Neural generators of the somatosensory evoked potentials: Recording from the cuneate nucleus in man and monkeys

The neural generators of the somatosensory evoked potentials (SEPs) elicited by electrical stimulation of the median nerve were studied in man and in rhesus monkeys. Recordings from the cuneate nucleus were compared to the far-field potentials recorded from electrodes placed on the scalp. It was found that the shape of the response from the surface of the human cuneate nucleus to stimulation of the median nerve is similar to that of the response recorded more caudally in the dorsal column, i.e., an initially small positivity followed by a negative wave that is in turn followed by a slow positive wave. The beginning of the negative wave coincides in time with the N14 peak in the SEP recorded from the scalp, and its latency is 13 msec. The response from the cuneate nucleus in the rhesus monkey has a similar shape and its negative peak appears with the same latency as the positive peak in the vertex response that has a latency of 4.5 msec; the peak negativity has a latency of about 6 msec and thus coincides with P6.2 in the vertex recording. Depth recordings from the cuneate nucleus and antidromic stimulation of the dorsal column fibers in the monkey provide evidence that the early components of the response from the surface of the cuneate nucleus are generated by the dorsal column fibers that terminate in the nucleus.The results support the hypothesis that the P14 peak in the human SEP is generated by the termination of the dorsal column fibers and that the cuneate nucleus itself contributes little to the far-field potentials.