Figure 6. Vulval muscle contractions are phased with respect to locomotor body bends and are limited by ERG.

(A) High-speed video recordings of freely behaving worms reveal four vulval muscle excitability states: smooth relaxation, smooth contraction, twitching, and egg laying. (top) Still frames are shown from an ERG null mutant recording where the twitches are more easily seen compared to wild type animals (Movie 1). Dashed arrows indicate worm direction and posture. Smooth relaxation occurs when the vulva passes through the ventral relaxation trough while smooth contraction occurs at the ventral contraction crest. Twitches are visible vulval muscle contractions that move and separate the tips of the vulva, while egg laying accompanies full vulval opening and egg release. Inset cartoons indicate position of the vulva within the idealized wave, and white boxes indicate regions of high magnification insets (bottom). Asterisk, vulva; arrowheads, opening of the vulva during twitching and egg laying. Scale bar, 10 μm.

(B) Vulval muscle twitching is phased with locomotion. High-speed video was used to record the timing of twitches in five wild-type and five ERG null mutant animals. Each video included 5 minutes prior to and 7 minutes after the first egg-laying event. Twitches were mapped relative to the timing of vulval muscle smooth contractions and relaxations (top) as it moves to its anterior on right (ventral arrow). Points indicate the proportion of twitches that occurred at each position; wild type: black circles, n=61 twitches; ERG null mutants: red squares, n=160 twitches.

(C) Vulval muscle twitching increases during egg laying and is more frequent in ERG null mutants. From the high-speed data in B, the frequency of body bends with twitches was determined. One minute prior to and three minutes after the first egg laying event was designated as the active phase for paired analyses; asterisks, p<0.0001 (Fisher’s exact test); n.s., not significant (p>0.05).