Close relation between TEA responses and Ca‐dependent membrane phenomena of four identified leech neurones

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Abstract
Tetraethylammonium chloride (TEA) was applied to 4 kinds of identified neurons in leech [Macrobdella decora] segmental ganglia; sensory cells responding to touch (T), pressure (P) and noxious (N) stimuli and the Retzius cell (R). TEA prolonged action potentials of these cells to characteristically different degrees, R > N > P > T, regardless of exposure time. This result was the same whether TEA was presented to whole ganglion via bathing fluid or injected iontophoretically into the soma of the cell. TEA in Na-free solution caused behavior of the N cell membrane to be dominated by a Ca-dependent, Mn-blockable event identical in every respect except smaller size to the previously described behavior of the R cell under the same conditions. The P cell displayed a still smaller event of the same kind, but none was detectable in the T cell. In the absence of TEA and Na, when Ca was the only extracellular cation available to carry current, active membrane responses to depolarization were present in the R cell (previous study) and the N cell; such responses were minimal in the P cell and absent from the T cell. Differences among the 4 cells in density of a divalent cation conductance mechanism are the simplest explanation for these observations, though a more complex explanation based on multiple, pharmacologically distinct K conductances is not excluded.