Histology Of Synapse And Neuroglia By Dr. Roomi 422r15

HISTOLOGY OF NERVOUS TISSUE BY DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)

SYNAPSES • Definition: it is the site of functional b/w two neurons at which an electric impulse is transmitted from one neuron to another.

Types of synapses- on the basis of site of 1. Axodendritic synapses (most common type) 2. Axosomatic synapses 3. Dendrodenritic synapses 4. axosaxonic synapses

Types of synapses- on the basis of method of signal transmission Chemical synapses: • Most common type • Signal transmission is delayed for about 0.5 ms in these synapses. Electrical synapses: • Less common • Flow of ions from one neuron to another via gap junctions. • Signal transmission is nearly instantaneous.

Anatomy of a typical synapse (synaptic morphology)

• Axon terminals • Pre-synaptic membrane • Post-synaptic membrane • Synaptic cleft (20-30nm wide) • Synaptic vesicles.

Events occurring at a chemical synapse during signal transmission

Neuroglia or ing cells •

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Glial cells are 10 times more abundant than neurons Don’t generate action potential Don’t make synapses Their main function is to provide ing framework for the neurons Neuroglia are best studied by silver or gold staining techniques

TWO MAIN TYPES OF NEUROGLIA: • Neuroglia proper: include astrocytes, oligodendrocytes and microglia • Ependyma: line the cavities in CNS • astrocytes+ oligodendrocytes = macroglia

astrocytes • • • • •

Astrocytes (Gr. astron, star, + kytos) have a large number of radiating processes and are unique to the CNS. Star shaped cells astrocytes are by far the most numerous glial cells ** Largest of all neuroglia** Contain lightly staining nucleus

• TWO TYPES OF ASTROCYTES: 1. fibrous astrocytes : These are with relatively few long processes and are located in the white matter; 2. protoplasmic astrocytes: these are with many short, branched processes, are found in the gray matter.

FUNCTION OF ASTROCYTES: 1. Astrocytes have ive 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 the CNS. 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.

BLLOD BRAIN BARRIER 1. Brain endothelial cells are ed by tight junctions 2. In peripheral endothelial cells there is good transcellular movement of molecules. There is no such movement in brain endothelial cells. 3. Brain capillaries are in with foot processes of astrocytes which essentially separate the capillaries from the neurones.

Clinical importance of astrocytes • Astrocytes are the most common source of the brain tumors • Tumors of astrocytes are called as astrocytomas. • The processes of all astrocytes are reinforced with bundles of intermediate filaments made of glial fibrillary acid protein (GFAP), which serves as a unique marker for astrocytomas.

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 as small cells with rounded, condensed nuclei. Function: produce the myelin sheath that provides the electrical insulation for neurons in the CNS. Oligodendrocytes extend processes that wrap around parts of several axons, producing a myelin sheath.

EPENDYMAL CELLS • Ependymal cells are low columnar or cuboidal cells that line the ventricles of the brain and central canal of the spinal cord. • In some CNS locations, the apical ends of ependymal cells have cilia, which facilitate the movement of cerebrospinal fluid (CSF), or long microvilli, which are likely involved in absorption. • Modified ependymal cells contribute to the formation of choroid plexus.

MIGROGLIA •

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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 and many 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).

MEDICAL APPLICATION • In multiple sclerosis, the myelin sheath is damaged by an autoimmune mechanism. • In this disease, microglia phagocytose and degrade myelin debris • AIDS dementia complex is caused by HIV-1 infection of the central nervous system. evidence indicates that microglia are infected by HIV-1. A number of cytokines, such as IL-1 and TNF, activate and enhance HIV replication in microglia. These cytokines are toxic to the CNS.

Schwann Cells (Neurolemmocytes) •

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Schwann cells, also called neurolemmocytes, are found only in the PNS and have trophic interactions with axons and allow for their myelination like the oligodendrocytes of the CNS. These are flat cells Derived from neural crest cells.*** These cells make myelinated or unmyelinated covering over neurons. One neurolemmocyte forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and sheath parts of more than one axon. a series of Schwann cells covers the full length of an axon.

Satellite Cells of Ganglia • Derived from the embryonic neural crest • small satellite cells form a covering layer over the large neuronal cell bodies in PNS ganglia. • Closely associated with the neurons, the satellite cells exert a trophic or ive role