Cutaneous dermatomes for initiation of three forms of the scratch reflex in the spinal turtle.

The turtle spinal cord produces three forms of the hindlimb scratch reflex. Each scratch form is initiated in response to gentle mechanical stimulation of a distinct set of sites in the periphery, termed the receptive field for that scratch form. The turtle spinal cord consists of 8 cervical segments (C1-C8), 10 dorsal segments (D1-D10), 2 sacral segments (S1, S2), and about 16 caudal segments (Ca1-Ca16). First, we recorded cutaneous afferents in peripheral nerves to reveal the tactile dermatomes of segments D3-D8. These segments innervate regions of the body between the forelimb and hindlimb, directly lateral to their spinal cord segments. Adjacent segments innervate adjacent and partially overlapping regions of the periphery. Second, we used successive spinal cord transections combined with either a) behavioral analysis in turtles with limb movements or b) electroneurographic recordings in immobilized turtles, and mapped the zone of remaining sensibility after each transection to measure the borders of dermatomes D2-Ca2. This technique revealed that adjacent dermatomes are innervated by non-adjacent spinal segments in regions near the hip. Segments D8 and Ca1 innervate adjacent and partially overlapping regions ventral to the hip. There is a similar discontinuity in the innervation of the shell and skin dorsal to the hip. These discontinuities correlate with the innervation of the hindlimb skin by segments D8-Ca1. The rostral scratch receptive field is innervated by sensory afferents entering spinal segments D3-D6; the pocket scratch receptive field is innervated by D6-D8; the caudal scratch receptive field is innervated by S2, Ca1, and more caudal segments. The rostral-pocket transition zone is innervated mainly by one segment, D6; the ventral part of the caudal-pocket transition zone is innervated by two non-adjacent segments, D8 and Ca1. Thus the motor pattern blends elicited by stimulation of sites within the rostral-pocket transition zone must be produced in response to a very different distribution of sensory inputs than the blends elicited by stimulation of sites within the caudal-pocket transition zone.