Fig. 1. The cell body locations, muscle targets, and axonal pathways of RP motoneurons discussed in this study.A, Schematic drawing of one abdominal hemisegment showing the pathway for RP3, and the neighboring motoneurons RP1 and RP4. The segmentally repeated RP3 motoneuron cell bodies are located on the dorsal aspect of the CNS (gray) on either side of the midline (vm). Each RP3 axon (black) crosses the midline in the anterior commissure (ac) and turns posteriorly in the longitudinal connective (lc) to exit the CNS via the intersegmental nerve tract (ISN). There it joins the segmental nerve (SN) to innervate muscle fibers7 and 6 in the next posterior segment. The axon projects within the cleft between the two fibers to establish a characteristic nerve branch and site of innervation at stage16 (Halpern et al., 1991; Sink and Whitington, 1991a,b;Broadie and Bate, 1993a) (for review, see Broadie et al., 1994). The second input to the 7/6 cleft (gray) also innervates other ventral muscle targets. Its cell body location is not established. RP1 and RP4 continue laterally to innervate muscle fiber13 (Halpern et al., 1991; Sink and Whitington, 1991a). The transverse nerve (TN) runs along the borders of each segment. B, The ventral musculature of a 3rd instar larval fillet stained for motoneurons with an antibody to horseradish peroxidase. There are at least three morphologically distinct ending types: type Ib (large boutons), type Is(intermediate-sized boutons), and type II (small boutons, more branched and extensive). These have been shown by bouton backfills to belong to distinct motoneurons. Note thatDrosophila muscle fibers can be polyinnervated. Scale bar, 50 μm.