Nerve Tissue pdf - د.عبير

First stage

Biology

Lec

/ 10

Abeer H. Alkhafaf

Nervous System

The Nervous system is the most complex system in the human body histologically

and physiologically. It is formed by interconnected a network of billions nerve cells

(neurons), assisted by much numerous glial cells. Each neuron has hundreds of

interconnections with other neurons, forming a very complex system for processing

communication and generating responses.

The nervous system is divided anatomically into:-

1- Central nervous system CNS consisting of:-

a- The brain (cerebrum and cerebellum).

b- The spinal cord.

2- Peripheral nervous system PNS consisting of:-

a- Nerves ( cranial, spinal, peripheral ).

b- Ganglia.

c- Nerve ending.

* Both Central and Peripheral tissue consists of two cell types:

1- Nerve cells or Neurons. 2- Glial cells.

Neurons

The Neuron or nerve cell is the functional unit in both CNS and PNS, which usually

show numerous long processes, they are responsible for the reception, transmission,

and processing of stimuli. Neurons respond to environmental changes (stimuli) by

altering the ionic gradient that exists between the inner and outer surfaces of their

membranes. Neurons and their processes vary in size and shape.

Functionally, neurons are classified according to the direction in which they transmit

impulses relative to the central nervous system into:-

Sensory (afferent) neurons: carry impulses from peripheral sense receptors to

the CNS.

Motor (efferent) neurons: transmit impulses from the CNS to effector organs

such as muscles and glands.

Interneuron: forming a communicating network between sensory and motor

neurons.

Neuron structure consists of three parts:-

1- The cell body or Perikaryon. 2- Dendrites. 3- Axon.

The cell body ( Perikaryon )

The cell body which is the part of neuron that contain the nucleus and surrounding

cytoplasm, it is primarily a trophic center for the entire nerve cell. It is the receptive to

stimuli and it receives a great number of nerve endings of other nerve cells, the nucleus

is large, spherical, euchromatic (pale staining) with prominent nucleolus. Cell body

contains rough endoplasmic reticulum (RER), Golgi apparatus, and polyribosomes and

contains Nissl bodies (chromatophilic substance).

Dendrites

Dendrites are elongated processes specialized to receive stimuli from the environment,

sensory epithelial cells, or other neurons. They are short and divided like branches of a

tree; they are covered with many synapses and are the principal signal reception and

processing site on the neurons. The arborization increases the receptive area of the

cell, and the branches become much thinner with further division.

Axon

Axons are long, single, cylindrical processes that originate from a pyramid- shaped

region of the perikaryon; the Axon hillock, which is devoid of nissl bodies and Golgi

cisternae, and it is the site where all other organelles pass to the axon.

The plasma membrane of the axon is called Axolemma and the contents are known as

the axoplasm, which contains mitochondria, microtubules, some cisternae of SER, and

neurofilaments. RER and polyribosome are absent. The axon has a constant diameter

and varies in length and diameter according to the type of neuron.

The axon is specialized in generating and conducting nerve impulses to other cell

(nerve, muscle, gland cells), also may receive information from other neurons. The

distal portion of the axon is usually branched as terminal arborization, Each branch

terminates on the next cell in dilatations called End bulbs (boutons), which interact

with other neurons or non nerve cells at structures called Synapses which are

specialized junctions for the transmission of nerve impulses from neuron to another

neuron or other effector cell (muscle and gland) in unidirectional way.

Collateral branches: - are branches of the axon that connect with other group of cells.

Structures of neuron

Neurons can be classified according to the number of processes extending from

the cell body into:-

a- Multipolar neurons, which have one axon and two or many dendrites, most

neurons are Multipolar.

b- Bipolar neurons, with one dendrite and one axon, found in the retina and the inner

ear.

d- Unipolar or pseudounipolar neurons, which have a single process that bifurcates

close to the cell body with the longer branch extending to a peripheral ending and the

other toward the CNS. Pseudounipolar found in spinal ganglia.

Structural classes of neurons

Glial cells ( Neuroglia )

Glial cells, which have short processes, support and protect neurons, and participate

in neural activity, neural nutrition, and the defence processes of the central nervous

system. They are 10 times more abundant in brain than the neurons. They are smaller

than neurons, and in the CNS they surround most of the neuronal cell bodies. They

provide an ideal environment for the neuronal activities.

In the CNS there is very little or no connective tissue. Instead, there is Neuropil, which

is dense network of fibers from processes of both neurons and glial cells fills the

interneuronal spaces of CNS.

Neuron, neuropil, and common glial cells

There are six kinds of glial cells:

1- Oligodendrocytes. 2- Astrocytes. 3- Ependymal cells.

4- Microglia cells. 5- Schwann cells. 6- Satellite cells.

1- Oligodendrocytes

Oligodendrocytes are small cells with rounded, condensed nuclei and unstained

cytoplasm. They are predominant glial cell in CNS white matter, they produce the

myelin sheath that provide the electrical insulation for neurons in the CNS, and these

cells extend processes that form the myelin sheath wrap around many axons.

2- Astrocytes

Astrocytes are star-shaped cells, because of their multiple radiating processes, and they

are unique to the CNS. Astrocytes compared to other glial cells are by far the most

numerous and exhibit considerable morphological and functional diversity. In addition

to their supporting function, Astrocytes have major roles in controlling the ionic

environment of neurons, and they have processes that extend between blood vessels

and neurons, the processes are expanded, forming perivascular feet that cover capillary

endothelial cells and contribute to the Blood – Brain barrier.

Two kinds of astrocytes have been identified:-

- Protoplasmic astrocytes, which are more prevalent in the gray matter, have short

and branched processes.

- Fibrous astrocytes, which are more common in the white matter, have few and long

processes.

3- Ependymal cells

Ependymal cells are low columnar or cuboidal cells lining the ventricles of the brain

and central canal of the spinal cord. ependymal cells have cilia, which facilitate the

movement of the cerebrospinal fluid (CSF), or long microvilli which are involved in

absorption.

4- Microglia

Microglia are small cells with short irregular processes and less numerous than

oligodendrocytes or astrocytes cells, but more distributed throughout gray and white

matter, have dense elongated nuclei which contrast with the spherical nuclei of other

glial cells. Microglia are phagocytic cells, originate from blood monocytes and belongs

to the same family as macrophages and antigen presenting cells.

Microglia secrete a number of immunoregulatory cytokines and constitute the major

mechanism of immune defense in CNS tissues.

5- Schwann cells (Neurolemmocytes)

They are found only in PNS, and have trophic interactions with axons and allow for

their myelination, one Schwann cell forms myelin around a segment of one axon, in

contrast to the ability of oligodendrocytes to branch and sheath parts of more than one

axon.

6- Satellite cells of ganglia

Small satellite cells form a covering layer over the large neuronal cell bodies in PNS

ganglia. Closely associated with the neurons, the satellite cells exert a trophic or

supportive role.

First stage

Biology

Lec

/ 11

Abeer H. Alkhafaf

Central Nervous System

The principal structures of the CNS are:

1- The brain ( cerebrum and cerebellum ) which is enclosed in the skull.

2- Spinal cord which is contained within the vertebral canal.

It is soft and gel like organ because there is no connective tissue, in freshly sectioned

tissues ( cerebrum cerebellum and spinal cord ) show regions of white (white matter)

and gray (gray matter). This difference in color is due to the distribution of the nerve

cell bodies, glial cells and the myelinated nerve axons.

The white matter is composed mainly of the myelinated axons and the myelin-

producing oligodendrocytes. It does not contain neuronal cell body, but microglia are

present.

Gray matter is composed mainly of the nerve cell bodies, dendrites, the initial

unmyelinated portions of axon, astrocytes and the glial cells. This is the region where

synapses occur. Aggregates of neuronal cell bodies forming island of gray matter

embedded in the white matter are called nuclei.

Gray matter prevalent mainly at cortex of cerebrum and cerebellum, and the white

matter present in the central region.

In cross section of the spinal cord, white matter is peripheral and gray matter is

internal and has general shape of an H (butterfly shape). In the center is an opening

of the central canal. The gray matter forms:

- The anterior horns, which contain the motor neurons whose axon make up the ventral

roots of spinal nerves.

- The posterior horns which receive sensory fibers from neurons in the spinal ganglia

(dorsal root).

Spinal cord neurons are large and multipolar, especially the motor neurons in the

anterior horns.

Spinal cord

. (P) horns (sensory) and two anterior (A) (motor) horns all joined by the gray commissure around the central canal. (a): The gray matter

(b): The white matter

Meninges

The skull and the vertebral column protect the CNS. The meninges are membranes of

connective tissue that present between the bone and nervous tissue, which are invest

and protect the brain and spinal cord. Three meningial layers distinguished; the dura

mater, the arachnoid and the delicate pia mater.

Blood Brain Barrier

Blood Brain Barrier (BBB) is a functional barrier that prevents the passage of some

substances, such as antibiotics and chemicals and bacterial toxic matter, from blood to

the nerve tissue, and maintains a constant environment. The main structural component

of the BBB is capillary endothelium.

Peripheral Nervous System

The main components of PNS are:-

1- Nerves. 2- Ganglia. 3- Nerve endings.

Nerves are bundles of nerve fibers (axons) surrounded by glial cells and C.T.

Nerve fibers

In PNS nerve fibers consist of axons sheathed by schwann cells and it is divided into:

1- Myelinated nerve fibers

The multiple layers of Schwann cell membrane unite as a myelin layer. Myelin sheath

is a lipid layer, It is composed of concentrically wrapped layers of plasma membrane

of Schwann cells (Neurolemmocytes), pushing the nucleus and cytoplasm to the

periphery. Between adjacent Schwann cells the myelin sheath shows small nodal gaps

along the axon called the node of Ranvier devoid of myelin. The distance between

two nodes is called an "Internode" and consists of one Schwann. The Myelin sheath is

segmented due to numerous Schwann cells arranged along the length of the axon.

Myelin sheath serves to protect axons and maintain a constant ionic environment

required for action potentials.

Myelination of PNS axons

1- Unmyelinated nerve fibers

The Unmyelinated nerve fibers are more abundant in the CNS, where axons run free,

while in the PNS the axons are enveloped within simple folds of Schwann cells.

Peripheral nerves

In PNS nerve fibers are grouped into bundles to form nerves ( Myelinated and

Unmyelinated ). They have a whitish, glistening appearance because of their myelin

and collagen content. The nerves establish communication between centers in the brain

and spinal cord and the sense organs and effectors (muscles, glands, etc) by containing

both afferent and efferent fibers.

Axons and Schwann cells of nerves are enclosed within connective tissue layers:

1- Epineurium: which is an external dense irregular fibrous coat of C.T., which

surrounded the bundles of nerve fibers.

2- Perineurium: a sleeve of specialized C.T. formed by layers of flattened

epithelium-like cells surrounded each bundle.

3- Endoneurium: is thin layer of C.T. within the perineurial sheath run the

Schwann cell – sheathed axons.

Peripheral nerve connective tissue

Ganglia

Ganglia are small group of nerve cells outside the CNS. They are nodular masses of

neuronal cell bodies, glial cells and Satellite cells supporting by C.T, ganglia transmit

nerve impulses. According the direction of the nerve impulse, there are two kinds of

ganglia in the PNS:

1- Sensory ganglia: It receives afferent impulses that go to the CNS. The neurons of

these ganglia are pseudounipolar neurons and relay information from the

ganglion's nerve endings to the gray matter of the spinal cord via synapses with

local neurons. Satellite cells surround ganglion cells with large number.

* The sensory ganglia include:

- Cranial ganglia, which are associated with some of the cranial nerves.

- Spinal ganglia, which are associated with dorsal root of the spinal nerve.

2- Autonomic ganglia: which are small bulbous dilatations in autonomic nerves. The

neurons of these ganglia are multipolar. Satellite cells are less in number than the

sensory ganglion. The autonomic nerves are effect the activity of smooth muscle,

secretion of some glands, and the modulation of cardiac rhythm. It’s function to

maintain constant internal environment ( homeostasis ).

Autonomic nerves comprise an autonomic nervous system, classified into:

- Sympathetic division.

- Parasympathetic division.

Sympathetic division it is that part of the autonomic nervous system consisting of

nerves that located in the thoracic and lumbar regions of the spinal cord, and

functioning in opposition to the parasympathetic division, in which neurons are

found in small ganglia located near or within the effector organs, for example in the

walls of stomach and intestines.