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H.I.I.D
Dr. Zana
Nervous system
Histology
Introduction
 The nervous system is designed to deliver rapid and precise communication
between different parts of the body by the action of specialized nerve cells
called neurons.
 These highly specialized cells are interconnected and function to gather and
process information and then generate appropriate response signals
The nervous system is divided into two main parts
 The central nervous system (CNS) comprising the brain and spinal cord.
 The peripheral nervous system (PNS) comprising the nerves which run
between the CNS and other tissues, together with nerve 'relay stations'
termed ganglia
 Functionally, the nervous system is divided into the somatic nervous system
which is involved in voluntary functions, and the autonomic nervous system
which exerts control over many involuntary functions. Histologically, however,
the entire nervous system merely consists of variations in the arrangement of
neurons and their supporting tissues
 The functions of the nervous system depend on a fundamental property of
neurons called excitability. As in all cells, the resting neuron maintains an ionic
gradient across its plasma membrane thereby creating an electrical potential.
 Excitability involves a change in membrane permeability in response to
appropriate stimuli such that the ionic gradient is reversed and the plasma
membrane becomes depolarised
 depolarized; a wave of depolarization, known as an action potential, then
spreads along the plasma membrane. This is followed by the process of
repolarization in which the membrane rapidly re-establishes its resting
potential.
 The sites of intercommunication between neurons are termed synapses.
Depolarization of one neuron causes it to release chemical transmitter
substances, neurotransmitters, which Initiate an action potential in the
adjacent neuron.
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 Within the nervous system, neurons are arranged to form pathways for the
conduction of action potentials from receptors to effector organs via
integrating neurons.
 neurotransmitters not only mediate neuron-to-neuron transmission but also
act as chemical intermediates between the nervous system and effector
organs which also exhibit the property of excitability
The Synapse
Concept: Synapses are highly specialized
intercellular junctions which link the neurons of
each nervous pathway.
Similar intercellular junctions link neurons
and their effector cells such as muscle fibers;
where neurons synapse with skeletal muscle they
are referred to as neuromuscular junction or
motor end plate.
Classification of synapses:
According the constitution:
axodendritic synapse
axosomatic synapse
axoaxonal synapse
dendro-axonic
dendro-dendritic
somato-somatic synapse
somato-dendritic synapse
 The effector organs of voluntary nervous pathways are generally skeletal
muscle while those of involuntary pathways are usually smooth muscle,
cardiac muscle and muscle-like epithelial cells (myoepithelial cells) within
some exocrine glands.
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Neuron
 Despite great variation in size and shape in different parts of the nervous
system, all neurons have the same basic structure.
 The neuron consists of a large cell body containing the nucleus surrounded by
cytoplasm known as the perikaryon. Processes of two types extend from the
cell body, namely a single axon and one or more dendrites
Cell body: Perikaryon
The cell body, soma, is the part of neuron
that contains nucleus and surrounding cytoplasm, also called perikaryon. It is the
trophic center of the neuron. The protein and enzymes synthesis in this area.
 Position: only in grey matter in CNS which also contains dendrites and
axons starting from or ending on the cell bodies,ganglia in PNS
 Shape:They can be pyramidal, spherical, ovoid or pear-shaped.
 Size: Measuring 5-150 um in diameter.
(1)Cell membrane: the structure is as the same as the normal cell. It functions
in getting the stimuli and integration and conducting the nerve impulse.
(2) The nucleus is large and pale with H-E stain, prominent nucleoli are very
clear.
(3)Cytoplasm: the cytoplasm has some distinctive characteristics not seen in
other cells. The cytoplasm is basophilic and full of neurofibrils.
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Nissl bodies: The cytoplasm shows the presence
of a granular material that stains intensely with
basic dyes; this material is the Nissl substance
(also called Nissl bodies or granules).
Neurofibrils are thin black fibers observed in LM
with silver nitrate slides, which is composed of
microtubule and filaments in EM.
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EM: rough surfaced endoplasmic reticulum.
 The presence of abundant granular endoplasmic reticulum is an indication
of the high level of protein synthesis in neurons. Mitochondria, SER,
lysosomes, Golgi complexes, ribosome etc.
 The proteins are needed for maintenance and repair, and for production of
neurotransmitters and enzymes.
 Dendrites are highly branched, tapering processes which either end in
specialized sensory receptors (as in primary sensory neurons) or form
synapses with neighboring neurons from which they receive stimuli. In
general, dendrites function as the major sites of information input into the
neuron
 Each neuron has a single axon arising from a cone-shaped portion of the
cell body called the axon hillock The axon is a cylindrical process up to 1
meter in length terminating on other neurons or effector organs by way of
a variable number of small branches which end in small swellings called
terminal boutons
 An axon may have not much branches than that of dendrites. If branches,
that arise near the cell body and lie at right angles to the axon are called
collaterals. At its termination the axon breaks up into a number of fine
branches called telodendria which may end in small swellings (terminal
boutons.
 The axon is identified according to the axon hillock with LM.The part of
the axon just beyond the axon hillock is called the initial segment.
Neurites or processes
Dendrites
many
short
irregular in thickness
Nissl granules
spines
impulse towards the soma
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Axons
one
long
uniform in diameter
No Nissl substance
axon hillock
away from the cell body
H.I.I.D
 Action potentials arise in the cell body as
a result of integration of afferent
(incoming) stimuli; action potentials are
then conducted along the axon to
influence other neurones or effector
organs.
 Axons are commonly referred to as nerve
fibres
 In general, the cell bodies of all neurones
are located in the central nervous
system; exceptions are the cell bodies of
most primary sensory neurones and the terminal effector neurones of the
autonomic nervous system where, in both cases, the cell bodies lie in
aggregations called ganglia in peripheral sites
Basic neuron types
 Throughout the nervous system, neurons have a wide variety of shapes which
fall into three main patterns according to the arrangement of the axon and
dendrites with respect to the cell body.
 The most common form is the multipolar neuron (90% of neurons) in which
numerous dendrites project from the cell body; the dendrites may all arise from
one pole of the cell body or may extend from all parts of the cell body
 In general, intermediate, integratory and motor neurons conform to this
pattern.
 Bipolar neurons have only a single dendrite which arises from the pole of the
cell body opposite to the origin of the axon. These unusual neurons act as
receptor neurons for the senses of smell, sight and balance.
 Most other primary sensory neurons are described as pseudo-unipolar neurons
since a single dendrite and the axon arise from a common stem of the cell body;
this stem is formed by the fusion of the first part of the dendrite and axon of a
bipolar type of neuron during embryological development
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According to the size of cell body and the
length of axon:
Golgi type I neurons: long axon
Golgi type II neurons: short axon
According to their function:
Sense (afferent) neurons
Interneurons
Motor (efferent) neurons
According to the neurotransmitter they release
Cholinergic neurons: acetylcholin
Aminergic neurons: adrenaline, non
Peptidergic neurons: neuropeptids
Neuroglia or glia: neurons are supported by a special kind of connective tissue
within the brain and spinal cord, that is called neuroglia,it also located in the PNS.
• Neuroglia:
Within the central nerve system:
Oligodendrocytes
astrocytes
microglia
ependymal cells
Oligodendrocytes
 small cells that are active in the formation and maintenance of myelin in
the CNS.
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



Round bullous cells with slender cytoplasm wrap around nerve axon.
Smaller than astrocytes with fewer processes.
found in both grey and white matter of CNS
particularly in white matter, processes from these cells form the myelin
sheaths that are around many axons
 analogous to Schwann cells of peripheral nervous system
Astrocytes
 Star shape and most numerous type
 provide physical support for neurons
 store glycogen
 isolate synaptic areas from one and other
 In the brain, processes abut against the basement membrane of capillary
endothelium (pedicles) forming the blood-brain barrier
 other processes are closely applied to neurons (pedicles)
 may form a conduit for nutrients from blood vessels to neurons
Two types of astrocytes
1. Protoplasmic astrocytes
 granular cytoplasm, many branches on short processes
 found mainly in gray matter
2. Fibrous Astrocytes
 have longer slender processes
 found mainly in white matter (but also occur in gray matter).
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Microglia
 small cell body that is usually elongated and stains densely
 sometimes an elongate nucleus with mostly heterochromatin (Other glia have
spherical nucleus)
 many of what were thought to be microglia under the light microscope, have
turned out to be oligodendroglia when cells were examined with the electron
microscope.
 microglial cells are derived from mesoderm and originate from monocytes.
 microglial cells function in phagocytosis - components of immune system, act
as brain macrophages.
 known to migrate and accumulate at the site of nerve damage within the
central nervous system.
Ependymal cells
 ciliated cells forming single layer of simple cuboidal to low columnar
epithelium that lines the entire neurocoel
 Epithelial cells that line ventricles and central cavities of brain and spinal cordsecrete CSF
 ciliary action acts to circulate cerebral spinal fluid.
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