It has been suggested that the distinctive capacity of some nonhuman primates to use tools may reflect a well-developed corticospinal system and, in particular, direct cortico-motoneuronal (CM) connections to hand muscles. We investigated the activity of corticospinal neurons in the primary motor cortex hand area during the use of a tool bytwo adult macaque monkeys. They used a light raketoretrieve food rewards placedintheir extrapersonal space.AnanalysisofEMG activity showed that the rake task involvedacomplex interaction of muscles actingon the digits, hand, and arm. Sixty-nine corticospinal neurons were identified antidromicallyaspyramidal tract neurons (PTNs). When testedonthe rake task, most (64of69; 93%) showedasignificant modulationoftheir discharge duringatleast oneofthree task periods: grasping the rake, projecting it beyond the food reward, and then pulling it back to retrieve the reward. Discharge patterns were heterogeneous, and many PTNs showed significant suppression of discharge during raking. Seventeen of the 69 PTNs recorded during the rake task were further identified as CM cells, exerting clear postspike facilitation on digit muscles, demonstrating that the CM system contributes tothe skilled use oftools.Wecompared the activityofeach PTNonthe rake task with that during precision grip. Most PTNs (90%) modulated their activity significantly for both tasks, demonstrating that PTNs activated by a task involving fractionated movementsofthedigits arealsorecruitedduringrakeuse,although therewere oftencontrastingpatternsofPTN recruitment andmuscle activity for the two tasks.
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