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.
Rossi
S
S
AFaR-Ospedale Fatebenefratelli-Isola Tiberina, Rome, 00136, Italy.
Pasqualetti
P
P
Tecchio
F
F
Sabato
A
A
Rossini
P M
PM
D003265:Contingent Negative Variation / Q000502:physiology; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005246:Feedback / Q000502:physiology*; D006225:Hand / Q000294:innervation*; D006801:Humans; D008475:Median Nerve / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation*; D009473:Neuronal Plasticity / Q000502:physiology; D011712:Pyramidal Tracts / Q000502:physiology*; D011826:Radial Nerve / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D012683:Sensory Deprivation / Q000502:physiology*; D011984:Sensory Receptor Cells / Q000502:physiology