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.
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 vesidles to the surface of the knob. The synaptic vesicles are fused with the presynaptic membrane and get ruptured to discharge their
contents (neurotransmitter) 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 postsynaptic 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. Thus, the impulse is transferred to next neuron.
