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 magnetic brain stimulation, before and after anesthetic block of right median (sensory + motor) and radial (sensory) nervous fibers at the wrist. MEPs were simultaneously recorded from two ulnar-supplied muscles during full relaxation and voluntary contraction. These muscles maintained an intact strength following anesthesia, but they were in a remarkably different condition with respect to the surrounding skin: the first dorsal interosseous muscle (FDI) was totally 'enveloped' within the anesthetized area but was still dispatching a normal proprioceptive feedback; the abductor digiti minimi (ADM) was preserving both cutaneous and proprioceptive information. Spinal and peripheral nerve excitability were monitored as well. The sensory deprivation induced short-term changes which selectively took place within the hemisphere connected to the anesthetized hand. The physiological latency 'anticipation' of MEPs recorded during active contraction versus relaxation was reduced (P < 0.001) in the FDI, but not in the ADM, when values during anesthesia were compared with preanesthesia values. The FDI cortical representation-as analyzed by a mapping procedure of the motor cortex via focal stimuli of several scalp positions-was significantly (P < 0.002) reduced, while the ADM representation remained either unchanged or enlarged. MEP and F-wave variability significantly decreased in the FDI but not in the ADM. F-waves were also affected due to changes in the 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.