Ionic mechanisms of electrical activity in somatic muscle of the nematode Ascaris lumbricoides

Abstract

The ionic dependence of the myogenic spike potentials and slow waves recorded fromAscaris lumbricoides somatic muscles has been investigated. Spikes appear to be mediated exclusively by calcium ions; the spike active potential varies with calcium concentration as expected for a calcium electrode and spikes persist in sodium-free media (Fig. 2). Slow waves can be mediated either by sodium or calcium; they persist when calcium or sodium are removed separately, but not when both are removed together (Figs. 3, 4, 6). In rhythmically active preparations, a burst of slow waves and spikes accompanies each contraction. Two phenomena may be related to the mechanism of this modulation: 1) TEA, although it does not prolong slow waves or spikes, induces rhythmic bursts of activity similar to spontaneous modulation (Fig. 5). This TEA-induced modulation appears to be myogenic. 2) Under conditions where calcium influx is reduced (either by addition of EGTA to the bath or by replacement of calcium with barium or strontium), very long-duration "square waves" are observed (Figs. 4. 7. 8). The square waves resemble slow waves in their ionic dependence, but differ in their sensitivity to TEA and to variation in the external potassium concentration. It is suggested that modulation and square waves involve the same channels. The significance of these results in understanding the role of myogenic activity in nematode locomotion is discussed.

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