The inhibitory chemical transmitter substance postulated by Eccles
has never been detected in spite of numerous efforts to do so. The
mechanism(s) of inhibition is perhaps the key to the question of
cellular interaction and, in one form or another, must be accounted for
in any adequate theory.
Other rather specific forms of excitation and inhibition interaction
have been proposed at one time or another. Perhaps the best example is
the polar neuron of Gesell (8) and, more recently, Retzlaff (18). In
such a concept, excitatory and inhibitory couplings differ basically
because of a macroscopic structural difference at the cellular level;
that is, various arrangements or orientation of intimate cellular
structures give rise to either excitation or inhibition.