Axonal Morphometric Correlates of Evoked Potentials in Experimental Spinal Cord Injury

Cortical somatosensory-evoked potentials recorded at a succession of intervals after contusion injury of the cat spinal cord were compared with the numbers of axons surviving in transverse sections of the lesion at 3 months postinjury. In a sample of 25 animals, with injuries varying from ca.87% to 99% loss of myelinated axons in the white matter, there was a significant correlation between chronic axonal survival and the amplitude of the SEP recorded from 30 min to 1 day postinjury. There was little or no correlation between axon numbers and SEPs recorded at 3 months, within this narrow but crucial range of injury intensity. Robust, relatively normal waveforms could be obtained from animals with as few as 2% of the original axon population surviving at the lesion site, and from animals in which all the surviving axons in the dorsal columns were restricted to the outer 100–200 um of tissue. In other cases, repeatable SEPs could not be recorded from animals with >5% survival of axons. SEPs were lost immediately after contusion injury, but partially recovered within the first 4 hours in ca.30% of cases. There was a secondary severe reduction of SEP amplitude within the first day in most examples of early recovery. Most of the final recovery of chronic SEP amplitude occurred within 2–4 weeks.