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

1. In the isolated CNS of the pond snail Planorbis corneus, spontaneou s bursts of activity in the motor neurons (MNs) supplying the columell ar muscle were occasionally observed. The biphasic pattern of this act ivity, with a shorter (3-5 s) initial burst and longer (20-40 s) subse quent burst, was similar to that of the motor output during the genera l (''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 pneumo stome with a temporal pattern characteristic of the defense reaction w ere 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) trig gered by the sensory input, rather than generated by ongoing processin g of sensory input. The CPG consists of two components responsible for generation of two phases of the defense reaction. A characteristic fe ature of the CPG is that the magnitude of its response depends in a gr aded fashion on the strength of the initial stimulus. 2. In the pleura l 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 thro ugh all (except pedal and buccal) ganglia. The DRN1s received an excit atory input when a peripheral nerve was stimulated. They generated act ion potentials of long (0.2-2 s) duration. The DRN1 from the right gan glion was studied in more detail. When stimulated intracellularly, thi s neuron was found to evoke excitation of columellar motor neurons typ ical 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 th e 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 n eurons of the columellar muscle electrically connected with DRN1s.