The selective effect of the accelerator nerves on ventricular systole

Object of Investigation.—Acceleration of the heart in man is chiefly due to a varying balance of control exerted through the vagi and the accelerator nerves. The only hopeful method of determining which mechanism is at least predominantly concerned is suggested by the observations of Baxt, Pavlow, Frank and Reid Hunt and others 1 that the accelerator nerves exert a predominant effect on the length of systole, whereas the vagi nerves affect chiefly diastole. This, on superficial examination, of course appears contrary to any mechanical conception of the cardiac regulation, such, for example, as the “law of Uniformity of Behavior” advanced by Henderson and his coworkers. 2

It is quite obvious that, if, as appears from volume curves recorded by many different investigators, the rate of ejection diminishes late in systole, then, on the basis of the uniformity law enunciated by Henderson, the length of systole will be only slightly altered during the longer cycles but will be progressively more and more abbreviated in a mechanical way as the heart cycles become shorter and shorter. Inasmuch as vagus section and vagus stimulation ordinarily do not alter the heart rate beyond the range where slight variations might be expected, whereas accelerator stimulation quickens the beat so that pronounced shortening of systole might be anticipated, the mere demonstration that accelerator stimulation shortens the systole is proof neither of any specific influence of these nerves over ventricular contraction, nor does it prove that the heart deviates in its beat from a mechanical scheme. Only if it can be shown that the periods of systole during accelerator nerve stimulation vary materially from those which may be accounted for on the basis of volume curves, can any inference be drawn as to a selective action of the accelerator nerves on the ventricle.