What is the correct pathway of a nerve impulse during a spinal reflex arc, such as withdrawing a hand from a sharp object?
Answer and explanation
Correct answer: A. Receptor -> Sensory neuron -> Relay neuron in spinal cord -> Motor neuron -> Effector
- Step 1: Trace the stimulus-response sequence in an involuntary somatic reflex: stimulus detected by specialized receptors generates an action potential.
- Step 2: The impulse travels along the afferent sensory neuron into the dorsal horn of the spinal cord, crosses a synapse to an interneuron (relay neuron), exits via an efferent motor neuron, and reaches the effector muscle to trigger withdrawal.
Answer
Receptor -> Sensory neuron -> Relay neuron in spinal cord -> Motor neuron -> Effector.
Why not the others: 'Receptor -> Motor neuron -> Brain -> Sensory neuron -> Effector' reverses sensory and motor pathways; 'Effector -> Motor neuron -> Spinal cord -> Sensory neuron -> Receptor' starts at the muscle rather than the receptor; 'Receptor -> Brain -> Relay neuron -> Sensory neuron -> Effector' bypasses direct spinal integration.
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