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LO: Nervous System Histology
1. Identify and describe the major cellular components of the central and
peripheral nervous systems
Central nervous system
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 meninges and in the walls of large blood
vessels. Collagenous fibres or fibrocytes/blasts are consequently not observed. The two major
classes of cells that make up the nervous tissue are nerve cells, neurones, and supporting
cells, glia.
The cerebrum, cerebellum and spinal cord consist of white and grey matter:
- White matter: does not contain neuronal cells bodies, but consists of myelinated axons
+ myelin producing oligodendrocytes.
- Grey matter: contains neuronal cell bodies, dendrites and initial unmyelinated portion
of axons and glial cells. Synapses occur in this region.
The Meninges include;
- Dura mater: outermost layer composed of dense CT continuous with periosteum of
the skull.
 Dura mater separated from periosteum of vertebrae by epidural space – contains
thin walled veins, loose CT and adipose tissue.
 Dura mater separated from arachnoid mater by thin subdural space.
- Arachnoid mater: has two components (i) a layer in contact with dura mater & (ii) a
system of trabeculae connecting the layer with the pia mater. Cavities between the
trabeculae are the subarachnoid space (communicates with ventricles of the brain)
which is filled with CSF and completely separated from subdural space. It functions
to protect CNS from trauma. Composed of CT devoid of BV’s.
- Pia mater: loose CT containing BV’s. Close to nerve tissue but not indirect contact as
it is separated by a thin layer of neuroglial processes which keeps the CSF away.
Blood brain barrier: functional layer that prevents the passages of substances (antibiotics,
chemical and bacterial toxins) from blood to nerve tissue. Results from reduced permeability
of blood capillaries in nervous tissue due to occluded cell junctions, unfenestrated cytoplasm
of endothelial cells and very few pinocytotic vesicles
Choroid plexus: consists of invaginated folds of pia mater rich in dilated fenestrated
capillaries that penetrate the interior of the brain ventricles. Found in the roofs of the 3rd + 4th
ventricles and partly in walls of the lateral ventricles. Produces CSF that acts as a protective
device against mechanical shocks.
 CSF: it is clear, has a low density and protein content. Few desquamated cells and
lymphocytes are present. Choroid plexus  ventricles  subarachnoid space  venous
circulation.
Peripheral nervous system
The PNS comprises all nervous tissue outside the brain and spinal cord. It consists of groups
of neurones (ganglion cells), called ganglia, feltworks of nerve fibres, called plexuses, and
bundles of parallel nerve fibres that form the nerves and nerve roots.
Nerve fibres, which originate from neurones within the CNS and pass out of the CNS in
cranial and spinal nerves, are called efferent or motor fibres. Nerve fibres which originate
from nerve cells outside the CNS but enter the CNS by way of the cranial or spinal nerves are
called afferent or sensory nerve fibres.
The principal neurotransmitters in the PNS are acetylcholine and noradrenalin.
2. Describe the microscopic structure of peripheral nerves, ganglia and
synapses
Peripheral Nerve
A peripheral nerve is a bundle of nerve fibres held together by connective tissue.
Fibre types
o Type A fibres (myelinated) are 4 - 20 µm in diameter and conduct impulses at high
velocities (15 - 120 m per second). E.g. motor fibers, which innervate skeletal
muscles, and sensory fibres.
o Type B fibres (myelinated) are 1 - 4 µm in diameter and conduct impulses with a
velocity of 3 - 14 m per second. E.g. preganglionic autonomic fibres.
o Type C fibres (unmyelinated) are 0.2 - 1 µm thick and conduct impulses at velocities
ranging from 0.2 to 2 m per second. E.g. autonomic and sensory fibres.
Connective tissue components
o Endoneurium: the loose connective tissue surrounding each individual nerve fibre
 Consists of collagen fibres secreted by Schwann cells.
 Fibrils run parallel and around nerve fibres binding them together into a fascicle
 Fibrocytes, macrophages and mast cells are present in the endoneurium
o Perineurium: the specialised connective tissue surrounding each nerve fascicle

Serves as a metabolically active diffusion barrier, “blood nerve barrier”, which
maintains the ionic milieu of the ensheathed nerve fibres
 Cells in this layer are squamous, have an external basal lamina on both surfaces,
are contractile (contain actin filaments), and have tight junctions.
o Epineurium: the dense irregular connective tissue that surrounds and binds nerve
fascicules into a common bundle.
 Outermost tissue of peripheral nerves, may contain adipose tissue in larger
nerves.
 BV supplying the nerves travel in the epineurium and then the branches
penetrate into the nerve and travel within the Perineurium.
 Tissue is poorly vascularised, metabolic exchange relies on diffusion from the
BV through perineurial sheath.
Cell bodies of peripheral nerves may be located within the CNS or outside the CNS in the
peripheral ganglia. Ganglia contain cluster of neuronal cell bodies and the nerve fibres
leading to and from them
o Cell bodies in dorsal root ganglia + cranial nerve ganglia belong to sensory neurons
(somatic and visceral afferents).
o Cells bodies in paravertebral, prevertebral and terminal ganglia belong to postsynaptic
“motor” neurons (visceral efferents) of the ANS.
o Motor neuron cells bodies of the PNS lies in the CNS
 Cell bodies of motor neurons that innervate sk mm (somatic efferent) are located
in the brain, brain stem and spinal cord. The axons leave the CNS and travel in
peripheral nerves to the sk mm they innervate. A single neuron conveys impulses
from the CNS to the effector organ.
o In the ANS a chain of two neurons connect the CNS to sm mm, cardiac mm, and
glands (visceral efferents). The cell bodies of presynaptic neurons of the ANS are
located in the CNS. Their axons leave the CNS and travel in the peripheral nerves to
synapse with the postsynaptic neurons in the peripheral ganglia.
o Sensory neuron cell bodies are located in the ganglion outside of, but close to the
CNS.
 In the sensory system, (both somatic and visceral afferent components), a single
neuron connects to the receptor, through a sensory ganglion, to the spinal cord or
brainstem. Sensory ganglion re located along the dorsal roots of spinal nerves
(DRG) and in association with the sensory components of CN V, VII, VIII, IX,
and X.
Ganglia
Ganglia are ovoid structures containing neuronal cell bodies and glial cells supported by CT.
Sensory ganglia: receive afferent impulses going to CNS. There are two types
o Cranial ganglia: associated with cranial nerves
o Spinal ganglia: associated with dorsal root of spinal nerves that comprise of large
neuronal cell bodies with prominent fine Nissl bodies surrounded by abundant small
glial cells called satellite cells.
o A CT framework and capsule support ganglion cells. The neurons of ganglia are
pseudounipolar and relay information from the ganglion’s nerve endings to the grey
matter of the spinal cord via synapses with local neurons.
Autonomic ganglia: appear as bulbous dilatations in autonomic nerves. Some are located
within viscera especially their walls i.e. intramural ganglia. They are devoid of CT capsule
and their cells are supported by a stroma. They usually have multipolar neurons, and neuronal
perikaryon’s with fine Nissal bodies. A layer of satellite cells frequently envelops autonomic
ganglia.
Synapses
 Synapses are specialised junction between neurons that facilitate transmission of impulses
from one (presynaptic) neuron to another (postsynaptic) neuron. Synapses also occur
between axons and effector (target) cells such as muscles and glands.
 Synapses between neurons can be classified morphologically as:
o Axodendritic: occurring between axons and dendrites
o Axosomatic: occurring between axons and the cell body
o Axoaxonic: occurring between axons and axons.
 Chemical synapses: in which conduction of impulses is achieved by the release of
chemical substances (neurotransmitters) from the presynaptic neuron. Neurotransmitters
then diffuse across the narrow intracellular space that separates a presynaptic neuron from
a postsynaptic neuron or target cells