Key words: somatosensory evoked potential --gating --voluntary movement --afferent somatosensory information --inputThe inflow of somatosensory information to the cerebral cortex is modified before and during active movement in animals. This phenomenon has been termed 'gating' and occurs at several levels of the sensory pathway. We studied somatosensory evoked potentials (SEPs) to stimulation of the median nerve at the wrist during voluntary movement of the ipsilateral thumb in man. Results indicate that SEPs are attenuated shortly after a command to move (approximately 100 ms before the onset of the electromyogram (EMG)), become maximally attenuated with maximum EMG and return to normal size when movement is finished.

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Article abstract-We recorded potentials evoked by specific somatosensory stimuli over peripheral nerve, spinal cord, and cerebral cortex. Vibration attenuated spinal and cerebral potentials evoked by mixed nerve and muscle spindle stimulation; in one subject that was tested, there was no effect on cutaneous input. Presynaptic inhibition ofla input in the spinal cord and muscle spindle receptor occupancy are probably the responsible mechanisms. In contrast, muscle contraction attenuated cerebral potentials to both cutaneous and muscle spindle afferent volleys; central mechanisms modulating neurons in the dorsal columns nuclei, thalamus, or cerebral cortex are probably responsible.

SUMMARYSomatosensory cerebral evoked potentials were recorded in man to natural forms of somatosensory stimulation of the lower extremity including stretching of the muscle tendons, tapping on muscle bellies and tapping on cutaneous surfaces. These potentials were compared with those evoked by electrical stimulation of peripheral nerves measuring the amplitudes and latencies of the evoked potential components and defining the effects of stimulus variables on these parameters. Spinal cord potentials could only be detected to electrical stimuli Mechanical stimulation of tendons and muscle bellies evoked scalp potentials at latencies earlier than those evoked by electrical stimulation of the peripheral nerve and by cutaneous stimulation at the same level of the leg. M1USCle receptors, most probably muscle spindles, are the source of the short latency components obtained by the stretching of tendons and tapping on muscle bellies. The proximal location of these receptolfS as well as very rapid spinal conduction account for the latency difference.The potentials were larger to electrical stimulation of nerve trunks than to mechanical stimulation of tendons or skin, suggesting the asynchronous activation of a smaller number of fibres by the latter. Individuals with the largest potentials to one form of stimulation usually had the largest potentials to the other modes of stimulation.The use of physiological stimuli such as muscle stretch to test the transmission in specific neural pathways might be useful in investigating the processing of relatively selective afferent volleys using noninvasive evoked potential recordings.

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