Medullary Inspiratory Activity During Opossum Development
Discharge properties of medullary neurons associated with inspiration were assessed during opossum development. Location of these neurons and projections to the spinal cord were also evaluated. Studies were performed on suckling opossums, from 15-20 days of age to weaning (approximately 100 days), as well as adult animals, anesthetized with Inactin. Most cells either discharged during inspiration (I cells) or were phase-spanning between late expiration and inspiration (EI cells). Number of spikes per breath for I and EI cells significantly increased as the animal matured. For younger opossums (up to approximately 60 days of age), bulbospinal I and EI cells discharged an average of 1.9 spikes per breath, with a minimum interspike interval of 59 ms. Their conduction velocities, determined by use of antidromic stimulation, were in the unmyelinated range (less than 2 m/s). The occurrence of few spikes per breath with long interspike interval, in bulbospinal cells of the younger opossums, suggests that inspiration is not subject to fine feedback control and that rate of high-frequency motor bursts is relatively slow.