The action of atropine and acetylcholine on the pace maker ganglion cells of limulus heart

On the median nerve trunk-heart muscle preparation of Limulus the authors studied the effects of atropine and acetylcholine upon the pace maker ganglion cells. The results are summarized as follows: (1) Atropine exerts an excitatory action on the pace maker ganglion cells in a concentration of 1-2 per cent. resulting in an increase of the heart rate. No effect is recognized on the heart beats, where the drug is applied to the heart muscle. (2) Acetylcholine exerts an excitatory action in a lower concentration (0.001-0.10 %) and produces a transitory excitation followed by an inhibition in a higher concentration (1-5 %). No effect is perceptible on the heart beats, when the drug is applied to the heart muscle. (3) Where atropine has been previously applied to the median nerve trunk, acetylcholine applied to the same spot produces always an inhibition of the heart beats. Conversely, when the ganglion cells activated previously by acetylcholine, a subsequent administration of atropine suppresses the activity of the ganglion cells, resulting in an inhibition of the heart beats.