The Spinal Reflex Arc
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A spinal reflex arc produces a fast, unconscious motor response through as few as one synapse between a sensory and a motor neuron, without the brain's involvement.
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The knee-jerk reflex a doctor tests runs through a single synapse in the spinal cord.
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A reflex can still send information to the brain afterward, through interneurons, though the response does not wait for it.