Question

Four events in the transmission of nerve impulses across synapses are given below.

(i) Depolarisation of the presynaptic membrane.

(ii) Propagation of postsynaptic action potential.

(iii) Hydrolysis of transmitter substance.

(iv) Rupturing of synaptic vesicles.

In which sequence do these events occur?
A) (i) → (iii) → (ii) → (iv)
B) (i)→(iv)→(ii) → (iii)
C) (iv)→(i)→(iii)→(ii)
D) (iv)→(iii)→(i)→ (ii)

Answer 1

Hint:
The space between the axon terminal of one neuron and dendrites or cell body of the consecutive neuron is known as a synapse. When a nerve impulse arrives at the terminal of the presynaptic neuron transmission of the nerve impulse to the next neuron occurs by chemical neurotransmitters.

Complete step by step answer:
A nerve impulse arrives at the axon terminal of a presynaptic axon. The change in membrane potential caused by the arrival of action potential leads to the opening of voltage-gated calcium channels in the presynaptic membrane. Calcium moves inside by the open channels as calcium concentration is more in the extracellular fluid. An increase in the concentration of calcium inside the presynaptic neuron serves as a signal that triggers exocytosis/rupturing of the synaptic vesicles. A vesicle membrane merges with the plasma membrane, and neurotransmitter molecules placed within the vesicles, are released into the synaptic cleft. These neurotransmitters move through the synaptic cleft and bind to the neurotransmitter receptors present on the postsynaptic neuron.
The binding of neurotransmitter molecules to their receptor causes the opening of channels and the movement of ions across the membrane. When the ions move through the opened channels, the voltage across the membrane changes, this change in membrane voltage is postsynaptic potential. Firstly the presynaptic membrane is depolarised which mediates the release of neurotransmitter which then goes into postsynaptic receptors and propagate the signal, lastly the neurotransmitter is hydrolyzed.

Option ‘B’ is correct

Note:
A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some of which release inhibitory neurotransmitters. The effect on the postsynaptic neuron Is determined by the total excitatory and inhibitory signals.