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HISTOLOGY OF NERVOUS TISSUE
BY
DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)
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SYNAPSES
Definition: it is the site of functional contact
b/w two neurons at which an electric impulse
is transmitted from one neuron to another.
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Types of synapses- on the basis of site of
contact
1. Axodendritic synapses
(most common type)
2. Axosomatic synapses3. Dendrodenritic
synapses
4. axosaxonic synapses
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Types of synapses- on the basis of method
of signal transmission
Chemical synapses:
Most common type
Signal transmission isdelayed for about 0.5 ms
in these synapses.Electrical synapses:
Less common
Flow of ions from one
neuron to another via gapjunctions.
Signal transmission isnearly instantaneous.
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Anatomy of a typical synapse (synaptic morphology)
Axon terminals
Pre-synaptic membrane
Post-synapticmembrane
Synaptic cleft (20-30nm
wide)
Synaptic vesicles.
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Events occurring at a chemical synapse during
signal transmission
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Neuroglia or supporting cells
Glial cells are 10 times moreabundant than neurons
Dont generate action potential
Dont make synapses
Their main function is to providesupporting framework for the
neurons Neuroglia are best studied by
silver or gold staining techniques
TWO MAIN TYPES OF NEUROGLIA:
Neuroglia proper: include
astrocytes, oligodendrocytesand microglia
Ependyma: line the cavities inCNS
astrocytes+ oligodendrocytes =macroglia
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astrocytes
Astrocytes (Gr. astron, star, + kytos)have a large number of radiatingprocesses and are unique to the CNS.
Star shaped cells
astrocytes are by far the mostnumerous glial cells **
Largest of all neuroglia** Contain lightly staining nucleus
TWO TYPES OF ASTROCYTES:
1. fibrous astrocytes : These are withrelatively few long processes and
are located in the white matter;2. protoplasmic astrocytes: these are
with many short, branchedprocesses, are found in the graymatter.
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FUNCTION OF ASTROCYTES:
1. Astrocytes have supportive roles for
neurons and are very important for
proper formation of the CNS during
embryonic and fetal development.
2. have major roles in controlling the ionic
environment of neurons.
3. Some astrocytes develop processes with
expanded perivascular feet that cover
capillary endothelial cells and contribute
to the formation of blood-brain barrier.
4. Their cellular processes form the
superficial glial limiting membrane which
acts as sealed barrier b/w pia mater theCNS.
5. Furthermore, when the CNS is damaged,
astrocytes proliferate to form scar tissue
and thus fill in the gaps after tissue is lost
due to injury or disease.
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BLLOD BRAIN BARRIER
1. Brain endothelial cells arejoined by tight junctions
2. In peripheral endothelial cells
there is good transcellularmovement ofmolecules. There is no suchmovement in brain endothelialcells.
3. Brain capillaries are in contact
with foot processes ofastrocytes which essentiallyseparate the capillaries fromthe neurones.
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Clinical importance of astrocytes
Astrocytes are the mostcommon source of thebrain tumors
Tumors of astrocytes are
called as astrocytomas. The processes of all
astrocytes are reinforcedwith bundles ofintermediate filaments
made ofglial fibrillaryacid protein (GFAP),which serves as a uniquemarker for astrocytomas.
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Oligodendrocytes or
oligodendroglia
Oligodendrocytes (Gr. oligos, small,few + dendron, tree + cytos, cell).They have only a few short processes.
No perivascular feet
Located in grey and white matter.
oligodendrocytes usually appear assmall cells with rounded, condensednuclei.
Function: produce the myelin sheaththat provides the electrical insulationfor neurons in the CNS.
Oligodendrocytes extend processesthat wrap around parts of severalaxons, producing a myelin sheath.
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EPENDYMAL CELLS
Ependymal cells are lowcolumnar or cuboidal cells thatline the ventricles of the brainand central canal of the spinalcord.
In some CNS locations, theapical ends of ependymal cellshave cilia, which facilitate themovement of cerebrospinalfluid (CSF), or long microvilli,which are likely involved in
absorption. Modified ependymal cells
contribute to the formation ofchoroid plexus.
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MIGROGLIA
Somewhat less numerous than
oligodendrocytes or astrocytes but more
evenly distributed throughout gray and white
matter
microglia are small cells with short irregular
processes.
They have condensed, elongated nucleus andmany short branching processes.
Unlike other glial cells microglia migrate
through the neuropil, analyzing the tissue for
damaged cells and invading microorganisms.
They secrete a number of immunoregulatory
cytokines and constitute the major
mechanism of immune defense in CNS
tissues.
Microglia originate from blood monocytes,
belonging to the same family as
macrophages and other antigen-presenting
cells (APCs).
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Satellite Cells of Ganglia
Derived from the
embryonic neural crest
small satellite cells form a
covering layer over thelarge neuronal cell bodies
in PNS ganglia.
Closely associated with
the neurons, the satellite
cells exert a trophic or
supportive role
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