Activity in a Crustacean Ganglion. I. Cardio-Inhibition and Acceleration in Panulirus Argus

The morphology and aspects of the physiology of the cardiac ganglion and the extrinsic cardiac nerves of Panulirus argus have been studied.1. The extrinsic nerves consist of two pairs of cardio-accelerators and one pair of cardio-inhibitors which arise in the anterior portion of the thoracic ganglion. These run dorsally and laterally to join at the pericardial plexus where fibers from each come together to form the dorsal nerve. This runs to the cardiac ganglion. The thick fibers (2-4) of the dorsal nerve which ramify over the large cells of the ganglion are probably inhibitors. The identification of the accelerator fibers remains unclear.2. Upon electrical stimulation of the extrinsic nerves, two or three levels of response were obtained with increasing pulse duration and voltage. This was probably because of an increase in the number of fibers stimulated.3. Complete inhibition was usually obtained by frequencies of 35, 50 or 90 stimuli per second, depending upon the level of response of the heart. Usually no inhibition was obtained below a frequency of 15 per second. Between the minimum and maximum there was a gradation of both frequency and amplitude of heart beat. The stimulus-response curves approximated an exponential curve. Facilitation and an after-effect were present but of short duration.4. The maximum increase in frequency and amplitude of the heart beat was usually obtained by stimulation of the accelerators at 60 per second or more. No response was ordinarily obtained below 5-10 stimuli per second. The stimulus-response curve was usually hyperbolic, though in some cases it was sigmoid. The actions of the two accelerators appear to be identical. Facilitation and after-effect have a much longer time constant than in the case of the inhibitor.5. From a consideration of the participation of individual cells in the cardiac ganglion discharge for each beat, it is probable that the cardio-inhibitor and accelerators depress or enhance the excitabilities of the ganglion cells.