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The human CNS contains about 100 billion
It also contains 10-50 times this number of glial
The neurons, the basic building blocks of the
nervous system have evolved from primitive
A Specialized cell transmitting nerve impulses
Otherwise called as nerve cell.
Differ from other cells-
Neuron has branches called axons and dendrites.
Neuron does not have centrosome so it cannot undergo
A whitish fibre or bundle of fibres in the body that
transmits impulses of sensation to brain or spinal
cord,& impulses from these to muscles & organs.
DEPENDING UPON THE NUMBER OF
UNIPOLAR NEURONS- have only one pole from
where both the dendrites and axons arise.
Present only in embryonic stage in human beings.
BIPOLAR NEURONS – two poles, one for axons and
the other for dendrites.
MULTIPLE NEURONS – have many poles. One of
the poles gives rise to axon and all the other poles
give rise to dendrites.
DEPENDING UPON THE FUNCTION :
MOTOR NEURON – also known as efferent nerve cells.
Carry the motor impulses from CNS to the peripheral
effector organs like muscles, glands, blood vessels.
The motor neurons have long axons and short dendrites.
SENSORY NEURONS – also called as afferent
Carry the impulses from periphery to the CNS.
They have short axons and long dendrites.
DEPENDING UPON THE LENGTH OF THE AXON :
GOLGI TYPE 1 NEURONS – have long axons.
The cell body is in CNS and their axons reach the
remote peripheral organs.
GOLGI TYPE 2 NEURONS – have short axons.
Present in the cerebral cortex and spinal cord.
In the early embryonic disc,3rd
week of I.U life,a
layer of ectodermal cells called neuroectodermal
cells form a plate called the NEURAL PLATE or
the medullary lamina.medullary lamina.
A median groove called the neural groove
appears in this plate as its lateral edge raises as 2
folds called NEURAL FOLDSNEURAL FOLDS..
The neural folds starts uniting in the middle and
the union proceeds in both cranial and caudal
direction connecting the neural groove into a
CNS at 3rd
week of IU
Neuro Epithelial Cells
As the neural tube is being formed, cells at the
junction of the neural fold and the skin ectoderm
gets detached to lie as clusters on the dorsolateral
side of the neural tube. The cluster of cells are called
Neural crest or Neuro epithelial cells. DerivativesNeural crest or Neuro epithelial cells. Derivatives
of the neural crest –of the neural crest –
1. Sheath cells of schwann- neurolemmal cells of the
2.2. Dorsal root ganglia of the spinal nerves,
sensory ganglia of the cranial nerves
3. sympathetic and parasympathetic ganglia.
-The caudal part of the neural tube remains
relatively narrow and gives rise to the spinal
The neural tube in the beginning is lined by a
single layer of pseudostratified epithelial cells
which proliferate and migrate outward to form a
second layer called mantle layer.mantle layer.
The cells which migrate from the mantle layer
are of two types :
1.1. NeuroblastsNeuroblasts which differentiate into nerve cells
2.2. SpongioblastsSpongioblasts which differentiate into
neuroglial cells, astrocytes and
3. Microglia are mesodermal in origin andare mesodermal in origin and
migrate into the neural tube along with themigrate into the neural tube along with the
blood vesselsblood vessels
The neuron is made up of three parts :
1. NERVE CELL BODY : irregular in shape,
constituted by a mass of cytoplasm called
neuroplasm covered by a cell membrane.
• NUCLEUS : present in the central part of the nerve
• Has one or two nucleoli.
• No centrosome.
NISSIL BODIES : small basophilic granules.
Present throughout the soma except in axon hillock.
Are called TIGROID SUBSTANCES since these
bodies are responsible for tigroid or spotted
appearance of soma after suitable staining.
Are membranous organelles containing ribosomes -
concerned with protein synthesis in the neurons.
During injury of neuron, these bodies fragment
disappear by a process called chromatolysis.
NEUROFIBRILS : thread like structure present in the
soma and the processes.
Consist of neurofilament and microtubules.
MITOCHONDRIA : form the powerhouse of the cell
where ATP is produced.
GOLGI APPARATUS : concerned with the package
of proteins into the granules.
Nerve cell body also contain lipofuscin pigment and
2. DENDRITE : are the branched process of the
Have nissl bodies and neurofibrils.
Conductive in nature and transmit impulse
towards the nerve cell body.
3. AXON : is the longer process of the nerve cell.
Arises from axon hillock of the nerve cell body
and devoid of NISSIL GRANULES
The distal ends of the terminal branches of the
axons are often enlarged – TERMINALS OR
Some axons near their termination show a
series of swelling resembling a string of beads
and are called as VARICOSITIES.
Structure of axon : the axons are arranged in
different bundles called FASCICULI
The whole nerve is covered by tubular sheath
which is formed by areolar membrane. This sheath
is called as EPINEURIUM.
Each fasciculus is covered is by perineurium and
each nerve fibre is covered by endoneurium.
The axon has a long central core of cytoplasm
called AXOPLASM, covered by the tubular sheath
like membrane called AXOLEMMA
Synapse is the junction between the two
The presynaptic membrane has two important
• Mitochondria which help in the synthesis of
• Synaptic vesicles which store neurotransmitter
The membrane of the postsynaptic neuron is
called postsynaptic membrane – contains
The small space b/w pre and postsynaptic
membrane is called synaptic cleft(20-30nm) –
Conduct electrical impulses along the plasma
Produce nerve impulse
Produce action potential
Longevity: can live and function for a life time
Do not divide: fetal neurons lose their ability to
High metabolic rate: require abundant oxygen
The Nerve is typically
most WAVY in section
although not elastic,
because it must
increased length when
the tissue in which it
resides is strecthed.
Wavy nucleus is seen
Mass of Support cells in the Central Nervous
System (CNS) are grouped together as neuroglia
Neuroglia literally means “nerve glue”.
The cells are nonexcitable hence called the
nonneural cells ,glial cells.
Play an important role in the reaction of nerve
Provide both mechanical and metabolic support
The function of neuroglia is to support, insulate,
and protect the delicate neurons of the brain
Half of all brain tissue
Brace neurons; they provide Mechanical support as
well as mediating the exchange of metabolites between
Neurons and the vascular system,thus regulating the
composition of intercellular environment of CNS.
They also form a part of Blood-Brain barrier
Play an important role in repair of CNS tissue after
Astrocytes are identified at molecular level by IHC
staining for a Protein called Glial Fibrillary Acidic
All Astrocytes contain bundles of intermediate
filaments . The intermediate filaments are formed
of GFAP, which is characteristic of Astrocytes.
Many of astrocytes process end
in terminal expansions adjacent
to the non-synaptic regions of
neurons. Other processes of the
same astrocyte terminate upon
the basement membrane of the
capillaries ,these Perivascular
feet cover most of the surface of
capillary basement membrane
& form part of Blood-Brain
Spiderlike phagocytes (white blood cells) derived
Dispose of debris like dead brains cells and bacteria
Lines the cavities of the brain and spinal cord
Circulate cerebrospinal fluid by beating their cilia
Cerebrospinal fluid, fills the space that the brain
does not take up and forms a protective cushion
around the brain and spinal chord
Wrap around nerve cells in the brain and spinal chord
Produce myelin sheaths
Myelin is a fatty, insulation covering the nerve cells;
allows for the electrical signal to transmit faster (like
A single oligodendrocyte can contribute to the
myelination of upto 50 axons from the same or the
different fibre tracts . Conversly,any one axon will reqire
the service of numerous different oligodendrocyte
because of limited length of myelin segments (internodes)
produced by oligodendrocytes.
Form myelin sheath in the
peripheral nervous system.
A nerve is basically a bundle of
axons (long projections) from
various neurons. Around these
axons (note the dark centers in the
middle of the circular structures
on the high power image on the
right) are glial cells called
Schwann cells. These make up the
white myelin which extends
around the each axon.
Protects neuron cell bodies which is where the
nucleus of the cell if found
TANCYTESTANCYTES : long basal processes with end feet on
line the floor and the third ventricle.
Transport the substances from CSF to
hypophyseal portal system.
Thoroidal epithelial cellsThoroidal epithelial cells : secrete CSF.
MS affects the
ability of nerve cells
in the brain and
spinal cord to
In MS, the body's
system attacks and
damages the myelin
Neuroprexia :: it transient blocks and the paralysis
is incomplete, recovery is rapid and complete and
no microscopic evidence of degeneration.
Pressure is the most important cause.Pressure is the most important cause.
Axonotmesis :: term applied to the nerve lesion in
which the axons are damaged but the
surrounding connective tissues sheaths remain
more or less intact.
Crush injuries, traction and compression are theCrush injuries, traction and compression are the
most common causesmost common causes
Neurotmesis :: term applied to the lesion of
complete section of the nerve trunk.
Sensory recovery is indicated by the
appearance of tingling sensation in the areas of
cutaneous distribution called Tinnel’s sign.Tinnel’s sign.
Light, touch and tactile sensation are the lastLight, touch and tactile sensation are the last
sensation to return and are often incomplete.sensation to return and are often incomplete.
1. K. SMITH - NEURONS PHYSIOLOGY
2. ARTHUN BUTT - NEUROGLIA
3. N.A. GREGSON - CNS MYELIN
4. Wheater's Functional Histology
5. The Neuron , by Richard H.Hall(1998)
6. TORTORA - TEXTBOOK OF PHYSIO AND
7. HAM’S HISTOLOGY