Give a brief account of Mechanism of synaptic transmission.
A nerve impulse is transmitted from one neuron to another through junctions called synapses. There are two types of synapses: electrical synapses and chemical synapses.
At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other. Transmission across electrical synapses is always faster than across chemical synapses, but electrical synapses are rare in our system.
At chemical synapses, the membranes are separated by a fluid-filled space called synaptic cleft. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors present on the post-synaptic membrane. This binding opens ion channels, allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential may be either excitatory or inhibitory.
Explanation
This answer covers both types of synapses (electrical and chemical) as mentioned in the NCERT textbook context. For a 3-mark question, the mechanism of chemical synaptic transmission is the primary focus, which includes: arrival of impulse at axon terminal, movement and fusion of synaptic vesicles, release of neurotransmitters, binding to receptors on post-synaptic membrane, opening of ion channels, and generation of new potential. The distinction between electrical and chemical synapses adds completeness to the answer.