Organisation of the Larval Motor System

<div><p>(A) Diagrams of abdominal body wall muscles of one half segment (top) and of the VNC (bottom). These diagrams are included in most subsequent figures as reference for the relative positions of muscles and their innervating motor neurons. Internal muscles are red; external muscles, green. Muscle nomenclature is according to <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000041#pbio.0000041-Bate2" target="_blank">Bate (1993</a>): muscle position (D, dorsal; L, lateral; V, ventral), followed by orientation (A, acute; L, longitudinal; O, oblique; T, transverse) and SBM. In the VNC, cell bodies of motor neurons innervating an abdominal half-segment are indicated: red shows ISN with internal muscle targets; green shows SN with external muscle targets. The TN (brown) coincides approximately with the segment boundary. TN exit glia (asterisk) located on the ventral midline are also shown. The neuropile is indicated in black.</p> <p>(B and C) Retrograde fills of ISN and SN motor neurons in 15-h-old wild-type embryos. The neuropile (blue) was visualised with anti-HRP. ISN and SN motor neurons innervating muscles in the same segment elaborate their dendrites (arrows) in distinct regions. In (B), groups of ISN and SN motor neurons are labelled in two consecutive segments. In (C), individual ISN and SN motor neurons of a segment are shown.</p> <p>(D) The separation between ISN and SN motor neuron dendrites appears to be maintained, at least until 19 h AEL, when the motor system is functional.</p> <p>Anterior is left and dorsal is up. Symbols and abbreviations: triangles, ventral midline; AC, anterior commissure; PC, posterior commissure; asterisks, TN exit glia in (A) and dorsoventral channels, which are landmarks for the segment borders, in (B)–(D). Scale bar (not applicable to diagrams of CNS and muscle field): (B) 18 μm; (C–D) 14 μm. <i>n</i> numbers refer to the number of segments in which the same types of motor neurons (e.g., RP3 and SBM in [C]) were labelled.</p>