Diaphragm muscle function following midcervical contusion injury in rats

摘要

Midcervical spinal cord contusion injury results in tissue damage, disruption of spinal pathways, and motor neuron loss. Unilateral C4 contusion results in loss of 40%-50% of phrenic motor neurons ipsilateral to the injury (similar to 2.5% of the total phrenic motor neuron pool). Over time after unilateral C4 contusion injury, diaphragm muscle (DIAm) electromyogram activity increases both contralateral and ipsilateral to the side of injury in rats, suggesting compensation because of increased activation of the surviving motor neurons. However, the impact of contusion injury on DIAm force generation is less clear. Transdiaphragmatic pressure (Pdi) was measured across motor behaviors over time after unilateral C4 contusion injury in adult male SpragueDawley rats. Maximum Pdi (Pdi(max)) was elicited by bilateral phrenic nerve stimulation at 7 days postinjury. We hypothesized that Pdi(max) is reduced following unilateral C4 contusion injury, whereas ventilatory behaviors of the DIAm are unimpaired. In support of our hypothesis, Pdi(max) was reduced by similar to 25% after unilateral C4 contusion, consistent with the extent of phrenic motor neuron loss following contusion injury. One day after contusion injury, the Pdi amplitude during airway occlusion was reduced from similar to 30 to similar to 20 cmH(2)O, but this reduction was completely reversed by 7 days postinjury. Ventilatory behaviors (similar to 10 cmH(2)O), DIAm-specific force, and muscle fiber crosssectional area did not differ between the laminectomy and contusion groups. These results indicate that the large reserve capacity for DIAm force generation allows for higher-force motor behaviors to be accomplished despite motor neuron loss, likely reflecting changes in motor unit recruitment.