Sensory-motor processing in substantia nigra pars reticulata in conscious cats

Extracellular recordings were made with chronically implanted micro-electrodes from 109 substantia nigra neurones in conscious cats. Ninety-six of 109 neurones met the criteria of presumed non-dopaminergic pars reticulata (s.n.r.) neurones. Background discharge, in animals in a state of relaxed wakefulness and in the absence of overt movements, was in the range of 11-37 impulses/s, mean 19.2 impulses/s. The discharges of fifty-two of ninety-six neurones tested were modified by innocuous mechanical skin stimulation. Neurones responded chiefly to stimuli delivered to the contralateral body side. Responses generally comprised net excitation and occurred with short latency (range 10-34 ms; mean 17.3 ms). Convergence from both forelimbs or the contralateral fore- and hind limbs was evident in a few cases. One-fourth (twenty-four out of ninety-six) of the s.n.r. neurones tested were sensitive to passive manipulation of limb joints in the quiet, conscious cat and responded exclusively to angular displacement of one contralateral joint. Responses were directional and phasic. None of the s.n.r. neurones tested responded to clicks and/or light flashes. However, stimuli moving across the contralateral visual field substantially modified the discharge rate of ten out of ninety-six s.n.r. neurones. Responses were directional and invariably associated with eye movements. Animals were also trained to walk on a treadmill and to perform certain self-generated limb movements. S.n.r. neurones with a receptive field on a limb regularly showed modulations in discharge during locomotion, phase-related to the step cycle, and also short-latency responses during disturbance of such movements. Ten out of ninety-six s.n.r. neurones discharged almost exclusively prior to and during self-generated movements of a single limb. Their most powerful modulations in firing rate occurred, whenever an animal tried to overcome an external impediment or to resist an imposed movement. These observations on s.n.r. neurones, taken together with previous findings on nigral influences on spinal motor circuitry, indicate that the s.n.r. represents an output station of the basal ganglia which is involved in the subconscious processing of convergent multimodal sensory information and which participates in setting appropriate gains and biasses of spinal motor neuronal systems to adequately deal with changing motor requirements.