The neurochemical basis of backward inhibition in a defense reaction reflex arc.

An investigation was carried out on a preparation of the isolated nervous system of the edible snail of the mechanism of habituation in the network of defense behavior neurons. It was established that the intracellular activation of one of the systems of the defense behavior command neurons, as in the case of the application of the neuropeptide FMRF-amide, leads to a decrease in the amplitude of the postsynaptic potentials in the remaining neurons of this class. An inference is reached, when account is taken of the presence of endogenous FMRF-amide in the defense behavior command neurons, regarding the possibility of active inhibition of the behavioral response to an appropriate stimulus through this neurochemical mechanism of the backward connection. The possible interrelationships with other modulating systems and the significance of such an association in the organization of behavior are discussed.