Muscle afferent contributions to tibial nerve somatosensory evoked potentials investigated using knee stimulations

Objective To investigate the contribution of muscle afferents to tibial nerve somatosensory evoked potentials (SEPs). Methods The left tibial nerve was stimulated at the knee and ankle in eight normal subjects. We tried to selectively stimulate Ia fibers from the calf muscles at the popliteal fossa by subtly changing the stimulation site while monitoring the H-waves of the calf muscles and sensory events. Results Selective or predominant Ia stimulation at the knee was achieved in seven subjects, and evoked a significantly smaller first cortical component (labeled as P38 for both ankle and knee stimulations) than that evoked by ankle stimulation or by mixed stimulation of the foot branch and muscle afferents at the knee. The P38 following mixed stimulation at the knee was smaller than that following ankle stimulation in six out of eight subjects, which must be due to a partial gating mechanism and also indicates that calf Ia afferent SEPs are not extremely large. Conclusions Physiologically important muscle afferents from the large calf muscles evoked rather small cortical components. Significance It seems reasonable to infer that the contribution of muscle afferents from the small intrinsic foot muscles to routine tibial nerve SEPs following ankle stimulation is even smaller.