DISCHARGE PATTERNS OF CAROTID BODY CHEMORECEPTORS

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The discharge patterns of carotid body chemoreceptors will be studied. An avian animal model was chosen because avian carotid body chemoreceptors are nearly identical in structure and function to mammalian carotid bodies, and because the avian lung offers the powerful experimental advantage of unidirectional ventilation (UDV) for controlling arterial blood gases. Experiments are proposed that combine (UDV), on-line blood gas measurement, single unit neural recording techniques, and computerized on-line data acquisition to test the physiological responses of carotid body chemoreceptors in ways difficult or impossible with a mammalian model. Carotid body chemoreceptors are multimodal, responding to arterial PO2, PCO2, and other stimuli. This project will determine the response of single receptors to arterial pH and blood pressure. This project will also analyze the temporal occurrence of action potentials from single chemoreceptors exposed to different static levels of stimuli cause different receptor discharge patterns. Pattern differences could be a neural encoding mechanism for carrying differential stimulus information to the central respiratory controller, and may represent fundamental differences in transduction mechanisms for O2 and CO2. Dynamic oscillations of arterial PCO2 associated with tidal breathing are hypothesized to cause a feed-forward control signal via the carotid bodies for ventilatory control during exercise. This project will use UDV-induced ramp oscillations of arterial PCO2, and PO2 to test the rate sensitivity of carotid body chemoreceptors (which would amplify the feed-forward signal in exercise). UDV-induced sinusoidal oscillations of arterial PO2 and PCO2 will be used to test the frequency response of the receptors (determining the receptor response to changes in respiratory rate), and it will test for phase differences between oscillating PO2 or PCO2 receptor response (which affects the efficacy of chemoreceptor discharge arriving at the central controller). Many aspects of the normal dynamic and static carotid body chemoreceptor sensitivity remain uncertain. It is important to study these normal physiological responses so that we can define the roles of carotid body chemoreceptors in the control of pulmonary ventilation in health and disease.