His research uncovered fundamental properties of synapses, the junctions across which nerve cells signal to each other and to other types of cells. By the 1950s, he was studying the biochemistry and action of acetylcholine, a signalling molecule found in synapses linking motor neurons to muscles, used to stimulate contraction. Katz won the Nobel for his discovery with Paul Fatt that neurotransmitter release at synapses is "quantal", meaning that at any particular synapse, the amount of neurotransmitter released is never less than a certain amount, and if more is always an integral number times this amount. Scientists now understand that this circumstance arises because, prior to their release into the synaptic gap, transmitter molecules reside in like-sized subcellular packages known as synaptic vesicles, released in a similar way to any other vesicle during exocytosis.