The crossed phrenic phenomenon and recovery of function following spinal cord injury.

This review will focus on neural plasticity and recovery of respiratory function after spinal cord injury and feature the "crossed phrenic phenomenon" (CPP) as a model for demonstrating such plasticity and recovery. A very brief summary of the earlier literature on the CPP will be followed by a more detailed review of the more recent studies. Two aspects of plasticity associated with the CPP that have been introduced in the literature recently have been spontaneous recovery of ipsilateral hemidiaphragmatic function following chronic spinal cord injury and drug-induced persistent recovery of the ipsilateral hemidiaphragm lasting long after animals have been weaned from drug treatment. The underlying mechanisms for this plasticity and resultant recovery will be discussed in this review. Moreover, two new models involving the CPP have been introduced: a mouse model which now provides for an opportunity to study CPP plasticity at a molecular level using a genetic approach and light-stimulated induction of the CPP accomplished by transfecting mammalian cells with channelrhodopsin. Both models provide an opportunity to sort out the intracellular signaling cascades that may be involved in motor recovery in the respiratory system after spinal cord injury. Finally, the review will examine developmental plasticity of the CPP and discuss how the expression of the CPP changes in neonatal rats as they mature to adults. Understanding the underlying mechanisms behind the spontaneous expression of the crossed phrenic pathway either in the developing animal or after chronic spinal cord injury in the adult animal may provide clues to initiating respiratory recovery sooner to alleviate human suffering and eventually eliminate the leading cause of death in human cases of spinal cord injury.