Further Studies on the Heart and Median Cardiac Nerve of Limulus Polyphemus

Abstract

The heart of Limulus consists essentially of 2 divisions, an atrium and a ventricle, each of which are contractile. The electromyograms obtained with the cathode ray oscillograph as a recording mechanism indicate that the fully contracted state is reached by a process of successive additions. In the atrial and ventricular muscle fibers the greatest amount of shortening occurs at the onset of a contraction, the further additions being of smaller and more nearly equal degree. The electromyogram consequently has a form somewhat comparable to the RST complex of the mammalian electrocardiogram. In contrast the electromyogram of a small group of isolated ventricular muscle fibers after stimulation with a single shock has the form of an ordinary diphasic muscle record, the rate of progression being 1 to 2 cm. per second.

The electroneurogram of the median cardiac nerve shows 2 groups of oscillatory discharges beginning 30 to 80 sigmas before the start of electrical activity in the musculature of the atrium and ventricle. The start of the ventricular electromyogram precedes the start of the visible ventricular contraction by several hundred sigmas. The oscillatory discharge can be shown to consist of potentials derived from 2 sources. From a correlation of the electrical and histological studies it is inferred that these 2 sources of potential are the axons of the large ganglion cells and the axons of the small ganglion cells. For reasons previously presented (Heinbecker) the large ganglion cells are considered normally to be the pace-maker cells, the small ganglion cells being responsible for the nerve impulses which directly innervate the cardiac muscle. Normally activity in the adult heart of Limulus is directly neurogenic in origin. The heart muscle can, however, be shown also to be rhythmically contractile without the intervention of cardiac ganglion cells.