Recovery of airway protective behaviors after spinal cord injury

Abstract

Pulmonary morbidity is high following spinal cord injury and is due, in part, to impairment of airway protective behaviors. These airway protective behaviors include augmented breaths, the cough reflex, and expiration reflexes. Functional recovery of these behaviors has been reported after spinal cord injury. In humans, evidence for functional recovery is restricted to alterations in motor strategy and changes in the frequency of occurrence of these behaviors. In animal models, compensatory alterations in motor strategy have been identified. Crossed descending respiratory motor pathways at the thoracic spinal cord levels exist that are composed of crossed premotor axons, local circuit interneurons, and propriospinal neurons. These pathways can collectively form a substrate that supports maintenance and/or recovery of function, especially after asymmetric spinal cord injury. Local sprouting of premotor axons in the thoracic spinal cord also can occur following chronic spinal cord injury. These mechanisms may contribute to functional resiliency of the cough reflex that has been observed following chronic spinal cord injury in the cat.

Fig. 1

Rectus abdominis (RA) EMGs recorded on the injured and uninjured sides and esophageal pressure (P_ES) during repetitive cough from an anesthetized cat 6 months following spinal cord hemisection at T10. Robust activity is present in the rectus abdominis muscles bilaterally and esophageal pressures indicate strong cough efforts. Preliminary observations indicate that vigorous cough efforts with bilateral EMGs in the rectus abdominis muscles that are similar to those recorded before injury can be observed up to many months post-injury. Rectus abdominis EMG electrode were placed percutaneously caudal to the pelvic crest approximately 1 cm lateral to the midline. Cough was elicited by mechanical stimulation of the vocal folds and epiglottis with a rigid plastic coated rod inserted orally. Some spared tissue at the lesion epicenter is present in the medial aspect of the ventral funiculus can be observed ventromedially on the injured side of this animals spinal cord.

Fig. 2

Intercostal and abdominal EMGs recorded on the injured and uninjured sides in an anesthetized cat during repetitive cough 5 months following spinal cord hemisection at T9/T10. Arrow indicates the occurrence of an expiration reflex near the onset of stimulation. Following the expiration reflex, five coughs were produced. Robust activity was present bilaterally in the transversus abdominis muscles. The external intercostal EMG on the injured side at T11 was robust. Cough was elicited by mechanical stimulation of the intrathoracic airway with a flexible polyethylene catheter. The solid bar above the EMG records indicates the duration of mechanical stimulation. The hemisection extended across the midline both dorsally and ventrally. TA: transversus abdominis, PS: parasternal muscle, EIC: external intercostal muscle, MA: moving average 50 ms time constant.