Anatomic and physiologic bases of posterior tibial nerve somatosensory evoked potentials.

Following stimulation of the posterior tibial nerve, lumbar electrodes record a response that is the composite of two signals, one (PV) corresponding to the afferent volley in the cauda equina and gracile tract, and another (N22) generated postsynaptically in the gray matter of the lumbar cord. Subcortical structures generate two distinct, widely distributed signals, recordable from scalp electrodes using a noncephalic reference, P31 and N34. P31 is most likely generated by the afferent volley in the caudal medial lemnicus. N34 probably reflects subcortical postsynaptic activity in brain stem and/or thalamus, respectively. The "primary" cortical response, P38/N38, has a complex scalp distribution reflecting the location of the leg area on the mesial aspect of the postcentral gyrus, within the interhemispheric fissure. It is most likely a composite waveform with multiple cortical generators.