Gating of somatosensory evoked potentials during voluntary movement of the lower limb in man
Somatosensory evoked potentials (SEPs) evoked by stimulation of the tibial nerve (TN) in the popliteal fossa, the sural nerve (Sur) at the lateral malleole, and an Achilles tendon (Achilles) tap were recorded before and during voluntary plantarflexion, dorsiflexion, and cocontraction of the ipsi- and contralateral foot in normal subjects. Suppression (gating) of the TN-SEP began around 60 ms before the onset of electromyographic activity (EMG), and became maximal 50–100 ms after the onset of EMG. Similar gating was observed for the SEP evoked by activation of muscle afferents (Achilles) and cutaneous afferents (Sur). The TN-SEP was similarly depressed at the onset of a plantarflexion as at the onset of dorsiflexion. A depression, although much smaller, was also observed at the onset of movement of the contralateral limb. The depression of the TN-SEP after the onset of EMG decreased when fast-conducting afferents were blocked by ischemia below the knee joint. The TN-SEP was equally depressed during tonic dorsiflexion, plantarflexion, and cocontraction of dorsi- and plantarflexors. The TN-SEP was depressed for up to 300 ms when preceded by stimulation of Sur or a biceps femoris tendon tap. Gating of lower limb SEPs thus appears to have both central and peripheral components of which neither seems to be specific for the muscle being contracted or the sensory afferents being stimulated. We encourage that caution is taken when drawing functional conclusions regarding movement-specific modulation of afferent inflow to the somatosensory cortex based on observations of gating of lower limb SEP.