Membrane properties and firing characteristics of rat cardiac neurones in vitro.

Electrophysiological characteristics of neurones in isolated cardiac ganglia from the left atrium and interatrial septum of the rat were studied with intracellular microelectrodes. At rest the neurones were characterized by a membrane potential of -52.6 +/- 0.83 mV, an input resistance of 85.6 +/- 7.6 M omega, a membrane time constant of 4.6 +/- 0.24 ms and an input capacitance of 63.1 +/- 5.25 pF. Removal of Ca2+ ions from the external solution resulted in a membrane depolarisation of 5.5 +/- 0.70 mV and an increase in input resistance of 96 +/- 52% which indicated that a substantial Ca(2+)-sensitive component contributed to resting membrane potential. A prolonged after-hyperpolarization (AHP) was recorded following a train of spikes; this was inhibited in a Ca(2+)-free solution, indicating that a Ca(2+)-sensitive component of potassium conductance contribute to it. On the basis of the duration of the AHP following a single spike, two types of neurones, I and II, were tentatively identified, having short (less than 300 ms) and long (greater than 300 ms) AHPs, respectively. Type I neurones responded to prolonged membrane depolarization with bursts of firing (Ib neurones) or multiple discharges (Im neurones). Type II neurones also responded with single spikes or multiple discharges to prolonged membrane depolarization. In some Im neurones, tonic firing was recorded which was inhibited by a hyperpolarizing current and accelerated by a depolarizing current injected through the recording microelectrode. Thus, neurones of isolated cardiac ganglia of the rat from the region studied here are heterogeneous in their electrical behaviour, suggesting the existence of functionally different groups within the ganglia.