<Li> Ion channel linked receptor </Li> <Li> Ligand (such as acetylcholine) </Li> <P> Acetylcholine is a neurotransmitter synthesized from dietary choline and acetyl - CoA (ACoA), and is involved in the stimulation of muscle tissue in vertebrates as well as in some invertebrate animals . In vertebrate animals, the acetylcholine receptor subtype that is found at the neuromuscular junction of skeletal muscles is the nicotinic acetylcholine receptor (nAChR), which is a ligand - gated ion channel . Each subunit of this receptor has a characteristic "cys - loop", which is composed of a cysteine residue followed by 13 amino acid residues and another cysteine residue . The two cysteine residues form a disulfide linkage which results in the "cys - loop" receptor that is capable of binding acetylcholine and other ligands . These cys - loop receptors are found only in eukaryotes, but prokaryotes possess ACh receptors with similar properties . Not all species use a cholinergic neuromuscular junction; e.g. crayfish and fruit flies have a glutamatergic neuromuscular junction . </P> <P> AChRs at the skeletal neuromuscular junction form heteropentamers composed of two α, one β, one ɛ, and one δ subunits . When a single ACh ligand binds to one of the α subunits of the ACh receptor it induces a conformational change at the interface with the second AChR α subunit . This conformational change results in the increased affinity of the second α subunit for a second ACh ligand . AChRs therefore exhibit a sigmoidal dissociation curve due to this cooperative binding . The presence of the inactive, intermediate receptor structure with a single - bound ligand keeps ACh in the synapse that might otherwise be lost by cholinesterase hydrolysis or diffusion . The persistence of these ACh ligands in the synapse can cause a prolonged post-synaptic response . </P>

The neurotransmitter used at the neuromuscular junction (nmj) is