DIFFERENT TYPES OF GANGLION-CELL IN THE CARDIAC PLEXUS OF GUINEA-PIGS

1. Intracellular recordings were made from the parasympathetic ganglio n cells that lie in the epicardium of the left atrium of guinea-pig he art near the interatrial septum. 2. Three distinct types of neurone we re identified on the basis of their electrophysiological properties. I n one group of neurones, S cells, somatic action potentials were follo wed by brief after-hyperpolarizations. In the other two sets of neuron es, somatic action potentials were followed by prolonged after-hyperpo larizations. The neurones with prominent afterhyperpolarization were f urther subdivided: one group of neurones, P cells, showed inward recti fication at membrane potentials near the resting membrane potential wh ilst neurones in the other group, SAH cells, did so only at more negat ive potentials. 3. In the group of neurones that displayed inward rect ification at potentials near rest, rectification resulted from the act ivation of an inward current, which resembled the hyperpolarization-ac tivated inward current present in cardiac muscle pacemaker cells. 4. T he three different types of neurone received different patterns of syn aptic input. Each SAH cell received a synaptic excitatory connection f rom the vagus which in most cells released sufficient transmitter to i nitiate an action potential in that cell; several SAH cells also recei ved a separate connection, which could be activated by local stimulati on. 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 stimul ation failed to initiate synaptic potentials in P cells. 5. When the s tructure of cardiac ganglion cells was determined, by loading the cell s with either biocytin or neurobiotin, it was found that most cells la cked extensive dendritic processes. S cells were invariably monopolar, most P cells were dipolar or pseudodipolar, whereas many SAH cells we re multipolar. 6. In many neurones an on-going discharge of action pot entials was detected in the absence of obvious stimulation. In S and S AH 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 excitator y synaptic potentials was not detected. 7. The results are discussed i n 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 su ch a way as to permit the local handling of neuronal information withi n the heart.