Inhibitory receptive fields of primate spinothalamic tract cells

The inhibitory cutaneous receptive fields of 62 spinothalamic tract (STT) neurons located on the left side of the lumbosacral enlargement were examined in anesthetized monkeys. Only cells with excitatory receptive fields on the left hindlimb were included in the sample. Cells were classified according to their excitatory receptive-field properties and according to the destinations of their axons within either the caudal portion of the ventral posterior lateral (VPL(c)) nucleus or the intralaminar complex of the thalamus. Most of the neurons (94%) sending axons to the VPL(c) nucleus were inhibited by noxious mechanical or thermal stimulation of widespread areas of the body, such as the tail, contralateral foot, hands, face, or trunk. Inhibition was not obtained following innocuous stimulation of these same cutaneous areas. STT cells classified either as weide dynamic range or high threshold were inhibited. Electrical stimulation of the right sural nerve often inhibited those STT neurons that had inhibitory receptive fields on the right foot. Stimulation of A-α,β-afferents had little or no inhibitory effect, while A-δ-fibers were very effective in inhibiting background or evoked activity. In some cases, a separate late inhibitory pause was produced following stimulation of C-fibers. Short trains of stimuli were more effective than single pulses. In 10 experiments trains of stimuli were delivered to the right sural nerve to condition the responses of STT neurons to excitatory volleys in the left sural nerve. Conditioning trains that activated just A-α,β-fibers had a minimal inhibitory effect. The late excitatory responses to C-fiber volleys in the left sural nerve were more readily inhibited by conditioning volleys in A-δ- or C-fibers of the right sural nerve than were the excitatory responses to A-fiber volleys. Strong inhibition remained in six of seven animals after complete transection of the cervical spinal cord. This finding led to the conclusion that there are propriospinal inhibitory pathways, although additional involvement of a supraspinal loop was not ruled out.