A re-examination of the vesicle hypothesis of transmission in relation to its applicability to the mammalian central nervous system.

The methods used to detect transmitters in the central nervous system, and their accuracy, are reviewed. The applicability of each of the seven different criteria used to define a transmitter is then examined, particularly in respect of substances not generally accepted as transmitters, but fulfilling some of these criteria. Twenty-three separate assumptions inherent in the vesicle hypothesis of transmission are identified; some of them are unproved and some unprovable. It is concluded that too many of these assumptions are in the latter two categories, which, from a Popperian viewpoint, render the hypothesis as a whole unwarrantable.