The effect of electrical stimulation of cardioaccelerator and cardioinhibitor nerves on the mechanically recorded heart beat of crayfish was studied. Similar experiments were performed with the lobster, Homarus americanus.

Quantitative relationships between uni- and bilateral accelerator and/or inhibitor nerve stimulation and the resulting change in frequency and amplitude of the heart beat were established.

With increasing frequency of stimulation the accelerator nerves show a relative decrease in their action, while that of the inhibitor nerves increases. It appears that left and right regulator nerves have synaptic contacts at the same areas of the postsynaptic cells within the heart ganglion. Similar results are obtained whether all impulses arrive over one, or over the other, or over both accelerator (or inhibitor) nerves; the resulting acceleration or inhibition depends strictly on the number of accelerator, or inhibitor impulses arriving at the ganglion.

The ganglion cells can adapt to the inhibitor action. This is shown to be a postsynaptic phenomenon. Adaptation to accelerator stimulation is virtually absent.

Characteristic after-effects of the accelerator and inhibitor action were observed and quantitatively evaluated.

The interpretation of the results is based on the assumption of chemical transmitter substances. It is concluded that the accelerating transmitter decays slowly while the inhibitory transmitter is inactivated relatively rapidly.

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