Neural regulation of the heart muscle in an isopod crustacean: acceleration and peripheral inhibition.

1. In the neurogenic heart of the isopod crustacean Porcellio dilatatus, repetitive electrical stimulation of the cardiac nerves elicted either cardio-acceleratory or cardio-inhibitory effects depending on the stimulation parameters. 2. Acceleratory effects were accompanied by a decrease of membrane potential and by changes in the contour of the spontaneous electrical responses: increase in the speed of the rising phase and enhancement of the plateau phase. 3. Inhibitory stimulation acted on rhytjmicity and/or contour of spontaneous responses. At stimulation pulse frequencies beyond 25/s a hyperpolarization appeared after the cessation of the inhibitory train. 4. Inhibitory stimulation elicted IJPs in the myocardium. Their reversal potential was found to be close to the value of the resting membrane potential. During inhibitory stimulation, the membrane resistance of the heart muscle was frequently decreased. 5. The effects of changing the external chloride content, and of adding GABA and picrotoxin support the hypothesis that the inhibitory impulses increased the myocardium permeability to CL-. 6. On the basis of these findings it is assumed that cardio-inhibitory fibres act on both cardiac ganglion and myocardium. 7. Comparisons are established between the wood-louse's heart and the skeletal or heart muscle of some arthropods. The functional significance of peripheral inhibition is further discussed in relation to the nature of the spontaneous electrical responses and to contraction.