Extracellular Action Potentials

Exploration of the cytoplasm of the giant esophageal cell of Ascaris with a recording microelectrode shows the existence of shallow spaces where the microelectrode tip becomes extracellular in spite of being in the interior of the cell. When the microelectrode penetrates into these spaces from the cytoplasm, the resting potential shifts to a different level or entirely disappears. At the same time the large intracellular spikes are replaced by small transients similar to extracellularly recorded action potentials. It is concluded that such spaces are in communication with the external solution, and separated from the cytoplasm by an electrically active membrane; i.e., able to generate action potentials. Measurement of the potential differences between the interior of the spaces and the external solution shows that although some are not polarized, many spaces have a resting potential of the same polarity as that of the cytoplasm. It is suggested that although they are of larger size these spaces may be equivalent to the tubular systems which in other muscle cells are known to be involved in the spread of excitation into the cytoplasm. As described in the previous paper (del Castillo and Morales, 1967), if the cytoplasm of the esophageal cell of Ascaris is minutely explored with an intracellular electrode, shallow layers are revealed where the tip of the micropipette becomes extracellular, at least from an electrical viewpoint. Indeed, the large intracellular spikes disappear when the tip of the microelectrode enters these spaces; instead, smaller transients, similar to extracellularly recorded action potentials, follow the applied pulses. The experiments described in this paper suggest that such deflections are generated by the flow of action currents within a system of spaces which communicate with the cell exterior by pathways which bypass the membrane which generates the resting and action potentials. Since extracellular "action potentials" are actually voltage drops caused by the flow of action currents in the resistance of the external solution, we shall use the word "currents" as a synonym for "extracellular action po-