[The neuronal mechanisms of the defensive reaction to stimulation of the cutaneous nerve in the freshwater snail].

The whole body withdrawal reaction of freshwater snail Planorbarius corneus consists of two phases. In the first phase the shell is rapidly moved down to cover the head, in the second one the body is slowly retracted into the shell. The columellar muscle is involved in this behaviour. Motoneurons of the columellar muscle are identified in the cerebral, parietal and pedal ganglia. In the preparation of the central nervous system connected with the columellar muscle it is demonstrated that stimulation of the lip nerve evoked a biphasic motoneuron excitation responsible for two phases of the muscle contraction. A similar biphasic excitation of the motoneurons could arise spontaneously. This implies that the whole body withdrawal reaction is, at least partly, a fixed act generated by a central mechanism (a central program) which is triggered by a sensory stimulus. The central mechanism of the withdrawal reaction could be also activated by a depolarization of some columellar motoneurons. This suggests that the central mechanism received a feedback from the motoneurons.