A heterogeneous group of disorders characterised by a congenital defect in neuromuscular transmission at the neuromuscular junction. This includes presynaptic, synaptic, and postsynaptic disorders (that are not of autoimmune origin). The majority of these diseases are caused by mutations of various subunits of the nicotinic acetylcholine receptor (nicotinic receptors) on the postsynaptic surface of the junction.