RIM is a set of two motoneurons that innervate muscles in the head via NMJs in the nerve ring. The cell bodies are situated in the lateral ganglia and send processes into the ventral cord via the amphidial commissures. These run anteriorly through the neuropile of the ventral ganglion and then make one complete circuit round the anterior regions of the nerve ring near the outside surface. They are closely associated with the processes of AIB for most of the way round the ring. RIM is the only motoneuron class in the nerve ring that does not have axons situated on the inner surface of the nerve ring and so is only accessible to muscle arms where they penetrate the ring at the four muscle spurs (figure 14). Large NMJs are formed on the anterior side of each of the muscle spurs (a) and are usually intercepted by dendrites from RMD motoneurons (d). The processes of RIM and RMD form characteristic structures, which are seen just anterior to the NMJ region in each quadrant, where the RMD process flattens into a sheet and covers the anterior surface of a varicosity in the RIM process near the NMJ (b, c). The main neuron classes that are postsynaptic to RIM are: RMD (d), SAA (g), SMD(e) and AVB (f). The main synaptic inputs are from AIB (*a), AIZ (*g), SAA (*b) and RIS (*b). There are several gap junctions to AVE (h) and some to AVA, RIS and AIY (*d) . Magnifications: (a) x 25500, (b, c, g, h) x 12750, (d-f) x 17000.
Web adaptation, Thomas Boulin, for Wormatlas, 2001, 2002