Activity of Cardiac Vagal Preganglionic Neurones during the Pulmonary Chemoreflex in the Anaesthetized Cat
The pulmonary chemoreflex is a primitive stereotyped response which occurs when phenylbiguanide (PBG) is injected into the pulmonary circulation. This is a chemical which activates receptors located near the pulmonary capillaries and supplied by small unmyelinated nerve fibres running in the vagus. The reflex response of the animal to this chemical flooding the pulmonary circulation is a cessation of breathing and a slowing of the heartbeat. With smaller doses of PBG there is little effect upon respiration although the cardiac effect is still obvious. This bradycardia is of interest because in the cat, it appears to unmodulated by central inspiratory activity (Daly, 1991). This is in marked contrast to the bradycardia of the carotid body chemoreflex which demonstrates such tight cardiorespiratory coupling. This result is still obtained after cardiac sympathetic denervation, indicating that cardiac vagal motoneurones are responsible. It has been postulated that these two reflexes utilize two different populations of cardiac vagal motoneurones, and that one population may not be coupled to the central respiratory network. There is some anatomical support for this two population hypothesis. When horseradish peroxidase is applied to the cardiac branch of the cat (Bennett et al., 1981; Jordan et al., 1986), labelled cells are seen in two very different locations. One group is found in the vicinity of the nucleus ambiguus in the ventral part of the medulla oblongata, and the other group is near the dorsal vagal motor nucleus. The experiments of McAllen & Spyer (1978) have clearly demonstrated that the ventral group play an important part in cardioinhibitory reflexes. This group of neurones exhibit strong respiratory related activity and have axons in the B-fíbre range. The function of the dorsal group is unknown. The question arises then, how do these two populations behave during the pulmonary chemoreflex ?
KeywordsNucleus Ambiguus Preganglionic Neurone Cardiac Sympathetic Denervation Ventral Group Cardiac Branch
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