The Electrical Activity of the Cricothyroid Muscles of the Cat

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Abstract
In the lightly anaesthetized preparations, the electrical activity of the cricothyroid ((IT) muscle of the cat was mainly of the expiratory type (figs. 1A, 6 and 8). Upon increasing the anaesthesia depth, expiratory activity was depressed or abolished and inspiratory discharges appeared, especially after increasing the respiratory dead space, after the occlusion of the tracheal cannula (fig. 1), or after inhalation of 8-10 % CO2 in oxygen. Discrete mechanical stimulation of the nasal and laryngeal mucosa and noxious stimuli applied to the skin of the face induced expiratory burst discharges in these conditions (fig. 2). Single unit analysis showed that the discharge patterns of the CT muscle may vary from the purely inspiratory type to the purely expiratory one, with all possible; intermediate combinations (fig. 3). Asphyxia and hypercapnia increased the number of inspiratory CT units (figs. 1 and 4). Expiratory activity was also augmented after strong asphyxia. Lung inflation generally decreased inspiratory (fig. 5) and increased expiratory activity (fig. 6). Lung deflation had the opposite effects although exceptions to this behaviour were frequently observed (Tabic; 1). Trigeminal stimulation evoked spike reflex discharges in units of all types, the response patterns being closely related with the discharge patterns in time. Inspiratory units were more susceptible to reflex activation during inspiration and expiratory units during expiration (figs.7, 8 and 9). The physiological implications of the CT muscle being innervated both by inspiratory and expiratory mofoneurones are discussed.