Sensitivity of the short-range spinal interneurons of the cat to experimental spinal cord trauma.

The technique of retrograde axoplasmic transport was used to demonstrate the effect of experimental spinal cord injury on the spinal interneurons in the upper lumbar and lower thoracic segments of cats. Force of varied intensity was applied to the dorsal surface of the spinal cord and horseradish peroxidase (HRP) was injected into the next caudal segment. A large impact (250 to 350 gm-cm) inducing permanent paraplegia of the hind legs blocked the axoplasmic transport instantaneously in both cranial and caudal directions. If 1 week elapsed between the trauma and injection, neurons cranial to the trauma did not show any evidence for retrograde axoplasmic transport, while few neurons in the caudal direction were labeled with HRP. A moderate impact (150 gm-cm) which rendered the animals only transiently paraplegic spared the axoplasmic transport in some neurons both cranially and caudally to the injection. No obvious recovery or additional loss in the number of HRP-labeled neurons could be found in the cats if the injections followed the trauma by 1 week. The loss of spinal cord neurons following the injury seems to be the immediate mechanical consequence of the trauma.