Passive membrane properties of motorneurons and their role in long- distance signaling in the nematode Ascaris

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Abstract
In the motornervous system of the large parasitic nematode, Ascaris suum, the dorsal and ventral nerve cords are connected by a repeating pattern of single identified motorneuron processes, called commissures (Stretton et al., 1978). By making microelectrode penetrations of the commissures, we here report the first successful intracellular recordings of nematode neurons. These cells, like muscle cells of Ascaris, exhibit resting potentials of approximately -30 to -40 mV. Several tests indicate that these are the normal resting potentials of the cells and are not low due to damage. Using 2 intracellular microelectrodes (one for stimulation and one for recording), we have determined the input resistance and cable properties of commissural motorneurons. Over the physiological voltage range, the steady-state I- V plots are linear with little indication that voltage-sensitive conductances are contributing substantially to signaling. The membrane capacitance is comparable to that of single biological membranes (range, 0.4–0.9 microF/cm2) and the internal resistivity (range, 79–314 omega cm) is similar to that found in other cells. Because of unusually large membrane resistances (range, 61–251 k omega cm2), the space constants, lambda, are high (range, 4–10 mm). Such membrane properties produce cells that are well-designed for conducting passive signals over long distances. This long-distance signaling ability appears to be due to the intrinsic properties of the motorneuron membrane itself.