Concept: This question tests your understanding of how nerve impulses are transmitted and the structural components that ensure their unidirectional flow.

Why (C) is correct: The synapse is the specialized junction between two neurons (or between a neuron and an effector cell). Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron. This mechanism ensures that the impulse travels only from the presynaptic neuron to the postsynaptic neuron, thus maintaining unidirectional transmission.

Why other options are wrong:

• (A) Interneurons: Interneurons are a type of neuron that connect other neurons; they don't inherently ensure unidirectional flow.
• (B) Myelin sheath: The myelin sheath insulates the axon and increases the speed of impulse conduction (saltatory conduction), but it doesn't dictate the direction.
• (D) Membrane polarity: Membrane polarity (resting potential, action potential) is essential for impulse generation and propagation, but it doesn't inherently ensure unidirectional flow across neurons.

NEET Tip: Remember that the synapse acts like a one-way valve for nerve impulses, preventing them from traveling backward.

Correct Answer: (C)