Responses of rat substantia nigra pars reticulata units to cortical stimulation

In order to study the function of multiple pathways between the sensorimotor cortex (Cx) and the substantia nigra pars reticulata (SNr), responses of SNr units to stimulation of the Cx were studied in anesthetized rats. Most of the units (229 of 236) exhibited repetitive firing with fairly short, regular intervals. The other 7 units displayed long duration spikes, irregular firing intervals and slow spontaneous firing. Stimulation of the Cx usually resulted in a short latency excitation and occasionally a long latency inhibition in both types of units. When strong stimulation was applied, multiple excitatory and inhibitory responses alternating each other with about a 25 ms interval were observed. SNr units responded with different patterns and latencies to stimulation of different sites of the sensorimotor Cx. The results indicate that signals derived from the sensorimotor Cx reach the SNr via multiple pathways and converge on many SNr neurons.