Diaphragm and Intercostal Muscle Activity after Mid-Cervical Spinal Cord Contusion in the Rat

The present study was designed to investigate the diaphragm and intercostal muscle activity after unilateral mid-cervical spinal cord contusion in rats. Electromyogram (EMG) activity of the bilateral diaphragm and T2 intercostal muscle was measured in anesthetized and spontaneously breathing rats. Unilateral mid-cervical contusion caused an immediate reduction in inspiratory bursting in the bilateral diaphragm and intercostal muscles. From 3 days to 8 weeks post-contusion, contused animals displayed significantly lower tidal volume than uninjured animals, regardless of the time point after injury. The burst amplitude of the contralateral diaphragm EMG was augmented in contused animals at 3 days post-injury. When the data were normalized by the maximal response during hypoxic-hypercapnic challenge (12-13% O₂, 3-4% CO₂), the ipsilateral diaphragm EMG of contused animals was greater than that of uninjured animals at 3 days and 2 weeks post-injury. Moreover, hypoxia-hypercapnia-induced increases in ipsilateral diaphragm EMG activity were blunted in contused animals at 2 weeks post-injury, but recovered at 8 weeks post-injury. Bilateral diaphragm EMG activity in contused animals was comparable to uninjured animals at 8 weeks post-injury. Notably, intercostal muscle activity was not substantially changed by mid-cervical spinal cord contusion from 3 days to 8 weeks post-contusion. These results suggest that mid-cervical spinal contusion induces a compensatory increase in contralateral diaphragmatic activity and greater utilization of a percentage of maximal inspiratory activity in the ipsilateral diaphragm. The maintenance of intercostal muscle activity may enable the animal to sustain essential breathing capacity after cervical spinal cord injury.