Modulation of sural nerve somatosensory evoked potentials during stance and different phases of the step-cycle

In order to investigate the modulation of somatosensory processing during stance and locomotion, sural nerve somatosensory evoked potentials were recorded during both stance and different phases of the step-cycle. Characteristic sequences of negative-positive waves were elicited, consisting of an early component, N40, presumably of subcortical origin, followed by a P50-N80-P220 complex of cortical origin. The N40 and N40-P50 components had similar amplitudes in both gait and stance. However, the P50-N80 component was attenuated whereas the N80-P220 complex became biphasic during gait. Within the step-cycle, amplitudes of the cortical components P50-N80 and N80-P110 were larger prior to footfall and smaller at the beginning of the support phase. The results demonstrate that locomotion produces a modulatory effect on somatosensory input at a cortical level. Within the step-cycle, excitability of the somatosensory cortex is increased during the middle and late swing phases and decreased during the support phase. Such modulation may contribute to an improved detection of foot contact at touchdown.