Characteristics of Somatosensory Evoked Potentials Recorded over the Spinal Cord and Brain of Man

Evoked potentials were recorded from the skin over the lumbar and cervical portions of the spinal cord, and the scalp over the sensory cortex of the brain, using averaging techniques. Responses could be identified over the cauda equina and root entry zone in the lumbar spine to stimulation of the tibial nerve at the popliteal fossa. These responses had characteristics of nerve root and spinal cord events in their thresholds, timing, duration, and refractoriness. Stimulation of the median nerve at the wrist likewise resulted in recognizable responses over root entry portions of the cervical spinal cord. These later waves had a morphology suggestive of components arising from nerve plexus, nerve roots, and spinal cord. Responses recorded over the spinal cord were in the 1-10 μV amplitude range. Tibial, peroneal and median nerve stimulation were used to elicit 1-20 μV responses recorded over the cortex, which were found to be sensitive to the site, amplitude, and rate of stimulation.