Responses of phrenic motoneurones of the cat to stimulation of medullary raphe nuclei

Abstract

Responses of phrenic motoneurones to stimulation of the three medullary raphe nuclei (raphe magnus (r. magnus), raphe obscurus (r. obscurus) and raphe pallidus (r. pallidus] were recorded in anaesthetized and decerebrated cats. Stimulation of r. magnus or r. obscurus depressed phrenic motoneurones. Stimulation at 100 Hz reduced action potential frequency within each inspiratory burst, without appreciable changes in inspiratory duration, or number of inspiratory bursts per unit time. The depression was proportional to the stimulus intensity (40-160 microA) and frequency (12-100 Hz) and lasted throughout the period of stimulation. Intracellular recording revealed concomitant depression of central respiratory drive potentials (c.r.d.p.s) and increased membrane input resistance during r. obscurus or r. magnus stimulation. In motoneurones which discharged action potentials during expiratory as well as inspiratory phases following intracellular chloride injection, stimulation of r. magnus or r. obscurus depressed cell firing during both phases. Both c.r.d.p.s and reversed inhibitory post-synaptic potentials (i.p.s.p.s) were depressed. These findings indicate that the depression is not related to post-synaptic inhibition of phrenic motoneurones. Stimulation (100 Hz) of r. pallidus produced discharges of action potentials in phrenic motoneurones. Stimulation lengthened the duration of each inspiratory discharge in proportion to stimulus intensity. Continuous firing occurred throughout the period of stimulation with maximal intensities. Intracellular recordings revealed sustained depolarization and reduction in membrane input resistance during the discharge. Responses were recorded extracellularly from medullary inspiratory neurones of the dorsal respiratory group (d.r.g.) and ventral respiratory group (v.r.g.) and from vagal axons which fired in phase with phrenic nerve activity. Responses to raphe stimulation were similar to those recorded from phrenic motoneurones. Evidence is presented that the responses are not related to stimulation of decussating bulbo-spinal axons from d.r.g. or v.r.g. neurones. It is suggested that medullary respiratory neurones receive inhibitory and excitatory synaptic inputs from medullary raphe neurones. Hypercapnia (5% CO2 in O2) or hypoxia (15% O2 in N2) reduced markedly the inhibition produced during stimulation of r. obscurus or r. magnus, and restored expiratory-linked silent periods during stimulation of r. pallidus. Activation of Hering-Breuer or baroreceptor reflexes did not alter responses to r. pallidus stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)