Question

Receptor sites for neurotransmitters are present on-
A. Post-synaptic membrane
B. Membrane of synaptic vesicles.
C. Pre-synaptic membrane
D. Tips of axon

Answer 1

Hint: The axon is usually a long fibre at the distal end of which is branched. Each branch then terminates as a bulb-like structure which is called a synaptic knob. It possesses synaptic vesicles which contain the chemicals called as the neurotransmitters.

Complete answer:
The axon then transmits nerve impulses away from the cell body into a synapse or to the neuro-muscular junction. There are 2 types of axons, which are namely, myelinated and the unmyelinated. The myelinated nerve fibres which are enveloped within the Schwann cells, then form a myelin sheath around the axon. The gaps b/w 2 adjacent myelin sheaths are usually called nodes of Ranvier.

Chemical synapses:
At a chemical synapse, the membranes of the pre- and postsynaptic neurons are then separated by a fluid-filled space which is called a synaptic cleft. The presynaptic neuron then transmits an impulse (or action potential) across the synaptic cleft to the postsynaptic neurons. When an impulse (or action potential) arrives at an axon terminal, it can then stimulate the movement of the synaptic vesicles to the membrane where they then usually fuse with the plasma membrane and then release their neurotransmitters in the synaptic cleft. The released neurotransmitters then bind to their specific receptors, which is present on the postsynaptic membrane.

When an impulse (or action potential) arrives at the terminal of the axon, it then stimulates the movement of the synaptic vesicles towards the membrane where they usually fuse with the plasma membrane and then it releases their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors which are present on the postsynaptic membrane. This binding opens the ion channels and allows the entry of ions which can then generate a new potential in the postsynaptic neuron. The new potential developed can be either excitatory or inhibitory.

Hence, the correct answer is option (A).

Note: Chemicals which are called as the neurotransmitters are then involved in the transmission of impulses at these synapses. The axon terminals which contain vesicles are filled with these neurotransmitters.