State‐ and vagal‐dependent modulation of augmenting pattern of phrenic nerve discharge during eupnea in rats (726.8)

Abstract

Pontomedullary nuclei interact to generate and shape the respiratory rhythm and pattern, which is relayed to and modified in cranial and spinal respiratory motor outputs, including the phrenic nucleus, the motoneurons of which form the phrenic nerve and innervate the diaphragm. Under normal conditions, phrenic nerve discharge during eupnea is typified by an initial augmenting (ramp‐like) rising phase of inspiratory activity peaking in late‐inspiration, representing an orderly spatiotemporal recruitment of phrenic motoneurons. We tested the hypothesis that phrenic ramp is state‐ and vagal‐dependent. Experiments were performed on twenty‐one Sprague‐Dawley adult male rats. Groups included anesthetized vagus‐intact (n=6), decerebrate initially vagus‐intact (n=14), and decerebrate initially vagotomized (n=1) animals. Of the decerebrate initially vagus‐intact animals, a group was vagotomized (n=5) and another (n=7) was re‐anesthetized with isoflurane (2%) during continuous phrenic nerve recordings. This allowed comparisons to be made between animal groups as well as within the same animal to examine the effects of anesthesia and vagotomy on phrenic pattern. Three measures were primarily used to quantitatively represent phrenic burst shape ‐ 1) the fractional time of occurrence of peak integrated activity relative to inspiratory duration, 2) the peak‐to‐root‐mean‐square of burst amplitude, and 3) the relative proportion of total burst power contained in the fraction of the inspiratory epoch preceding peak integrated activity. Both anesthesia and vagotomy resulted in a statistically significant decrease in all three measures of phrenic ramp as well as an increase in cycle‐to‐cycle variability of phrenic burst shape. We conclude that the augmenting pattern of phrenic nerve discharge and its cycle‐to‐cycle consistency is both state‐ and vagal‐dependent.