Reflex arcs are evolved in animals for quick and efficient responses.

Reflex arcs are evolved in animals for quick and efficient responses. Which one of the following sequences correctly represents a reflex arc?

Receptor—Sensory neuron—Relay neuron in spinal cord—Brain—Motor neuron—Effector

Receptor—Sensory neuron—Brain—Relay neuron in spinal cord—Motor neuron—Effector

Receptor—Motor neuron—Relay neuron in spinal cord—Sensory neuron—Effector

Receptor—Motor neuron—Brain—Sensory neuron—Effector

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UPSC CDS-1 – 2023

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A reflex arc is the pathway followed by nerve impulses during a reflex action. The typical sequence involves a Receptor detecting the stimulus, a Sensory neuron transmitting the signal to the central nervous system (spinal cord), an interneuron (Relay neuron) in the spinal cord processing the signal and synapsing with a Motor neuron, which carries the command to an Effector (muscle or gland) to produce a response. While simple spinal reflexes occur without conscious brain processing for the motor response, the sensory information is often simultaneously transmitted to the brain. Option A, despite including ‘Brain’ in the serial path (which is not how a simple spinal reflex motor command originates), is the only option that correctly orders the initial steps (Receptor -> Sensory neuron) and subsequent components (Relay neuron in spinal cord, Motor neuron, Effector) in a biologically plausible sequence among the given choices, relative to the flow of information and action.

The fundamental components of a reflex arc are Receptor, Sensory neuron, Integration center (often involving interneurons in the spinal cord), Motor neuron, and Effector. The speed of reflex action is due to the short pathway, often bypassing higher brain centers for the rapid motor command.

Reflex arcs can be monosynaptic (one synapse between sensory and motor neuron) or polysynaptic (involving one or more interneurons).

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