Characterization of the cutaneous input to the ventral horn in vitro using the isolated spinal cord-hind limb preparation.

Intracellular recordings were made from 21 ventral horn neurones including 7 flexor motoneurones in a 10-12-day-old rat isolated spinal cord-hind limb preparation. The cutaneous input to these neurones was assessed using natural mechanical stimulation within the cutaneous mechanoreceptive field or electrical stimulation of the sural nerve. The receptive fields of 10 ventral horn neurones including 3 flexor motoneurones were characterized: 60% of cells responded to both low (touch) and high (pinch) threshold mechanical stimulation of the skin while the remaining 40% responded only to noxious mechanical stimuli. The postsynaptic response consisted of either purely subthreshold polysynaptic EPSPS (n = 8) or graded sub- and suprathreshold EPSPS (n = 2). The duration of the EPSP was typically prolonged by as much as a factor of ten compared to duration of the mechanical stimulation. In another 11 neurones (4 flexor motoneurones) the pattern of the postsynaptic response was related to the intensity of sural nerve stimulation. A low intensity single shock produced a short latency (30 ms), short duration EPSP (less than 500 ms) while higher intensities elicited a longer duration (greater than 1 s), more complex EPSP. In 36% of cells tested the EPSP remained subthreshold for cell firing even at the highest stimulus intensity.