Spinal circuitry and respiratory recovery following spinal cord injury.

Numerous studies have demonstrated anatomical and functional neuroplasticity following spinal cord injury. One of the more notable examples is return of ipsilateral phrenic motoneuron and diaphragm activity which can be induced under terminal neurophysiological conditions after high cervical hemisection in the rat. More recently it has been shown that a protracted, spontaneous recovery also occurs in this model. While a candidate neural substrate has been identified for the former, the neuroanatomical basis underlying spontaneous recovery has not been explored. Demonstrations of spinal respiratory interneurons in other species suggest such cells may play a role; however, the presence of interneurons in the adult rat phrenic circuit - the primary animal model of respiratory plasticity - has not been extensively investigated. Emerging neuroanatomical and electrophysiological results raise the possibility of a more complex neural network underlying spontaneous recovery of phrenic function and compensatory respiratory neuroplasticity after C2 hemisection than has been previously considered.