Influence of Some Ions

The influence of several ions on the membrane potential of the somatic muscle of Ascaris has been investigated by changing their concentration in the surrounding solution. When [K]o is increased at the expense of [Na]o leaving [C1] constant, the membrane potential is first seen to increase. [K] higher than 45 m reduces the membrane potential with a slope of 23 my for a tenfold change in [K] o. However, when [K] is increased keeping [Na] and [Cl] 0 low and constant, the line relating the membrane potential with log [K] has a slope of almost 50 my. If [C1] 0 is reduced in the absence of external Na, after the [K] is increased to 45 mM, the membrane potential decreases with a slope of 59 my per tenfold change in [Cl] in close agreement with the Nernst equation. If C1is replaced by S0 42-, a depolarization is produced, while chloride replacement by NO3 -, Br-, and Iresults in a hyperpolarization of the membrane. Removal of the external Na+ ions increases the average membrane potential by 17 my. Microelectrode recordings from the somatic musculature of Ascaris lumbricoides reveal a relatively low resting potential interrupted by rhythmic graded spikes up to 45 to 50 mv in amplitude. These spikes are generated in a specialized region of the muscle cells in contact with the nerve cord. At this level, the terminal arborizations of the muscle cell arms interlace with one another forming a structure which exhibits functional properties similar to those of visceral smooth muscle of vertebrates. The individual mononucleated, somatic muscle cells are electrically interconnected throughout that web by low impedance pathways; accordingly, this region of the muscle has been called the syncytium (DeBell, del Castillo, and Sanchez, 1963). Electron microscopy has shown areas in which the surface membranes of adjacent muscle arms come into close contact, with obliteration of the intervening extracellular space (Rosenbluth, 1963).