Projection of thenar muscle afferents to frontal and parietal cortex of human subjects

Abstract

There is some controversy about the projection of muscle afferents from the human upper limb to cerebral cortex and about their contribution to somatosensory evoked potentials. In 8 normal volunteers, the somatosensory projections of muscle and cutaneous afferents from the hand were recorded at 21 scalp sites, using a non-cephalic reference. Low-threshold thenar muscle afferents were selectively activated by intramuscular microstimulation. In addition, the averaged data for the projections were mapped for each individual. In each subject a focal parietal negativity was detected over the contralateral parietal cortex at a mean latency of 20.8 msec (S.D. 1.15 msec) following stimulation of thenar muscle afferents. The amplitude of the parietal ‘N20-P25’ was relatively small (mean 0.49 μV, range 0.18–1.56 μV). A small focal positivity was detected, maximal over contralateral frontal cortex at 22.8 msec (S.D. 2.05 msec) but recorded bilaterally. In all subjects subcortical positive waves (P9 and P14) were defined for the muscle afferent volley. This pattern of cortical activity was similar to that for the projection from the digital nerves of the index finger. For the cutaneous input the latency of the parietal ‘N20’ was 21.7 msec (S.D. 1.17 msec) and of the frontal ‘P22’ was 24.2 msec (S.D. 3.09 msec). The amplitude of the parietal ‘N20-P25’ was larger for the cutaneous projection (mean 1.59 μV; range 0.65–4.28 μV).