MODULATION OF CORTICOSPINAL OUTPUT TO HUMAN HAND MUSCLES FOLLOWING DEPRIVATION OF SENSORY FEEDBACK

Excitability and conductivity of corticospinal tracts of 10 volunteers were investigated by motor-evoked potentials (MEPs) to transcranial m agnetic brain stimulation, before and after anesthetic block of right median (sensory + motor) and radial (sensory) nervous fibers at the wr ist. MEPs were simultaneously recorded from two ulnar-supplied muscles during full relaxation and voluntary contraction, These muscles maint ained an intact strength following anesthesia, but they were in a rema rkably different condition with respect to the surrounding skin: the f irst dorsal interosseous muscle (FDI) was totally ''enveloped'' within the anesthetized area but was still dispatching a normal propriocepti ve feedback; the abductor digiti minimi (ADM) was preserving both cuta neous and proprioceptive information. Spinal and peripheral nerve exci tability were monitored as well. The sensory deprivation induced short -term changes which selectively took place within the hemisphere conne cted to the anesthetized hand, The physiological latency ''anticipatio n'' of MEPs recorded during active contraction versus relaxation was r educed (P < 0.001) in the FDI, but not in the ADM, when values during anesthesia were compared with preanesthesia values. The FDI cortical r epresentation-as analyzed by a mapping procedure of the motor cortex v ia focal stimuli of several scalp positions-was significantly (P < 0.0 02) reduced, while the ADM representation remained either unchanged or enlarged. MEP and F-wave variability significantly decreased in the F DI but not in the ADM. F-waves were also affected due to changes in th e motoneuronal excitability at spinal level. Peripheral nerve and root stimulation showed no modifications, Results are discussed in view of the short-term modifications of the corticospinal pathway somatotopy produced by the selective reduction of the sensory flow, Implications of the sensory feedback in motor control are also discussed. (C) 1998 Academic Press.