GapjunctFIG 4: Gap junctions link muscle cells into functional units.

A. Pharyngeal muscles fall into 8 segmental sets, pm1-pm8, with gap junctions (red symbols) linking neighboring segments to one another. Additional gap junctions link all muscles within a segment indirectly for pm2-pm7, via local gap junctions to marginal cells (not shown). Waves of radial contractions pass quickly along the pharynx, even without chemical synaptic input, causing widening of the central lumen. Each segment has been pulled slightly apart graphically to show where gap junctions link them to neighboring segments. Anterior is to the left in each panel.
B. Head and bodywall muscles are arranged in almost segmental fashion, with gap junctions connecting all neighboring muscles, both in L/R groups, and linearly along the head and bodywall. Most gap junctions occur on extended “muscle arms”, as highlighted in panel C. Waves of contraction pass along the body, causing local shortening of either dorsal or ventral muscles, while the opposing quadrants relax in the same locale. View from left aspect at low power, showing only the left side BWM quadrants; compare to panel C showing all four BWM quadrants.
C. Bodywall muscles (BWMs) contact neighbors via specialized long thin muscle arms extending medially, where they exchange gap junctions with other muscles, and receive neuromuscular junctions from motor axons lying in the two motor nerve cords (shown in red). White circles mark locales where neuromuscular junctions and gap junctions occur at dorsal and ventral muscle plates.
D. Three types of specialized (non-equivalent) muscles for defecation in hermaphrodite tail extend muscle arms to the surface of pre-anal ganglion (shown in pale red). White dotted circle indicates zone where overlapping muscle arms form gap junctions with one another, and receive neuromuscular junctions from motor axons.

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