EXCITABILITY CYCLE OF THE HYPOTHALAMUS-SYMPATHETIC NEURONE SYSTEM

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Abstract
Stimulation of either side of the hypothalamus of the cat with low intensity, high frequency shocks caused preganglionic neurones of the cervical sympathetic to fire rhythmically and repetitively at frequencies proportional to the intensity and frequency of stimulation. If both sides of the hypothalamus were stimulated simultaneously, the neurones fired at higher frequencies which were greater than, equal to or slightly less than the sum of the frequencies obtained on stimulation of the 2 sides separately. Following a testing period of hypothalamic stimulation, the rate of firing of sympathetic neurones to constant hypothalamic stimulation decreased initially and gradually recovered to the control value. This reduction in excitability of sympathetic neurones to hypothalamic stimulation was proportional to the frequency and intensity of the testing stimulation. The duration of depressed excitability was related to frequency, intensity and also duration of the testing stimulation. The site at which the frequency of firing of a neurone is limited, is caudal to the point of convergence of pathways from the two sides of the hypothalamus, but cephalic to that portion of the system accessible to antidromic excitation. The pathways connecting the hypothalamus with the frequency limiting mechanism, as well as the more peripheral parts of the system, showed relatively less depression of excitability as a result of activity than did the frequency limiting mechanism itself. The suggestion is made that this mechanism lies in the medulla at a point accessible to afferent impulses carried by the buffer nerves.

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