Rapid modulation of human cortical motor outputs following ischaemic nerve block

The amplitudes of motor evoked potentials to transcranial magnetic stimulation from muscles immediately proximal to a temporarily anaesthetized (Bier's block) human forearm increase in minutes after the onset of anaesthesia and return to control values after the anaesthesia subsides. In order to determine the level at which the early modulation of human motor outputs takes place, we recorded maximal H reflexes, peripheral M responses, motor evoked potentials to transcranial magnetic stimulation, and motor evoked potentials to transcranial electrical stimulation and spinal electrical stimulation from a muscle immediately proximal to a limb segment made ischaemic by a pneumatic tourniquet. The amplitudes of motor evoked potentials to transcranial magnetic stimulation, but not to transcranial electrical stimulation and spinal electrical stimulation, were larger during ischaemia, implying that the site of change was in the motor cortex. The maximal H/M ratios were unaffected by ischaemia, indicating that alpha-motor neuron excitability to segmental Ia inputs remained unchanged. The map of cortical representation areas for this muscle obtained with transcranial magnetic stimulation was also enlarged. Taken together, our findings suggest that the temporary removal by ischaemic nerve block of myelinated afferent inputs reduces inhibition at the motor cortical level and that this disinhibition is responsible for the increased excitability of the corticospinal system.