The withdrawal response (reflex), also known as the nociceptive flexion reflex, is an automatic response of the spinal cord that is critical in protecting the body from harmful stimuli. The first known definition of a reflex dates back to 1649 when René Descartes noted that specific bodily movements occurred instantaneously and independent of the process of thought. Modern definitions state that a reflex is an involuntary response of effector tissue caused by the stimulation of specific receptors.
The reflex arc is the basic unit of a reflex, which involves neural pathways acting on an impulse before that impulse has reached the brain. Instead of directly traveling to the brain, sensory neurons of a reflex arc synapse in the spinal cord. This is an important evolutionary adaptation for survival, which allows faster actions by activating spinal motor neurons instead of delaying reaction time by signals first having to go to the brain.
The withdrawal reflex can occur in either the upper or lower limbs and is a polysynaptic reflex, which means that interneurons mediate the reflex between the afferent (sensory) and efferent (motor) signals. In contrast, the deep tendon reflex is monosynaptic and does not utilize interneurons to transmit information. Additionally, the withdrawal response is an intersegmental reflex arc, meaning that the outcomes of the reflex are mediated by the stimulation or inhibition of motor neurons from multiple levels of the same spinal cord.
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