Transmission Through Synaps | Biology | NEET

Transmission Through Synapse

The junction between two neurons , across which the impulse passes from one neuron to the next is called synapse .

A typical ( generalised ) synapse consists of a bulbous expansion of a nerve terminal called a pre - synaptic knob lying close to the membrane of a dendrite . The cytoplasm of the synaptic knob contains numerous synaptic vesicles . Each vesicle contains neurotransmitter ( chemical substance ) .

The membrane of the synaptic knob nearest to the synapse forms the presynaptic membrane . The membrane of the dendrite is called the postsynaptic membrane . These membranes are separated by a gap , the synaptic cleft . The post synaptic membrane contains receptor sites for neurotransmitter. The two main neurotransmitters in vertebrate nervous system are acetylcholine ( ACh ) and noradrenaline although other neurotransmitters also exist .

Types of synapse

It is of two types :

( 1 ) Electrical synapse : At electrical synapse there is continuity between the pre - synaptic and post - synaptic neurons . The continuity is provided by the gap junction between the two neurons . In electrical synapse there is minimal synaptic delay because of the direct flow of electrical current from one neuron to the other through gap junctions . Thus , impulse transmission across an electrical synapse is always faster than that across a chemical synapse . Impulse transfer through electrical synapse does not involve any neurotransmitter . Electrical synapses are rare . They are found in the cardiac muscle fibres , smooth muscle fibres of intestine and the epithelial cells of lens .

( 2 ) Chemical synapse : At a chemical synapse , when an impulse arrives at a pre - synaptic knob , calcium ions from the synaptic cleft enter the cytoplasm of the pre synaptic knob . The calcium ions cause the movement of synaptic vesicles to the surface of the knob . The synaptic vesicles are fused with the pre - synaptic membrane and get ruptured to discharge their content ( neurotransmitters ) into the synaptic cleft.

The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post - synaptic membrane . This binding action changes the membrane potential of the post - synaptic membrane , opening channels in the membrane and allowing sodium ions to enter the cell . This causes the depolarisation and generation of action potential in the post - synaptic membrane . The new potential develop may be either excitatory or inhibitory. Thus , the impulse is transferred to the next neuron .

Manish Mevada

M.Sc, M.Phil, B.Ed