4 nervous tissue

NERVOUS TISSUE
• Nervous tissue consists of two main cell types:
neurons (nerve cells) and neuroglia (non-neuronal,
glial cells).
• Neurons are the structural and functional units of
the nervous system specialized for rapid
communication.
• A neuron is composed of a cell body with processes
(extensions) called dendrites and an axon.
Compiled by Dr. D. Simon

• The cytoplasm contains a large central nucleus with a
prominent nucleolus, numerous mitochondria,
lysosomes and a Golgi complex.
• The cytoplasm also shows the presence of a granular
material that stains intensely with basic dyes called
Nissl substance or Nissl bodies or Nissl granules.
• The dendrites terminate near the cell body.
• They are irregular in thickness, and Nissl granules
extend into them.
• The axon may extend for a considerable distance
away from the cell body.

• The longest axons may be as much as a meter long.
• Each axon has a uniform diameter, and is devoid of
Nissl substance.
• The Nissl-free zone of the axon extends for a short
distance into the cell body: this part of the cell body
is called the axon hillock.
• The dendrites carry impulses to and the axons away
from the cell body.

• The most common type of neuron gives off several
processes and the cell body is therefore multipolar.
• Some neurons have only one axon and one dendrite
called bipolar.
• Another type of neuron has a single process. After a
very short course this process divides into two.
• One of the divisions represents the axon; the other is
functionally a dendrite. This type is called as
unipolar.

Neurons

• During development, each axon comes to be associated
with certain cells that provide a sheath for it.
• The cells providing this sheath for axons lying outside
the central nervous system are called Schwann cells.
• Axons lying within the central nervous system are
provided a similar covering by a kind of neuroglial cell
called an oligodendrocyte.
• An axon lying near a Schwann cell invaginates into the
cytoplasm of the Schwann cell.

• In this process the axon comes to be suspended by a
fold of the cell membrane of the Schwann cell. This
fold is called the mesaxon.
• The mesaxon becomes elongated and comes to be
spirally wound around the axon.
• Thus the axon is surrounded by several layers of cell
membrane.
• Lipids and proteins are deposited between adjacent
layers of the membrane.
• These layers of the mesaxon, along with the lipids
and proteins, form the myelin sheath.
• Outside the myelin sheath a thin layer of Schwann
cell cytoplasm persists to form an additional sheath
which is called the neurolemma.

• There are some axons that are devoid of myelin
sheaths.
• These unmyelinated axons invaginates into the
cytoplasm of Schwann cells, but the mesaxon does
not spiral around them.
• Another differnce is that several such axons may
invaginate into the cytoplasm of a single Schwann
cell.

• Neurons communicate with each other at synapses,
points of contact between neurons.
• The communication occurs by means of
neurotransmitters, chemical agents released or
secreted by one neuron, which may excite or inhibit
another neuron, continuing or terminating the relay
of impulses or the response to them.

Nodes of Ranvier:
• The myelin sheath is in the form segments. Each
segment is formed by one Schwann cell.
• At the junction of any two such segments there is a gap
in the myelin sheath called a node of Ranvier.
• The part of the nerve fiber between two such nodes is
called the internode.
• When an impulse travels through a nerve fiber, it does
not proceed uniformly along the length axis, but jumps
from one node to the next. This is called saltatory
conduction.

• In unmyelinated neurons the impulse travels along
axolemma. Such conduction is much slower than the
saltatory conduction and consumes more energy.
• Immediately next to a node the myelin sheath shows
an expansion called the paranodal bulb.
• There are longitudinal furrows on the surface of the
paranodal bulb.
• These furrows are filled in by Schwann cell cytoplasm.
• Finger-like processes of this cytoplasm extend towards
the axon and come in contact with it.
• They interdigitate with processes from the
neighbouring Schwann cell.

Node of Ranvier

• In the intervals between these processes the axon is
covered by a gap substance that plays a role in
regulating the flow of the nerve impulse by
influencing the passage of ions into, and out of, the
axon.
• At a node of Ranvier the axon itself is much thinner
than the internode.
• The part of the axon passing through paranodal bulb
shows infoldings of its axolemma (cell membrane)
that correspond to the grooves on the surface of the
paranodal bulb.

• Blood-Nerve Barrier:
• Peripheral nerve fibers are separated from circulating
blood by a blood-nerve barrier.
• Capillaries in nerves are non-fenestrated and
endothelial cells are united by tight junctions.
• There is a continuous basal lamina around the
capillary.
• This barrier is reinforced by cell layers present in the
perineurium.

• Neuroglia (glial cells or glia) are approximately five
times as abundant as neurons, are non-neuronal,
non-excitable cells that form a major component of
nervous tissue.
• Neuroglia support, insulate, and nourish the
neurons.

• Synapse:
• Synapses are sites of junction between neurons.
• In the most common, an axon terminal establishes
contact with the dendrite of a receiving neuron to
form an axo-dendritic synapse.
• Synapses on dendrites may be located on spines or
on the smooth areas between spines.
• The axon terminal may synapse with the cell body
axo-somatic synapse.

• Axo-axonal synapse
• Dendr-oaxonic synapse
• Dendro-dendritic synapse
• Somato-somatic synapse
• Somato-dendritic

• A synapse transmits an impulse only in one
direction.
• The two elements taking part in a synapse can,
therefore, be of presynaptic and postsynaptic.
• In an axo-dendritic synapse, the terminal
enlargement of the axon called as presynaptic
bouton or synaptic bag.
• The region of the dendrite receiving the axon
terminal is the postsynaptic process.
• The two are separated by a space called the synaptic
cleft.
• Delicate fibers or granular material may be seen
within the cleft.

Synapse

• On either side of the cleft there is a region of dense
cytoplasm.
• On the presynaptic side this dense cytoplasm is
broken into several bits.
• On the postsynaptic side the dense cytoplasm is
continuous.
• The thickened areas of membrane on the presynaptic
and postsynaptic sides constitute the active zone of
a synapse. Neurotransmission takes place through
this region.
• Within the presynaptic bouton numerous synaptic
vesicles can be seen.

• The transmission of impulses through synapses
involves the release of chemical substances called
neurotransmitters, that are present within synaptic
vesicles.
• When a nerve impulse reaches a terminal bouton
neurotransmitter is released into the synaptic cleft.
• Under the influence of the neurotransmitter the
postsynaptic surface becomes depolarized resulting
in a nerve impulse in the postsynaptic neuron.

• Central Nervous System:
• The central nervous system consists of the brain and
spinal cord.
• Grey matter and white matter:
• Sections through the spinal cord or through any part of
the brain show certain regions that appear darker
greyish colour, and others that have a whitish.
• These constitute the grey matter and white matter
respectively.
• Cell bodies of neurons are located only in grey matter
that also contains dendrites and axons starting from or
ending on the cell bodies.

• Most of the fibers within the grey matter are
unmyelinated.
• On the other hand the white matter consists
predominantly of myelinated fibers.
• Isolated masses of grey matter anywhere in the CNS
are referred to as nuclei.
• Aggregations of cell bodies outside the CNS are
referred to as ganglia.
• Aggregations of axons in the CNS are called as tracts.
• Neuroglia and blood vessels are present in both grey
and white matter.

• In transverse sections of the spinal cord, the grey
matter appears roughly as an H-shaped area
embedded in a matrix of white matter.
• The struts (supports) of the H are horns; therefore,
there are right and left posterior (dorsal) and
anterior (ventral) grey horns.

Spinal cord and meninges

• There are paired (right and left) intermediolateral
cell columns (IMLs) are a part of the grey matter,
extending between the first thoracic (T1) and the
second or third lumbar (L2 or L3) segments of the
spinal cord.
• In horizontal sections of this part of the spinal cord,
the IMLs appear as small lateral horns of the H -
shaped grey matter, looking somewhat like an
extension of the cross-bar of the ‘H’ between the
posterior and the anterior horns of grey matter.

• Structure and Components of a Typical Spinal
Nerve:
• A typical spinal nerve arises from the spinal cord by
nerve rootlets, which converge to form two nerve
roots:
• the anterior (ventral) root consists of motor
(efferent) fibers passing from nerve cell bodies in the
anterior horn of the spinal cord grey matter to
effector organs located peripherally.

• The posterior (dorsal) root consists of sensory
(afferent) fibers that convey neural impulses to the
CNS from sense organs (e.g., the eyes) and from
sensory receptors in various parts of the body (e.g.,
in the skin).
• Both types of sensory fibers - visceral sensory and
general sensory have their cell bodies in spinal
ganglia (sensory ganglia) (dorsal nerve root ganglia)
of spinal nerves.

• The anterior and posterior roots unite at the
intervertebral foramen to form a spinal nerve, which
immediately divides into two rami (branches):
• an anterior ramus and a posterior ramus.
• As branches of a mixed spinal nerve, the anterior
and posterior rami also carry both motor and
sensory nerves, as do all their branches.

• The anterior rami supply nerve fibers to a much
larger area, consisting of anterior and lateral regions
of the trunk and the upper and lower limbs arising
from them.
• The posterior rami supply nerve fibers to synovial
joints of the vertebral column, deep muscles of the
back, and the overlying skin.

• Sensory Ganglia:
• Neurons are large and arranged in groups.
• The neurons are unipolar.
• The groups of cells are separated by groups of
myelinated nerve fibers.
• The cell body is surrounded by a layer of flattened
capsular cells or satellite cells.
• The satellite cells are continuous with the Schwann
cells covering the processes arising from the neuron.
• Outside the satellite cells there is a layer of delicate
connective tissue.
• The connective tissue covering each neuron is
continuous with the endoneurium.

Sensory Ganglia (Neurons in groups)

Sensory Ganglia (Neurons Unipolar)

• Autonomic Ganglia:
• The neurons are smaller than those of sensory
ganglia.
• Neurons are multipolar.
• The neurons are not arranged in groups. But
scattered throughout the ganglion.
• The nerve fibers are non-myelinated and thinner.
• Satellite cells are present but not so well defined.
• The ganglion is permeated by connective tissue that
also provide a capsule for it (just as in sensory
ganglia).
• The Nissl substance of the neurons is much better
defined.

Autonomic Ganglia (Neurons scattered)

Autonomic Ganglia (Neurons Multipolar)

• Neuroglia:
• Supporting cells in the nervous system.
• (a) Neuroglial cells found in the parenchyma of the
brain and spinal cord.
• (b) Ependymal cells lining the ventricular system.
• (c) Schwann cells forming sheaths for axons of
peripheral nerves.
• (d) Capsular cells (satellite cells) that surround
neurons in peripheral ganglia.
• (e) Various types of supporting cells found in relation
to motor sensory terminals of nerve fibers.

• Neuroglial cells are in two major categories.
• (i) Macroglia (or large glial cells)
• Two types.
• (a) Astrocytes which may be subdivided into fibrous
astrocytes and protoplasmic astrocytes.
• (b) Oligodendrocytes
• (ii) Microglia (or small glial cells)
• All neuroglial cells are much smaller in size than
neurons, but the number is much more than the
neurons.

• Astrocytes:
• Small star shaped cells give of number of processes.
• The processes are often flattened into leaf-like
laminae that may partly surround neurons and
separate them from other neurons.
• Fibrous astrocytes are seen mainly in the white
matter.
• Their processes are thin and are asymmetrical.
• Protoplasmic astrocytes seen mainly in grey matter.
• Their processes are thicker and are symmetrical.

Astrocytes and Micoglial cells

• Oligodendrocytes:
• These cells have rounded or pear shaped cell bodies
with relatively few processes.
• These cells provide myelin sheaths to axons that lie
within the CNS.
• An oligodendrocyte may enclose several axons
(Schwann cell that ensheaths only one axon).
• Microglia:
• Smallest neuroglial cells; The cell body is flattened;
The cell processes are short; Seen frequently in
relation to capillaries; More numerous in grey matter.

Oligodendrocyte

• Three membranous layers –
• pia mater, arachnoid mater, and dura mater
collectively constitute the meninges.
• The meninges and the cerebrospinal fluid (CSF)
surround and protect the CNS.
• The brain and spinal cord are intimately covered on
their outer surface by the innermost meningeal layer,
a delicate, transparent covering, the pia mater (pia).
• The CSF is located between the pia and the
arachnoid mater (arachnoid), in the subarachnoid
space.

• External to the pia and arachnoid is the thick, tough
dura mater (dura), which is intimately related to the
internal aspect of the bone of the surrounding
neurocranium (braincase).
• The dura of the spinal cord is separated from the
vertebral column by a fat-filled space, the
epidural space.

• Peripheral Nervous System:
• The peripheral nervous system is made up of nerve
fibers and nerve cell bodies that connect the CNS
with peripheral structures.
• Peripheral nerves consist of bundles of nerve fibers,
their connective tissue coverings, and blood vessels
(vasa nervorum = blood vessels supplying nerve).
• A peripheral nerve fiber consists of an axon; its
neurolemma, the neurolemma (Schwann) cells that
immediately surround the axon separating it from
other axons; and its endoneurium, a connective
tissue sheath.

• In the PNS, the neurolemma may take two forms,
creating two classes of nerve fibers:
• The neurolemma of myelinated nerve fibers have a
neurolemmal (myelin) sheath that consists of a
continuous series of neurolemma (Schwann) cells
enwrapping an individual axon, forming myelin.

• The neurolemma of unmyelinated nerve fibers
consist of multiple axons separately embedded
within the cytoplasm of each neurolemma
(Schwann) cell.
• These neurolemma cells do not produce myelin.
• Most fibers in cutaneous nerves (nerves that supply
sensation to the skin) are unmyelinated.

• Peripheral nerves are protected by three connective
tissue coverings.
• Endoneurium, a delicate connective tissue sheath
that surrounds the neurolemma cells and axons.
• Perineurium, a layer of dense connective tissue that
encloses a fascicle (bundle) of peripheral nerve
fibers.

• Epineurium, a thick connective tissue sheath that
surrounds and encloses a bundle of fascicles,
forming the outermost covering of the nerve; it
includes fatty tissues, blood vessels, and lymphatics.
• A collection of nerve cell bodies outside the CNS is a
ganglion. There are both motor (autonomic) and
sensory ganglia.

• Peripheral nerves are both cranial and spinal nerves.
• Of the 12 pairs of cranial nerves (CN), only 11 arise
from the brain; the Spinal accessory nerve(CN XI)
arises mostly from the superior part of the spinal
cord.
• All cranial nerves exit the cranial cavity through
foramina in the cranium (G. kranion, skull).
• All 31 pairs of spinal nerves - 8 cervical (C), 12
thoracic (T), 5 lumbar (L), 5 sacral (S), and 1
coccygeal (Co) arise from the spinal cord and exit
through intervertebral foramina in the vertebral
column.

• Peripheral Nerve Degeneration:
• When peripheral nerves are crushed or severed,
their axons degenerate distal to the lesion because
they depend on their cell bodies for survival.
• A crushing nerve injury damages or kills the axons
distal to the injury site; however, the nerve cell
bodies usually survive and the connective tissue
coverings of the nerve are intact.
• No surgical repair is needed for this type of nerve
injury because the intact connective tissue sheaths
guide the growing axons to their destinations.

• Surgical intervention is necessary if the nerve is cut
because the regeneration of axons requires
apposition of the cut ends by sutures through the
epineurium.
• The individual fascicles (bundles of nerve fibers) are
realigned as accurately as possible.
• Compromising a nerve's blood supply for a long
period, produces ischemia by compression of the
vasa nervorum, which can also cause nerve
degeneration.

4 nervous tissue

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