STUDIES ON HUMAN EVOKED ELECTROSPINOGRAM .1. ORIGIN OF SEGMENTAL EVOKED-POTENTIALS

Evoked potentials from the human spinal cord were studied in 39 normal volunteers. Intrathecal recordings from lower cervical and lower thoracic intervertebral levels were made after the supramaximal stimulation of the median, ulnar and posterior tibial nerves, respectively. The segmental cord potential varied in shape and size according to the spatial relationship between the position of the electrode tip and the spinal cord and roots within the vertebral canal. Three main types of segmental evoked responses were obtained. One was recorded behind the cord dorsum and around the midline, and was composed of fast, sharp early, and slow late components. This was called a cord dorsum potential and its 1st component was related to the activity of the ascending dorsal funiculus fibers. The 2nd evoked response was the dorsal root potential, and this triphasic compound action potential of very high amplitude and longer duration had no remarkable late component. It was recorded when the tip of the intrathecal electrode was lateral to the midline within the vertebral canal, and was then related mostly to activity of the spinal roots. Another kind of potential was called posterior horn potential. It had a very small triphasic spike and 2 later components with prominent negativities being higher than the 1st spike. This potential might be related to the electrode tip position facing, and close to, the posterior horn of the spinal gray matter. Late components of the segmental evoked potentials were related to the pre- and post synaptic activity of the horizontally oriented fibers within the segmental gray matter of the posterior horn.