Concept: This question tests your understanding of synaptic transmission and the factors ensuring the unidirectional flow of nerve impulses.

Why (D) is correct: The unidirectional transmission of a nerve impulse across a synapse is primarily due to the specialized structure and function of the synapse. Neurotransmitters, the chemical messengers, are stored in vesicles and released exclusively from the presynaptic axon terminals. These neurotransmitters then bind to specific receptors located only on the postsynaptic membrane (dendrite or cell body). This structural and functional asymmetry ensures that the signal can only travel from the presynaptic neuron to the postsynaptic neuron, not in reverse.

Why other options are wrong:

(A) Nerve fibre is insulated by a medullary sheath: While the medullary (myelin) sheath insulates the axon and speeds up impulse conduction, it doesn't directly explain unidirectional transmission across a synapse.

(B) Sodium pump starts operating only at the cyton and then continues into the nerve fibre: The sodium-potassium pump operates along the entire neuron membrane to maintain resting potential, not just starting at the cyton, and it's not the primary reason for unidirectional synaptic transmission.

(C) Neurotransmitters are released by dendrites and not by axon endings: This is incorrect. Neurotransmitters are released by axon endings, not dendrites.

Common Mistake: Students sometimes confuse the role of myelin sheath in speeding up conduction with the mechanism of unidirectional synaptic transmission.

NEET Tip / Mnemonic: Remember that the 'A' in Axon comes before 'D' in Dendrite, just like the signal goes from Axon to Dendrite across the synapse. Also, 'Release' (neurotransmitters) happens at the Axon terminal, and 'Receive' (receptors) happens at the Dendrite.

Correct Answer: (D)