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Baroreceptor and chemoreceptor activity during nasal stimulation in the muskrat (Ondatra zibethica) Douse, Mark Alan


Diving muskrats (Ondatra zibethica) invoke a series of cardiovascular and respiratory adjustments in response to stimulation of the nares with water. This dive response is characterized by apnoea, a decrease in cardiac output and an increase in peripheral resistance. The result is that blood flow is maintained to those organs most susceptible to oxygen deprivation, the heart and the brain. The initiation of the dive response in mammals is primarily the result of nasal stimulation with water. In addition, the baroreceptors acting via the baroreflex have been suggested to be involved in either the initiation or the maintenance of this response. The chemoreceptors, acting via the chemoreflex, have also been implicated in the maintenance of the dive response, although the importance of this contribution is controversial. The purpose of this thesis was to examine the role of the baroreceptors and chemoreceptors in the diving response of the muskrat. Changes in input from these receptors recorded from the cut carotid sinus nerve and their modulation by the carotid sinus efferent activity during nasal stimulation may have important implications for the role of the baroreceptors and chemoreceptors in the diving response. In the initial part of the dive, baroreceptor activity decreased, while chemoreceptor activity did not change. Subsequently, baroreceptor and chemoreceptor activity increased, exceeding pre-dive levels. This increase was not due to a change in receptor threshold or sensitivity induced by the nasal stimulation, but was a reflection of the increase in the usual stimulus modality of both receptor groups. The efferent activity recorded from the central end of the cut carotid sinus nerve was of two types, both of which responded to nasal stimulation. This change in the efferent discharge has the potential to modify afferent activity. Nasal stimulation caused one type of efferent activity (type A) to stop. The second type of efferent activity (type B) responded with an initial increase in discharge, returning to pre-dive levels after 6.6 seconds. Based-on the similar characteristics of these efferents to those of previous work it is postulated that the actions of the efferents would be to inhibit the baroreceptors and chemoreceptors during the initiation of the nasal stimulation, but to be less effective as the dive progressed. It is concluded that there is no contribution from the baroreceptors to the initiation of the diving bradycardia, although the lack of baroreceptor activity may contribute to the increase in peripheral resistance. Later in the dive, both heart rate and arterial blood pressure increase, despite a concomitant elevation in baroreceptor activity. The baroreceptors therefore have no role in the maintenance of the diving response. The initial inhibition of the chemoreceptors may be important to permit the full expression of the dive response, including a decrease in central respiratory output. Later in the dive the chemoreceptors may contribute to the maintenance and termination of the diving response.

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