The neuromuscular system of the Drosophila larva. (A) The larval CNS is comprised of the brain and ventral nerve cord (VNC), which is segmentally reiterated and bilaterally symmetrical with respect to the ventral midline (dotted line). A representative MARCM clone (MN9-Ib) is shown. (B) The stereotyped organization of the peripheral body wall muscles. In each abdominal hemisegment, motor axons from the six main nerve branches (ISN, ISNb, ISNd, SNa, SNc, and TN) innervate the 30 muscles.

Identification of individual motor neurons. (A) The stereotyped pattern of Fasciclin II expression in the VNC. Axon fascicles in the VNC provide a frame of reference to characterize the dorsal-ventral and mediolateral position of the motor neuron cell body. (A′) Orthogonal view of (A). Fascicles are named according to (D=dorsal, C=central, V=ventral, L=lateral, I=Intermediate). (B) Merged image of the GFP-labeled MN9-Ib motor neuron and FasII. (B′) The soma of MN9-Ib resides in the ventral cortex, while its dendrites extend into the dorsal neuropil. (C) Peripheral muscle field innervated by MN9-Ib. MN9-Ib innervates muscle 9 and terminates with type Ib synaptic boutons. Anterior is up in this and all subsequent Figures.

Dendritic morphologies of motor neurons in the ISN branch. Single-cell motor neuron clones in the ISN branch. (A-L) Morphologies of individual motor neurons in the ISN branch. These neurons include MN1-Ib (A), MN2-Ib (B), MN3-Ib (C), MN4-Ib (D), MN9-Ib (E), MN10-Ib (F), MN11-Ib (G), MN18-Ib (H), MN19-Ib (I), MN20-Ib (J), MNISN-Is (K), and VUM (L). Both MNISN-Is and VUM innervate multiple dorsal muscles (not shown). The MN3-Ib neuron (C) is off-center. The soma of MN3-Ib is normally positioned in the lateral cortex. The dashed line represents the midline of the VNC, asterisks indicate position of segment borders. Arrows indicate position of cell body, arrowheads indicate axon.

Dendritic morphologies of motor neurons in the ISNb and ISNd branches. Single-cell motor neuron clones in the ISNb and ISNd branches. (A-F) Morphologies of individual motor neurons in the ISNb branch. These neurons include MN6/7-Ib (A), MN12-Ib (B), MN13-Ib (C), MN14-Ib (D), MN30-Ib (E), and MN28-Ib (F). (G,H) Morphologies of ISNd motor neurons MN15/16-Ib (G) and MN15/16/17-Ib (H). (I-K) ISNb/d motor neurons innervate muscles normally innervated by both ISNb and ISNd motor neurons including MNISNb/d-Is (I), MNISNb/d-II (J), and VUM (K). The dashed line represents the midline of the VNC, asterisks indicate position of segment borders. Arrows indicate position of cell body, arrowheads indicate axon.

Dendritic morphologies of motor neurons in the SNa and SNc branches. Single-cell motor neuron clones in the SNa and SNc branches. (A,B) Morphologies of individual motor neurons in the SNa branch including MN22/23-Ib (A) and VUM (B). (C) Dendritic morphology of MNSNc, which innervates multiple ventral muscles (not shown). The MNSNc motor neuron has two distinct cell bodies in MARCM clones. The dashed line represents the midline of the VNC, asterisks indicate position of segment borders. Arrows indicate position of cell body, arrowheads indicate axon.

Motor dendrites project to distinct neuropilar territories. The AP and mediolateral positioning of motor dendrites is largely stereotyped. (A,A′) The cell body of the SNa motor neuron MN22/23-Ib resides in the ventrolateral cortex. MN22/23-Ib extends its axon and dendrite into the dorsolateral neuropil before its axon exits the VNC. MN22/23-Ib dendrites normally project anteriorly along the DL fascicle and extend slightly medial beyond this fascicle. (B,B′) The cell body of the ISN motor neuron aCC is positioned within the dorsal cortex, and the motor neuron projects it dendrites to both ipsilateral and contralateral regions in the neuropil. Its dendritic field is normally restricted to the neuropil between the LI domain and DL fascicle. (C,C′) The dendritic field positions of the ISN motor neuron MN9-Ib is similar to aCC, although the soma of MN9-Ib resides in the ventral cortex. (D,D′) The cell body of the ISN motor neuron MN11-Ib resides in the lateral cortex. MN11-Ib extends its dendrites posteriorly between the LI domain and DL fascicle. (E,E′) Both MN19-Ib and MN2-Ib dendrites project their dendrites between the LI domain and DL fascicle. (F,F′) The cell body of the ISNb motor neuron MN13-Ib is located in the dorsal cortex, and the motor neuron extends its dendrites into the contralateral neuropil between the MI domain and DL fascicle. (G,G′) The soma position and dendrite field of the ISNb motor neuron MN30-Ib is similar to MN13-Ib. (H,H′) The ISNd motor neuron MN15/16-Ib is the only motor neuron that extends its dendrites to the ventral midline. (I) Schematic view of representative SNa (magenta), ISN (green), ISNb (red), and ISNd (blue) motor dendrites. Dendrites from motor neurons from each nerve branch occupy distinct territories to provide full coverage of the neuropil. (J) Average mediolateral positions and widths of representative SNa (magenta), ISN (green), ISNb (red), and ISNd (blue) motor dendrites relative to the ventral midline and DL fascicle. Lateral (L), lateral-intermediate (LI), medial-intermediate (MI), and medial (M) domains for each hemisegment are labeled. Values are normalized as described in Materials and methods and mean values ± standard deviation are shown. Vertical dashed lines represent the midline of the VNC and DL fascicle, horizontal dashed lines represent segment borders in A-H. Arrows indicate position of cell body, arrowheads indicate axon.

Dendrites of different motor neurons show extensive overlap. (A,B) Dendrites from different motor neurons within the same nerve branch show extensive overlap. (A) Dendrites from two adjacent ISN motor neurons, MN9-Ib (left) and MN11-Ib (right), do not exhibit tiling behavior. (B) Dendrites from two adjacent ISNb motor neurons, MN6/7-Ib (left) and MN12-Ib (right), show extensive overlap. (C,D) Different motor neurons overlap with respect to segment boundaries regardless of whether they are from different nerve branches (C) or the same nerve branch (D).