2 Synapse.pptx for first year physiology

2 Synapse.pptx for 1sy year physiology

1. SYNAPSE DR POTNURU SRINIVASA SUDHAKAR

2. Syapse is the junctional region where one neuron ends and the other begins . The terminal branches of the axon ( presynaptic terminals ) of other neurons ( presynaptic cell ) come in contact with the cell body ( soma ) or the dendrites of another ( postsynaptic cell ). Many presynaptic neurons may converge on any single postsynaptic neurons and the axon of any presynaptic neuron may divide into multiple branches and may diverge to end on multiple postsynaptic neurons.

3. CLASSIFICATION According to the nature of connections, the synapses can be classified as such: 1. Axosomatic synapse (Figs 89.1 and 89.2B )

4. 2. Axodendritic synapse

5. 3 . Axo-axonic synapse.

6. In axosomatic synapses, the presynaptic terminal of the axon ends in the cell body (soma) of the neuron . In the cerebellum synaptic connections in between the basket cell and the Purkinje cells are of axosomatic type . The axons of basket cell make synapses with the soma of the Purkinje cells. This type of synapse is also present in cerebral cortex where basket cell makes synapse with the soma of the pyramidal cells.

7. Axo-axonic synapse is also claimed to be present and in such synapse, presynaptic fibres of any axon ends in the axon of the postsynaptic cells.

8. ANATOMY OF THE SYNAPSE 1. Microscopic anatomy of a motor neuron of the anterior horn cells of the spinal cord shows the main body of the neuron-soma, dendrites, axon and multiple presynaptic terminals (synaptic knobs ) ending in the soma and dendrites. These presynaptic terminals are the ends of the neurofibrils of other neurons.

9. 2. Electron microscopic structure shows that there are two types of presynaptic terminals. In one type the ends of the presynaptic terminals are enlarged to form a knob or button which is known as synaptic knob.

10. 3. The synaptic knob has an intact membrane. The membrane of the synaptic knob is known as presynaptic membrane and that of the soma, the postsynaptic ( subsynaptic ) membrane. The synaptic knob has got intimate contact with a portion of the membrane of the postsynaptic cell. The synaptic knobs are separated from the soma of the postsynaptic cell by a synaptic cleft having an average width of approximately 200 Angstroms ( lA = m μ ) .

11. 4. The synaptic cleft actually represents the real discontinuity of cell cytoplasm in the synaptic junction . The synapse of brain cortex ( central synapse ) has parallel intersynaptic filaments ( canaliculi ) of about 50 Angstroms crossing the synaptic cleft. These filaments are fixed at both ends with the presynaptic and subsynaptic ( postsynaptic) membranes . At present the functions of these filaments are not clear.

12. 5. Besides these there is a web of filaments which is implanted on the subsynaptic membrane and extended at a varying and considerable distance in the postsynaptic cytoplasm. This web of filaments ( canaliculi ) is known as subsynaptic web (SSW). This subsynaptic web has been observed in the central synaptic knob contains mitochondria and a large number of synaptic vesicles.

13. 6. The synaptic vesicles are more concentrated towards the sites fronting the synaptic clefts. The tubular (vesicular ) neuroprotofibrils and the vesicles of the endoplasmic reticulum are present but less conspicuous . The synaptic vesicles contain the excitatory transmitter materials that mediate transmission of impulses from the presynaptic neurons to the postsynaptic neurons.

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