Epidurally recorded cervical somatosensory evoked potential in humans

Three slow wave components, P10, N13 and P18, can be seen in the cervical somatosensory evoked potential (CSEP) in response to median nerve stimulation recorded by an electrode in the epidural space at the dorsal aspect of the cervical spinal cord referenced to an electrode at the suprasternal notch. In the region of high CSEP amplitude, which extends over several cervical segments, the peak-to-peak amplitude is more than 10 microV, permitting observation of the CSEP slow waves in single, unaveraged records. The CSEP to finger nerve stimulation had a similar wave form and the same latencies (referred to the Erb's potential) as the CSEP to median nerve stimulation. The P10 activity is of presynaptic origin; it is generated in the brachial plexus, spinal roots and terminal branches of the primary sensory fibers. The N13 slow wave is of postsynaptic origin; however, the small wave on the ascending phase of this main postsynaptic component represents superimposed presynaptic activity. In bipolar epidural recordings, 3-5 fast waves are superimposed on the slow CSEP waves, which are of lower amplitude than the slow waves in unipolar recordings. The fast waves show a slight but progressive delay at the more rostral recording sites and are present even with high frequency stimulation, presumably reflecting activity in long ascending tracts. The surface recorded CSEP to median nerve stimulation is 4-7 times lower in amplitude than the CSEP in unipolar epidural recordings. The small wave on the ascending phase of N13 and the N13 peak of the unipolar epidural recordings had the same latencies as the surface N11 and N13 peaks.