A fast nociceptive subsystem mediating rapid reflexive behavior but not affective pain

Spinal nociceptive withdrawal reflexes are widely believed to rely on unmyelinated and thinly myelinated nociceptive fibers that also signal affective and motivational aspects of pain. Here we discover a population of myelinated mechanoreceptive nociceptor that forms free nerve endings as well as circumferential endings around hair follicles, and exclusively terminate in the deep spinal dorsal horn. Optogenetic activation of these fibers triggers rapid withdrawal reflexes that are precise and selective for the targeted limb, while silencing increases the threshold of mechanical nociceptive withdrawal reflexes. By contrast, optogenetic stimulation of the fibers is not associated with place aversion nor with changes in facial expression. Thus, we conclude that this nerve fiber population is uniquely positioned to rapidly respond to mechanical threats via selective withdrawal of the targeted body part, whereas other fast and slow nociceptive pathways are required for affective-motivational aspects of pain.