THE ACTIVITY OF THE CARDIAC SYMPATHETIC CENTERS

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Abstract
The activity of the cardiac sympathetic centers was investigated by recording the action potentials in the cardiac nerves from the stellate ganglia of the cat. There is a fairly continuous discharge of impulses which exert a "tonic" augmentor and accelerator influence upon the heart. This discharge is largely modified by changes in the chem. composition of the blood and by afferent impulses. The principal pathways of the impulses from the cord to the stellate ganglion are the 3d and 4th and to a lesser extent the 2d and 5th thoracic rami. The impulse frequency from the individual sympathetic motor nerve cells seldom exceeds 10 or 15 per sec, and is usually considerably less. This contrasts with the much higher frequency of discharge from somatic motor nerve cells. The potential pulses in the postganglionic nerves are of considerable magnitude because of the grouping of impulses which results from the innervation of many postganglionic fibers by a single preganglionic fiber. There are also much larger potential waves caused by the synchronous activity in very many nerve fibers. This is due to the coordinated and rhythmic discharge from large numbers of nerve cells in the centers. This activity is bilaterally synchronous. The grouped activity is of 4 types: the volleys may come (i) at irregular intervals; (ii) periodically, with frequencies varying from 5 to 20 a sec. but unrelated to any other obvious rhythm of the organism; (iii) synchronous with the pulse or (iv) with the respiratory cycle. The latter 2 forms of rhythmic cellular activity are largely due to afferent impulses from the viscera: bursts of impulses from the blood vessels initiated by the systolic rise in pressure or the impulses from dis-tention receptors in the lungs. An example of the marked effect of such afferent impulses upon the activity of the sympathetic centers is found in the observation that it is possible to drive those centers by repetitive stimulation of the central ends of the carotid sinus or aortic nerves, thus causing the motor nerve cells to discharge periodically with the frequency of the afferent impulses. This can be done within a limited range of stimulus frequencies. The characteristically grouped discharges from the cardiac sympathetic centers cause periodic variations in heart rate only if the bursts of efferent impulses are separated by some sec. This is due to the inertia of the effector mechanism.