Q.During synaptic transmission of nerve impulse, neurotransmitter (P) is released from synaptic vesicles by the action of ions (Q). Choose the correct P and Q.
a. P = Acetylcholine, Q = Ca++
b. P = Acetylcholine, Q = Na+
c. P = GABA, Q = Na+
d. P = Cholinesterase, Q = Ca++

In synaptic transmission, the neurotransmitter acetylcholine (P) is released from synaptic vesicles when calcium ions (Q = Ca++) rush into the presynaptic axon terminal, triggering vesicle fusion with the membrane.

When a nerve impulse (action potential) reaches the axon terminal (presynaptic knob), it depolarizes the membrane and opens voltage-gated calcium channels. Ca++ ions flow into the terminal from the extracellular fluid, and this rise in intracellular Ca++ concentration is the trigger that causes the synaptic vesicles (containing the neurotransmitter acetylcholine, in a typical cholinergic synapse) to move to, and fuse with, the presynaptic membrane, releasing acetylcholine by exocytosis into the synaptic cleft. Acetylcholine then diffuses across the cleft and binds receptors on the postsynaptic membrane, generating a new signal.

The other options are wrong: GABA is an inhibitory neurotransmitter, not the general 'P' in this classic excitatory pathway; cholinesterase (option b) is the enzyme that breaks DOWN acetylcholine after signalling, it is not itself released from vesicles; Na+ (option d) is responsible for depolarization/action-potential propagation along the axon, not for triggering vesicle release at the synapse — that specific role belongs to Ca++.