Nerve Tissue & Nervous System
Assoc. Prof Dr. Karim Al-Jashamy
Nerve Tissue & the Nervous System:
The human nervous system, by far the most complex system in the
human body, is formed by a network of more than 100 million nerve
cells (neurons), assisted by many more glial cells.
Each neuron has, on average, at least 1000 interconnections with other
neurons, forming a very complex system for communication.
Neurons are grouped as circuits. Like electronic circuits, neural circuits
are highly specific combinations of elements that make up systems of
various sizes and complexities.
Nerve tissue is distributed throughout the body as an integrated
Anatomically, the nervous system is divided into the central nervous
system, consisting of the brain and the spinal cord, and the peripheral
nervous system, composed of nerve fibers and small aggregates of
nerve cells called nerve ganglia
Structurally, nerve tissue consists of two cell types: nerve
cells, or neurons,
Usually show numerous long processes, and several types of
glial cells which have short processes, support and protect
neurons, and participate in neural activity, neural nutrition,
and the defense processes of the central nervous system.
Neurons react promptly to stimuli with a modification of
electrical potential that may be restricted to the place that
received the stimulus or may be spread (propagated)
throughout the neuron by the plasma membrane. This
propagation, called the action potential, or nerve impulse,
is capable of traveling long distances; it transmits information
to other neurons, muscles, and glands.
Central Nervous System (CNS)
The CNS consists of the brain (encephalon), which is
enclosed in the skull, and the spinal cord, which is contained
within the vertebral canal.
Nervous tissue of the CNS does not contain connective
tissue other than that in the three meninges (dura mater,
arachnoid membrane and pia mater) and in the walls of large
Collagenous fibers or fibrocytes/blasts are consequently not
observed, which is quite unlike other tissues. Because of the
absence of connective tissue, fresh CNS tissue has a very
soft, somewhat jelly-like consistency.
The major classes of cells that make up the nervous tissue
are nerve cells, neurones, and supporting cells, glia.
The Central Nervous System
The central nervous system consists of the cerebrum,
cerebellum, and spinal cord. It has almost no
connective tissue and is therefore a relatively soft, gel-
When sectioned, the cerebrum, cerebellum, and spinal
cord show regions that are white (white matter) and
that are gray (gray matter).
The differential distribution of myelin in the central
nervous system is responsible for these differences: The
main component of white matter is myelinated axons
and the myelin-producing oligodendrocytes.
White matter does not contain neuronal cell bodies.
The delicate, innermost, mesh-
like layer of the meninges. The
pia mater closely envelops the
entire surface of the brain,
running down into the fissures
of the cortex.
It joins with the ependyma
which lines the ventricles to
form choroid plexuses that
produce cerebrospinal fluid. In
the spinal cord, the pia mater
attaches to the dura mater by
the denticular ligaments
through the arachnoid
Gray and White Matter
Microscopically, the CNS
contains 2 neural elements:
Neuron cell bodies (clusters are
known as nuclei)
Nerve fibers (axons) in bundles
Viewed macroscopically, CNS
tissues can be distinguished by
Gray matter consists of somata,
dendrites, and unmyelinated
White matter consists primarily of
Gray matter contains neuronal cell bodies, dendrites, and the
initial unmyelinated portions of axons and glial cells.
Gray matter is prevalent at the surface of the cerebrum and
cerebellum, forming the cerebral and cerebellar cortex
whereas white matter is present in more central regions.
Aggregates of neuronal cell bodies forming islands of gray
matter embedded in the white matter are called nuclei
In the cerebral cortex, the gray matter has six layers of cells
with different forms and sizes. Neurons of some regions of
the cerebral cortex register afferent (sensory) impulses; in
other regions, efferent (motor) neurons generate motor
impulses that control voluntary movements.
Cells of the cerebral cortex are related to the integration of
sensory information and the initiation of voluntary motor
Allows for sensation,
voluntary movement, self-
recognition, and more.
40% of brain mass, but only
2-3 mm thick.
Each cerebral hemisphere is
concerned with the sensory
and motor functions of the
opposite side (contralateral
side) of the body.
Silver-stained section of
cerebral cortex showing
neurons with their
processes and a few glial
The cerebellum (H&E) does not reveal
the unusually large dendritic of the
Section of the
shows part of
its rich dendritic
The gray matter of the spinal
cord showing several motor
neurons with their basophilic
bodies (Nissl bodies). Nucleoli
are seen in some nuclei. The
neurons are surrounded by a
mesh of neuronal and glial
processes. PT stain. Medium
• Lies inferior to the cerebrum and
occupies the posterior cranial fossa.
• 2nd largest region of the brain.
• 10% of the brain by volume, but it
contains 50% of its neurons
• Has 2 primary functions:
1. Adjusting the postural muscles of the body
• Coordinates rapid, automatic adjustments, that maintain balance and
2. Programming and fine-tuning movements controlled at the
subconscious and conscious levels
• Refines learned movement patterns by regulating activity of both the
pyramidal and extrapyarmidal motor pathways of the cerebral cortex
• Compares motor commands with sensory info from muscles and joints
and performs any adjustments to make the movement smooth
contains huge, highly
branched Purkinje cells
dendrites can receive up
to 200,000 synapses.
Internally, the white
matter forms a
branching array that in a
resembles a tree – for
this reason, it’s called
the arbor vitae
In cross sections of the spinal cord, white matter is
peripheral and gray matter is central, assuming the shape
of an H.
In the horizontal bar of this H is an opening, the central
canal, which is a remnant of the lumen of the embryonic
It is lined with ependymal cells. The gray matter of the
legs of the H forms the anterior horns. These contain
motor neurons whose axons make up the ventral roots of
the spinal nerves.
Gray matter also forms the posterior horns (the arms of
the H), which receive sensory fibers from neurons in the
spinal ganglia (dorsal roots).
Spinal cord neurons are large and multipolar,
especially in the anterior horns, where large motor
neurons are found
Cross section of the spinal
cord in the transition between
gray matter (below) and white
The gray matter contains
neuronal bodies and abundant
cell processes, whereas the
white matter consists mainly of
nerve fibers whose myelin
sheath was dissolved by the
histological procedure. PT
stain. Medium magnification.
Section of spinal cord gray
matter. The meshwork of
cell neuron and glial
The small nuclei are from
glia cells. Note that these
cells are more numerous
than neurons. H&E stain.
Ganglion cells will typically be
several times larger than other
cells in the ganglia
The perikaryon is very large and
surrounds a large and light
nucleus. Only the cells
immediately surrounding the
ganglion cells as one flattened
layer are satellite cells.
Ganglion cells are of course in
contact with other parts of the
nervous system and with the
peripheral tissues which they
innervate. Consequently, nerve
fibers will be visible close to or
within the ganglion.
longitudinal H&E stained sections it is
possible to identify the axon running
in its myelin sheath, nodes of Ranvier
and Schwann cell nuclei.
Components of the connective tissue
elements, which accompany the
nerve, should be visible and
identifiable in both longitudinal and
transversely cut preparations give a
good picture of the axon in the middle
of a ring-like structure (sometimes
fussy), which represents the remains
of the myelin sheath.
Due to their small size and the lack of
a myelin sheath, type C fibres are very
difficult to detect in either osmium or