The hypothesis, first formulated by Bernard Katz (1911–2003) and colleagues in the 1950s, that nerve impulses are transmitted across junctions (synapses) in the nervous system by the release of discrete packets, or quanta, of neurotransmitter. It has since been verified that each quantum represents the contents of a single synaptic vesicle. The size of the vesicle, and hence the number of molecules of neurotransmitter it contains (i.e. the quantum size), is characteristic for any nerve cell terminal, although the number of vesicles discharged (i.e. the quantum content) can vary considerably. For example, at the neuromuscular junction of a frog, each vesicle contains about 7000 molecules of the neurotransmitter acetylcholine, and discharge of about 200 such vesicles is needed to generate the end-plate potential that triggers contraction of the muscle.