The effects of hypoxia on sleep, breathing, and medullary respiratory neurons in the cat

Date

2003-08

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

Low oxygen (hypoxia) causes perturbations in all physiological systems. We investigated the perturbations in sleep, breathing and the ventral medullary respiratory network caused by hypoxia in the intact unanesthetized cat. Our first objective examined the effect of hypoxia on sleep. We found that the low arterial carbon dioxide (CO2) levels, associated with hyperventilation resulting from hypoxia, were the primary cause of the sleep disruption that occurs in hypoxic environments. To investigate further the effects of low carbon dioxide on sleep, we mechanically hyperventilated cats and altered the level of CO2. As with hypoxia-induced hyperventilation, we found that sleep was decreased by low levels of COj. Our second objective examined the effects of hypoxia on breathing during sleep and wakefulness. We found that hypoxia caused sustained hyperventilation in sleep and wakefulness. We found also that hypoxia increased the activity of the diaphragm during inspiration and expiration, and that these changes in diaphragmatic activity were associated with significant changes in the airflow waveform from sawtooth waveforms in normoxia to square waveforms in hypoxia. We speculate that waveform changes may represent a previously unknown adaptation to hypoxia. Our third objective investigated the response of nine major classes of medullary respiratory neurons to hypoxia during sleep and wakefulness. We found that only about one-half of both inspiratory and expiratory neurons were excited by hypoxia despite the fact that breathing was significantly increased. Of particular interest, respiratory neurons changed their profile of activity during exposure to hypoxia, which may be the mechanism by which selective activation of medullary respiratory neurons results in a significant increase in ventilation. Total activation of the respiratory system would significantly increase oxygen consumption whereas selective activation of respiratory neural activity during conditions of hypoxia would limit increased oxygen consumption. Thus, selective activation of the respiratory neurons may represent also an adaptive mechanism.

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Keywords

Cats -- Research, Neurochemistry -- Experiments, Toxicological chemistry -- Observations, Medulla Oblongata -- cytology

Citation