A SURVEY OF SPINAL DORSAL HORN NEURONS ENCODING THE SPATIAL-ORGANIZATION OF WITHDRAWAL REFLEXES IN THE RAT

The withdrawal reflex pathways to hindlimb muscles have an elaborate s patial organization in the rat. In short, the distribution of sensitiv ity within the cutaneous receptive field of a single muscle has a spat ial pattern that is a mirror image of the spatial pattern of the withd rawal of the skin surface ensuing on contraction in the respective mus cle. In the present study, a search for neurones encoding the specific spatial input-output relationship of withdrawal reflexes to single mu scles was made in the lumbosacral spinal cord in halothane/nitrous oxi de-anaesthetized rats. The cutaneous receptive fields of 147 dorsal ho rn neurones in the L4-5 segments receiving a nociceptive input and a c onvergent input from A and C fibres from the hindpaw were studied. The spatial pattern of the response amplitude within:the receptive fields of 118 neurones was quantitatively compared with those of withdrawal reflexes to single muscles. Response patterns exhibiting a high simila rity to those of withdrawal reflexes to single muscles were found in 2 7 neurones located in the deep dorsal horn. Twenty-six of these belong ed to class 2 (responding to tactile and nociceptive input) and one be longed to class 3 (responding only to nociceptive input). None of the neurones tested (n=20) with reflex-like response patterns could be ant idromically driven from the upper cervical cord, suggesting that they were spinal interneurones. With some overlap, putative interneurones o f the withdrawal reflexes to the plantar flexors of the digits, the pl antar flexors of the ankle, the pronators, the dorsiflexors of the ank le, and a flexor of the knee, were found in succession in a mediolater al direction. It is concluded that neurones that are able to encode th e specific spatial input-output organization of the withdrawal reflexe s to single muscles do exist in the deep dorsal horn. Such reflex enco ders appear to have a ''musculotopic'' organization. A hypothesis of t he organization of the withdrawal reflex system is presented.