The theory of classical conditioning is one of several learning theories which explains how behaviours can result from stimulation without, however, taking into consideration the processes in the brain (black box).
Stimuli differ considerably. They can be, for example, chemical, mechanical, acoustic or temperature stimuli.
The stimulus makes contact with a sensory organ, this leads to excitation formation. The excitation is transferred to the brain via sensory nerves and processed there. The command from the brain is passed on via motor nerves. The command is executed and a response occurs.