NERVOUS TISSUE
Dr. Atikah Abdul Latiff
University of Cyberjaya
Learning outcomes
At the end of the lecture the student will be able to
1. Describe the features of a typical neuron and cross section of
the spinal cord with the help of a diagram.
2. Describe the different types of neuroglial cells and their
functions.
3. Classify neurons according to morphology, function and size of
the axon.
4. Describe the types of synapses with examples
5. Describe the characteristic microscopic features of peripheral
nerves and the types of ganglia
6. Describe the various lesions in injuries of nerve tissue.
Nervous tissue – one of 4 basic tissue types
• The protoplasmic properties of irritability (react to stimuli) and
conductivity are well developed.
• The basic properties of nervous tissue are –
1. To receive stimuli from within and outside the body
2. To conduct stimuli from Receptor to CNS (afferent)
3. To transmit impulses from CNS to (efferent)
i) muscles for contraction
ii) glands for secretion
Subdivisions of the nervous tissue
1. CNS - Central nervous system
Brain and Spinal cord
2. PNS - Peripheral nervous system
i) Cranial nerves (12 pairs) from brain
ii) Spinal nerves (31 pairs) from spinal cord
3. ANS
– Autonomic nervous system
i) Sympathetic and
ii) Parasympathetic
Nervous tissue (histological aspect of
the NS)
Composed of two types of cells
1. Neurons (or ) Nerve cells
2. Nervous connective tissue –
Neuroglial cells
Neurons
•
Neurons are structural and
functional units of nervous tissue.
•
Adult neurons do not undergo
mitosis
Nerve cells or neurons are responsible for the reception, transmission,
and processing of stimuli, the triggering of
certain cell activities, and release of
neurotransmitters.
Neurons :- Consist of 3 parts
1. Cell body or perikaryon - which is the trophic center for the whole
cell and also receptive to stimuli.
2. Dendrites - Multiple elongated processes specialized in receiving
stimuli from environment, sensory epithelial
cells, or other neurons. They possess Nissl granules
3. Axons - Single process specialized in conducting nerve impulse
to other cells (nerve, muscle and glands).
Axon hillock- initial segment of an axon, devoid of Nissl granules.
Photomicrograph of a
Motor neuron from
spinal cord
Classification of neurons
(A) According to morphology
Based on the number of processes emerging from the cell body into
1. Bipolar neuron – One dendrite and one axon emerging from the cell
body at opposite ends. Found in special sense organs example
retina, olfactory neuron.
2. Pseudo unipolar or unipolar neuron – single process dividing into
two. One goes to peripheral ending and the other to the CNS.
Found in posterior root ganglion.
3. Multipolar neuron – more than one dendrite and single axon . Most
neurons of the brain and spinal cord.
Retina,
Olfactory neuron.
Posterior root ganglion Most neurons of the
brain and spinal cord
(B) According to function (OR) Functional types of neurons
1.Motor neurons – Send motor impulses to muscles and glands and
bring about movements of muscles and secretion of
glands.
Found in
i) Brain – Cranial motor nuclei
ii) Spinal cord – Ventral horn of the grey matter
2.Sensory neurons- receive impulses from peripheral receptors
Found in
i) Brain – Cranial sensory nuclei
ii) Spinal cord- 1. Posterior root ganglia
2. Dorsal horn grey matter
3.Connector neuron - connect and integrate the motor and sensory
neurons
i)- gray mater of spinal cord
i) Brain – Cranial sensory
nuclei
ii) Spinal cord- 1. Posterior
root ganglia
2. Dorsal horn grey matter
Connector (inter)
neuron:
- gray mater of
spinal cord
i)
Brain – Cranial motor
nuclei
ii)
Spinal cord – Ventral
horn of the grey matter
( C ) According to size of the axon found in the CNS
1. GOLGI TYPE I NEURONS – Have long axon. Axons form long
fiber tracts (ascending and descending) of the brain and spinal cord
and nerve fibers of peripheral nervous system.
Examples are
i) Pyramidal cells of the cerebral cortex
ii) Purkinje cells of cerebellar cortex and
iii) Motor cells of the spinal cord.
2. GOLGI TYPE II NEURONS – have a short axon that terminates
near the cell body.
They are more numerous than the Golgi type I neurons.
They are numerous in the
i) cerebral cortex
ii) cerebellar cortex
and are often inhibitory in function.
Synapses
• Site of functional contact between
two neurons at which nerve impulses pass
from one neuron to another.
• There is no anatomical continuity.
• The function of the synapse is to convert
an electrical impulse from the presynaptic
cell into a chemical signal that acts on the
post synaptic cell.
• This is done by releasing
neurotransmitters.
Neurotransmitters are chemicals
that when combined with a receptor
protein initiate second–messenger
cascades.
• The synapse is formed by an axon
terminal (presynaptic terminal)
that delivers the signal.
• A region of another cell where a
new signal is generated is
postsynaptic terminal.
•The thin intercellular space
between them is the synaptic cleft.
• The presynaptic terminal always
contain synaptic vesicles with
neurotransmitters and numerous
mitochondria.
• Neurotransmitters are synthesized
in the cell body and then stored in
vesicles in the presynaptic region.
• During transmission they are
released into the synaptic cleft by
exocytosis.
If an axon forms a synapse
• With a cell body it is called an –
Axosomatic.
• With a Dendrite – Axodendritic
•
With an axon -
Axoaxonic
3 types of synapses (structurally)
Two types of synapses (variation in the messengers)
1 Chemical synapses (use chemical messengers)- commonest
2 Electrical synapses (transmits ionic signals through gap junctions).
The two types differ in structure and in the mechanism of impulse
transmission.
Impulse transmission is faster at electrical synapses. Eg: CNS
Neuroglial cells or Glial
cells
Non-neuronal nervous
connective tissue cells
• Consists of cell and
processes
• Their number is more
than the neurons
• They provide a
microenvironment suitable
for the neuronal activity.
Drawings of neuroglial cells
• Astrocytes have vascular end-feet
that attaches to the wall of blood
capillaries.
Two types:
1. Protoplasmic astrocyte
2. Fibrous astrocyte
• Microglia
• Oligodendrocytes
(A) Astrocytes are star shaped cells with multiple radiating processes.
There are 2 types of astrocytes
1. Fibrous astrocytes located in the white matter
2. Protoplasmic astrocytes with many short branched
processes found in the grey matter.
Function of astrocytes
1. Structural support- hold blood
vessels and neurons in place
2. Metabolic exchanges- increases
blood supply when need arises
3. Blood brain barrier- perivascular feet
4. Repair processes – scar tissue
can form
Photomicrograph of Fibrous Astrocyte
(B) Microglial cells
•
Are small elongated cells with short
irregular processes and dense elongated
nuclei.
• Found in the CNS.
• Phagocytic cells of the mononuclear
phagocytic system in the nerve tissue.
• They are involved in inflammation and
repair in the adult CNS.
•When activated
1. Microglia retract their processes and
become macrophages which are
phagocytic
2. Act as antigen presenting cell
3. Dispose off unwanted cellular
debris caused by CNS lesions.
(C) Ependymal cells
•
Are low columnar epithelial cells,
lining the
i) ventricles of the brain and
ii) central canal of the spinal cord
•
Secreting epithelium which produces the
cerebrospinal fluid.
•
In some locations ependymal cells are
ciliated which facilitates the movement
of cerebrospinal fluid .
(D) Oligodendrocytes
• Small cell with less number of
processes.
• Function is formation of
myelin and electric insulation in
CNS. Oligodendrocytes can
branch and serve one neuron and
its processes
(E) Schwann Cell
• Myelin production and
electric insulation in the PNS.
• One Schwann cell forms
myelin around a segment of one
axon by wrapping of the Schwann
cell membrane around the axon.
Oligodendrocytes
Single oligodendrocyte forms
myelin sheath for several
nerve fibers in the CNS
NR- gap in myelin sheath
produced by adjacent
Schwann cells.
SL- pockets of residual
cytoplasm left after
myelination.
Nerve fibers
• Consist of an axon
enveloped by a special sheath
In PNS the sheath
cell is the schwann cell.
In CNS the sheath
cell is the oligodendrocyte.
Neurolemmocyte= Schwann cell
Connective tissue components
of a peripheral nerve
A peripheral nerve consist of nerve fibers
and their supporting Schwann cells held
together by connective tissue organized into
3 distinct components
• Endoneurium – loose connective tissue
surrounding each individual nerve fiber.
• Perineurium- specialized connective
tissue surrounding each nerve bundle or
fascicle. Serves as a diffusion barrier
that contributes to the formation of the
blood-nerve barrier.
• Epineurium – dense irregular
connective tissue that surrounds a
peripheral nerve and fills the spaces
between nerve bundles or fascicle
Neurapraxia
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Nerve Injuries
Mildest form of nerve damage.
No disruption of nerve or its sheath.
Only interruption in conduction of impulse in the nerve.
Full recovery with true regeneration within hours to months of injury.
Eg: compression of nerve, disruption of blood supply, numbness
Axonotmesis
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More severe injury
Disruption of neuronal sheath sparing myelin sheath.
Can cause paralysis of motor, sensory and autonomic functions.
Eg: Stretching of fractured bones and joints causing nerve injury
Neurotmesis
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Most severe lesion.
Disruption of axon and myelin sheath.
Involves perineurium, endoneurium, axons and myelin sheaths.
Complete loss of motor, sensory and autonomic functions.
Eg: severe contusion, stretch, laceration, local anaesthetic toxicity.
Reactions of neurons to injury
1. Damage of the cell body results in death of
the neuron.
Neurons cannot be replaced postnatally
because
i) neurons cannot undergo cell division
ii) no neural precursor cells are present
after birth.
2. Severing or crushing of an axon (axonal
injury) leads to characteristic changes.
a) Wallerian (anterograde) degeneration
occurs in the distal portion of the of the axon
i) The axon swells and degenerates, the
myelin sheath fragments and phagocytes
remove cellular debris.
ii) After 3 weeks Schwann cells proliferate
and form a tube of cells distal to the injury.
Recovery and regeneration of damaged
axons
If the lesion in the peripheral nerve is in a
sufficient distance from the cell body, the
nerve may recover
i) Axonal sprouts are produced at the
distal end of the axon stump using materials
synthesized in the cell body.
ii) The axon stump elongates and seek the
tubes formed by Schwann cells.
iii) If sprouts penetrate the Schwann cell
tube and reestablish contact with the
appropriate effector cell, regeneration is likely
to occur.
b) If axonal injury is close to cell body, reactional changes occur in the
neuron cell body (retrograde degeneration)- loss of trophic support
i) The cell body swells due to edema and nucleus is displaced
peripherally
ii) Nissl bodies disperse known as chromatolysis (absent
cytoplasmic basophilia)
Demyelinative diseases of the nervous system
Multiple sclerosis (MS) is an immune-mediated inflammatory
disease that attacks myelinated axons and oligodendrocytes in the
central nervous system, destroying the myelin and the axon in
variable degrees.
Its counterpart in the peripheral nervous system is inflammatory
demyelinative polyradiculoneuropathy (Guillain-Barré syndromeGBS) and its chronic variants.
Pathophysiology of MS:
BBB disruption (The blood–brain barrier is a protective barrier that
denies the entrance of foreign material)→ penetration of the barrier
by lymphocytes and activated → result in direct attacks on myelin
sheaths and oligodendrocytes within the CNS →characteristic
demyelination