The ionic basis of action potentials in petrosal ganglion cells of the cat.

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Abstract
The ionic conductances underlying the action potential and after-hyperpolarization of the cat petrosal ganglion neurons with myelinated axons in the carotid nerve were studied in vitro. Neurons were divided into 2 groups based on the presence or absence of an inflexion or hump on the spike falling phase. The application of tetrodotoxin (TTX, 3 .times. 10-7-3 .times. 10-6 M) revealed the presence of a TTX-resistant component in spikes with a hump, which was abolished in Na+-free solution. The action potential without a hump was blocked by TTX. The spike hump decreased or was abolished when Ca2+-channel blockers (Mn2+, 3-4 mM or Co2+, 5 mM) or low-Ca2+ solutions (0.1-0.2 mM) were applied to the preparation. In neurons with a hump on the spike, regenerative responses were obtained in Na+-free, high-Ca2+ (8.8 mM) solution. These responses were antagonized by Mn2+ and their amplitude was proportional to the external Ca2+ concentration. Apparently, the action potential with a hump was produced by a Na+ current, a part of which was TTX-resistant, and by a Ca2+ current which is responsible for the hump. Neurons without a hump had a TTX-sensitive Na+ spike. The spike with a hump was followed by a long-lasting after-hyperpolarization which reversed polarity at about -82 mV. During the hyperpolarization an increase in membrane conductance was observed. The amplitude and duration of the long hyperpolarizing potential decreased when Ca2+-channel blockers or low-Ca2+ solutions were applied. In Na+-free solution, regenerative responses were followed by a long hyperpolarization associated with an increase in membrane conductance. Evidently, the long after-hyperpolarization is produced by activation of the Ca2+-dependent K+ conductance.