

and (3) the cortex itself. At each of these levels, the

medulla, (2) the ventrobasal nuclei of the thalamus,

instance, in (1) the dorsal column nuclei of the

In the case of the dorsal column system, lateral

lateral spread of the excitatory signals and, therefore,

The importance of

secrete an inhibitory transmitter.

inhibitory signals to the surrounding neurons. That is,

excitatory signal, short lateral pathways transmit

in a dorsal column nucleus. Aside from the central

neurons. For instance, consider an excited neuron

signals; these spread to the

sensory pathway, when excited, gives rise simultane-

As pointed out in Chapter 46, virtually every

, as

point. The capability of the sensorium to distinguish

ence of two stimulatory points, rather than a single

a valley, allow the sensory cortex to detect the pres-

has two separate peaks. These two peaks, separated by

taneously. Note that the resultant zone of excitation

The blue curve shows the spatial pattern of cortical

are excited by signals from the two stimulated points.

mation. This figure shows two adjacent points on the

Figure 47–10 shows the mechanism by which the

areas.

detected. The reason for this difference is the different

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

593

millimeters before two separate points can be

numbers of specialized tactile receptors in the two

dorsal column pathway (as well as all other sensory
pathways) transmits two-point discriminatory infor-

skin that are strongly stimulated as well as the areas
of the somatosensory cortex (greatly enlarged) that

excitation when both skin points are stimulated simul-

this presence of two points of stimulation is strongly
influenced by another mechanism, lateral inhibition
explained in the next section.

Effect of Lateral Inhibition (Also Called Surround Inhibition) to
Increase the Degree of Contrast in the Perceived Spatial
Pattern.

ously to lateral inhibitory
sides of the excitatory signal and inhibit adjacent

these signals pass through additional interneurons that

lateral inhibition is that it blocks

increases the degree of contrast in the sensory pattern
perceived in the cerebral cortex.

inhibitory signals occur at each synaptic level—for

lateral inhibition helps to block lateral spread of the

separation of the peaks when the intensity of lateral
inhibition is great.

tance in apprising the sensorium of rapidly changing

that occur in as little as 1/400 of a second.

repetitive and can be detected as vibration up to 700

signals originate from the pacinian corpuscles in the

(below about 200 per second) can originate from

fork”) to different peripheral parts of the body is an
important tool used by neurologists for testing func-

Stimulus Intensity

apprise the psyche of the state of the body and its sur-

briefly some of the principles related to transmission of
sensory stimulus intensity to the higher levels of the
nervous system.

sible for the sensory system to transmit sensory experi-

the auditory system can detect the weakest possible
whisper but can also discern the meanings of an explo-

eyes can see visual images with light intensities that vary

in the previous chapter shows the relation of the recep-
tor potential produced by the pacinian corpuscle to the

Discharges per second

Cortex

Two adjacent points
strongly stimulated

sent the pattern when “surround” inhibition does occur.

tion without “surround” inhibition, and the two red curves repre-

stimuli. The blue curve represents the pattern of cortical stimula-

Transmission of signals to the cortex from two adjacent pinpoint

Figure 47–10

of change. Therefore, multiple different types of recep-

static and dynamic, depends on knowing the degrees

Knowledge of position, both

Position Sensory Receptors.

, also called

movement sense

the body with respect to one another, and (2)

, which means conscious per-

. They can be divided into two subtypes:

The

, K, and k are found, a linear relation

Figure 47–11, and when appropriate quantitative values

using double logarithmic coordinates, as shown in

When this power law relation is plotted on a graph

In this formula, the exponent

formula, known as the power law.

Power Law.

be for the psyche to detect the change.

sensory intensity, the greater an additional change must

Fechner principle is still a good one to remember,

other types of sensory experience. Yet the Weber-

higher intensities of visual, auditory, and cutaneous

Fechner principle is quantitatively accurate only for

More recently, it has become evident that the Weber-

means. To express this mathematically.

about 1 to 30, which is what the logarithmic principle

instance, the

barely detect a 10-gram increase in weight. Thus, in this

And, when already holding 300 grams, he or she can

That is, a person

strength are discriminated approximately in proportion

In the mid-1800s, Weber first and Fechner later

Weber-Fechner Principle—Detection of “Ratio” of Stimulus

Judgment of Stimulus Intensity

sunlight or at twilight; the camera cannot do this without

light. Yet, that person’s own eyes are capable of dis-

sity, a person almost always overexposes the film on

a light meter. Left to intuitive judgment of light inten-

with a camera, to adjust the light exposure without using

the attempts of most people, when taking photographs

operating in the wrong range. This is demonstrated by

range of sensory reception that we can experience, the

Were it not for the tremendous intensity

Importance of the Tremendous Intensity Range of Sensory

intensity levels changing as much as millions of times.

other mechanisms, makes it possible for some sensory

sity on impulse rate in each nerve fiber, as well as several

This mechanism, plus the direct effect of stimulus inten-

fibers, which is another mechanism by which stimulus

point also become stimulated. Thus, signals are trans-

sound intensity increases, many more hair cells in each

at the point of maximum sound vibration. But as the

membrane, weak sound stimulates only those hair cells

When sound stimulates a specific point on the basilar

method for separating gradations of stimulus intensity.

The transduction mechanism for detecting sound by

change

intensity levels, but at high-intensity levels, the change

changes

further increases in receptor potential are slight. Thus,

markedly, whereas at high levels of stimulus intensity,

sity, slight changes in intensity increase the potential

intensity of the sensory stimulus. At low stimulus inten-

The Nervous System: A. General Principles and Sensory Physiology

594

Unit IX

the pacinian corpuscle is capable of accurately measur-
ing extremely minute

in stimulus at low-

in stimulus must be much greater to cause the same
amount of

in receptor potential.

the cochlea of the ear demonstrates still another

direction farther away from the maximum vibratory

mitted over progressively increasing numbers of nerve

intensity is transmitted to the central nervous system.

systems to operate reasonably faithfully at stimulus

Reception.

various sensory systems would more often than not be

bright days and greatly underexposes the film at twi-

criminating with great detail visual objects in bright

very special manipulation because of the narrow criti-
cal range of light intensity required for proper exposure
of film.

Strength.

proposed the principle that gradations of stimulus

to the logarithm of stimulus strength.
already holding 30 grams weight in his or her hand can
barely detect an additional 1-gram increase in weight.

ratio of the change in stimulus strength

required for detection remains essentially constant,

Interpreted signal strength = Log (Stimulus)

+ Constant

sensory experience and applies only poorly to most

because it emphasizes that the greater the background

Another attempt by physiopsychologists to

find a good mathematical relation is the following

Interpreted signal strength = K • (Stimulus

- k)

y and the constants K and

k are different for each type of sensation.

for the constants y
can be attained between interpreted stimulus strength
and actual stimulus strength over a large range for
almost any type of sensory perception.

Position Senses

position senses are frequently also called proprio-

ceptive senses
(1) static position sense
ception of the orientation of the different parts of

rate of

kinesthesia or dynamic

proprioception.

of angulation of all joints in all planes and their rates

tors help to determine joint angulation and are used

100

1000

10,000

Stimulus strength (arbitrary units)

Interpreted stimulus strength

(arbitrary units)

100

200

500

or very strong stimulus strengths.

to be. Note that the power law does not hold at either very weak

actual stimulus strength and strength that the psyche interprets it

Graphical demonstration of the “power law” relation between

Figure 47–11

nized in 10 to 20 gradations of strength, rather than as

far less accurate, most of the sensations being recog-

signals is poor; (3) the gradations of intensities are also

and 40 m/sec; (2) the degree of spatial localization of

column–medial lemniscal system, ranging between 8

lowing differences: (1) the velocities of transmission

column–medial lemniscal system, except for the fol-

In general, the same principles apply to transmission

Characteristics of Transmission in the Anterolateral Pathway.

pain signals are further processed, as discussed in

amus. Instead, most pain signals terminate in the retic-

Conversely, only a small fraction of the pain signals

along with the signals from the dorsal columns.

dorsal column tactile signals terminate. From here, the

transmitted mainly into the ventrobasal complex, ter-

. In general, the tactile signals are

of the thalamus, the

mainly twofold: (1) throughout the

The upper terminus of the two spinothalamic tracts is

As shown in Figure 47–13, the anterolateral fibers

These laminae are where many of the dorsal root

dorsal horn laminae I, IV, V, and VI (see Figure 47–2).

spinal cord anterolateral fibers

The

sensations will be discussed specifically.

sexual sensations. In Chapter 48, pain and temperature

include pain, heat, cold, crude tactile, tickle, itch, and

of fine gradations of intensity. These types of signals

trast to the dorsal column pathway, transmits sensory

signals up the spinal cord and into the brain, in con-

The anterolateral pathway for transmitting sensory

Anterolateral Pathway

Sensory Signals in the

Transmission of Less Critical

at minimal rotation. Thus, the signals from the indi-

to joint rotation are of two categories: (1) those

47–12, one sees that

Referring to Figure

Column–Medial Lemniscal Pathway.

especially adapted for detecting rapid rates of change.

The pacinian corpuscles and muscle spindles are

Golgi tendon receptors found in muscle tendons.

puscles, Ruffini’s endings, and receptors similar to the

tional important factor in determining position. Types

At the extremes of joint angulation, stretch of the

column system for deciphering joint angulations.

loosened, and the net stretch information from the

in Chapter 54. When the angle of a joint is changing,

in helping to control muscle movement, as we shall see

. They are also exceedingly important

motion, among the most important receptors are the

For determining joint angulation in mid ranges of

Conversely, for most of the larger joints of the body,

believed to be detected through the skin receptors.

abundance, as much as half of position recognition is

case of the fingers, where skin receptors are in great

and deep receptors near the joints are used. In the

together for position sense. Both skin tactile receptors

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

595

deep receptors are more important.

muscle spindles

some muscles are being stretched while others are

spindles is transmitted into the computational system
of the spinal cord and higher regions of the dorsal

ligaments and deep tissues around the joints is an addi-

of sensory endings used for this are the pacinian cor-

It is likely that these are the receptors most responsi-
ble for detecting rate of movement.

Processing of Position Sense Information in the Dorsal

thalamic neurons responding

maximally stimulated when the joint is at full rotation
and (2) those maximally stimulated when the joint is

vidual joint receptors are used to tell the psyche how
much each joint is rotated.

signals that do not require highly discrete localization
of the signal source and do not require discrimination

Anatomy of the
Anterolateral Pathway

originate mainly in

sensory nerve fibers terminate after entering the cord.

cross immediately in the anterior commissure of the
cord to the opposite anterior and lateral white columns,
where they turn upward toward the brain by way of the
anterior spinothalamic and lateral spinothalamic tracts.

reticular nuclei of the

brain stem and (2) in two different nuclear complexes

ventrobasal complex and the

intralaminar nuclei

minating in some of the same thalamic nuclei where the

signals are transmitted to the somatosensory cortex

project directly to the ventrobasal complex of the thal-

ular nuclei of the brain stem and from there are relayed
to the intralaminar nuclei of the thalamus where the

greater detail in Chapter 48.

in the anterolateral pathway as in the dorsal

are only one third to one half those in the dorsal

many as 100 gradations for the dorsal column system;

100

120

140

160

180

Degrees

Impulses per second

100

varied over an intensive continuum. J Neurophysiol 26:807, 1963.)

G: The relation of thalamic cell response to peripheral stimuli

its range of motion. (Data from Mountcastle VB, Poggie GF, Werner

mic ventrobasal complex when the knee joint is moved through

Typical responses of five different thalamic neurons in the thala-

Figure 47–12

late developments.

netic development of animals, whereas the critical

crimination of these sensibilities. It is interesting that

lower brain stem, the thalamus, and other associated

Therefore, there is much reason to believe that the

a moderate effect on the perception of temperature.

effect on one’s perception of pain sensation and only

Conversely, loss of the somatosensory cortex has little

to discriminate tactile sensation, even though the thala-

does return. Therefore, it must be assumed that the thal-

bilities, but a slight degree of crude tactile sensibility

destroyed, that person loses most critical tactile sensi-

When the somatosensory cortex of a human being is

Function of the Thalamus

Somatosensory Function

pressure.

and sexual sensations, in addition to crude touch and

niscal system. They are pain, temperature, tickle, itch,

column–medial lemniscal system. Even so, certain

Thus, it is evident that the anterolateral system is a

rapidly repetitive signals is poor.

The Nervous System: A. General Principles and Sensory Physiology

596

Unit IX

and (4) the ability to transmit rapidly changing or

cruder type of transmission system than the dorsal

modalities of sensation are transmitted only in this
system and not at all in the dorsal column–medial lem-

Some Special Aspects of

in Somatic Sensation

amus (as well as other lower centers) has a slight ability

mus normally functions mainly to relay this type of
information to the cortex.

basal regions of the brain play dominant roles in dis-

these sensibilities appeared very early in the phyloge-

tactile sensibilities and the somatosensory cortex were

Medulla oblongata

Medial lemniscus

Mesencephalon

Internal
capsule

Cortex

Spinal cord

Dorsal root and spinal
ganglion

Ventrobasal

and intralaminar

nuclei of the

thalamus

Lateral

division of the

anterolateral

pathway

division of the

anterolateral

pathway

Anterior and lateral divisions of the anterolateral sensory pathway.

Figure 47–13

T10

T11

T12

S4&5

T4
T5

T7
T8

T10

T12

T11

C Thomas, Publisher, Ltd., Springfield, Illinois.)

ed. Springfield, IL: Charles C Thomas, 1966. Courtesy of Charles

Dermatomes. (Modified from Grinker RR, Sahs AL: Neurology, 6th

Figure 47–14

human brain. Ann N Y Acad Sci 999:364, 2003.

Thaut MH: Neural basis of rhythmic timing networks in the

plasticity in the auditory system. Nat Rev Neurosci 4:783,

Suga N, Ma X: Multiparametric corticofugal modulation and

Curr Opin Neurobiol 13:663, 2003.

Sommer MA: The role of the thalamus in motor control.

sentations. Nat Rev Neurosci 3:741, 2002.

Pouget A, Deneve S, Duhamel JR: A computational per-

lular and systems physiology. Pflugers Arch 447:126, 2003.

Petersen CC: The barrel cortex—integrating molecular, cel-

touch are constant companions. Curr Biol 14:R349, 2004.

Pears S, Jackson SR: Cognitive neuroscience: vision and

Biol 13:R531, 2003.

and the body schema: close to hand and within reach. Curr

Maravita A, Spence C, Driver J: Multisensory integration

Physiol Sci 19:22, 2004.

tinguishes between escapable and inescapable pain. News

Lumb BM: Hypothalamic and midbrain circuitry that dis-

Science, 4th ed. New York: McGraw-Hill, 2000.

Kandel ER, Schwartz JH, Jessell TM: Principles of Neural

mechanoreceptors. Curr Opin Neurobiol 11:455, 2001.

Johnson KO: The roles and functions of cutaneous

Behav Brain Res 142:1, 2003.

Jeannerod M: The mechanism of self-recognition in humans.

explained? Lancet Neurol 2:687, 2003.

Janig W, Baron R: Complex regional pain syndrome: mystery

Curr Opin Neurobiol 14:225, 2004.

Ivry RB, Spencer RM: The neural representation of time.

Churchill Livingstone, 2003.

Haines DE, Lancon JA: Review of Neuroscience. New York:

Churchill Livingstone, 1997.

Haines DE: Fundamental Neuroscience. New York:

14:89, 2004.

plasticity in somatosensory cortex. Curr Opin Neurobiol

Foeller E, Feldman DE: Synaptic basis for developmental

gence in central processing.Annu Rev Neurosci 26:1, 2003.

Craig AD: Pain mechanisms: labeled lines versus conver-

Neurosci 3:553, 2002.

movement plans in the posterior parietal cortex. Nat Rev

Cohen YE, Andersen RA: A common reference frame for

tion following injury. Neuroscience 111(4):761, 2002.

Chen R, Cohen LG, Hallett M: Nervous system reorganiza-

lar perspective. Physiol Rev 81:539, 2001.

Bosco G, Poppele RE: Proprioception from a spinocerebel-

disturbed by the injury.

can use a dermatomal map as shown in Figure 47–14 to

ments, which is evident from the dermatomal map. One

ments (L2 to S3), rather than from the distal sacral seg-

most distal portion of the body. The legs originate

matome S5. In the embryo, this is the tail region and the

in the dermatome of the most distal cord segment, der-

The figure shows that the anal region of the body lies

matomes, which is far from true because much overlap

shown in Figure 47–14. They are shown in the figure as

. The different dermatomes are

Each spinal nerve innervates a “segmental field” of the

chapters.

only the somatic system, as explained in subsequent

gal sensory control is used by all sensory systems, not

ferentiate sensory patterns. This principle of corticofu-

sharpness in the signal pattern. Second, it keeps the

cent neurons and, therefore, increases the degree of

in the relay nuclei. This does two things: First, it

that when sensory input intensity becomes too great, the

Corticofugal signals are almost entirely inhibitory, so

mus, medulla, and spinal cord; they control the intensity

Signals

Somatic Sensations: I. General Organization, the Tactile and Position Senses

Chapter 47

597

Cortical Control of Sensory
Sensitivity—“Corticofugal”

In addition to somatosensory signals transmitted from
the periphery to the brain, corticofugal signals are trans-
mitted in the backward direction from the cerebral
cortex to the lower sensory relay stations of the thala-

of sensitivity of the sensory input.

corticofugal signals automatically decrease transmission

decreases lateral spread of the sensory signals into adja-

sensory system operating in a range of sensitivity that is
not so low that the signals are ineffectual nor so high
that the system is swamped beyond its capacity to dif-

Segmental Fields of Sensation—
The Dermatomes

skin called a dermatome

if there were distinct borders between the adjacent der-

exists from segment to segment.

embryologically from the lumbar and upper sacral seg-

determine the level in the spinal cord at which a cord
injury has occurred when the peripheral sensations are

References

spective on the neural basis of multisensory spatial repre-

2003.

type of pain in most of these areas.

other deep tissues are only sparsely supplied with pain endings; nevertheless,

in the cranial vault. Most

joint surfaces,

rial walls,

as well as in certain internal tissues, such as the

tissues are all free nerve endings. They are widespread in the superficial layers

The pain receptors in the skin and other

Pain Receptors Are Free Nerve Endings.

ing. It can occur both in the skin and in almost any deep tissue or organ.

It can lead to prolonged, unbearable suffer-

This type of pain is usually

chronic pain.

throbbing pain, nauseous pain,

slow burning pain, aching pain,

Slow pain also goes by many names, such as

pain is not felt in most deeper tissues of the body.

burned. It is also felt when the skin is subjected to electric shock. Fast-sharp

is stuck into the skin, when the skin is cut with a knife, or when the skin is acutely

This type of pain is felt when a needle

pricking pain, acute pain,

Fast pain is also described by many alternative names, such as

each of them has specific qualities.

and sometimes even minutes. During the course of this chapter, we shall see

is felt within about 0.1 second after a pain stimulus is applied, whereas slow pain

Fast pain

slow pain.

Types of Pain and Their Qualities—Fast Pain

skin at the areas of pressure.

fore, fails to shift. This soon results in total breakdown and desquamation of the

has lost the pain sense, as after spinal cord injury, fails to feel the pain and, there-

ischemia, the person normally shifts weight subconsciously. But a person who

the weight of the body. When the skin becomes painful as a result of the

damaged, and it causes the individual to react to remove the pain stimulus. Even

these reasons, the first part of this chapter is devoted

knowledge of the different qualities of pain. For

eases depends to a great extent on a physician’s

Furthermore, the ability to diagnose different dis-

Many, if not most, ailments of the body cause pain.

Thermal Sensations

II. Pain, Headache, and

598

Somatic Sensations:

mainly to pain and to the physiologic bases of some
associated clinical phenomena.

Pain Is a Protective Mechanism.

Pain occurs whenever any tissues are being

such simple activities as sitting for a long time on the ischia can cause tissue
destruction because of lack of blood flow to the skin where it is compressed by

and Slow Pain

Pain has been classified into two major types: fast pain and

begins only after 1 second or more and then increases slowly over many seconds

that the conduction pathways for these two types of pain are different and that

sharp pain,

and electric pain.

and

associated with tissue destruction.

Pain Receptors and Their Stimulation

of the skin

periosteum, the arte-

the

and the falx and tentorium

any widespread tissue damage can summate to cause the slow-chronic-aching

release of chemical pain-inducing substances.

ischemia even greater, creating ideal conditions for the

olism in the muscle tissue, thus making the relative

ischemia. Also, the spasm increases the rate of metab-

stimulating mechanosensitive pain receptors, but it

clinical pain syndromes. This pain probably results

a common cause of pain, and it is the basis of many

stimulate the pain nerve endings.

of cell damage and that these, in addition to lactic acid,

proteolytic enzymes, are formed in the tissues because

ble that other chemical agents, such as bradykinin and

olism (metabolism without oxygen). It is also proba-

tissues, formed as a consequence of anaerobic metab-

remains zero.

absence of muscle exercise, the pain may not appear

can cause muscle pain within 15 to 20 seconds. In the

ceases, exercise of the forearm muscles sometimes

For instance, if a blood pressure cuff is placed around

lism of the tissue, the more rapidly the pain appears.

within a few minutes. The greater the rate of metabo-

tissue is blocked, the tissue often becomes very painful

When blood flow to a

Tissue Ischemia as a Cause of Pain.

permeable to ions.

causing pain following tissue damage. Also, the inten-

pain receptors can be found in these extracts. One

pain when injected beneath the normal skin. Most of

Special Importance of Chemical Pain Stimuli During Tissue

tusion, and so forth.

such as bacterial infection, tissue ischemia, tissue con-

The intensity of pain is also closely correlated with

rate at which damage to the tissues is occur-

level indefinitely. Therefore, it is immediately apparent

to be damaged by heat; indeed, the tissues are even-

This is also the temperature at which the tissues begin

skin is heated above 45°C, as shown in Figure 48–1.

The average person begins to perceive pain when the

Rate of Tissue Damage as a Stimulus

persists.

adapt, because it allows the pain to keep the person

continues. This increase in sensitivity of the pain recep-

slow-aching-nauseous pain, as the pain stimulus

becomes progressively greater, especially so for

under some conditions, excitation of pain fibers

adapt very little and sometimes not at all. In fact,

other sensory receptors of the body, pain receptors

occurs after tissue injury.

tant in stimulating the slow, suffering type of pain that

them. The chemical substances are especially impor-

proteolytic enzymes.

ions, acids, acetylcholine,

bradykinin, serotonin, histamine, potassium

types.

stimuli, whereas slow pain can be elicited by all three

In general, fast pain

chemical pain stimuli.

mechanical,

types of stimuli. They are classified as

Three Types of Stimuli Excite Pain Receptors—Mechanical,

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

599

Thermal, and Chemical.

Pain can be elicited by multiple

thermal, and
is elicited by the mechanical and thermal types of

Some of the chemicals that excite the chemical type

of pain are

and

addition, prostaglandins and substance P enhance the
sensitivity of pain endings but do not directly excite

Nonadapting Nature of Pain Receptors.

In contrast to most

tors is called hyperalgesia. One can readily understand
the importance of this failure of pain receptors to

apprised of a tissue-damaging stimulus as long as it

for Pain

tually destroyed if the temperature remains above this

that pain resulting from heat is closely correlated
with the
ring and not with the total damage that has already
occurred.

the rate of tissue damage from causes other than heat,

Damage.

Extracts from damaged tissue cause intense

the chemicals listed earlier that excite the chemical

chemical that seems to be more painful than others is
bradykinin. Many researchers have suggested that
bradykinin might be the agent most responsible for

sity of the pain felt correlates with the local increase
in potassium ion concentration or the increase in pro-
teolytic enzymes that directly attack the nerve endings
and excite pain by making the nerve membranes more

the upper arm and inflated until the arterial blood flow

for 3 to 4 minutes even though the muscle blood flow

One of the suggested causes of pain during ischemia

is accumulation of large amounts of lactic acid in the

Muscle Spasm as a Cause of Pain.

Muscle spasm is also

partially from the direct effect of muscle spasm in

might also result from the indirect effect of muscle
spasm to compress the blood vessels and cause

Temperature (

∞

Number of subjects

ified from Hardy DJ: Nature of pain. J Clin Epidemiol 4:22, 1956.)

showing the minimal skin temperature that will cause pain. (Mod-

Distribution curve obtained from a large number of persons

Figure 48–1

for tactile sensations, as was discussed in Chapter 47.

interruption, terminating in the

stem, but most pass all the way to the thalamus without

Termination of the Neospinothalamic Tract in the Brain

columns.

turn upward, passing to the brain in the anterolateral

the neospinothalamic tract. These give rise to long

Figure 48–2, and there excite second-order neurons of

(lamina marginalis) of the dorsal horns, as shown in

thermal pain. They terminate mainly in lamina I

The fast type A

Neospinothalamic Tract for Fast Pain.

pathways to the brain, through (1) the

On entering the spinal cord, the pain signals take two

Paleospinothalamic Tract

Neospinothalamic Tract and the

as shown in Figures 48–2 and 48–3.

dorsal horns. Here again, there are two systems for

roots, the pain fibers terminate on relay neurons in the

pain tends to become greater over time. This sensation

remove himself or herself from the stimulus. The slow

a damaging influence and, therefore, plays an impor-

pathway. The sharp pain apprises the person rapidly of

fiber pathway, followed a second or so

brain by the A

sensation: a fast-sharp pain that is transmitted to the

a sudden painful stimulus often gives a “double” pain

C fibers at velocities between 0.5 and 2 m/sec.

persisting mechanical or thermal stimuli. This slow-

fibers at velocities between 6 and 30 m/sec. Conversely,

peripheral nerves to the spinal cord by small type A

or thermal pain stimuli; they are transmitted in the

The fast-

pain pathway.

slow-chronic

fast-sharp pain pathway

ting pain signals into the central nervous system. The

Even though all pain receptors are free nerve endings,

Nervous System

Transmission of Pain

The Nervous System: A. General Principles and Sensory Physiology

600

Unit IX

Dual Pathways for

Signals into the Central

these endings use two separate pathways for transmit-

two pathways mainly correspond to the two types of
pain—a

and a

Peripheral Pain Fibers—“Fast” and “Slow” Fibers.

sharp pain signals are elicited by either mechanical

the slow-chronic type of pain is elicited mostly by
chemical types of pain stimuli but sometimes by

chronic pain is transmitted to the spinal cord by type

Because of this double system of pain innervation,

later by a slow pain that is transmitted by the C fiber

tant role in making the person react immediately to

eventually produces the intolerable suffering of long-
continued pain and makes the person keep trying to
relieve the cause of the pain.

On entering the spinal cord from the dorsal spinal

processing the pain signals on their way to the brain,

Dual Pain Pathways in the
Cord and Brain Stem—The

neospinothala-

mic tract and (2) the paleospinothalamic tract.

pain fibers transmit mainly mechanical and acute

fibers that cross immediately to the opposite side of
the cord through the anterior commissure and then

Stem and Thalamus.

A few fibers of the neospinothal-

amic tract terminate in the reticular areas of the brain

ventrobasal complex

along with the dorsal column–medial lemniscal tract

A few fibers also terminate in the posterior nuclear

Fast-sharp
pain fibers

Spinal
nerve

C A

Slow-chronic

pain fibers

Anterolateral

pathway

IX VIII

VII

III

Substantia

gelatinosa

Lamina

marginalis

Tract of

Lissauer

into and through the spinal cord on their way to the brain.

Transmission of both “fast-sharp” and “slow-chronic” pain signals

Figure 48–2

To: Somatosensory areas

Ventrobasal

complex and

posterior

nuclear

group

"Fast" Pain

Fibers

"Slow" Pain

Fibers

Thalamus

Reticular

formation

Intralaminar
nuclei

Pain tracts

slow burning pain pathway.

cerebral cortex by way of the

Transmission of pain signals into the brain stem, thalamus, and

Figure 48–3

fast pricking pain pathway and the

he or she is in severe pain.

system,” which is discussed in Chapter 59.This explains

areas constitute part of the brain’s principal “arousal

activity throughout the entire brain. In fact, these two

pain terminates, has a strong arousal effect on nervous

, the areas where the slow-suffering type of

Excitability.

centers.

role in interpreting pain quality, even though pain per-

3 per cent of the points stimulated. However, it is

nothing to do with normal pain appreciation; electri-

pain. This does not mean that the cerebral cortex has

stem reticular formation, the thalamus, and other

fore, it is likely that pain impulses entering the brain

not destroy an animal’s ability to perceive pain. There-

nectivity of this pathway. It explains why patients often

This is in keeping with the multisynaptic, diffuse con-

arm or leg but not to a specific point on the arm or leg.

ized only to a major part of the body, such as to one

For instance, slow-chronic pain can usually be local-

ted by way of the paleospinothalamic pathway is poor.

Slow-Chronic Pathway.

ize Precisely the Source of Pain Transmitted in the

Very Poor Capability of the Nervous System to Local-

matized. From the brain stem pain areas, multiple

of Sylvius. These lower regions of the brain appear to

periaqueductal gray region

deep to the superior and inferior colliculi; or (3) the

cephalon; (2) the

of the medulla, pons, and mesen-

mus. Instead, most terminate in one of three areas: (1)

shaded area shown in Figure 48–3. Only one tenth to

terminates widely in the brain stem, in the large

The slow-chronic paleospinothalamic pathway

Chronic Pain Signals) into the Brain Stem and Thala-

Projection of the Paleospinothalamic Pathway (Slow-

nervous system, and substance P is concerned with

tion. Regardless of the yet unknown details, it seems

transmitter gives a faster pain sensation, whereas the

“double” pain sensation one feels after a pinprick

even minutes. In fact, it has been suggested that the

onds. Substance P is released much more slowly, build-

substance P transmitter. The glutamate transmitter

mitter of Type C Nerve Endings.

Substance P, the Probable Slow-Chronic Neurotrans-

eral pathway.

of the cord, then upward to the brain in the anterolat-

join the fibers from the fast pain pathway, passing first

mainly lamina V, also in the dorsal horn. Here the last

fiber in Figure 48–2. Most of the signals then pass

horns, which together are called the

pathway, the peripheral fibers terminate in the spinal

fibers as well. In this

mit some signals from type A

slow-chronic type C pain fibers, although it does trans-

The paleospinothalamic pathway is a much older

Paleospinothalamic Pathway for Transmitting Slow-Chronic

action lasting for only a few milliseconds.

central nervous system, usually having a duration of

pain nerve fiber endings. This is one of

at the type A

Glutamate, the Probable Neurotransmitter of the Type

are simultaneously stimulated, the localization can be

or so of the stimulated area. Yet when tactile receptors

be poorly localized, often only within 10 centimeters

ous stimulation of tactile receptors, even fast pain may

pain receptors are stimulated, without the simultane-

body than can slow-chronic pain. However, when only

The fast-sharp type of pain can be local-

in the Body.

Capability of the Nervous System to Localize Fast Pain

group of the thalamus. From these thalamic areas, the

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

601

signals are transmitted to other basal areas of the brain
as well as to the somatosensory cortex.

ized much more exactly in the different parts of the

that excite the dorsal column–medial lemniscal system

nearly exact.

Fast Pain Fibers.

It is believed that glutamate is the

neurotransmitter substance secreted in the spinal cord

the most widely used excitatory transmitters in the

Pain.

system and transmits pain mainly from the peripheral

cord almost entirely in laminae II and III of the dorsal

substantia gelati-

nosa, as shown by the lateral most dorsal root type C

through one or more additional short fiber neurons
within the dorsal horns themselves before entering

neurons in the series give rise to long axons that mostly

through the anterior commissure to the opposite side

Research experi-

ments suggest that type C pain fiber terminals entering
the spinal cord secrete both glutamate transmitter and

acts instantaneously and lasts for only a few millisec-

ing up in concentration over a period of seconds or

might result partly from the fact that the glutamate

substance P transmitter gives a more lagging sensa-

clear that glutamate is the neurotransmitter most
involved in transmitting fast pain into the central

slow-chronic pain.

mus.

one fourth of the fibers pass all the way to the thala-

the reticular nuclei

tectal area of the mesencephalon

surrounding the aqueduct

be important for feeling the suffering types of pain,
because animals whose brains have been sectioned
above the mesencephalon to block pain signals from
reaching the cerebrum still evince undeniable evi-
dence of suffering when any part of the body is trau-

short-fiber neurons relay the pain signals upward into
the intralaminar and ventrolateral nuclei of the thala-
mus and into certain portions of the hypothalamus and
other basal regions of the brain.

Localization of pain transmit-

have serious difficulty in localizing the source of some
chronic types of pain.

Function of the Reticular Formation, Thalamus, and Cerebral
Cortex in the Appreciation of Pain.

Complete removal of

the somatic sensory areas of the cerebral cortex does

lower brain centers cause conscious perception of

cal stimulation of cortical somatosensory areas does
cause a human being to perceive mild pain from about

believed that the cortex plays an especially important

ception might be principally the function of lower

Special Capability of Pain Signals to Arouse Overall Brain

Electrical stimulation in the reticular areas

of the brain stem and in the intralaminar nuclei of the
thalamus

why it is almost impossible for a person to sleep when

pain signals, especially withdrawal reflexes described

the initial entry point to the spinal cord. In fact, it can

Thus, the analgesia system can block pain signals at

where they synapse in the dorsal horns.

of incoming type C and type A

to secrete enkephalin as well. The enkephalin is

their endings. The serotonin causes local cord neurons

Fibers originating in this area send signals to the

shown in Figure 48–4, the endings of many fibers in

gray area secrete enkephalin at their endings. Thus, as

analgesia system; especially involved are

thalamus.

medial forebrain bundle,

extent, the

adjacent to the third ventricle, and (2) to a lesser

periventricular nuclei in the hypothalamus,

area can also suppress pain. Some of these areas are

spinal roots. Also, stimulation of areas at still higher

At this point, the

the dorsal horns of the spinal cord.

the medulla. From these nuclei, second-order signals

reticularis paragigantocellularis,

the lower pons and upper medulla, and the

magnus nucleus,

Sylvius and portions of the third and fourth ventricles.

ductal gray

consists of three major components: (1) The

The analgesia system is shown in Figure 48–4. It

tremendously. This results partly from a capability of

The degree to which a person reacts to pain varies

Brain and Spinal Cord

Pain Suppression

pain, an important protective mechanism.

pain while leaving intact one’s appreciation of “acute”

in the thalamus, which often relieves suffering types of

pathways in the dorsolateral cord). Another experi-

that normally are too weak to be effectual (e.g., sparse

later, partly as a result of sensitization of other pathways

fibers. Second, pain frequently returns several months

the brain, so that the cordotomy does not transect these

relieving pain, for two reasons. First, many pain fibers

A cordotomy, however, is not always successful in

to interrupt the anterolateral sensory pathway.

to a few months. To do this, the spinal cord on the side

cordotomy

part of the body, a

at any one of several points. If the pain is in the lower

pain. To do this, the pain nervous pathways can be cut

rapidly spreading cancer), it is necessary to relieve the

When a person has

The Nervous System: A. General Principles and Sensory Physiology

602

Unit IX

Surgical Interruption of Pain Pathways.

severe and intractable pain (sometimes resulting from

in the thoracic region of

the spinal cord often relieves the pain for a few weeks

opposite to the pain is partially cut in its anterolateral
quadrant

from the upper part of the body do not cross to the
opposite side of the spinal cord until they have reached

mental operative procedure to relieve pain has been to
cauterize specific pain areas in the intralaminar nuclei

(“Analgesia”) System in the

the brain itself to suppress input of pain signals to the
nervous system by activating a pain control system,
called an analgesia system.

periaque-

and periventricular areas of the mesen-

cephalon and upper pons surround the aqueduct of

Neurons from these areas send signals to (2) the raphe

a thin midline nucleus located in

nucleus

located laterally in

are transmitted down the dorsolateral columns in the
spinal cord to (3) a pain inhibitory complex located in

analgesia signals can block the pain before it is relayed
to the brain.

Electrical stimulation either in the periaqueductal

gray area or in the raphe magnus nucleus can suppress
many strong pain signals entering by way of the dorsal

levels of the brain that excite the periaqueductal gray

(1) the

lying

also in the hypo-

Several transmitter substances are involved in the

enkephalin

and serotonin. Many nerve fibers derived from the

periventricular nuclei and from the periaqueductal

the raphe magnus nucleus release enkephalin when
stimulated.

dorsal horns of the spinal cord to secrete serotonin at

believed to cause both presynaptic and postsynaptic
inhibition

d pain fibers

also block many local cord reflexes that result from

in Chapter 54.

Brain’s Opiate System—Endorphins
and Enkephalins

More than 35 years ago it was discovered that injec-
tion of minute quantities of morphine either into the
periventricular nucleus around the third ventricle or

Periventricular
nuclei

Periaqueductal gray

Mesencephalon

Enkephalin neurons

Pons

Raphe magnus
nucleus

Medulla

Serotonergic
neurons

Enkephalin
neurons

Anterolateral
somatosensory tract

Aqueduct

Fourth

ventricle

Third

ventricle

Pain fibers

Presynaptic

pain inhibition

both the cord and the brain stem.

that suppress pain signals in

enkephalin-secreting neurons

bition of incoming pain signals at the cord level and (2) presence

Analgesia system of the brain and spinal cord, showing (1) inhi-

Figure 48–4

causes pain that can be severe. For instance, ischemia

cant pain. Conversely, any stimulus that causes

pain. For instance, a surgeon can cut the gut entirely in

differs from surface pain in several important aspects.

modalities of sensation besides pain. Also, visceral pain

Often, the viscera have sensory receptors for no other

ceral infectious disease, and other visceral ailments.

can be used for diagnosing visceral inflammation, vis-

In clinical diagnosis, pain from the different viscera of

Visceral Pain

sensations originate in the skin itself.

from the skin, and the person has the feeling that the

skin. When the visceral pain fibers are stimulated, pain

figure, branches of visceral pain fibers are shown to

able mechanism by which most pain is referred. In the

Figure 48–5 shows the prob-

surface. Knowledge of the different types of referred

For instance, pain in one of the

fairly remote from the tissue causing the pain. This is

Referred Pain

been reported in some instances. Also, pain relief has

control the degree of stimulation. Dramatic relief has

the diencephalon. The patient can then personally

In some patients, electrodes have been placed stereo-

to stimulate the dorsal sensory columns.

on occasion, implanted over the spinal cord, supposedly

electrodes are placed on selected areas of the skin or,

suppressing pain by electrical stimulation. Stimulating

Treatment of Pain by Electrical

acupuncture.

This mechanism and the simultaneous psychogenic

iments are often useful for pain relief.

in relieving pain. And it probably also explains why lin-

the spinal cord. It explains why such simple maneuvers

This presumably results from local lateral inhibition in

the discovery that stimulation of large type A

by Simultaneous Tactile

Inhibition of Pain Transmission

peripheral nerves.

by morphine-like drugs, can almost totally

pathways

gray and periventricular areas, or

Thus, although the fine details of the brain’s opiate

but in much lower quantities.

is found mainly in the same areas as the enkephalins,

the hypothalamus and the pituitary gland. Dynorphin

described earlier, and

and spinal cord, in the portions of the analgesia system

The two enkephalins are found in the brain stem

-endorphin, met-enkephalin, leu-

opiomelanocortin, proenkephalin,

ent points of the nervous system; all are breakdown

for the natural opiate of the brain. About a dozen such

brain. Therefore, an extensive search was undertaken

assumed that the “morphine receptors” of the analge-

of neurons do so by acting on synaptic receptors, it was

spinal cord. Because most drugs that alter excitability

analgesia system, including the dorsal horns of the

mainly the opiates, also act at many other points in the

studies, it has been found that morphine-like agents,

causes an extreme degree of analgesia. In subsequent

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

603

into the periaqueductal gray area of the brain stem

sia system must be receptors for some morphine-like
neurotransmitter that is naturally secreted in the

opiate-like substances have now been found at differ-

products of three large protein molecules: pro-

and prodynorphin.

Among the more important of these opiate-like
substances are

enkephalin, and dynorphin.

b-endorphin is present in both

system are not understood, activation of the analgesia
system by nervous signals entering the periaqueductal

inactivation of pain

suppress many pain signals entering through the

Sensory Signals

Another important event in the saga of pain control was

b sensory

fibers from peripheral tactile receptors can depress
transmission of pain signals from the same body area.

as rubbing the skin near painful areas is often effective

excitation of the central analgesia system are probably
also the basis of pain relief by

Stimulation

Several clinical procedures have been developed for

taxically in appropriate intralaminar nuclei of the thal-
amus or in the periventricular or periaqueductal area of

been reported to last for as long as 24 hours after only
a few minutes of stimulation.

Often a person feels pain in a part of the body that is

called referred pain.
visceral organs often is referred to an area on the body

pain is important in clinical diagnosis because in many
visceral ailments the only clinical sign is referred pain.

Mechanism of Referred Pain.

synapse in the spinal cord on the same second-order
neurons (1 and 2) that receive pain signals from the

signals from the viscera are conducted through at least
some of the same neurons that conduct pain signals

the abdomen and chest is one of the few criteria that

One of the most important differences between

surface pain and visceral pain is that highly localized
types of damage to the viscera seldom cause severe

two in a patient who is awake without causing signifi-

diffuse

stimulation of pain nerve endings throughout a viscus

Skin nerve

fibers

Visceral

nerve fibers

Mechanism of referred pain and referred hyperalgesia.

Figure 48–5

Parietal Pathway for Transmission of Abdominal and Thoracic

organs, representing in general the areas in the embryo

racic segments. Figure 48–6 shows several other surface

gastrium above the umbilicus, which is the surface area

Therefore, stomach pain is referred to the anterior epi-

seventh to ninth thoracic segments of the embryo.

The stomach originated approximately from the

quently, the pain is on the left side rather than on the

nerve fibers into the C-3 to T-5 cord segments. Most fre-

sternal area of the upper chest. These are the areas of

the pectoral muscles, down the arm, and into the sub-

referred to the side of the neck, over the shoulder, over

fore, as shown in Figure 48–6, pain from the heart is

the spinal cord between segments C-3 and T-5. There-

upper thorax, so that the heart’s visceral pain fibers pass

lies. For instance, the heart originated in the neck and

embryo, not necessarily where the visceral organ now

When visceral pain is referred to the surface of the body,

Localization of Referred Pain Transmitted via Visceral Pathways.

directly over the painful area.

localized

cardium, and these sensations are usually

nerves from the parietal peritoneum, pleura, or peri-

often far from the painful organ. Conversely, parietal

sensory fibers within the autonomic nerve bundles, and

True visceral pain is transmitted via pain

etal pathway.

true visceral pathway

Second, sensations from the abdomen and thorax are

originates internally can be localized only generally.

different internal organs exist; therefore, any pain that

localize, for a number of reasons. First, the patient’s

Transmission Pathways

Pain

and the

Visceral

Localization of Visceral Pain

wall is not very painful, if painful at all.

peritoneum is very painful, whereas a similar

pain and true visceral pain: a knife incision through the

etal wall overlying a viscus is frequently sharp. An

peripheral spinal nerves. Therefore, pain from the pari-

pericardium. These parietal surfaces, like the skin, are

often spreads to the parietal peritoneum, pleura, or

When a disease affects a viscus, the disease process

Visceral Disease

Caused by

bronchi

sitive, both the

to pain. In the lungs, even though the alveoli are insen-

trauma and stretch, and the

parenchyma of the liver and the alveoli of the lungs. Yet

pletely insensitive to pain of any type. These include the

encircle the viscus or that pass into its wall, thus perhaps

because of overstretch of the tissues themselves.

hollow viscus also can result in pain, presumably

turition, gallbladder disease, or ureteral obstruction.

dicitis, gastroenteritis, constipation, menstruation, par-

The cramping type of pain frequently occurs in appen-

along an overly excitable spastic gut, a cramp occurs.

muscle. For instance, each time a peristaltic wave travels

cycles result from periods of contraction of smooth

intermittently, once every few minutes. The intermittent

severity and then subsiding. This process continues

cramps,

muscle’s increased metabolic need for nutrients, thus

diminished blood flow to the muscle, combined with the

tion of the pain nerve endings. Or the spasm might cause

viscus can cause pain, possibly by mechanical stimula-

the gallbladder, a bile duct, a ureter, or any other hollow

usually excruciatingly severe.

thus stimulating broad areas of pain fibers. The pain is

through a ruptured gastric or duodenal ulcer. This juice

For instance, proteolytic acidic gastric juice often leaks

from the gastrointestinal tract into the peritoneal cavity.

On occasion, damaging substances leak

enzymes, or others that stimulate pain nerve endings.

degenerative products such as bradykinin, proteolytic

that it does in other tissues, presumably because of the

mitted through small type C pain fibers and, therefore,

within the viscus. Essentially all visceral pain that orig-

of a hollow viscus, excess distention of a hollow viscus,

the surfaces of the viscera, spasm of the smooth muscle

include ischemia of visceral tissue, chemical damage to

areas of the viscera can cause visceral pain. Such stimuli

Causes of True Visceral Pain

The Nervous System: A. General Principles and Sensory Physiology

604

Unit IX

caused by occluding the blood supply to a large area of
gut stimulates many diffuse pain fibers at the same time
and can result in extreme pain.

Any stimulus that excites pain nerve endings in diffuse

and stretching of the connective tissue surrounding or

inates in the thoracic and abdominal cavities is trans-

can transmit only the chronic-aching-suffering type of
pain.

Ischemia.

Ischemia causes visceral pain in the same way

formation of acidic metabolic end products or tissue-

Chemical Stimuli.

causes widespread digestion of the visceral peritoneum,

Spasm of a Hollow Viscus.

Spasm of a portion of the gut,

causing severe pain.

Often pain from a spastic viscus occurs in the form of

with the pain increasing to a high degree of

Overdistention of a Hollow Viscus.

Extreme overfilling of a

Overdistention can also collapse the blood vessels that

promoting ischemic pain.

Insensitive Viscera.

A few visceral areas are almost com-

the liver capsule is extremely sensitive to both direct

bile ducts are also sensitive

and the parietal pleura are very

sensitive to pain.

“Parietal Pain”

supplied with extensive pain innervation from the

example can emphasize the difference between this

parietal
cut through the visceral peritoneum or through a gut

—

“

”

“Parietal”

Pain from the different viscera is frequently difficult to

brain does not know from firsthand experience that the

transmitted through two pathways to the central
nervous system—the

and the pari-

the sensations are referred to surface areas of the body

sensations are conducted directly into local spinal

the person generally localizes it in the dermatomal
segment from which the visceral organ originated in the

upward along the sympathetic sensory nerves and enter

the body surface that send their own somatosensory

right because the left side of the heart is much more fre-
quently involved in coronary disease than the right.

of the body subserved by the seventh through ninth tho-

areas to which visceral pain is referred from other

from which the respective organs originated.

Pain.

Pain from the viscera is frequently localized to two

surface areas of the body at the same time because of

sensitive area. The sensory portion of the fifth nerve is

The pain of tic douloureux can usually be blocked by

mandibular portion of the fifth nerve.

tonsil, it might set off a severe lancinating pain in the

patient swallows a bolus of food, as the food touches a

rather than a pain stimulus. For instance, when the

on the surface of the face, in the mouth, or inside the

few seconds at a time or may be almost continuous.

sudden electrical shocks, and it may appear for only a

). The pain feels like

(or part of the area) of the fifth or ninth nerves; this phe-

Tic Douloureux

and then crusts over within another few days, all of this

ronal peripheral axons to their cutaneous origins. Here

neuronal cytoplasmic flow outward through the neu-

virus. In addition to causing pain, the virus is carried by

The cause of the pain is presumably infection of the

eruption that often ensues.

or “shingles,” because of a skin

herpes zoster,

of pain that circles halfway around the body. The disease

served by the ganglion, thus eliciting a segmental type

This causes severe pain in the dermatomal segment sub-

sequent sections.

thalamus. Several of these lesions are discussed in sub-

prostaglandins, perhaps others. Secondary hyperalgesia

products from the burn—perhaps histamine, perhaps

sensitivity of sunburned skin, which results from

(2) facilitation of sensory transmission, which is called

themselves, which is called

means hypersensitivity to pain. Possible causes of hyper-

sively excitable; this gives rise to

Some Clinical Abnormalities

the abdominal wall. These cause pain of the sharp type

umbilicus and is of the aching, cramping type. Pain

T-10 or T-11; this pain is referred to an area around the

nerve bundles, and then into the spinal cord at about

appendix. Pain impulses pass first from the appendix

Figure 48–7 shows dual transmission from an inflamed

ceral pathway and the direct parietal pathway. Thus,

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

605

the dual transmission of pain through the referred vis-

through visceral pain fibers located within sympathetic

impulses also often originate in the parietal peritoneum
where the inflamed appendix touches or is adherent to

directly over the irritated peritoneum in the right lower
quadrant of the abdomen.

of Pain and Other
Somatic Sensations

Hyperalgesia

A pain nervous pathway sometimes becomes exces-

hyperalgesia, which

algesia are (1) excessive sensitivity of the pain receptors

primary hyperalgesia, and

secondary hyperalgesia.

An example of primary hyperalgesia is the extreme

sensitization of the skin pain endings by local tissue

frequently results from lesions in the spinal cord or the

Herpes Zoster (Shingles)

Occasionally herpesvirus infects a dorsal root ganglion.

is called

pain neuronal cells in the dorsal root ganglion by the

the virus causes a rash that vesiculates within a few days

occurring within the dermatomal area served by the
infected dorsal root.

Lancinating pain occasionally occurs in some people
over one side of the face in the sensory distribution area

nomenon is called tic douloureux (or trigeminal neural-
gia or glossopharyngeal neuralgia

Often it is set off by exceedingly sensitive trigger areas

throat—almost always by a mechanoreceptive stimulus

surgically cutting the peripheral nerve from the hyper-

H e a r t

E s o p h a g u s

Liver and
gallbladder

Stomach

Pylorus

Umbilicus

Appendix and
small intestine

Right kidney

Left kidney

Colon

Ureter

Surface areas of referred pain from different visceral organs.

Figure 48–6

Visceral pain

T-10

L-1

Parietal pain

Visceral and parietal transmission of pain signals from the

Figure 48–7

appendix.

cause “occipital headache” referred to the posterior

and slightly below the ear. Subtentorial pain stimuli

cal nerves, which also supply the scalp above, behind,

through the glossopharyngeal, vagal, and second cervi-

Conversely,

pain impulses from beneath the

cranial nerve, as shown in Figure 48–9.

portion of the fifth nerve and, therefore, causes referred

tentorium itself, initiates pain impulses in the cerebral

above the tentorium, including the upper surface of the

cially sensitive structure is the middle meningeal artery,

can cause headache. An espe-

tizing, crushing, or stretching stimulus to the

ognized as headache. Also, almost any type of trauma-

stretching the dura at

brain, damaging the tentorium,

Conversely,

damage within the brain itself.

sensory cortex stimulated. Therefore, it is likely that

instead, it causes prickly types of paresthesias on the

themselves are almost totally insensitive to pain. Even

The brain tissues

Pain-Sensitive Areas in Cranial Vault.

as from the nasal sinuses.

others result from pain arising outside the cranium, such

result from pain stimuli arising inside the cranium, but

the head from deep head structures. Some headaches

localized, still persists because of partial transmission in

medulla of the brain. “Crude touch,” which is poorly

column, is transected. That is, the fibers in this column

pathway for the transmission of light touch, the dorsal

of the transection. Discrete “light touch” is impaired

ization, and two-point discrimination—are lost

position sensations, vibration sensation, discrete local-

By contrast, the sensations that are transmitted only in

opposite side. The sensations of pain, heat, and cold—

lost on the transected side, and others are lost on the

tion. Yet only some of the modalities of sensation are

knowledge of the cord fiber tracts shown in Figure 48–8.

occurs. The

Brown-Séquard syndrome

one side, the

are blocked, but if the spinal cord is transected on only

If the spinal cord is transected entirely, all sensations

and not in the peripheral nerves.

unsuccessful, indicating that the lesion that causes the

be annoying. Furthermore, sometimes the operation is

leaves the side of the face anesthetic, which in itself may

the sensory elements are destroyed. This operation

needed for many jaw movements, can be spared while

from each other, so that the motor portions, which are

often sectioned immediately inside the cranium, where

The Nervous System: A. General Principles and Sensory Physiology

606

Unit IX

the motor and sensory roots of the fifth nerve separate

pain might be in the sensory nucleus in the brain stem

Brown-Séquard Syndrome

and motor functions distal to the segment of transection

effects of such transection can be predicted from a

All motor functions are blocked on the side of the tran-
section in all segments below the level of the transec-

sensations served by the spinothalamic pathway—are
lost on the opposite side of the body in all dermatomes
two to six segments below the level of the transection.

the dorsal and dorsolateral columns—kinesthetic and

on the

side of the transection in all dermatomes below the level

on the side of the transection because the principal

do not cross to the opposite side until they reach the

the opposite spinothalamic tract.

Headache

Headaches are a type of pain referred to the surface of

Headache of Intracranial Origin

cutting or electrically stimulating the sensory areas
of the cerebral cortex only occasionally causes pain;

area of the body represented by the portion of the

much or most of the pain of headache is not caused by

tugging on the venous sinuses around the

the base of the brain can cause intense pain that is rec-

blood

vessels of the meninges

and neurosurgeons are careful to anesthetize this artery
specifically when performing brain operations under
local anesthesia.

Areas of the Head to Which Intracranial Headache Is Referred.

Stimulation of pain receptors in the cerebral vault

headache to the front half of the head in the surface
areas supplied by this somatosensory portion of the fifth

tentorium enter the central nervous system mainly

part of the head.

Lateral

corticospinal

Rubrospinal

Olivospinal

Tectospinal

Vestibulospinal

Descending

Tracts

Ventral

corticospinal

Fasciculus cuneatus

Fasciculus gracilis

Dorsal
spinocerebellar

Ventral
spinocerebellar

Ventral spinothalamic

Spinotectal

Ascending

Tracts

Lateral
spinothalamic

Cross section of the spinal cord, showing principal ascending

Figure 48–8

tracts on the right and principal descending tracts on the left.

Brain stem and
cerebellar vault
headaches

Cerebral vault

headaches

Nasal sinus

and eye

headaches

Areas of headache resulting from different causes.

Figure 48–9

spots.

The cold and warmth receptors are located imme-

“burning hot” sensations.

cold and warmth receptors, for “freezing cold” and

cold and, therefore, are responsible, along with the

warmth receptors, and pain receptors. The pain recep-

three types of sensory receptors: cold receptors,

Thermal gradations are discriminated by at least

cold and heat, from

The human being can perceive different gradations of

Thermal Sensations

cause of the headache.

retina can also burn the retina, and this could be the

the surface of the head or retro-orbitally. However,

irritation of the conjunctivae, and the pain is referred to

hours. The headache sometimes results from “actinic”

tion by light rays, especially ultraviolet light. Looking at

headache.

and extraocular muscles, which is a possible cause of

orbital headache. Also, excessive attempts to focus

Even though these muscles are extremely small, it is

one’s eyes clearly may cause excessive contraction of

illary sinuses, can be felt in the face.

Also, pain from the lower sinuses, such as from the max-

faces of the forehead and scalp, as shown in Figure 48–9.

in the case of frontal sinus infection, to the frontal sur-

and cause headache that is referred behind the eyes or,

Nevertheless, infection or other irritative processes in

nasal sinuses are sensitive to pain, but not intensely so.

The mucous membranes of the nose and

Headache Caused by Irritation of Nasal and Accessory Nasal

lesions do.

headache. The pain of the spastic head muscles suppos-

muscles attached to the occiput, to become spastic, and

often causes many of the muscles of the head, especially

Extracranial Types of Headache

impulses from the colon. Therefore, it may result from

pain sensory tracts in the spinal cord have been cut, we

headache in many people. Because it has been shown

is most likely that alcohol, because it is toxic to tissues,

enced, a headache usually follows an alcoholic binge. It

migraine has been reported in 65 to 90 per cent of cases.

headaches, because a positive family history for

There may be a genetic predisposition to migraine

pression, psychological abnormalities, and vasospasm

of migraine headaches. Other theories of the cause

of the arteries—including some extracranial arteries,

vessels causes them to dilate and pulsate intensely, and

lar tone for 24 to 48 hours. The blood pressure in the

smooth muscle contraction, to allow the blood vessels

happens to the vascular walls, perhaps exhaustion of

Then, as a result of the intense ischemia, something

and this is responsible for the prodromal symptoms.

including arteries that supply the brain. The vasospasm

symptoms.

beginning of the headache. Any theory that explains

of sensory hallucinations. Ordinarily, the prodromal

part of the field of vision, visual aura, and other types

prodromal sensations, such as nausea, loss of vision in

unknown. Migraine headaches often begin with various

cular phenomena, although the exact mechanism is

headache.

by the cerebrospinal fluid. The weight of the brain

headache. Removing this quantity of fluid removes part

upright position, often causes intense intracranial

spinal canal, particularly if the person remains in an

around the venous sinuses. Such intense damage can

causes inflammation of all the meninges, including the

headaches of all is that resulting from meningitis, which

Types of Intracranial Headache

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

607

Headache of Meningitis.

One of the most severe

sensitive areas of the dura and the sensitive areas

cause extreme headache pain referred over the entire
head.

Headache Caused by Low Cerebrospinal Fluid Pressure.

Removing as little as 20 milliliters of fluid from the

of the flotation for the brain that is normally provided

stretches and otherwise distorts the various dural
surfaces and thereby elicits the pain that causes the

Migraine Headache.

Migraine headache is a special type

of headache that is thought to result from abnormal vas-

symptoms begin 30 minutes to 1 hour before the

migraine headache must also explain the prodromal

One of the theories of the cause of migraine

headaches is that prolonged emotion or tension causes
reflex vasospasm of some of the arteries of the head,

theoretically produces ischemia of portions of the brain,

to become flaccid and incapable of maintaining vascu-

it is postulated that the excessive stretching of the walls

such as the temporal artery—causes the actual pain

of migraine headaches include spreading cortical de-

caused by excess local potassium in the cerebral extra-
cellular fluid.

Migraine headaches also occur about twice as fre-
quently in women as in men.

Alcoholic Headache.

As many people have experi-

directly irritates the meninges and causes the intracra-
nial pain.

Headache Caused by Constipation.

Constipation causes

that constipation headache can occur in people whose

know that this headache is not caused by nervous

absorbed toxic products or from changes in the circula-
tory system resulting from loss of fluid into the gut.

Headache Resulting from Muscle Spasm.

Emotional tension

those muscles attached to the scalp and the neck

it is postulated that this is one of the common causes of

edly is referred to the overlying areas of the head and
gives one the same type of headache as intracranial

Structures.

widespread areas of the nasal structures often summate

Headache Caused by Eye Disorders.

Difficulty in focusing

the eye ciliary muscles in an attempt to gain clear vision.

believed that tonic contraction of them can cause retro-

the eyes can result in reflex spasm in various facial

A second type of headache that originates in the eyes

occurs when the eyes are exposed to excessive irradia-

the sun or the arc of an arc-welder for even a few
seconds may result in headache that lasts from 24 to 48

focusing intense light from an arc or the sun on the

Thermal Receptors and
Their Excitation

freezing cold to cold to cool to

indifferent to warm to hot to burning hot.

tors are stimulated only by extreme degrees of heat or

diately under the skin at discrete separated

heated room to the out-of-doors on a cold day.

temperature if it were constant. The response to

versely, if the temperature is actively rising, the person

the temperature remains cold at the same level. Con-

actively falling, a person feels much colder than when

This means that when the temperature of the skin is

being able to respond to steady states of temperature.

changes in temperature,

Thus, it is evident that the thermal senses respond

extent, but never 100 per cent.

more. In other words, the receptor “adapts” to a great

becomes strongly stimulated at first, but this stimula-

denly subjected to an abrupt fall in temperature, it

When a cold receptor is sud-

Stimulatory Effects of Rising and Falling Temperature—Adap-

feel almost alike.

tion—that is, freezing cold and burning hot sensations

enough, may give almost the same quality of sensa-

painful and why both these sensations, when intense

different types of endings. One can also understand

One can understand from Figure 48–10 that a

ulated again, possibly because of damage to the cold

paradoxically, some of the cold fibers begin to be stim-

also fade out at about 49°C. Finally, at around 45°C,

warmth receptors begin to be stimulated, but these

out slightly above 40°C. Above about 30°C, the

cease, but the cold receptors begin to be stimulated,

perature rises to +10° to 15°C, the cold-pain impulses

freeze, these fibers cannot be stimulated). As the tem-

even colder, so that it nearly freezes or actually does

cold region, only

temperature. For instance, in the

pain fiber stimulated by heat. Note especially that

by cold, (2) a cold fiber, (3) a warmth fiber, and (4) a

four types of nerve fibers: (1) a pain fiber stimulated

Figure 48–10 shows the

Indifferent, Warm, and Hot.

as cold receptors.

transmitted in type C nerve fibers as well, which sug-

20 m/sec. Some cold sensations are believed to be

tors via type A

dermal cells. Signals are transmitted from these recep-

ending that branches a number of times, the tips of

tified. It is a special, small type A

Conversely, a definitive cold receptor has been iden-

at transmission velocities of only 0.4 to 2 m/sec.

presumed to be free nerve endings, because warmth

they have not been identified histologically. They are

endings is quite certain, based on psychological tests,

cold spots as warmth spots, and the number in differ-

most areas of the body, there are 3 to 10 times as many

The Nervous System: A. General Principles and Sensory Physiology

608

Unit IX

ent areas of the body varies from 15 to 25 cold spots
per square centimeter in the lips to 3 to 5 cold spots
per square centimeter in the finger to less than 1 cold
spot per square centimeter in some broad surface
areas of the trunk.

Although the existence of distinctive warmth nerve

signals are transmitted mainly over type C nerve fibers

d myelinated nerve

which protrude into the bottom surfaces of basal epi-

d nerve fibers at velocities of about

gests that some free nerve endings also might function

Stimulation of Thermal Receptors—Sensations of Cold, Cool,

effects of different temperatures on the responses of

these fibers respond differently at different levels of

very

the cold-pain fibers are stimulated (if the skin becomes

reaching peak stimulation at about 24°C and fading

the heat-pain fibers begin to be stimulated by heat and,

endings caused by the excessive heat.

person determines the different gradations of thermal
sensations by the relative degrees of stimulation of the

why extreme degrees of both cold and heat can be

tation of Thermal Receptors.

tion fades rapidly during the first few seconds and pro-
gressively more slowly during the next 30 minutes or

markedly to

in addition to

feels much warmer than he or she would at the same

changes in temperature explains the extreme degree
of heat one feels on first entering a tub of hot water
and the extreme degree of cold felt on going from a

Warmth receptors

Cold

Cool

Warm

Hot

Freezing

cold

Indiffer-

ent

Burning

hot

Cold-pain

Heat-pain

Cold-receptors

Temperature (

∞

Impulses per second

heat-pain fiber.

fiber,

warmth

cold fiber,

cold-pain fiber,

tures of a

Discharge frequencies at different skin tempera-

Figure 48–10

and a

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tinguish gradations of temperature.

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complex. Occasionally a neuron in cortical somatic

the ventrobasal complex of the thalamus.

enter long, ascending thermal fibers that cross to the

processing by one or more cord neurons, the signals

horns—the same as for pain. After a small amount of

terminate mainly in laminae I, II, and III of the dorsal

spinal cord, the signals travel for a few segments

parallel to those for pain signals. On entering the

In general, thermal signals are transmitted in pathways

Transmission of Thermal Signals

1 square centimeter in size.

versely, temperature changes 100 times as great often

the entire surface of the body simultaneously. Con-

summate. For instance, rapid changes in temperature

all at once, the thermal signals from the entire area

lated. However, when a large skin area is stimulated

area of the body is slight, it is difficult to judge grada-

as modified by temperature.

words, thermal detection probably results not from

more than twofold for each 10°C change. In other

stimulated by changes in their metabolic rates, and

Thermal Receptors

Somatic Sensations: II. Pain, Headache, and Thermal Sensations

Chapter 48

609

Mechanism of Stimulation of

It is believed that the cold and warmth receptors are

that these changes result from the fact that tempera-
ture alters the rate of intracellular chemical reactions

direct physical effects of heat or cold on the nerve
endings but from chemical stimulation of the endings

Spatial Summation of Thermal Sensations.

Because the

number of cold or warm endings in any one surface

tions of temperature when small skin areas are stimu-

as little as 0.01°C can be detected if this change affects

will not be detected when the affected skin area is only

in the Nervous System

upward or downward in the tract of Lissauer and then

opposite anterolateral sensory tract and terminate in
both (1) the reticular areas of the brain stem and (2)

A few thermal signals are also relayed to the cere-

bral somatic sensory cortex from the ventrobasal

sensory area I has been found by microelectrode
studies to be directly responsive to either cold or warm

of the entire cortical postcentral gyrus in the human

being reduces but does not abolish the ability to dis-

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immune and glial cells contribute to pathological pain