So far, the discussion has focused on reflexes driven by sensory receptors located within muscles or tendons. Other reflex circuitry, however, mediates the withdrawal of a limb from a painful stimulus, such as a pinprick or the heat of a flame. Contrary to what might be imagined, given the speed with which we are able to withdraw from painful stimuli, this flexion reflex involves several synaptic links (Figure 16.13). As a result of activity in this circuitry, stimulation of nociceptive sensory fibers leads to excitation of ipsilateral flexor muscles and reciprocal inhibition of ipsilateral extensor muscles. Flexion of the stimulated limb is also accompanied by an opposite reaction in the contralateral limb (i.e., the contralateral extensor muscles are excited while flexor muscles are inhibited). This crossed extension reflex serves to enhance postural support during withdrawal of the affected limb from the painful stimulus.

Figure 16.13

Spinal cord circuitry responsible for the flexion reflex. Stimulation of cutaneous receptors in the foot leads to activation of spinal cord local circuits that withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory (more...)

Like the other reflex pathways, local circuit neurons in the flexion reflex pathway receive converging inputs from several different sources, including cutaneous receptors, other spinal cord interneurons, and upper motor neuron pathways. Although the functional significance of this complex pattern of connectivity is unclear, changes in the character of the reflex following damage to descending pathways provides some insight. Under normal conditions, a noxious stimulus is required to evoke the flexion reflex; following damage to descending pathways, however, other types of stimulation, such as squeezing a limb, can sometimes produce the same response. This observation suggests that the descending projections to the spinal cord modulate the responsiveness of the local circuitry to a variety of sensory inputs.