REVIEW ■ : Mechanisms of Cortical Control of Hand Function

The direct cortico-motoneuronal (CM) projection from motor cortex to spinal motoneurons is a distinctive primate feature, and appears to be of particular importance for the control of fine hand and finger move ments. This review considers recent neurophysiological evidence showing that the degree of dexterity in different nonhuman primates is reflected in the strength of the CM connections to hand motoneurons. It also shows that phylogenetically older, indirect pathways may have been superseded by the direct CM system. The unique contribution of the CM system to hand function may reside in the focused and powerful facilitation it exerts on selective groups of muscles, as shown by recordings from identified CM neurons during performance of precision grip by conscious, trained monkeys. In man, damage by stroke to the corticospinal system often results in weakness and poverty of hand movement. New evidence from the use of transcranial magnetic stimulation (TMS) in stroke patients demonstrates that any recovery in the voluntary activation of hand muscles is always accompanied by recovery of short-latency responses to TMS, which are probably mediated by the fast corticospinal system, suggesting that the integrity of this system is es sential for recovery of hand function. NEUROSCIENTIST 3:389-398, 1997