The cortical distribution of muscle and cutaneous afferent projections from the human foot

Somatosensory evoked potentials to electrical stimulation of muscle and cutaneous afferents from the foot were recorded in normal human subjects using multiple channels centred on the vertex and referenced to the contralateral earlobe. Low-threshold muscle afferents were selectively activated by an insulated microelectrode inserted percutaneously at the motor point of abductor hallucis. Low-threshold cutaneous and joint afferents of the hallux or second toe were stimulated with ring electrodes. The posterior tibial and sural nerves were stimulated at the ankle through surface electrodes. The cerebral distribution of the initial cortical response (N33-P40) to stimulation of muscle afferents largely paralleled that to stimulation of its parent nerve, the posterior tibial nerve (which contains afferents of muscle, cutaneous and joint origin). They were maximal slightly posterior and ipsilateral to the vertex. The cutaneous-joint afferent projection from the hallux paralleled that from the sural nerve and both were less lateralized than the tibial and abductor hallucis projections.