A study of peripheral cervical and cortical evoked potentials and afferent conduction times in the somatosensory pathway.

Somatosensory evoked potentials were recorded from Erb's point (N9), the cervical spine (N14) and the scalp (N20) in 47 volunteer patients to establish the normal impulse conduction time between these recording sites. Either the ulnar or the median nerve or both were stimulated percutaneously, and pure sensory as well as mixed sensorimotor nerve fibres were used. No significant differences in either the N9-N14 or N14-N20 conduction times were found for different sets of peripheral stimuli. The N9-N14 conduction time evidently reflects impulse propagation in the proximal part of the brachial plexus, the cervical roots and the dorsal column. Cross-correlation analysis, however, suggested that the main contribution to this conduction time is a central one, probably the dorsal column. The N14-N20 conduction time represents a pure central conduction time probably between the dorsal column nuclei and the cortex. While the amplitudes, morphologies and latencies of the somatosensory evoked responses N9, N14 and N20 varied significantly, the N9-N14 and N14-N20 conduction times were fairly constant. The results indicate that diagnostic use of the somatosensory evoked potentials based exclusively on the response latencies can be very misleading, both when latencies are normal and when pathologically delayed. In such cases the N9-N14 and N14-N20 conduction times can be conclusive, and we suggest that these times should be included in all SEP tests.