Abstract
In the cat it is known that the distribution and modulation of the so-called P2 responses are similar, irrespective of whether they are obtained with electrodes implanted in the different skin areas or in the various leg muscles. In man it is known that the specific stimulation of cutaneous afferents from different parts of the foot evokes P2 responses, the phase-dependent modulation pattern of which exhibits both location-specific and common features. Responses generally differ, but one striking feature is the occurrence of suppressive responses in the tibialis anterior (TA) of the ipsilateral (i) leg at the end of the swing phase independent of the nerve stimulated. The question arises of whether this aspecificity is limited to the foot. Can similar suppressive P2 responses in iTA be obtained when afferents outside the region of the foot are stimulated during walking? If so this would indicate that there is a very general suppression occurring of input to the TA motor neuron pool, for example through presynaptic inhibition of a corticospinal drive. To answer this type of question the motor responses following transcutaneous stimulation of the rectus femoris (RF) and the motor responses following stimulation of the femoral nerve branch innervating the skin area above the quadriceps were determined during human locomotion. Electromyographic (EMG) activity in iTA was recorded by means of surface electrodes. In all subjects (N=10), the first consistent responses following RF stimulation occurred at about 80 ms poststimulus. The amplitude of these responses showed a clear phase-dependent modulation pattern. Facilitatory responses occurred during the end stance and early swing phase and turned into suppressive responses at the end of the swing phase. To investigate whether cutaneous afferents overlying the RF determined some of the responses following transcutaneous RF stimulation, the experiments were repeated following local anesthesia of the skin under the stimulation electrodes. This did not affect the responses substantially, indicating that most of the RF stimulation results were related to activation of muscle afferents. A similar phase-dependent modulation pattern was found following stimulation of cutaneous afferents of the femoral nerve (Fn). However, this phase-dependent modulation pattern was less pronounced and less consistent over the subjects when compared to the one found following RF stimulation. Our first conclusion is that the results show that P2 reflexes can be elicited both by stimulation of cutaneous afferents in the foot and by proximal cutaneous nerve and muscle stimulation. Secondly, it can be concluded that the suppressive responses at the end of the swing phase are present for both RF stimulation and stimulation of cutaneous afferents of the foot. This result indicates that a wide variety of afferent inputs have a suppressive influence on the input drive to TA motor neurons just prior to heel strike.
