Question

Synaptic vesicles of pre synaptic neurons release neurotransmitter in synaptic cleft due to ________

• A. opening of \(\text{Ca}^{++}\) channels due to change in voltage.
• B. \(\text{K}^{+}\) ions which diffuse out of the axon.
• C. \(\text{Na}^{+}\) which diffuses into the neuron.
• D. \(\text{Na}^{+}\text{- K}^{+}\) pumps become operational.

Hint:

Calcium entry triggers fusion of synaptic vesicles.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
An action potential at the axon terminal depolarises the presynaptic membrane.

Step 2: Sequence of events
The voltage change opens voltage-gated \(\text{Ca}^{2+}\) channels and calcium ions enter the terminal.
The rise in \(\text{Ca}^{2+}\) makes the synaptic vesicles fuse with the membrane and release neurotransmitter by exocytosis.

Step 3: Why the other options fail
\(\text{K}^{+}\) outflow, \(\text{Na}^{+}\) inflow and the \(\text{Na}^{+}\)-\(\text{K}^{+}\) pump relate to the impulse itself, not to vesicle release.

Final Answer:
Release is due to opening of calcium channels by a change in voltage. \[ \boxed{\text{(A) }\text{opening of }\text{Ca}^{++}\text{ channels due to change in voltage.}} \]