Essentially there are two hypotheses which attempt to explain how transmitters are released from axon terminals (cf. 14,16,20). Exocytosis assumes that transmitters are packed in vesicles which, after having fused with the membrane of the axon terminal, discharge their contents into the external cleft. The first observation of the location of a transmitter in vesicles at nerve endings dates back to 1952; Blaschko and Welch for noradrenaline (7) and Fatt and Katz for acetylcholine (ACh) (16). Another hypothesis is that the transmitter is released from the cytoplasm (7,13,27,28,38,39). However, none of them can explain how transmitters cross the membrane and what kind of membrane mechanism operates to make the membrane permeable to the transmitters.