PATHWAYS AND MECHANISMS SUBSERVING CORTICAL CONTROL OF THE HAND IN PRIMATES

A unique feature of the primate motor system is the direct cortico mot oneuronal (CM) projections from motor areas of the cerebral cortex to spinal motoneurones, In sub-primates, anatomical evidence suggests tha t these projections are relatively weak or absent, and good physiologi cal evidence for their existence is so far lacking. In primates, the d ensity of CM projections to hand muscle motor nuclei is correlated wit h the degree of manual dexterity, While in the Old World macaque monke y, projections are dense and EPSPs are large and fast, in the New Worl d squirrel monkey, with poor dexterity, CM projections are scarce and CM-EPSPs are small and slowly rising, Indirect pathways, including tho se utilizing propriospinal transmission of corticospinal commands, are not well developed in Old World primates, which appear to rely to an increasing extent on direct CM control of the upper limb, The importan ce of the CM connections in man is strikingly illustrated by recoverin g stroke patients studied with transcranial magnetic stimulation (TMS) , There is little or no recovery of hand function unless there is spar ing of the fast corticospinal pathway; in contrast, there can be subst antial recovery of shoulder and girdle movements without any sign of r esponses to TMS.