DIRECT AND INDIRECT CORTICOSPINAL CONTROL OF ARM AND HAND MOTONEURONSIN THE SQUIRREL-MONKEY (SAIMIRI-SCIUREUS)

Anatomic evidence suggests that direct corticomotoneuronal (CM) projec tions to hand motoneurons in the New World squirrel monkey (Saimiri sc iureus) are weak or absent, but electrophysiological evidence is lacki ng. The nature of the corticospinal linkage to these motoneurons was t herefore investigated first with the use of transcranial magnetic stim ulation (TMS) of the motor cortex under ketamine sedation in five monk eys. TMS produced early responses in hand muscle electromyogram, but t hresholds were high (compared with macaque monkey) and the onset laten cy was variable. Second, stimulation of the pyramidal tract (PT) was c arried out with the use of chronically implanted electrodes in ketamin e-sedated monkeys; this produced more robust responses that were marke dly facilitated by repetitive stimulation, with little decrease in lat ency on the third compared with the first shock. Finally, postsynaptic potentials were recorded intracellularly from 93 arm and hand motoneu rons in five monkeys under general chloralose anesthesia. After a sing le PT stimulus, the most common response was a small, slowly rising ex citatory postsynaptic potential (EPSP), either alone (35 of 93 motoneu rons) or followed by an inhibitory postsynaptic potential (39 of 93). The segmental delay of the early EPSPs was within the monosynaptic ran ge (mean 0.85 ms); however, the rise time of these EPSPs was slow (mea n 1.3 ms) and their amplitude was small (mean 0.74 mV). These values a re significantly slower and smaller than EPSPs in a comparable sample of Old World macaque monkey motoneurons. The results show that CM conn ections do exist in the squirrel monkey but that they are weak and pos sibly located on the remote dendrites of the motoneurons. The findings are consistent with earlier anatomic studies. Repetitive PT stimulati on produced large, late EPSPs in some motoneurons, suggesting that, in this species, there are relatively strong nonmonosynaptic pathways li nking the corticospinal tract to hand motoneurons.