Cervical spinal cord hemisection impacts sigh and the respiratory reset in male rats

Cervical spinal cord injury impacts ventilatory and non-ventilatory functions of the diaphragm muscle (DIAm) and contributes to clinical morbidity and mortality in the afflicted population. Periodically, integrated brainstem neural circuit activity drives the DIAm to generate a markedly augmented effort or sigh-which plays an important role in preventing atelectasis and thus maintaining lung function. Across species, the general pattern of DIAm efforts during a normal sigh is variable in amplitude and the extent of post-sigh "apnea" (i.e., the post-sigh inter-breath interval). This post-sigh inter-breath interval acts as a respiratory reset, following the interruption of regular respiratory rhythm by sigh. We examined the impact of upper cervical (C₂ ) spinal cord hemisection (C₂ SH) on the transdiaphragmatic pressure (P_di ) generated during sighs and the post-sigh respiratory reset in rats. Sighs were identified in P_di traces by their characteristic biphasic pattern. We found that C₂ SH results in a reduction of P_di during both eupnea and sighs, and a decrease in the immediate post-sigh breath interval. These results are consistent with partial removal of descending excitatory synaptic inputs to phrenic motor neurons that results from C₂ SH. Following cervical spinal cord injury, a reduction in the amplitude of P_di during sighs may compromise the maintenance of normal lung function.