DESCENDING RESPIRATORY PATHWAYS IN THE CERVICAL SPINAL CORD

Abstract

In acute expts. on decerebrate or anesthetized cats and rabbits, a spinal lesion approaching or exceeding a hemisection was made at the 2d cervical level. The discharge of the phrenic motoneurons on each side of the cord was then measured by simultaneous action potential records from each hemidiaphragm. Controls established the absence of significant interaction between the 2 recordings. In each of 8 cats significant crossed (i.e., ipsilateral to lesion) diaphragmatic activity occurred in the control state, even when the spinal lesion far exceeded a hemisection. This crossed activity could be completely and differentially abolished by procedures which decrease respir- atory effort (e.g., hyperventilation), and augmented by procedures which enhance respiratory effort (e.g., rebreathing, negative intra-tracheal pressure). Similar results obtained for those rabbits (4 to 8) which exhibited crossed respiration in the control state. Procedures which enhance respiratory effort elicited significant crossed activity in the remaining rabbits. In conclusion, bulbo-spinal fibers of respiratory function descend on the intact side of the cord and make connections with phrenic motoneurons of the opposite side. These connections are quantitatively less powerful than the uncrossed ones. Intensity of discharge from the respiratory center is an important factor determining whether impulses in the crossing pathways alone will effectively excite the phrenic motoneurons.