The next recorded event reflects the entrance of the signal into the spinal cord, normally 13 ms after the stimulus is triggered, and this potential has a broader, almost mountainous appearance. Various peaks give salient academic neuroanatomic information, but the simple existence of the response reveals transmission to the central nervous system. The initial peak is thought to be the dorsal root entry zone of the cervical cord, while the second two mini-peaks are felt to correspond to the nucleus cuneatus in the dorsal columns and the medial lemniscus in the brainstem where signals cross to the contralateral side. It has a broad morphology Rising above baseline then diving below with a later rebound. This slope across the baseline is measured from the initial peak to the following trough to determine the amplitude of the response.
Cortical amplitudes range from less than one up to seven or more microvolts.
Latency of the primary cortical peak is about 20 ms after median nerve stimulation. Many procedures on the spine can potentially result in neurologic compromise, though the actual incidence of catastrophic complication is low. Ulnar nerve stimulation can be utilized for lower cervical procedures, but if the goal is to provide spinal cord monitoring, and posterior tibial nerve responses can be obtained, then those responses will also serve. For spinal procedures below the cervical level, the region at risk is best monitored by stimulating the lower extremity. The posterior tibial nerve is stimulated by placing electrodes posterior to the medial malleolus.