NeuroFIG 21: Chemical synapses and NMJs occur en passant in C. elegans.

A. Synapses can occur in a monadic (single postsynaptic partner) or polyadic (multiple post-synaptic partners for a single presynaptic neuron) fashion. (Top) Schematic rendition of a monadic and a dyadic synapse. (Bottom) TEM of a dyadic NMJ between the VA4 motor neuron and the DD2 neuron and a ventral body wall muscle arm. (Image source: [MRC] N2U [VC].)
B. Schematic rendition of an NMJ. The presynaptic motor neuron and post-synaptic muscle arm are separated by the basal lamina. When there is another motor neuron at the receiving end at (polyadic) NMJs, this post-synaptic neuron is on the same side of the basal lamina as the presynaptic neuron. (SV) Synaptic vesicle.
C. TEM shows an NMJ between a VD neuron and several muscle arms, transverse section. Basal-lamina-associated proteins nidogen/entactin (NID-1) (yellow line) and type XVIII (CLE-1) collagen (brown line) are drawn on the original image. Additional presynaptic sites occur on this VD process as it extends to the anterior and posterior of this region. (Left inset) The same synaptic region, magnified. At the point of contact with the post-synaptic elements, the presynaptic process enlarges into a varicosity with a specialized darkly staining bar at the active zone (thick arrow) and contains many SVs (thin arrows) close to MTs (arrowheads). (Right inset) The approximate section level, schematically. Bar, 1 μm. (Image source: [MRC] N2U[VC]-2167-8.)
D. TEMs of sublateral cords, transverse sections. (Top panel) Dense core vesicles (DCV), which may contain neuropeptides capable of modulating muscle contractility, clustered in a sublateral cord process. (Image source: N501C [Hall] 3538.) (Bottom panel) Synapse between two sublateral cord processes (arrowhead). Occasionally, sublateral cord processes also make NMJs onto nearby body wall muscles (not shown). (M) Muscle; (Cu) cuticle. (Image source: [MRC] N2U [ring and cords].)
E. TEM of the NR, transverse section. Two neurons are seen synapsing onto a third neuron sandwiched between them. Synaptic varicosities (V) contain synaptic vesicles clustered at the active zones featuring presynaptic dense bars (arrows). MTs (arrowhead) are placed near the vesicle clusters and DCVs are seen at the periphery. Bar, 1 μm. (Image source: Hall archive.)

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