Quantal

Bernard Katz won the Nobel for his discovery that neurotransmitter release at synapses is "quantal"--that is, 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. This circumstance arises, scientists now know, because, prior to their release into the synaptic gap, transmitter molecules reside in like-sized subcellular packages known as synaptic vesicles (more at exocytosis).