Cortico-motoneuronal system and dexterous finger movements.

Cortico-motoneuronal system and dexterous finger movements To the Editor: A recent paper in the Journal of Neurophys-iology by Sasaki et al. (2004) reports that in macaque monkeys, skilled movements of the digits can survive a large unilateral lesion of the lateral corticospinal tract (LSCT) at the C 5 level. This important report forces us to reconsider our views about the pathways mediating control of these movements and to consider the processes involved in plastic recovery mechanisms after spinal injury. In assessing the evidence presented by Sasaki et al. (2004), the first question is whether the lesion interrupted all of the corticospinal input to the lower cervical cord. The authors carefully addressed this question both by histolog-ical reconstruction and by electrophysiological recordings at the end of the experiment. In these recordings, they found evidence for field potentials in C 6 –C 8 and excitatory postsynaptic potentials (EPSPs) in forelimb motoneurons that were evoked by single stimuli to the pyramidal tract. These potentials and EPSPs had longer latencies than the fast cortico-motoneuronal input interrupted by the lesion and are interpreted as arising via disynaptic action of C 3 –C 4 propriospinal neurons (PNs). However, it seems unlikely that these late effects solely reflect this pathway: first, some of them are too early to be disynaptic; second, such effects are not normally evoked by single shocks, and finally, they show little temporal facilitation with repetitive pyramidal tract (PT) stimuli. Indeed, similar effects were reported some years ago by Maier et al. (1998; Fig. 7) in recordings from macaque motoneurons below an acute lesion of the LCST at C 5 ; these authors attributed them to the action of slowly conducting CST fibers that had survived the lesion. Such fibers represent the vast majority of axons in the LCST, are less susceptible to injury than fast conducting axons, and make monosynaptic connections with motoneu-rons (Porter and Lemon 1993). Thus it may not be safe to conclude " that the CST lesions in the three monkeys were complete. " The authors suggest that because recovery of precision grip after LCST lesions at C 5 is rapid compared with earlier reports of permanent loss of fine finger movements after lesions at a higher, i.e., PT level, then the key structures must be those innervated at an intermediary level, such as the C 3 –C 4 PNs. However, in earlier reports (Hepp-Reymond et al. 1974; Law-rence and …