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Department of Zoology, University of Melbourne, Parkville, Victoria, Australia.

1. Intracellular recordings were made from the parasympathetic ganglion cells that lie in the epicardium of the left atrium of guinea-pig heart near the interatrial septum. 2. Three distinct types of neurone were identified on the basis of their electrophysiological properties. In one group of neurones, S cells, somatic action potentials were followed by brief after-hyperpolarizations. In the other two sets of neurones, somatic action potentials were followed by prolonged after-hyperpolarizations. The neurones with prominent after-hyperpolarization were further subdivided: one group of neurones, P cells, showed inward rectification at membrane potentials near the resting membrane potential whilst neurones in the other group, SAH cells, did so only at more negative potentials. 3. In the group of neurones that displayed inward rectification at potentials near rest, rectification resulted from the activation of an inward current, which resembled the hyperpolarization-activated inward current present in cardiac muscle pacemaker cells. 4. The three different types of neurone received different patterns of synaptic input. Each SAH cell received a synaptic excitatory connection from the vagus which in most cells released sufficient transmitter to initiate an action potential in that cell; several SAH cells also received a separate connection, which could be activated by local stimulation. Although most S cells failed to receive a synaptic input from the vagus, all of those tested received an excitatory synaptic input which could be activated by local stimulation. Virtually all P cells failed to receive a synaptic input from the vagus; in addition, local stimulation failed to initiate synaptic potentials in P cells. 5. When the structure of cardiac ganglion cells was determined, by loading the cells with either biocytin or neurobiotin, it was found that most cells lacked extensive dendritic processes. S cells were invariably monopolar, most P cells were dipolar or pseudodipolar, whereas many SAH cells were multipolar. 6. In many neurones an on-going discharge of action potentials was detected in the absence of obvious stimulation. In S and SAH cells, the action potentials resulted from an on-going discharge of excitatory synaptic potentials. However, when a spontaneous discharge of action potentials was detected in P cells a discharge of excitatory synaptic potentials was not detected. 7. The results are discussed in relation to the idea that the three different types of cell may have different functions and that some of the cells may be organized in such a way as to permit the local handling of neuronal information within the heart.

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