DEFENSE REACTION IN THE POND SNAIL PLANORBIS-CORNEUS .2. CENTRAL PATTERN GENERATOR

1. In the isolated CNS of the pond snail Planorbis corneus, spontaneous bursts of activity in the motor neurons (MNs) supplying the columellar muscle were occasionally observed. The biphasic pattern of this activity, with a shorter (3-5 s) initial burst and longer (20-40 s) subsequent burst, was similar to that of the motor output during the general (''whole-body'') defense reaction. In preparations consisting of the CNS isolated with the columellar muscle or with the lung, spontaneous biphasic contractions of the muscle as well as openings of the pneumostome with a temporal pattern characteristic of the defense reaction were observed. These findings demonstrated that the efferent pattern of the defense reaction in the snail is, to a large extent, produced by a special neuronal mechanism (the central pattern generator, CPG) triggered by the sensory input, rather than generated by ongoing processing of sensory input. The CPG consists of two components responsible for generation of two phases of the defense reaction. A characteristic feature of the CPG is that the magnitude of its response depends in a graded fashion on the strength of the initial stimulus. 2. In the pleural ganglia there are at least two electrically connected interneurons (DRN1s) that play an important role in generation of the first phase of the defense reaction. Processes of the DRN1s form a ring passing through all (except pedal and buccal) ganglia. The DRN1s received an excitatory input when a peripheral nerve was stimulated. They generated action potentials of long (0.2-2 s) duration. The DRN1 from the right ganglion was studied in more detail. When stimulated intracellularly, this neuron was found to evoke excitation of columellar motor neurons typical of that observed during the first phase of the defense reaction. With stronger stimulation of DRN1, the second phase could be triggered, including the long-lasting activation of the shell-moving system and opening of the pneumostome. Inactivation of the DRN1 (by injection of a hyperpolarizing current) resulted in a considerable reduction of the response in columellar motor neurons to stimulation of the cutaneous nerve. The manifestations of the defense reaction in the isolated CNS could also be evoked by intracellular stimulation of cerebral motor neurons of the columellar muscle electrically connected with DRN1s.