Somatosensory evoked potentials elicited by intraneural microstimulation of afferent nerve fibers.

Cortical somatosensory evoked potentials (SEPs) after intraneural microstimulation (IMS) of cutaneous and afferent muscle nerve fibers in the median nerve were recorded to study the contribution of different afferent fiber groups to the SEP. Thirty-seven cutaneous fiber bundles, 10 afferent muscle nerve fiber bundles, and 45 single mechanoreceptive afferents of FA I- (n = 12), FA II- (n = 8), SA I- (n = 13), and SA II-type (n = 12) were studied in 29 healthy subjects. IMS of cutaneous fiber bundles evoked cortical responses corresponding to the N20 component after median nerve stimulation in 86% of the fascicles studied, whereas IMS of muscle nerve fiber bundles elicited responses only in 20%. After IMS of single mechanoreceptive afferents of FA I-, FA II-, and SA I-type cortical responses were obtained in all groups in approximately 80% of the stimulated units. The latencies of the SEPs evoked by IMS were comparable to that after compound median nerve stimulation (mean difference 0.58 ms). The N20 amplitudes of SEPs after IMS of cutaneous fiber bundles were on average 28% (n = 32) and for single afferents 22% (n = 30) of that after compound nerve stimulation. It is concluded that the median nerve SEPs evoked by compound median nerve stimulation of the resting hand are dominated by cutaneous rather than muscle afferent input. Furthermore, the fact that selective stimulation of only a few cutaneous afferents produces cortical SEPs of rather high amplitude is compatible with the view that sensory information produced by selective afferent stimulation is differentially gated into the somatosensory cortex.