[Changes in the excitability of command neurons during the initial period of conditioned reflex formation in grape snails].

The procedure of elaboration of conditioned defensive pneumostomum closure was reproduced in neurophysiological experiments on half-intact preparation of the snail. Activity of identified command neurones involved in the reinforcing unconditioned reflex, was recorded intracellularly. Conditioned and unconditioned stimuli converged on these neurones. Excitability of the explored command neurones increased during the first stages of conditioning. Appearance and consequent intensification of the initially absent spike response to the conditioned stimulus was essentially due to this excitability change. Command neurones action potentials evoked by the conditioned stimulus elicited some effector reactions which were the first signs of conditioning. The conclusion is made that the excitability changes in certain nerve cells are essential for determining the initial phase of conditioning.