Sensory Receptors and Neuronal Circuits for Processing Information pdf - The Nervous System

fact that we experience these different modalities of sensation, nerve fibers

of sensation. Yet despite the

sight, sound, and so forth—is called a

Each of the principal types of sensation that we can experience—pain, touch,

enough to damage the tissues.

of the body fluids but have never been known to respond to sound. Finally, pain

on the eyeballs, or chemical changes in the blood. The osmoreceptors of the

are almost completely nonresponsive to normal ranges of heat, cold, pressure

stimuli. Thus, the rods and cones of the eyes are highly responsive to light but

sitivities.” That is, each type of receptor is highly sensitive to one type of stimulus

tors detect different types of sensory stimuli? The answer is, by “differential sen-

The first question that must be answered is, how do two types of sensory recep-

Differential Sensitivity of Receptors

types of mechanoreceptors found in the skin or in deep tissues of the body.

tion to the sensory systems that they subserve. Figure 46–1 shows some of the

which receptors operate. Other receptors are discussed in other chapters in rela-

primarily peripheral mechanoreceptors, to illustrate some of the principles by

In this chapter, we discuss the function of a few specific types of receptors,

up the chemistry of the body.

body fluids, carbon dioxide concentration, and perhaps other factors that make

mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the

, which detect taste in the

chemoreceptors

on the retina of the eye; and (5)

, which detect light

cal damage or chemical damage; (4)

(pain receptors), which detect damage occurring in the tissues, whether physi-

temperature, some receptors detecting cold and others warmth; (3)

, which detect changes in

tissues adjacent to the receptor; (2)

, which detect mechanical compression or stretching of the receptor or of

mechanorecep-

shows that there are five basic types of sensory receptors: (1)

Table 46–1 lists and classifies most of the body’s sensory receptors. This table

Types of Sensory Receptors and the Sensory

sound, light, pain, cold, and warmth. The purpose of

Sensory Receptors, Neuronal

572

Circuits for Processing Information

Input to the nervous system is provided by sensory
receptors that detect such sensory stimuli as touch,

this chapter is to discuss the basic mechanisms by
which these receptors change sensory stimuli into
nerve signals that are then conveyed to and
processed in the central nervous system.

Stimuli They Detect

tors

thermoreceptors

nociceptors

electromagnetic receptors

for which it is designed and yet is almost nonresponsive to other types of sensory

supraoptic nuclei in the hypothalamus detect minute changes in the osmolality

receptors in the skin are almost never stimulated by usual touch or pressure
stimuli but do become highly active the moment tactile stimuli become severe

Modality of Sensation—The “Labeled Line” Principle

modality

tor, its immediate effect is to change the membrane

Whatever the type of stimulus that excites the recep-

Stimuli into Nerve Impulses

Transduction of Sensory

This specificity of nerve fibers for transmitting only

areas.

terminate in the vision areas of the brain, fibers from

the brain. Similarly, fibers from the retina of the eye

or in any other way, the person perceives touch

the person perceives pain. Likewise, if a touch fiber is

by damage to the tissue cells. In all these instances,

of the fiber, or stimulation of the pain nerve ending

can be electricity, overheating of the fiber, crushing

of what type of stimulus excites the fiber. The stimulus

is stimulated, the person perceives pain regardless

to which the fiber leads. For instance, if a pain fiber

specific point in the central nervous system, and

The answer is that each nerve tract terminates at a

transmit only impulses. Therefore, how is it that

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

573

different nerve fibers transmit different modalities of
sensation?

the type of sensation felt when a nerve fiber is stimu-
lated is determined by the point in the nervous system

stimulated by electrical excitation of a touch receptor

because touch fibers lead to specific touch areas in

the ear terminate in the auditory areas of the brain,
and temperature fibers terminate in the temperature

one modality of sensation is called the labeled line
principle.

Local Electrical Currents at Nerve
Endings—Receptor Potentials

All sensory receptors have one feature in common.

Free nerve

endings

Expanded tip

receptor

Tactile hair

Pacinian

corpuscle

Meissner’s

corpuscle

Krause’s

corpuscle

Ruffini’s

end-organ

Golgi tendon

apparatus

Muscle
spindle

Several types of somatic sensory nerve endings.

Figure 46–1

Table 46–1

Blood glucose, amino acids, fatty acids

Taste

V. Chemoreceptors

Vision

IV. Electromagnetic receptors

Free nerve endings

III. Nociceptors

Warm receptors

Warmth

II. Thermoreceptors

Vestibular receptors

Ruffini’s endings

Free nerve endings

Krause’s corpuscles

Meissner’s corpuscles

Ruffini’s endings

Merkel’s discs

Free nerve endings

I. Mechanoreceptors

Classification of Sensory Receptors

Skin tactile sensibilities (epidermis and dermis)

Expanded tip endings

Plus several other variants

Spray endings

Encapsulated endings

Hair end-organs

Deep tissue sensibilities

Expanded tip endings
Spray endings

Encapsulated endings

Pacinian corpuscles
Plus a few other variants

Muscle endings

Muscle spindles
Golgi tendon receptors

Hearing

Sound receptors of cochlea

Equilibrium

Arterial pressure

Baroreceptors of carotid sinuses and aorta

Cold

Cold receptors

Pain

Rods
Cones

Receptors of taste buds

Smell

Receptors of olfactory epithelium

Arterial oxygen

Receptors of aortic and carotid bodies

Osmolality

Neurons in or near supraoptic nuclei

Blood CO

Receptors in or on surface of medulla and in aortic and

carotid bodies

Receptors in hypothalamus

of a pacinian corpuscle. Note that the amplitude

mechanical compression (increasing “stimulus

Figure 46–4 shows the changing amplitude of the

izes the fiber membrane at this node, which then sets

the pacinian corpuscle, the local current flow depolar-

node of Ranvier, which itself lies inside the capsule of

arrows, that spreads along the nerve fiber. At the first

of current flow, shown by the

“receptor potential.” The receptor potential in turn

increased positivity inside the fiber, which is the

diffuse to the interior of the fiber. This creates

brane, allowing positively charged sodium ions to

has been deformed by compression of the corpuscle,

cle. Observe the small area of the terminal fiber that

The figure also shows the mechanism by which a

puscle to enter a peripheral sensory nerve.

central fiber inside the capsule is unmyelinated, but

layers but one have been removed. The tip of the

Now study Figure 46–3, which shows only the

indent, or otherwise deform the central fiber.

anywhere on the outside of the corpuscle will elongate,

tiple concentric capsule layers, so that compression

extending through its core. Surrounding this are mul-

46–1. Note that the corpuscle has a central nerve fiber

cal structure of the pacinian corpuscle shown in Figure

The student should at this point restudy the anatomi-

receptor potential rises above the threshold level, the

trated in Figure 46–2. Note also that the more the

to the receptor, then action potentials occur, as illus-

When

becomes maximally permeable to sodium ions.

high intensity of sensory stimulus. This is about the

100 millivolts, but this level occurs only at an extremely

The maximum

change in membrane permeability of the receptor,

types of known sensory receptors. In all instances, the

and allows ions to flow through membrane channels.

retinal visual receptor, which either directly or indi-

effects of electromagnetic radiation, such as light on a

alters the permeability of the membrane; or (4) by the

change of the temperature of the membrane, which

the membrane, which also opens ion channels; (3) by

opens ion channels; (2) by application of a chemical to

receptor, which stretches the receptor membrane and

tor potentials: (1) by mechanical deformation of the

electrical potential of the receptor. This change in

The Nervous System: A. General Principles and Sensory Physiology

574

Unit IX

potential is called a receptor potential.

Mechanisms of Receptor Potentials.

Different receptors

can be excited in one of several ways to cause recep-

rectly changes the receptor membrane characteristics

It will be recognized that these four means of exciting
receptors correspond in general with the different

basic cause of the change in membrane potential is a

which allows ions to diffuse more or less readily
through the membrane and thereby to change the
transmembrane potential.

Maximum Receptor Potential Amplitude.

amplitude of most sensory receptor potentials is about

same maximum voltage recorded in action potentials
and is also the change in voltage when the membrane

Relation of the Receptor Potential to Action Potentials.

the receptor potential rises above the threshold for
eliciting action potentials in the nerve fiber attached

greater becomes the action potential frequency.

Receptor Potential of the Pacinian Corpuscle—
An Example of Receptor Function

central fiber of the pacinian corpuscle after all capsule

the fiber does become myelinated (the blue sheath
shown in the figure) shortly before leaving the cor-

receptor potential is produced in the pacinian corpus-

and note that ion channels have opened in the mem-

induces a local circuit

off typical action potentials that are transmitted along
the nerve fiber toward the central nervous system.

Relation Between Stimulus Intensity and the Receptor Poten-
tial.

receptor potential caused by progressively stronger

strength”) applied experimentally to the central core

100 120 140

Membrane potential (millivolts)

Milliseconds

Resting membrane potential

Receptor potential

Threshold

Action potentials

when the receptor potential rises above threshold level.

Typical relation between receptor potential and action potentials

Figure 46–2

- - - -

+ + + + + + + + + + + + + + +

+ + + +

Deformed

area

Receptor potential

Action

potential

Node of
Ranvier

Excitation and inactivation in a receptor membrane. Ann N Y Acad

duced in a pacinian corpuscle. (Modified from Loëwenstein WR:

Excitation of a sensory nerve fiber by a receptor potential pro-

Figure 46–3

Sci 94:510, 1961.)

tial is no longer elicited. Thus, the receptor potential

the corpuscle redistributes, so that the receptor poten-

within a few hundredths of a second, the fluid within

fiber, thus eliciting a receptor potential. However,

applied to one side of the corpuscle, this force is

ways. First, the pacinian corpuscle is a viscoelastic

corpuscle. Adaptation occurs in this receptor in two

In the case of the mechanoreceptors, the receptor

instance, in the eye, the rods and cones adapt by chang-

receptor potential is an individual property. For

tor, in much the same way that development of a

The mechanism of

completely.

pain receptors, for instance—probably never adapt

tid and aortic baroreceptors. Conversely, some of

2 days, which is the adaptation time for many caro-

adapting” receptors. The longest measured time for

or days to do so, for which reason they are called “non-

ally adapt almost completely, but some require hours

mechanoreceptors

the hairs adapt to extinction within a second or more.

dredths of a second, and the receptors at the bases of

corpuscles adapt to “extinction” within a few hun-

greater extent than others. For example, the pacinian

Furthermore, some sensory receptors adapt to a far

muscle spindle receptors adapt slowly.

within a second or so, whereas some joint capsule and

types of receptors. Note that the pacinian corpuscle

Figure 46–5 shows typical adaptation of certain

continuous sensory stimulus is applied, the receptor

stant stimulus after a period of time. That is, when a

Adaptation of Receptors

weak to very intense.

tor to have an extreme range of response, from very

sensory experience is extreme. This allows the recep-

cable to almost all sensory receptors. It allows the

This is an exceedingly important principle that is appli-

additional increase in numbers of action potentials.

in Figure 46–4, one can see that very intense stimula-

potential. Putting this principle together with the data

In turn, the

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

575

increases rapidly at first but then progressively less
rapidly at high stimulus strength.

frequency of repetitive action potentials

transmitted from sensory receptors increases approx-
imately in proportion to the increase in receptor

tion of the receptor causes progressively less and less

receptor to be sensitive to very weak sensory experi-
ence and yet not reach a maximum firing rate until the

Another characteristic of all sensory receptors is that
they adapt either partially or completely to any con-

responds at a high impulse rate at first and then at a
progressively slower rate until finally the rate of action
potentials decreases to very few or often to none at all.

adapts extremely rapidly and hair receptors adapt

It is probable that all other

eventu-

complete adaptation of a mechanoreceptor is about

the nonmechanoreceptors—the chemoreceptors and

Mechanisms by Which Receptors Adapt.

receptor adaptation is different for each type of recep-

ing the concentrations of their light-sensitive chemi-
cals (which is discussed in Chapter 50).

that has been studied in greatest detail is the pacinian

structure so that when a distorting force is suddenly

instantly transmitted by the viscous component of the
corpuscle directly to the same side of the central nerve

100

Amplitude of observed

receptor potential (per cent)

Stimulus strength

(per cent)

100

Loëwenstein WR: Excitation and inactivation in a receptor mem-

mechanical stimulus applied to a pacinian corpuscle. (Data from

Relation of amplitude of receptor potential to strength of a

Figure 46–4

brane. Ann N Y Acad Sci 94:510, 1961.)

Impulses per second

Seconds

250

Joint capsule receptors

200

150

100

Muscle spindle

Hair receptor

Pacinian corpuscle

tion of some receptors and slow adaptation of others.

Adaptation of different types of receptors, showing rapid adapta-

Figure 46–5

fibers into two subgroups; yet

separate the type A

Alternative Classification Used by Sensory Physiologists.

slowly as 0.5 m/sec, requiring about 2 seconds to go from

Conversely, the smallest fibers transmit impulses as

mit impulses at velocities as great as 120 m/sec, a

46–6. Note that a few large myelinated fibers can trans-

the different nerve fiber types are also given in Figure

The sizes, velocities of conduction, and functions of

well as all the postganglionic autonomic fibers.

at low velocities. The C fibers constitute more than one

fibers of spinal nerves. Type C fibers are the

Type A fibers are the typical large and medium-sized

fibers.

, and

and C, and the type A fibers are further subdivided into

general classification, the fibers are divided into types A

fication” of the different types of nerve fibers. In the

is a “general classification” and a “sensory nerve classi-

Shown in Figure 46–6

General Classification of Nerve Fibers.

conducting velocities is between 0.5 and 120 m/sec.

ter, the greater the conducting velocity. The range of

Figure 46–6, nerve fibers come in all sizes between 0.5

that slowly conducting fibers will suffice. As shown in

aching pain, do not need to be transmitted rapidly, so

sensory information, such as that depicting prolonged,

during running. At the other extreme, some types of

the information would be useless. An example of this is

central nervous system extremely rapidly; otherwise,

Different Types of Signals,

Nerve Fibers That Transmit

tion so that the person will not fall. Loss of this pre-

tion of the next second. Therefore, appropriate motor

instance, when one is running, information from the

of movement of the different parts of the body. For

to keep from losing balance. Likewise, recep-

around a curve. Using this information, a person can

later. For instance, the receptors of the semicircular

status is taking place, one can predict in one’s mind the

body.

of rapid tissue deformations, but it is useless for trans-

released. In other words, the pacinian corpuscle is

later, it transmits a signal again when the pressure is

tation is over even though the pressure continues. But

this receptor for a few milliseconds, and then its exci-

puscle, sudden pressure applied to the tissue excites

receptors. Thus, in the case of the pacinian cor-

receptors, or

movement

receptors,

. Therefore, these recep-

change is actually taking place

strength changes. Yet they react strongly

receptors.

to transmit information for many hours, they are called

chemoreceptors of the carotid and aortic bodies.

receptors, (3) baroreceptors of the arterial tree, and (4)

tors of the macula in the vestibular apparatus, (2) pain

its surroundings. For instance, impulses from the

or hours). Therefore, they keep the brain constantly

Strength—The “Tonic” Receptors.

readjustments in the structure of the receptor itself,

receptors. That is, part of the adaptation results from

Presumably, these same two general mechanisms of

or most cell membrane sodium channels, as was

ally to close, an effect that seems to occur for all

the nerve fiber membrane, which means that sodium

progressive “inactivation” of the sodium channels in

dated” to the stimulus. This probably results from

the nerve fiber itself gradually becomes “accommo-

core fiber should continue to be distorted, the tip of

nerve fiber itself. That is, even if by chance the central

, which occurs in the

ian corpuscle, but a much slower one, results from a

The second mechanism of adaptation of the pacin-

compression continues.

The Nervous System: A. General Principles and Sensory Physiology

576

Unit IX

appears at the onset of compression but disappears
within a small fraction of a second even though the

process called accommodation

current flow through the channels causes them gradu-

explained in Chapter 5.

adaptation apply also to the other types of mechano-

and part from an electrical type of accommodation in
the terminal nerve fibril.

Slowly Adapting Receptors Detect Continuous Stimulus

Slowly adapting recep-

tors continue to transmit impulses to the brain as long
as the stimulus is present (or at least for many minutes

apprised of the status of the body and its relation to

muscle spindles and Golgi tendon apparatuses allow
the nervous system to know the status of muscle con-
traction and load on the muscle tendon at each instant.

Other slowly adapting receptors include (1) recep-

Because the slowly adapting receptors can continue

tonic

Rapidly Adapting Receptors Detect Change in Stimulus
Strength—The “Rate Receptors,” “Movement Receptors,”
or “Phasic Receptors.”

Receptors that adapt rapidly

cannot be used to transmit a continuous signal because
these receptors are stimulated only when the stimulus

while a

tors are called rate
phasic

exceedingly important in apprising the nervous system

mitting information about constant conditions in the

Importance of the Rate Receptors—Their Predictive Function.

If one knows the rate at which some change in bodily

state of the body a few seconds or even a few minutes

canals in the vestibular apparatus of the ear detect the
rate at which the head begins to turn when one runs

predict how much he or she will turn within the next
2 seconds and can adjust the motion of the legs ahead
of time
tors located in or near the joints help detect the rates

joint rate receptors allows the nervous system to
predict where the feet will be during any precise frac-

signals can be transmitted to the muscles of the legs to
make any necessary anticipatory corrections in posi-

dictive function makes it impossible for the person to
run.

and Their Physiologic
Classification

Some signals need to be transmitted to or from the

the sensory signals that apprise the brain of the momen-
tary positions of the legs at each fraction of a second

and 20 micrometers in diameter—the larger the diame-

a, b, g

myelinated
small unmyelinated nerve fibers that conduct impulses

half of the sensory fibers in most peripheral nerves as

distance in 1 second that is longer than a football field.

the big toe to the spinal cord.

Certain recording techniques have made it possible to

intensity of pain. The different gradations of intensity

must be conveyed is signal intensity—for instance, the

Temporal Summation

Nerve Tracts—Spatial and

of Different Intensity in

Transmission of Signals

diameter; they are type C fibers in the general classifi-

Unmyelinated fibers carrying pain, itch, temperature,

-type A fibers in the general classification).

ter; they are

Fibers carrying temperature, crude touch, and pricking

-type A fibers in the general classification).

dles (average about 8 micrometers in diameter; these

and from the flower-spray endings of the muscle spin-

Fibers from most discrete cutaneous tactile receptors

micrometers in diameter; these also are

Fibers from the Golgi tendon organs (average about 16

(average about 17 microns in diameter; these are

Fibers from the annulospiral endings of muscle spindles

fibers. Therefore, the fol-

easily between A

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

577

these same recording techniques cannot distinguish

b and Ag

lowing classification is frequently used by sensory
physiologists:

Group Ia

a-type

A fibers in the general classification).

Group Ib

a-type A

fibers).

Group II

are

b- and g

Group III

pain sensations (average about 3 micrometers in diame-

Group IV

and crude touch sensations (0.5 to 2 micrometers in

cation).

One of the characteristics of each signal that always

2.0

0.5

6 2.0

0.5

III

1 2.0

0.5

Myelinated

Unmyelinated

Nerve fiber diameter (micrometers)

Conduction velocity (m/sec.)

Muscle spindle

(primary ending)

Muscle spindle

(secondary ending)

Muscle tendon

(Golgi tendon organ)

Hair receptors

Sensory functions

Sensory nerve classification

General classification

Diameter (micrometers)

Skeletal muscle

(type A

)

Muscle spindle

(type A

)

Sympathetic

(type C)

Motor function

Deep pressure

and touch

Pricking pain

Cold

Warmth

Aching pain

Tickle

Crude touch

and pressure

Vibration

(pacinian corpuscle)

High discrimination touch

(Meissner's expanded tips)

120

Figure 46–6

Physiologic classifications and functions of nerve
fibers.

that causes it to process signals in its own unique way,

long pool of neurons.

cephalon, pons, and medulla. Also, the entire dorsal

specific nuclei in the thalamus, cerebellum, mesen-

ered to be a single large neuronal pool. Other neuronal

instance, the entire cerebral cortex could be consid-

few neurons, while others have vast numbers. For

to millions of neuronal pools; some of these contain

The central nervous system is composed of thousands

of Signals in Neuronal Pools

Transmission and Processing

by the nerve fiber.

. Figure 46–8 demonstrates this,

of nerve impulses in each fiber, which is called

Temporal Summation.

stronger signals spread to more and more fibers. This

gressively more fibers being stimulated. Thus, the

moderate stimulus and a strong stimulus, with pro-

stimulated weakly (half-red fibers). The other two

red-colored fiber), whereas several adjacent fibers are

stimulus, with only a single nerve fiber in the middle

from the skin area. To the left is the effect of a weak

Thus, the lower part of Figure 46–7 shows three

field is much greater than at the periphery.

than when it is in the periphery of the field, because

center of the receptive field of a particular pain fiber,

fibers simultaneously. When the pinprick is in the

from other pain fibers. Therefore, a pinprick of the skin

diminishes toward the periphery. One can also see

of that fiber. The

This area is called the

an area of skin as large as 5 centimeters in diameter.

that serve as pain receptors. The entire

innervated by a large number of parallel pain fibers.

numbers of fibers. This figure shows a section of skin

, whereby increasing signal

Figure 46–7 shows the phenomenon

respectively,

spatial summation and temporal

along a single fiber. These two mechanisms are called,

The Nervous System: A. General Principles and Sensory Physiology

578

Unit IX

can be transmitted either by using increasing numbers
of parallel fibers or by sending more action potentials

summation.

Spatial Summation.

of spatial summation
strength is transmitted by using progressively greater

Each of these arborizes into hundreds of minute free
nerve endings
cluster of fibers from one pain fiber frequently covers

receptor field

number of endings is large in the center of the field but

from the figure that the arborizing fibrils overlap those

usually stimulates endings from many different pain

the degree of stimulation of that fiber is far greater

the number of free nerve endings in the middle of the

views of the cross section of the nerve bundle leading

of the bundle stimulated strongly (represented by the

views of the nerve cross section show the effect of a

is the phenomenon of spatial summation.

A second means for transmitting

signals of increasing strength is by increasing the fre-
quency
temporal summation
showing in the upper part a changing strength of signal
and in the lower part the actual impulses transmitted

pools include the different basal ganglia and the

gray matter of the spinal cord could be considered one

Each neuronal pool has its own special organization

Weak

stimulus

Moderate

stimulus

Skin

Nerve

Strong

stimulus

Pattern of stimulation of pain fibers in a nerve leading from an area

Figure 46–7

of skin pricked by a pin. This is an example of spatial summation.

Strength of signal

(impulses per second)

Impulses

Time

of nerve impulses, showing the strength of signal (

Translation of signal strength into a frequency-modulated series

Figure 46–8

above) and the

separate nerve impulses (below). This is an example of temporal
summation.

toward its edges.

endings in the center; it becomes progressively less

The degree of inhibition in the center

inhibitory zone.

exciting them. This is the opposite of facilitation, and

that some incoming fibers inhibit neurons, rather than

We must also remember

subliminal zone

zone

facilitated zone,

are facilitated but not excited, and these areas are

. To each side, the neurons

liminal zone

excited zone

of the incoming fiber, also called the

charge zone

incoming fiber. Therefore, this is said to be the

circled area, all the neurons are stimulated by the

portion of the field in this figure, designated by the

bution “field,” as shown in Figure 46–10. In the central

Figure 46–9 represents a highly condensed version

are subthreshold, but facilitating, stimuli.

is a suprathreshold stimulus, and the stimuli to

, the stimulus to

Similarly, for

through other incoming nerve fibers. Therefore, the

theless, discharge of these terminals makes both these

b and c, but not enough to cause excitation. Never-

; it is also called a

from input fiber 1 to this neuron is said to be an

to discharge. The stimulus

input fiber 1

on each one of the neurons from each input fiber, he

neurons. If the student counts the number of terminals

46–9, let us assume that six terminals must discharge

cession to cause excitation. For instance, in Figure

action potential in a postsynaptic neuron. Instead,

45, it will be recalled that discharge of a single

From the discussion of synaptic function in Chapter

farther away.

input fiber lie on the nearest neuron in its “field,” but

. Note in Figure

The neuronal area stimulated by each incoming

in the pool. The dendrites usually also arborize and

to thousands of times, providing a thousand or more

fibers to the right. Each input fiber divides hundreds

pool, showing “input” fibers to the left and “output”

Figure 46–9 is

Relaying of Signals Through

the following pages.

have many similar principles of function, described in

despite their differences in function, the pools also

the multitude of functions of the nervous system. Yet

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

579

thus allowing the total consortium of pools to achieve

Neuronal Pools

Organization of Neurons for Relaying Signals.

a schematic diagram of several neurons in a neuronal

terminal fibrils that spread into a large area in the pool
to synapse with dendrites or cell bodies of the neurons

spread hundreds to thousands of micrometers in the
pool.

nerve fiber is called its stimulatory field
46–9 that large numbers of the terminals from each

progressively fewer terminals lie on the neurons

Threshold and Subthreshold Stimuli—Excitation or Facilitation.

excitatory presynaptic terminal almost never causes an

large numbers of input terminals must discharge on
the same neuron either simultaneously or in rapid suc-

almost simultaneously to excite any one of the

or she will see that

has more than enough

terminals to cause neuron a

excitatory stimulus

suprathreshold

stimulus because it is above the threshold required for
excitation.

Input fiber 1 also contributes terminals to neurons

neurons more likely to be excited by signals arriving

stimuli to these neurons are said to be subthreshold,
and the neurons are said to be facilitated.

input fiber 2

neuron d

neurons

b and c

of a neuronal pool because each input nerve fiber
usually provides massive numbers of branching termi-
nals to hundreds or thousands of neurons in its distri-

dis-

called the

also called the subthreshold

Inhibition of a Neuronal Pool.

the entire field of the inhibitory branches is called the

of this zone is great because of large numbers of

Basic organization of a neuronal pool.

Figure 46–9

Facilitated zone

Input nerve

fiber

Discharge zone

Facilitated zone

“Discharge” and “facilitated” zones of a neuronal pool.

Figure 46–10

second output pathway from the pool. This type of

(neuron 2), which secretes

excitatory output pathway, but it stimulates an inter-

tion is achieved. The input fiber directly excites the

Figure 46–13 shows the means by which the inhibi-

ling all antagonistic pairs of muscles, and it is called the

ment. This type of circuit is characteristic for control-

spinal cord to cause forward movement of a leg, an

elsewhere. For instance, at the same time that an exci-

Inhibitory Output Signals

Neuronal Circuit with Both Excitatory and

mates, and sorts different types of information.

which the central nervous system correlates, sum-

tion. Convergence is one of the important means by

from different sources, and the resulting response is a

ing from the brain into the spinal cord.Then the signals

cortex, and (4) several other long pathways descend-

another, (3) corticospinal fibers from the cerebral

eral nerve fibers entering the cord, (2) propriospinal

For instance, the interneurons of the

in Figure 46–12

, as shown

to the threshold required for discharge.

tial from a single input terminal. But action potentials

the same neuron. The importance of this is that

. That is, multiple ter-

uniting to excite a single neuron. Figure 46–12

Convergence of Signals

mus, almost all sensory information is relayed both

thalamus and cerebral cortex. Likewise, in the thala-

the lower part of the brain: (1) into the cerebellum and

pool. For instance, information transmitted up the

. In this case,

The second type of divergence, shown in Figure

10,000 muscle fibers.

highly facilitated conditions, of exciting as many as

large pyramidal cell in the motor cortex capable, under

pathway in its control of skeletal muscles, with a single

through successive orders of neurons in its path. This

This means simply that an input signal spreads

type of divergence is shown in Figure

entirely different purposes.

Two major types of divergence occur and have

gence.

leaving the pool. This phenomenon is called

Neuronal Pools

Divergence of Signals Passing Through

The Nervous System: A. General Principles and Sensory Physiology

580

Unit IX

Often it is important for weak signals entering a neu-
ronal pool to excite far greater numbers of nerve fibers

diver-

An amplifying

46–11A.
to an increasing number of neurons as it passes

type of divergence is characteristic of the corticospinal

46–11B, is divergence into multiple tracts
the signal is transmitted in two directions from the

dorsal columns of the spinal cord takes two courses in

(2) on through the lower regions of the brain to the

into still deeper structures of the thalamus and at the
same time to discrete regions of the cerebral cortex.

Convergence means signals from multiple inputs

A shows

convergence from a single source
minals from a single incoming fiber tract terminate on

neurons are almost never excited by an action poten-

converging on the neuron from multiple terminals
provide enough spatial summation to bring the neuron

Convergence can also result from input signals (exci-

tatory or inhibitory) from multiple sources

spinal cord receive converging signals from (1) periph-

fibers passing from one segment of the cord to

from the interneurons converge on the anterior motor
neurons to control muscle function.

Such convergence allows summation of information

summated effect of all the different types of informa-

Sometimes an incoming signal to a neuronal pool
causes an output excitatory signal going in one direc-
tion and at the same time an inhibitory signal going

tatory signal is transmitted by one set of neurons in the

inhibitory signal is transmitted through a separate set
of neurons to inhibit the muscles on the back of the
leg so that they will not oppose the forward move-

reciprocal inhibition circuit.

mediate inhibitory neuron
a different type of transmitter substance to inhibit the

circuit is also important in preventing overactivity in
many parts of the brain.

Divergence in same tract

Divergence in multiple tracts

multiple tracts to transmit the signal to separate areas.

“Divergence” in neuronal pathways.

Figure 46–11

A, Divergence within a

pathway to cause “amplification” of the signal. B, Divergence into

Convergence from

single source

Convergence from

multiple sources

Source

multiple separate sources.

Input fibers from

Multiple input fibers from a single source.

“Convergence” of multiple input fibers onto a single neuron.

Figure 46–12

tions in the circuit. Fatigue beyond a certain critical

it suddenly ceases entirely. The cause of this sudden

beration and then decreases to a critical point, at which

can last for many milliseconds or even minutes. The

lus may last only 1 millisecond or so, and yet the output

from a typical reverberatory circuit. The input stimu-

Figure 46–15 shows output signals

Characteristics of Signal Prolongation from a Rever-

weak or strong, depending on how many parallel nerve

a system, the total reverberating signal can be either

cell station, the terminal fibrils spread widely. In such

ways are constituted of many parallel fibers. At each

Figure 46–14

intensity and frequency of reverberation, whereas an

reverberating circuit.A facilitatory signal enhances the

initial discharge and the feedback signal. Figure

feedback circuit, which causes a longer delay between

Figure 46–14

discharging for a protracted time thereafter.

charges, the feedback stimuli could keep the neuron

important one, theoretically, once the neuron dis-

itself. Although this type of circuit probably is not an

case, the output neuron simply sends a collateral nerve

involves only a single neuron. In this

Figure 46–14

are shown in Figure 46–14. The simplest, shown in

for a long time.

once stimulated, the circuit may discharge repetitively

re-excite the input of the same circuit. Consequently,

, or

Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolon-

for many milliseconds.

tic “afterdischarge” mechanism alone, it is possible for

explained in Chapter 45. Thus, as a result of this synap-

mit a continuous train of output impulses, as was

can continue to excite the neuron, causing it to trans-

stances are involved. As long as this potential lasts, it

the neuron and lasts for many milliseconds, especially

neuron, a postsynaptic electrical potential develops in

When excitatory synapses dis-

following.

the incoming signal is over. The most important

, lasting a

afterdischarge

output discharge, called

instances, a signal entering a pool causes a prolonged

relayed through neuronal pools. However, in many

Thus far, we have considered signals that are merely

Prolongation of a Signal by a

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

581

Neuronal Pool—“Afterdischarge”

few milliseconds to as long as many minutes after

mechanisms by which afterdischarge occurs are the

Synaptic Afterdischarge.

charge on the surfaces of dendrites or soma of a

when some of the long-acting synaptic transmitter sub-

a single instantaneous input signal to cause a sustained
signal output (a series of repetitive discharges) lasting

gation.

One of the most important of all circuits in the

entire nervous system is the reverberatory

oscilla-

tory, circuit. Such circuits are caused by positive feed-
back within the neuronal circuit that feeds back to

Several possible varieties of reverberatory circuits

fiber back to its own dendrites or soma to restimulate

B shows a few additional neurons in the

46–14C shows a still more complex system in which
both facilitatory and inhibitory fibers impinge on the

inhibitory signal depresses or stops the reverberation.

D shows that most reverberating path-

fibers are momentarily involved in the reverberation.

beratory Circuit.

figure demonstrates that the intensity of the output
signal usually increases to a high value early in rever-

cessation of reverberation is fatigue of synaptic junc-

level lowers the stimulation of the next neuron in the
circuit below threshold level so that the circuit feed-
back is suddenly broken.

Excitatory synapse

Inhibitory synapse

Input fiber

Excitation

Inhibition

Inhibitory circuit. Neuron 2 is an inhibitory neuron.

Figure 46–13

Input

Output

Input

Output

Input

Output

Input

Output

Inhibition

Facilitation

Reverberatory circuits of increasing complexity.

Figure 46–14

rhythmical signals.

dog or the walking movements of any animal, require

throughout life. Other rhythmical signals, such as those

pons. This respiratory rhythmical signal continues

signals—for instance, a rhythmical respiratory signal

berating neuronal pathway.

iris in the eye, and heart rate. That is, the nerve exci-

vascular tone, gut tone, degree of constriction of the

This type of information transmission is used by the

whereas up to this point, the types of information

in signal intensity as well as an increase,

intensity. Note that this carrier wave system allows a

output signal, but they do

information transmission. That is, the excitatory and

Those students who are familiar with radio trans-

greatly increases the output signal, whereas an

of reverberation. Note that an excitatory input signal

a pool of neurons. The pool may be emitting impulses

Figure 46–16 shows a continuous output signal from

increase the output signal, whereas inhibition can

ation is a source of continuous impulses. And excita-

a Means for Transmitting Information.

can decrease the rate of firing to zero.

decreased by inhibitory signals; inhibitory signals often

rons of the spinal cord. The rates at which these cells

of the cerebellum, as well as in most of the interneu-

tinually. This occurs especially in many of the neurons

potential rises above a certain threshold level. The

Neurons, like other excitable tissues, discharge

ity.

beratory signals.

two mechanisms can cause this effect: (1) continuous

ously, even without excitatory input signals. At least

Neuronal Circuits

Continuous Signal Output from Some

in Figure 46–18).

brain that inhibit or facilitate the circuit. Almost these

The duration of the total signal before cessation can

The Nervous System: A. General Principles and Sensory Physiology

582

Unit IX

also be controlled by signals from other parts of the

exact patterns of output signals are recorded from the
motor nerves exciting a muscle involved in a flexor
reflex after pain stimulation of the foot (as shown later

Some neuronal circuits emit output signals continu-

intrinsic neuronal discharge and (2) continuous rever-

Continuous Discharge Caused by Intrinsic Neuronal Excitabil-

repetitively if their level of excitatory membrane

membrane potentials of many neurons even normally
are high enough to cause them to emit impulses con-

emit impulses can be increased by excitatory signals or

Continuous Signals Emitted from Reverberating Circuits as

A reverberating

circuit that does not fatigue enough to stop reverber-

tory impulses entering the reverberating pool can

decrease or even extinguish the signal.

because of intrinsic neuronal excitability or as a result

inhibitory input signal greatly decreases the output.

mitters will recognize this to be a carrier wave type of

inhibitory control signals are not the cause of the

control its changing level of

decrease

transmission we have discussed have been mainly pos-
itive information rather than negative information.

autonomic nervous system to control such functions as

tatory signal to each of these can be either increased
or decreased by accessory input signals into the rever-

Rhythmical Signal Output

Many neuronal circuits emit rhythmical output

originates in the respiratory centers of the medulla and

that cause scratching movements by the hind leg of a

input stimuli into the respective circuits to initiate the

All or almost all rhythmical signals that have been

studied experimentally have been found to result from
reverberating circuits or a succession of sequential
reverberating circuits that feed excitatory or inhibitory
signals in a circular pathway from one neuronal pool
to the next.

Output pulse rate

Time

Inhibited

Input stimulus

Facilitated

Normal

after a single input stimulus, showing the effects of facilitation and

Typical pattern of the output signal from a reverberatory circuit

Figure 46–15

inhibition.

Impulses per second

Time

Output
Excitation
Inhibition

of excitatory or inhibitory input signals.

intrinsically discharging neurons. This figure also shows the effect

Continuous output from either a reverberating circuit or a pool of

Figure 46–16

of fatigue to other pathways in the brain. Those that

Automatic Short-Term Adjustment of Pathway Sensitivity by the

subsequent reflex response.

successive flexor reflexes, the less the intensity of the

circuit. Furthermore, the shorter the interval between

that is, its strength diminishes; much of this effect is

strength of contraction progressively “decrements”—

the footpad of the paw. Note in each record that the

Figure 46–18 shows three successive records of a flexor

Synaptic Fatigue as a Means for

brain—for instance, many of the basal ganglia exert

overly excited; and (2) some neuronal pools that exert

Two types of inhibitory circuits in widespread areas

Stabilizing Nervous System Function

Inhibitory Circuits as a Mechanism for

fatigue of synapses.

central nervous system: (1) inhibitory circuits and (2)

from happening all the time? The answer lies mainly

epileptic seizures.

signals could be transmitted. Such an effect occurs in

mation but, nevertheless, would be consuming the cir-

parts. If this should occur, the brain would be inun-

would set off a continuous cycle of re-excitation of all

finally the signal re-excites the first part, it is clear that

second the third, the third the fourth, and so on until

serious problem. If the first part excites the second, the

or indirectly with every other part, and this creates a

of Neuronal Circuits

progressively.

deficiency, both the frequency and the amplitude of

When the carotid body is stimulated by arterial oxygen

the respiratory signal output in the phrenic nerve.

output. Figure 46–17, for instance, shows changes in

Sensory Receptors, Neuronal Circuits for Processing Information

Chapter 46

583

Excitatory or inhibitory signals can also increase

or decrease the amplitude of the rhythmical signal

the respiratory rhythmical output signal increase

Instability and Stability

Almost every part of the brain connects either directly

an excitatory signal entering any part of the brain

dated by a mass of uncontrolled reverberating
signals—signals that would be transmitting no infor-

cuits of the brain so that none of the informational

widespread areas of the brain during
How does the central nervous system prevent this

in two basic mechanisms that function throughout the

of the brain help prevent excessive spread of signals:
(1) inhibitory feedback circuits that return from the
termini of pathways back to the initial excitatory
neurons of the same pathways—these circuits occur in
virtually all sensory nervous pathways and inhibit
either the input neurons or the intermediate neurons
in the sensory pathway when the termini become

gross inhibitory control over widespread areas of the

inhibitory influences throughout the muscle control
system.

Stabilizing the Nervous System

Synaptic fatigue means simply that synaptic transmis-
sion becomes progressively weaker the more pro-
longed and more intense the period of excitation.

reflex elicited in an animal caused by inflicting pain in

caused by fatigue of synapses in the flexor reflex

Fatigue Mechanism.

Now let us apply this phenomenon

Phrenic nerve output

Increasing carotid

body stimulation

respiration.

quency of the phrenic nerve signal to the diaphragm to increase

tion of the carotid body increases both the intensity and the fre-

respiratory center, showing that progressively increasing stimula-

The rhythmical output of summated nerve impulses from the

Figure 46–17

Flexor muscle contraction force (g)

Seconds

Flexor reflexes–decremental responses
Stimulus

the reflex pathway.

Successive flexor reflexes showing fatigue of conduction through

Figure 46–18

81:299, 2001.

adaptations mediating opioid dependence. Physiol Rev

Williams JT, Christie MJ, Manzoni O: Cellular and synaptic

synapse formation. Curr Opin Neurobiol 14:83, 2004.

Shen K: Molecular mechanisms of target specificity during

Physiol Rev 82:875, 2002.

Schwartz EA: Transport-mediated synapses in the retina.

Neurophysiol 90:1363, 2003.

mitter transporters: not just for reuptake anymore. J

Richerson GB, Wu Y: Dynamic equilibrium of neurotrans-

glycine receptors and taurine. Neuron 41:839, 2004.

Renteria RC, Johnson J, Copenhagen DR: Need rods? Get

mic behavior. Annu Rev Physiol 62:723, 2000.

Pearson KG: Neural adaptation in the generation of rhyth-

tonic neurons. Neuron 35:600, 2002.

Palmer MJ, von Gersdorff H: Phasic transmitter release from

and taste receptors. Nat Rev Neurosci 5:263, 2004.

Mombaerts P: Genes and ligands for odorant, vomeronasal

bution? Acta Physiol Scand 177:227, 2003.

thetic ganglia—modulation, integration or simply distri-

McLachlan EM: Transmission of signals through sympa-

tic plasticity? Trends Neurosci 27:62, 2004.

Krupa B, Liu G: Does the fusion pore contribute to synap-

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

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

bridge, MA: MIT Press, 1998.

Ivry RB, Robertson LC: The Two Sides of Perception. Cam-

duction in living cells. Physiol Rev 81:685, 2001.

Hamill OP, Martinac B: Molecular basis of mechanotrans-

Biobehav Rev 27:729, 2004.

Gebhart GF: Descending modulation of pain. Neurosci

muscle fatigue. Physiol Rev 81:1725, 2001.

Gandevia SC: Spinal and supraspinal factors in human

tion in vertebrate photoreceptors. Physiol Rev 81:117,

Fain GL, Matthews HR, Cornwall MC, Koutalos Y: Adapta-

Basar E: Brain Function and Oscillations. Berlin: Springer,

Baev KV: Biological Neural Networks. Boston: Birkhauser,

Nat Neurosci 7:446, 2004.

Buzsaki G: Large-scale recording of neuronal ensembles.

or other nervous disorders. But fortunately, the auto-

chotic disturbances, hallucinations, mental tension,

expect almost continual muscle cramps, seizures, psy-

of these circuits were abnormally high; one might then

level required for proper function. Think for a moment

anisms for adjusting synaptic sensitivity, continually

regulation of receptors, as well as other control mech-

tic membrane.

combine with the receptor proteins, many of these

synaptic membrane. However, when the synapses are

overactivity. The mechanism for this is the following:

tivity, and downregulating the receptors when there is

The

Long-Term Changes in Synaptic Sensitivity Caused by Automatic

cuits. These help to keep the circuits operating in a

increase. Thus, fatigue and recovery from fatigue con-

underused become rested, and their sensitivities

sensitivities decrease. Conversely, those that are

are overused usually become fatigued, so that their

The Nervous System: A. General Principles and Sensory Physiology

584

Unit IX

stitute an important short-term means of moderating
the sensitivities of the different nervous system cir-

range of sensitivity that allows effective function.

Downregulation or Upregulation of Synaptic Receptors.

long-term sensitivities of synapses can be changed
tremendously by upregulating the number of receptor
proteins at the synaptic sites when there is underac-

Receptor proteins are being formed constantly by the
endoplasmic reticular–Golgi apparatus system and are
constantly being inserted into the receptor neuron

overused so that excesses of transmitter substance

receptors are inactivated and removed from the synap-

It is indeed fortunate that upregulation and down-

adjust the sensitivity in each circuit to almost the exact

how serious it would be if the sensitivities of only a few

matic controls normally readjust the sensitivities of the
circuits back to controllable ranges of reactivity any
time the circuits begin to be too active or too
depressed.

References

1998.

2001.