Download Cellular structure of nervous system

Cellular structure of nervous system
The human nervous system contains at least 10 billion neurons.
These basic building blocks of the nervous system have evolved
from primitive neuro-effector cells that respond to various stimuli by
contracting . nerve tissue is distributed throughout the body as an
integrated communications network.
Anatomically ,the nervous system is divided in to:1- the central nervous system which it consist of the brain and
spinal cord
2- the peripheral nervous system ,composed of nerve fibers and
small aggregates of nerve cells.
Structurally, nervous tissue(system) consist of 2 classes of cell
types :1- Nerve cells ,or nurons, which usually show numerous long
processes.
2- several types of glial cells , or neuroglia , which support and
protect neurons and participate in neural activity ,neural nutrition,
and the defense processes of the central nervous system.
Neurons respond to environmental changes (stimuli) by 
altering electrical potential differences that exist between the
inner and outer membranes .
cells with this property : neurons ,muscle cells , some gland 
cells are called excitable or irritable .
Neurons react promptly to stimuli the modification of 
electrical potential may be restricted to the place that
rereceived the stimulus or may spread propagated throughout
the neuron by the membrane . this propagation called the
action potential or nerve impulse which transmits information
to other neurons ,muscles or gland
Na+ Na+ Na+ Na+ Na+ Na+ 
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K+k+ k+ k+ k + k+ k+ k+ k+
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Neuron structure
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Although neurons vary in size and shape , they have certain
features in common, these include cell body (perikaryon or
soma ) and cytoplasmic processes extensions or nerve fibers.
Which conduct impulses to ,or from the cell body . the perikaryon
cotntains:amass of granular cytoplasm ,cell membrane ,numerous
mitochondria ,lysosomes golgi apparatus and a net work of fine
threads called neurofibrils which extend in to nerve fibers.
Near the center of the cell body there is alarge spherical nucleus
with prominent nucleolus. This nucleus does not undergo mitosis
and consequently mature neurons seems .to be incapable to
reproduction.The cytoplasm of neurons has agranular material
that stain intenselywith basic dyes these granules are known as
Nissl bodies which represent sites of active protein synthesis .
Two kinds of processes arise from the cell body , ashorter
processes branching called dendrites and one long process called
Axon.
Dendrites
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Most nerve cells have numerous dendrites which
increase considrably the receptive area of the cell
.dendrites arborization (branching) makes it
possible for one neuron to receive and integrate
with a great number of axon terminals from other
nerve cells.
the composition of dendrites cytoplasm is very
similar to that of the perikaryon , however
dendrites are devoid (without ) golgi complex .
Nissl bodies and mitochondria are present except
in very thin dendrites .
usually its short and divide like the braches of a
tree and covered by a large number of thorny
spines which are small dendritic projections
representing sites of synaptic contact.
Axons
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Most neurons have only one axon ; a very few do not
have an axon at all. An axon is a cylindrical process
that varies in length and diameter according to the
type of neuron.
all axon originate from short pyramid shaped region
called the axon hillock which usually arises from the
perikaryon but in a few cases originates from the
stem of amajor dendrites , the axon hillock can be
differentiated from dendrites by distinctive cytologic
features.
the rough endoplasmic reticulum and ribosomes
found in perikaryons and dendrites but do not extend
in the axon hillock .
In the axon hillock , the microtubules are arranged in
fascicles or bundles . the plasma membrane of the
axon is called the axolema and its contents called
axoplasm
Types of neurons
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Neurons vary considerably in the size and shape of their cell
bodies and in the length and manner of branching of their
processes .according to these factors the neurons are
classified in to :1-Unipolar neurons :A neuron with single process arising from the cell body .it is
rare in adult vertebrates.
2- Pseudounipolar neuron :which has process that bifurcates , one branch passing
peripherally and as dendrites .and other passing centrally as
axon such as the neuron in the cranial and spinal ganglia .
3-Bipolar neurons:- it is un common also it has single axon
and single dendrites ,usually located at apposite sides of
spindle shaped soma,it is found in retina, olfactory
epithelium.
4- Multipolar neurons:with numerous dendrites and usually single axon ,preseeent
in autonomic ganglia.
According to the functional differences it
can be grouped s follows: Sensory neurons:-,are those that carry
nerve impulse from peripheral body parts
9skin) in the brain or spinal cord .
 Inter neurons : or association neurons, lie
within C.N.S.. They form links between
other neurons .Its function to transmit
impulses from one part of the brain or
spinal cord to another.
 Motor neurons: or efferent neurons ,carry
nerve impulses out from C.N.S. to
effectors ( parts of the body such as
muscles or glands ).
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Types of nerves Types of nerves
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Sensory nerves or afferent nerves
that conduct impulses in to C.N.S.
Motor or efferent nerves that carry
impulses out , to the muscles or
glands (effectors) most nerves
include both types, they are so
called
Neuroglia
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Neuroglia :- are supporting cells of C.N.S. , there
are several types and more numerous than
neurons .they fill spaces .suppor neurons and
generally bind together in the nervous tissue of
C.N.S.
Neuroglial cells are smaller than neurons , some
of glial cells are smaller than neurons ,some of
glial cells mobile and ,unlike neurons ,retainthe
ability to divide.
Neurogllial cells may be divided in to two major
categories.
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Macroglia: or large glial cells.
Microglia : or small glial cells.
Macroglial cells are derived from ectoderm of the
neural tube , in contrast,
Microglial cells are derived from the mesoderm.
Types of neuroglia
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Astrocytes ( macroglial cell ):
are star –shaped cells have many radiating cytoplasmic
processes or extensions with expanded tips called
astrocyte pedicles or feed that adhere both to capillaries
and to neurons .
Astrocytes have large ovoid or spherical nucleus, the
cytoplasm contains golgi complex, few ribosomes
,lysosomes and glycogen,and there are bundles of glial
filaments similar to neuro filaments.
Two types of astrocytes are recognized: Protoplasmic astrocytes : seen mainly in gray matter of
C.N.S., they have short ,thick processes with many
branches.
Fibrous astrocytes : are found mainly in white matter and
have only a few thin and large processes , with few or no
branches.the main functions are supporting the C.N.S.
and involve in repair mechanism after brain damage .
2- Oligodendrocyt(macroglial cells)
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Are smaller than astrocytes with fewer shorter
cell processes ,it has small and dar4k nuclei and
scanty cytoplasm with numerous free and
attached ribosomes ,an extensive golgi apparatus
and many mitochondria and microtubules.
They are found in two places :in the gray matter near the cell bodies of neurons
among bundles of axons in white matter.
Oligodendrocytes function in the formation of
myelin within the C.N.S. ;thus serving the same
function as schwann cells in the P.N.S.. however
un like schwann cells each oligodendrocytes has
several processes and these form myelin
sheath,around several adjacent nerve fibers.
3-Glioblast (microglia cells)
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are smallest neuroglial cell.
The cell body is flattened with short
spiny processes few in number and
scattered throughout the C.N.S.
 if THE C. BN.S. damaged they
increase in size and become actively
motile and phagocytic.
Ependymal cells
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are supporting cells lining the
ventricles of the brain and central
canal of spinal cord , where they
constitute continuous epithelial lining
known as the ependyma. these
cells are cuboidal to low columnar
in shape and have few cilia on their
free surface and they cover
specialized parts within the brain
(choroids plexus).