In a reflex arc, the receptor first detects the______.
Stimulus
A reflex arc is the pathway followed by a reflex action: receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector.
The receptor is the sense organ or nerve ending that first detects a change in the surroundings – the stimulus, such as heat or a pin prick. It converts the stimulus into a nerve impulse, which the sensory neuron carries to the spinal cord.
The effector (a muscle or gland) is at the other end of the arc and produces the response; the response is the action itself, which comes last; hormones are chemical messengers of the endocrine system and are not detected by the receptor in a reflex arc.
Hence, in a reflex arc the receptor first detects the stimulus.
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