During the elaboration of an instrumental reflex, it is not obligatory to use a conditioned stimulus, which signals the necessity to
generate an instrumental reaction in order to receive reinforcement. However, the presence of a conditioned stimulus simplifies
analysis of instrumental reaction, which in this case is the response to the conditioned stimulus. On the other hand, it is necessary to
distinguish between instrumental and classical conditioning, since in both cases the response to a conditioned stimulus increases. We
studied neuronal analogs of classical and instrumental conditioning in the identified neurons responsible for the defensive closure of
the pneumostome in the Helix mollusk under the same conditions. During classical conditioning, a mollusk received punishment
after a tactile stimulus. During instrumental conditioning, a mollusk received punishment when an identified neuron did not
generate an action potential in response to a tactile stimulus. The appearance of a painful stimulus did not depend on the generation
or failure of a spike in the related control neuron. Another tactile stimulus, which was never paired with an unconditioned stimulus,
was used as a discriminated stimulus. We also compared the behavior of such identified neurons during pseudoconditioning. The
experiments were carried out in a semi-intact preparation. We examined how responses to the tactile and painful stimuli changed
during different forms of training. It was shown that the dynamics of neuronal responses to a conditioned tactile stimulus were much
more complex during instrumental conditioning and consisted of several phases. Throughout a learning session, neural system
consecutively acquired information as to which kind of learning was presented, whether a reaction of the neural system must be
generated or inhibited and which instrumental reaction is correct. We have demonstrated that response to a painful stimulus during
classical conditioning decreases after short-term initial increase. However, during instrumental learning, the neurons controlling
instrumental action remained highly sensitive to the unconditioned stimulus. Meanwhile, foreign neurons decreased their responses
to the unconditioned stimulus. We may tentatively conclude that classical and instrumental paradigms are fundamentally different at
the cellular level