Figure 1. Structure of mdIV terminals through postembryonic development.

(A) Cartoon comparison of the dendritic fields of the three nociceptive mdIV sensory neurons from a single hemisegment at first and third instar stages; sagittal view; anterior to left. (B) Dorsal view of EM reconstructions of all mdIV terminals from a single abdominal segment in the L1v (left; segment A1, first instar larva) and L3v (right; segment A3, third instar larva) data. Colors are as in A. The vertical extent of the gray box indicates the width of the neuropil; anterior to left; dashed line indicates midline. (C) Morphology of the terminals of each mdIV subtype, presented as in B. Unbranched primary projections from the nerve are cropped. (D) Dorsal view of a single vdaB terminal from the L1v and L3v, shown with synapses (outputs, red; inputs, cyan). Dashed line indicates midline. (E) Number of synaptic outputs on each mdIV terminal. L1v (solid bars), L3v (empty bars); left/right bar corresponds to left/right neuron. (F) Fold-change in synaptic outputs in the L1v and L3v. For each mdIV subtype, left and right neurons were averaged. (G) A standard polyadic synapse. In this example, taken from the L3v, the single pre-synaptic site (red arrowhead) has four post-synaptic contacts (cyan arrowheads). (H) Normalized histogram of number of post-synaptic contacts per pre-synaptic site on mdIV terminals (No significant difference; p=0.5641, two-sided Kolmogorov-Smirnov test). n.s. not significant; *p$<$0.05. **p$<$0.01. ***p$<$0.001.

Figure 1—figure supplement 1. A new EM image volume from a third instar larva ventral nerve cord.

(A) Schematic of the region of the third instar larva CNS sectioned and imaged for the L3v. Anterior is up. (B) A single section of L3v includes the complete neuropil (region inside white outline) and all soma (region outside white outline). Dorsal is up. (C) Ventromedial neuropile indicated in the blue outline in B. Neurite cross-sections highlighted in orange correspond to ipsilateral mdIV axons. (D–F’) Example synapses from vdaB (E), v’ada (E,E’), and ddaC (F,F’) terminals. Vesicles and pre-synaptic specializations highlighted by the red arrowhead. Post-synaptic neurons from LNs described in the main text are highlighted. Note the combination of small and dense core vesicles found in all three mdIV neurons.

Figure 1—figure supplement 2. Reconstructions of mdIV terminals.

(A, A’), Dorsal view of all mdIV terminals from the L1v (A) and L3v (A’), identities as labeled. Views are at the same scale. Dashed lines indicate lateral neuropil boundaries, solid line the midline. (B, B’) ddaC terminals in the L1v (B) and L3v (B’), left and right shown separately for clarity, as in all subsequent panels. ddaC can be distinguished by a midline crossing where the axon initially approaches the midline from the nerve and a projection into the adjacent segment posterior with little to no midline crossing. (C, C’), v’ada terminals in the L1v (C) and L3v (C’) can be distinguished by a lack of midline crossings and a projection into the adjacent segments anterior and, typically, posterior. D, D’, vdaB terminals in the L1v (D) and L3v (D’) can be distinguished by a midline crossing both where the axon initially approaches the midline and a second midline crossing in the adjacent segment anterior. Note that for all mdIV types, there is some variability — extra or missing branches, such as the missing posterior branch of the right L1v v’ada, are true reflections of the data — although certain features remain typical across most cell types.