Responses of primate spinothalamic neurons located in the sacral intermediomedial gray (Stilling's nucleus) to proprioceptive input from the tail.

Extracellular an intracellular recordings were obtained from 24 spinothalamic neurons located in the intermediomedial gray (Stilling's nucleus) in the S2 and S3 spinal segments of anesthetized adult monkeys (Macaca fascicularis). Most units were spontaneously active. They lacked cutaneous receptive fields but could be excited by subcutaneous receptors. Bending the tail to one side of the animal's body axis excited cells located on the contralateral side of the cord an inhibited ipsilateral cells. The firing rate of each cell was a reproducible function of the angle of the portion of the tail distal to the innervated segment. Cells responded with a burst of spikes to electrical stimulation of low threshold ipsilateral primary afferent fibers in the homosegmental dorsal root. The latency of the underlying synaptic potential was 0.5-0.7 ms from the time of arrival of the afferent volley at the cord, indicating monosynaptic input. Low intensity stimulation of the contralateral homosegmental dorsal root inhibited back-ground activity. The latency of the underlying inhibitory synaptic potential was 0.8-1.1 ms, suggesting crossed disynaptic inhibition. We conclude that sacral spinothalamic tract neurons located in the intermediomedial gray participate in signalling the spatial orientation of the animal's tail.