Modular organization of human leg withdrawal reflexes elicited by electrical stimulation of the foot sole.

Human withdrawal reflex receptive fields were determined for leg muscles by randomized, electrical stimulation at 16 different positions on the foot sole. Tibialis anterior, gastrocnemius medialis, peroneus longus, soleus, rectus femoris, and biceps femoris reflexes, and ankle joint angle changes were recorded from 14 subjects in sitting position. Tibialis anterior reflexes were evoked at the medial, distal foot and correlated well with ankle dorsal flexion. Gastrocnemius medialis reflexes were evoked on the heel and correlated with plantar flexion. Stimulation on the distal, medial sole resulted in inversion (correlated best with tibialis anterior activity), whereas stimulation of the distal, lateral sole evoked eversion. Biceps femoris reflexes were evoked on the entire sole followed by a small reflex in rectus femoris. A detailed withdrawal reflex organization, in which each lower leg muscle has its own receptive field, may explain the ankle joint responses. The thigh activity consisted primarily of flexor activation.