Histology of Synapse and Neuroglia by Dr. Roomi

7/31/2019 Histology of Synapse and Neuroglia by Dr. Roomi

1/18

HISTOLOGY OF NERVOUS TISSUE

BY

DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)


2/18

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.


3/18

Types of synapses- on the basis of site of

contact

1. Axodendritic synapses

(most common type)

2. Axosomatic synapses3. Dendrodenritic

synapses

4. axosaxonic synapses


4/18

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.


5/18

Anatomy of a typical synapse (synaptic morphology)

Axon terminals

Pre-synaptic membrane

Post-synapticmembrane

Synaptic cleft (20-30nm

wide)

Synaptic vesicles.


6/18

Events occurring at a chemical synapse during

signal transmission


7/18

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


8/18

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.


9/18

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.


10/18

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.


11/18

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.


12/18

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.


13/18

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.


14/18

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).


15/18


16/18


17/18

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


18/18

Histology of Synapse and Neuroglia by Dr. Roomi

Documents

Histology of Synapse and Neuroglia by Dr. Roomi