Intraspinal localization of the somatosensory evoked potential.

Somatosensory evoked potentials (SEPs) are used widely for monitoring neurophysiological function in experimental spinal injury. Yet the spinal pathways for SEP conduction remain unclear. Consequently, we sought to define specific changes in the SEP after interruption of selected spinal pathways. We activated cortical SEPs with sciatic nerve stimulation in 11 anesthetized (25 mg of pentobarbital per kg) cats after a multilevel thoracic laminectomy. The most consistent wave from component was an initial positivity (IP) at a 17- to 19-ms onset latency. We then used a Cavitron ultrasonic surgical aspirator to interrupt specific spinal pathways. A unilateral dorsal column lesion abolished the ipsilateral IP, but did not affect conduction in the contralateral column. Bilateral dorsal column lesions obliterated the IP, but sometimes left some longer latency components. Interruption of all but the ventral columns abolished the SEPs. When we interrupted all spinal pathways but the dorsal columns, an intact IP remained. In fact, a distinct IP was conducted through a single dorsal column after the division of all other spinal cord pathways. We concluded that, in the barbiturate-anesthetized cat: (a) the most consistent SEP wave form is an initial positivity at a 17- to 19-ms onset latency, (b) the integrity of the dorsal columns is both necessary and sufficient to conduct a normal-appearing IP component of the SEP, (c) the lateral columns may carry some longer latency component of the SEP, (d) the ventral columns carry no component of the SEP, and (e) bilateral recording may be useful for detecting asymmetry of injury.