P15 in tibial nerve SEPs as an example of the junctional potential.
Tibial nerve stimulation at the ankle elicited a stationary, dipolar potential P15/N15 over the buttock with a reference electrode at the contralateral greater trochanter (GTc). P15 was distributed in the rostral and contralateral region and N15 in the caudal and ipsilateral region. The derivation from the contralateral iliac crest to the ipsilateral greater trochanter (ICc-GTi) registered a large P15 (= P15-N15) potential which was well free from artifacts. The Cz'-contralateral knee lead, which had been employed in previous studies, registered a smaller P15 of poor quality. Sequential bipolar recording along the course of the sciatic nerve indicated that P15 was generated around the greater sciatic foramen. Comparison with a simulation study suggested that P15 is a junctional potential which is generated when the sciatic nerve enters the bone at the greater sciatic foramen. P15 is expected to be a useful tool to evaluate the proximal segment of the tibial nerve.