Respiratory motor outputs following unilateral midcervical spinal cord injury in the adult rat

The present study was designed to investigate the impact of midcervical spinal cord injury on respiratory outputs and compare respiratory recovery following high- vs. midcervical spinal injury. A unilateral hemisection (Hx) in the spinal cord at C-2 or C-4 was performed in adult rats. Respiratory behaviors of unanesthetized animals were measured at normoxic baseline and hypercapnia by whole body plethysmography at 1 day and 1, 2, 4, and 8 wk after spinal injury. C(2)Hx and C(4)Hx induced a similar rapid shallow breathing pattern at 1 day postinjury. The respiratory frequency of C(4)Hx animals gradually returned to normal, but the tidal volume from 1 to 8 wk postinjury remained lower than that of the control animals. Linear regression analyses indicated that the tidal volume recovery was greater in the C(4)Hx animals than in the C(2)Hx animals at the baseline, but not at hypercapnia. The bilateral phrenic nerve activity was recorded in anesthetized animals under different respiratory drives at 8-9 wk postinjury. The phrenic burst amplitude ipsilateral to the lesion reduced following both high-and midcervical Hx; however, the ability to increase activity was lower in the C(4)Hx animals than in the C(2)Hx animals. When the data were normalized by the maximal inspiratory effort during asphyxia, the phrenic burst amplitude enhanced in the C(4)Hx animals, but reduced in the C(2)Hx animals compared with the control animals. These results suggest that respiratory deficits are evident following midcervical Hx, and that respiratory recovery and neuroplasticity of phrenic outputs are different following high-vs. midcervical spinal injury.