Type: Pharyngeal motor neuron
In MoW:
Male Wiring Project:
In Wormbase: M4
Lineage: MS paaaaaa
Location: Pharynx anterior bulb
Description: The cell body of M4 is located dorsally at the level of the nerve ring.


M4 sends two ventral branches, which run around the nerve ring on each side. These processes then cross to the other side and continue to run posteriorly in the subventral nerve cords. In the isthmus, M4 makes neuromuscular synapses on pm5. The two processes turn dorsally in the terminal bulb commissure, meet each other in gap junctions and run anteriorly in the dorsal nerve cord for some variable distance through the isthmus before ending. These also make synapses on pm5 on the dorsal side. Unlike other pharyngeal motor neurons, M4 contains darkly-staining vesicles, similar to the 65 nm dense core vesicles associated with some amphidial neurones (Albertson and Thomson, 1976)
Neurotransmitter/ Neuropeptide:
- Acetylcholine
- Glutamate - weak
- FLP-21; FMRFamide-related neuropeptide (FaRP)
(Pereira et al., 2015; Duerr et al., 2008; Rogers et al., 2003; Rand and Nonet, 1997-Appendix 2; O. Hobert pers comm.)
Innexin expression:
None, yet reported, although described to have gap junctions in adult animals (Albertson and Thomson, 1976)
Receptor expression:
- SER7b ; G-alpha(s) coupled 5-HT7-like receptor
- GLR-8; glutamate receptor subunit
(Hobson et al., 2003; Brockie et al., 2001)
-M4 is essential for feeding in wild-type worms. M4 synapses on the isthmus and the terminal bulb, and is necessary for posterior isthmus peristalsis (Avery and Thomas, 1997). Worms lacking M4 continue pumping, however, bacteria become trapped in corpus and anterior isthmus and these worms fail to grow since they can not pass the food to their intestines

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M4 (MS paaaaaa) development in the embryo. Dorsal view. Bottom is left side of the embryo. Spheres indicate individual nuclei. Black sphere: ancestors of M4; dark grey spheres: apoptotic cells; other cells follow the WA color code (after they acquire specific cell or tissue identities). 0 min is fertilization. Click on the movie for higher resolution rendition (by A. Santella & Z. Bao).