Untrained Movements Research Articles

The activity of pars compacta neurons of the monkey substantia nigra in relation to motor activation

The extracellular activity of single pars compacta neurons of the substantia nigra was recorded in awake monkeys. Animals were subjected to a behavioral paradigm consisting of separate phases of preparation for trained movements and their execution. Cell activity was also studied during untrained movements. Two basic firing patterns could be distinguished in the substantia nigra, depending on the subdivision examined. Pars compacta neurons at rest discharged impulses 1.0 to 2.0 ms in duration at a rate of 0 to 8/s, contrasting with the shorter pars reticulata discharges (0.6 to 1.0 ms long) occurring at higher frequencies (30 to 110 impulses/s). Forty of 68 tested and histologically-localized pars compacta neurons significantly changed, mostly increased, their discharge rate during and sometimes before the execution of trained or untrained individual large reaching movements of the contralateral arm. Distal movements of the arm or postural adjustments did not lead to detectable changes. Only a minority of these neurons showed modulated activity prior to onset of movement. None of the neurons responded to the sensory cues of moderate intensity employed in the paradigm. The modulations were of a slow and moderate nature but were reproducible from trial to trial. Peak frequencies never exceeded 20 impulses/s. The duration of altered activity in many cells exceeded that of the arm movements. These characteristics thus contrasted with the more dramatic electromyographic changes of the involved muscles. Sixteen of 23 neurons lying dorsally to the substantia nigra with electrophysiological characteristics similar to pars compacta neurons were modulated in the same fashion during performance in the paradigm. We conclude that the activity of nigral pars compacta neurons is modulated in relation to motor activation. They do not appear to encode detailed movement parameters, rather they may subserve a more general function that is associated with behavioral, particularly motor, activation.