Nervous Tissue & The Nervous System
The human nervous system, the most complex system in the human body
the system collect stimuli from the environment, transforms such stimuli into nervous
impulses , and passes them to a large highly organized reception and correlation area
where they are interpreted , and in turn, issued to effector organs to institute
appropriate responses.
These functions are performed by highly specialized cells called neurons which
together with their supporting or neuroglial cells and associated extracellular material
form this integrated communications network .
Anatomically, the nervous system can be divided into two parts : the central nervous
system (CNS), consisting of the brain and spinal cord, and the peripheral nervous
system (PNS), which includes all other nervous tissue . The CNS receives all stimuli
originating outside the body (exteroceptive) and all nervous impulses from the body
(interoceptive) and act as an integrating and communications center. The PNS serves
to interconnect all other tissues and organs with the CNS.
Functionally , the nervous system is divided into somatic and autonomic parts ,with
central & peripheral divisions.
The somatic part is concerned with structures derived from the embryological
somites,i .e muscles , bones and skin. The autonomic nervous system innervates
smooth & cardiac muscle & the gland of the body , a great extent, its functions are
independent of the rest of the nervous system.
In the neuron, two properties of protoplasm are highly developed . These are
irritability , the capacity for response to physical & chemical agents with the initiation
of an impulse , and conductivity , the ability to transmit that impulse.
Neurons thus are excitable and react to stimuli , by stimulating or inhibiting other
neurons with which they are in contact Such specialized contacts between neurons are
called synapses.
Structurally, nerve tissue consists of two cell types : nerve cells or neurons , which
usually show numerous long processes , and several types of glial cells, which have
short processes , support & protect neurons, and participate in neural activity , neural
nutrition , defense processes of the CNS .
The typical neuron consists of a cell body (perikaryon or soma) with an axon and
usually several dendrites.
The perikaryon , formed by the nucleus and surrounding cytoplasm, has a receptive
function . In most neurons , it receives stimuli generated in other nerve cells, but its
main role is as the trophic center of the cells supplying organelles & macromolecules
to its processes .
Usually the perikaryon is large up to (135) micrometers μm in diameter , although
some are only (4) μm .
In most nerve cells, the nucleus is large (up to 20 μm) in diameter spherical , and
centrally situated in the soma . Chromatin is fine and dispersed , and there is one or
more large nucleoli, indicating active synthesis in the cell. The nuclear envelope is
distinct and shows numerous pores. Within cytoplasm, the mitochondria are usually
small , ovoid or spherical , the Golgi apparatus usually is large , paranuclear in
position .Centriols are not prominent in nerve cells (Nerve cells are in capable of cell
division) .
A characteristic feature of the perikaryon of larger neurons is the presence of Nissl
bodies . These represent the basophilic components of the cytoplasm and are stainable
by basic aniline dyes . The bodies are formed by stacks of granular endoplasmic
reticulum with associated ribosomes , both free and attached . They also are present in
dendrites , but are absent in the axon & the axon hillock , a clear conical area at the
origin of the axon from the soma. In small soma , only a diffuse basophilia is seen .
The reaction of the Nissl bodies to injury or exhaustion after prolonged stimuli is
characteristic , the Nissl bodies braking up and diffusing throughout the cytoplasm .
This is called chromatolysis . The presence of extensive granular reticulum ,
prominent nucleoli and numerous mitochondria is concerned with synthesis of new
protein that passes to all regions of the neuron, including its axon & dendrites .
A second prominent feature of neurons is the presence of neurofibrils visible by light
microscopy in silver stains and found throughout the perikaryon , dendrites & axons .
These bundles contain both microtubules (neurotubules) and microfilaments
(neurofilaments).
Nerve cell processes :
These processes are cytoplasmic extentions of the nerve cell body developed to
provide conduction pathways & to provide greater surface area for contact .
These processes are of two types :
Dendrites : Most nerve cells have numerous dendrites that structurally resemble the
perikaryon . Most dendrites show branching, the branches being of smaller diameter
than the main stem. Main stem dendrites contain Nissl bodies , mitochondria and
small Golgi apparatus, but the content of endoplasmic reticulum & ribosomes is
reduced with branching until these are absent from the smaller, more slender twigs.
A prominent feature is the present of many neurotubules & neurofilaments ,aligned
along the axis of the dendrite and in some ,extending nearly to the tips. Functionally ,
there is considerable evidence that the microtubules are involved in transport of
organelles such as mitochondria and proteins from the perikaryon to the terminations
of the dendrites.
In many neurons, the dendrites appear to be beaded, the dendrites covered by
numerous , small , spine-like processes called dendritic spines that are specialized for
synaptic contact. Generally , spines have a short, slender stem (0.5-.0 ) μm in length
with an expanded tip (0.5-2.0) μm in diameter. Spines are prominent , particularly on
major dendrites , less so near the perikaryon and at dendritic tips . They represent the
main synaptic surface of the dendrites.
Axon : The axon is a single , cylindrical process arising from the nerve cell body at
a region called the axon hillock . Its initial portion is often the narrowest part . The
axon hillock contains no chromophil substance , but contain numerous neurotubules
& neurofilaments . Axons generally have a smooth contour and are of uniform
diameter , usually more slender , longer and straighter than dendrites varying from a
fraction of a millimeter to more than a meter in length . Along its course , an axon
may or may not show side branches or collaterals , these leaving the axon at aright
angle near the axon hillock .
Axons usually terminate in twig-like branchings , the telodendria , which contact the
perikaryon , dendrites ,or axon of one or more neurons at synapses . At their
terminations the axonal twigs show small swellings , called boutons terminaux .
Types of Neurons :
According to the size and shape of their processes , there are three types of neurons :
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Pseudounipolar neurons : which have a single processes that is close to
perikaryon and divide form a T- shape with one branch extending to peripheral
ending & the other toward the CNS. Pseudounipolar neurons are found in the
spinal ganglia & also found in the most cranial ganglia . The nerve cell body is
globular in shape.

Bipolar neurons : Are found in the retina & olfactory mucosa & inner ear . The
nerve cell body is spindle-shaped.
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Multipolar neurons : have more than two processes , one axon and many (At
least two dendrites ) . Most neurons in the body (over 99 %) are multipolar .
There are great variation in size & shape among the multipolar type . For
example :
Anterior horn cell or motor neuron cell of the spinal cord , has a large cell
body with dendrites radiating in all directions
Smaller neurons with dendrites that radiate in all directions are termed stellate

Purkinje cells of the cerebellar cortex . The cell body is flask-shaped with a
single or more frequently branched dendrites arising from the pointed pole and
branching extensively , but in a single plane , a small axon leaves the opposite
broader pole of the perikaryon.
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Pyramidal cells of the cerebral cortex have an apical dendrite and 4 or more
branching dendrites passing outward the base of the pyramid , the axon also
leaves the base centrally .
According to the length of the axons , there are two types of neurons :
Golgi type 1 neurons : have a well-developed dendritic tree and along axon
that leaves gray matter enters white matter , and run in a major fiber tract of CNS
to terminate in other area such as muscle or skin. This type include anterior horn
cell ( motor .n ) Purkinje cell & pyramidal cell.
Golgi type 11 neurons : have short axons that do not leave the area of their
perikarya often passing from their dendrite field for only a short distance >
Examples are the interneurons , especially neurons in cerebral & cerebellar
cortices.
Neurons can also be classified a according to their functional roles :
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Motor (efferent) neurons control effector organs such as muscle fibers and
exocrine & endocrine glands.
Sensory (afferent) neurons , are involved in the reception of sensory stimuli
from the environment and from within the body.
Interneurons , establish relationship among other neurons forming complex
functional networks . Interneurons are multipolar neurons between the sensory
& motor neurons . the vast majority of neurons are interneurons .
Neuroglia : Although neurons are the principle cells nerve tissue , glial cells play
important supporting role . This tissue functions to bind together the nervous tissue of
CNS. Neuroglial cells generally are small and only nuclei are seen in routine
preparations , thy are studied best by special silver & gold staining that demonstrate
the entire cell.
Neuroglia include Macroglia , Microglia and Ependymal cells.
Macroglia include :- Astrocytes and oligodendrocytes
Astrocytes : are star-shaped cells with many branching cytoplasmic processes .
Nuclei are large , ovoid or spherical and pale staining . There are bundles of glial
filaments, similar to neurofilaments extending into the cytoplasmic processes.
Cytoplasmic processes pass between neurons and their processes and extend to blood
vessels as pedicle or perivascular feets and to the surface of the brain and spinal cord
to the pial surface forming a layer beneath it .
Two types of astrocytes are recognized : * Protoplasmic astrocytes , located mainly in
gray matter of the brain & spinal cord . They have short thick processes with many
branching and lie around neurons , synaptic area , and blood vessels .
* Fibrous astrocytes : are found mainly in white matter and have long , slender
processes with few or no branches . Glial filaments, are prominent in this type.
Astrocytes are important as a supporting or structural elements . In the CNS , after
brain damage , astrocytes remove neuronal debris .
Oligodendrocytes : ( oligodendroglia ) are smaller than astrocytes with fewer ,
shorter cell processes . Their nuclei are small ovoid in shape . It occur mainly in two
locations : in gray matter closely associated with the perikarya of neurons
(perineuronal satellite cells) , and among bundles of axon in white matter
(interfascicular oligodendrocytes). Others lie in aperivascular position around blood
vessels.
Oligodendrocytes are responsible for myelin formation thus serving the same function
as Schwann cells in the PNS . Unlike Schwann cell , each oligodendrocyte has several
processes and thus forms myelin sheaths around several adjacent nerve fibers .
Microglia : (Mesoglia) are small , somewhat elongated cells with short spiny
processes . Few in number, they are derived from mesoderm , whereas all other types
of neuroglia are ectodermal . Microglia lie in both white & gray matter usually near
blood vessels . They were believed to be the main source of phagocytic cells in the
CNS . ( aperivascular phagocytic cell ) .
Ependyma : are low columnar epithelial cells lining the ventricles of the brain and
central canal of the spinal cord . In some locations the cells are ciliated , which
facilitates the movement of cerebrospinal fluid .
Ganglia : A collection of nerve cell bodies located outside the CNS is called a
ganglion . A similar collection in the substance of the CNS is termed a nucleus .
Ganglia are of two main types : those of the craniospinal group (sensory ganglia) and
those of the autonomic nervous system (Visceral , Motor ganglia). Ganglia vary in
size , ranging from very small ones containing only a few nerve cell bodies to very
large ones with 50,000 or more cells . Each ganglion has a con . t capsule , which may
be quite dense a round large ganglia . Continuous with capsule is a fine con . t
network This network , found throughout the substance of the ganglion , is composed
of fine collagenous & reticular fibers . Blood vessels run in the con .t . In addition to
nerve cell bodies , nerve fibers (axons & dendrites) are present with their supporting
sheaths and each ganglionic cell has a capsule , composed of a single layer of small
cuboidal cells called satellite cells .
Craniospinal Ganglia : are fusiform or globular swelling of the posterior roots of
spinal nerves, and cranial ganglia are similar swelling of some cranial nerves . The
ganglionic cells are pseudounipolar, globular . Histologically , both have asimillar
structure . The perikarya are arranged in groups at the periphery or cortical zone of the
ganglion separated by fiber bundles. The perikarya may be only 15-25μm in diameter
with processes that are unmyelinated or up to 100μm with myelinated processes. Each
perikarya has a capsule formed by asingle layer of small, low cuboidal cells, the
satellite cells or amphicytes around which is a thin layer of vascular con.t . Centrally,
located in the ganglion are bundles of nerve fibers, forming amedullary zone, where
perikarya are few.
Autonomic Ganglia : are swellings located along the sympathetic chain and it
branches , and within the walls of organs supplied by the autonomic system, where
they may be very small. Like craniospinal ganglia, a con.t capsule envelops the
ganglion, except in very small peripheral ganglia. Ganglion cells are multipolar and
smaller than those of sensory ganglia being 15- 45μm in diameter. Perikarya do not
show a tendency to group, nor are the axons grouped into bundles, perikarya and
fibers are intermingled in the ganglion.
Nerve Fibers : consists of axons enveloped by a special sheath derived from cells
of ectodermal in origin . Nerve fibers exhibit differences in their enveloping sheaths
related to whether the fibers are part of CNS or PNS. Most axons in adult nerve tissue
are covered by single or multiple folds of a sheath cell. In PNF, the sheath cell is
Schwann cell, and in CNF it is oligodendrocytes. Axons of small diameter are usually
unmyelinated n.f . Thick axons are generally sheathed by concentric wrapping of the
enveloping cell, forming the myelin sheaths . These fibers are known as myeliated n.f.
Myelin consists of many layers of modified cell membrane. These membrane have a
higher proportion of lipids . Myelin is a lipoprotein complex whose lipid component
removed by histological procedures.
Each axon is surrounded by myelin formed by a sequential series of Schwann cells .
The myelin sheath shows gaps along its path called the nodes of Ranvier , these
represent the spaces between adjacent Schwann cells along the length of the axon .
The distance between two nodes is called an internode and consists of one Schwann
cell . Internode length varies between 1 & 2 mm .
In the CNS, the myelin sheath is formed by the processes of the oligodendrocytes .
Oligodendrocytes differ from Schwann cells in that different branches of one cell can
envelop segments of several axons .
Unmyelinated Fibers : In both the CNS & PNS , not all axons are sheathed in
myelin . In the PNS , all unmyelinated axons are enveloped within simple clefts of
Schwann cells . Each Schwann cell can sheathed many unmyelinated axons .
unmyelinated nerve fibers do not have nodes of Ranvier .
The CNS is rich in unmyelinated axons , these axons are not sheathed . In brain and
spinal cord , unmyelinated axonal processes run free among the other neuronal
processes .
Schwann cells are essential to the vitality & function of PNF . They form myelin ,
also are necessary for regeneration of axons, also Schwann cells can become
phagocytic after nerve injury, removing cellular debris .
Peripheral Nerve : are composed of bundles of nerve fibers held together by
connective tissue and include spinal nerves connected to the spinal cord and cranial
nerves connected with the brain. Most nerves are mixed containing both sensory
(afferent) and motor (efferent) fibers . Surrounding the entire nerve is a sheath of
dense con.t called epineurium . It is composed of fibroblasts and collagenous fibers
mainly longitudinal in orientation and few elastic fibers . The epineurium contains the
major blood vessels to the nerve. Within the epineurium , nerve fibers are grouped
into bundles or fascicles. Each fascicle is surrounded by con.t sheath called
perineurium. This sheath is formed by concentric layers or sleeves of flattened
fibroblast-like cells, each sleeve being one cell thick. The number of sleeves decreases
as the nerve branches becomes smaller. The perineurium provides a barrier to the
passages of materials into or out of the nerve fascicle. Within the perineurium are
strands of delicate con.t extending around and between individual nerve fibers. This is
the endoneurium, composed of delicate collagenous and reticular fibers and flattened ,
elongated fibroblasts. The peripheral nerves establish communication between brain
and spinal cord centers and the sense organs and effectors (muscles , glands, etc) .
Meninges : The CNS is protected by the skull and the vertebral column . It is also
encased in membranes of con. t called the meninges. Starting with the outermost
layer, the meninges are the dura mater , arachnoid , and pia mater . The arachnoid and
the pia mater are linked together and are often considered a single membrane called
the pia-arachnoid .
Dura mater: is the external layer (meninx), and is composed of dense con.t
continuous with the periosteum of the skull. The dura mater that envelops the spinal
cord is separated from the periosteum of the vertebra by the epidural space , which
contains thin-walled veins , loose con.t and adipose tissue. The dura mater is always
separated from the arachnoid by thin subdural space . The internal space of all dura
mater, as well as its external surface in the spinal cord is covered by simple squamous
epi of msenchymal origin .
Arachnoid : The arachnoid has two components : alayer in contact with the dura
mater , and a system of trabeculae connecting the layer with the pia mater . The
cavities between trabeculae form the subarachnoid space , which is filled with CSF
(cerebrospinal fluid) , and is completely separated from the subdural space. This space
form a hydraulic cushion that protects the CNS from trauma. The subarachnoid space
communicates with the ventricles of the brain. The arachnoid is composed of delicate
spider web like con.t devoid of blood vessels.
In some areas, the arachnoid perforates the dura mater , forming protrusions that
terminate in venous sinuses in dura mater. These protrusions are called arachnoid
villi, and covered by endothelial cells of the veins. Their function is to reabsorb CSF
into the blood of venous sinuses.
Pia mater : Is a loose con.t containing many blood vessels . It is located quite close to
the nerve tissue , it is not in contact with nerve cells or fibers. Between the pia mater
and the neural elements is a thin layer of neuroglial processes forming a physical
barrier at the periphery of CNS . This barrier separates the CNS from CSF.
The pia mater follows all irregularities of the surface of the CNS and penetrates it to
some extent a long with the blood vessels. Pia mater is covered by squamous cells of
mesenchymal origin .
Blood vessels penetrate the CNS through tunnels covered by pia mater-the
perivascular space . The pia mater disappears before the blood vessels are transformed
into capillaries . In the CNS the blood capillaries are completely covered by
expansions of the neuroglial cell processes.