Q.Give a brief account of the mechanism of synaptic transmission.

Synaptic transmission is the handing over of a nerve impulse from one neuron to the next across a junction called a synapse. At a chemical synapse the two neurons are separated by a narrow, fluid-filled gap, the synaptic cleft, so the signal is carried across by chemical messengers called neurotransmitters.

• An action potential travels down the pre-synaptic neuron and reaches its axon terminal.
• Its arrival lets calcium ions (Ca2+) enter the terminal, which makes the synaptic vesicles move towards the pre-synaptic membrane.
• The vesicles fuse with that membrane and release their neurotransmitters into the synaptic cleft.
• The neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane. …

A nerve impulse does not remain trapped inside a single neuron. For information to travel through the nervous system, the impulse must be handed over from one neuron to the next, and this handover takes place at specialised junctions called synapses. A synapse is formed where the membrane of one neuron meets the membrane of the next. The neuron that brings the impulse up to the junction is the pre-synaptic neuron, and the one that receives it on the other side is the post-synaptic neuron.

Synapses are of two kinds. At an electrical synapse the two membranes lie extremely close together, so electrical current flows straight from one neuron into the next, much as an impulse moves along a single continuous axon; this route is very fast but is rare in the human body. The far more common route is the chemical synapse, and this is what the mechanism below describes.

At a chemical synapse the two neurons are not in direct contact. Their membranes are separated by a narrow, fluid-filled gap called the synaptic cleft. Electricity cannot jump this gap on its own, so the impulse is carried across by chemical messengers called neurotransmitters. These are stored in tiny sacs, the synaptic vesicles, packed into the axon terminals of the pre-synaptic neuron.

The transmission proceeds step by step:

• An impulse, in the form of an action potential, travels down the pre-synaptic neuron and arrives at its axon terminal.
• The arrival of the impulse triggers the movement of the synaptic vesicles towards the pre-synaptic membrane; this is set off by the entry of calcium ions (Ca2+) into the terminal.
• The vesicles fuse with the pre-synaptic membrane and release their neurotransmitters into the synaptic cleft.
• The released neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane. …