Defense reaction in the pond snail Planorbis corneus. II. Central pattern generator

Abstract

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroanatomical, immunocytochemical, and physiological studies of the pharyngeal retractor muscle and its putative regulatory neurons playing a role in withdrawal and feeding in the snail, Helix pomatia

“…This type of suppression requires multiple interactions between the withdrawal and the feeding CPG networks. Since the PRM neurons and the CM neurons are located in the same loci in Helix, we suggest that they have common elements and that the PRM is regulated by a network of neurons that behaves as a CPG and that is able to produce a graded response similarly to that observed in Planorbis (Arshavsky et al 1994b).…”

“…In Planorbis, the whole body withdrawal response executed by the CM complex has been proposed to be mediated by a central pattern generator (CPG) network, suggesting that some of the CM neurons are interneurons belonging to elements of the CPG network, whereas others are motorneurons belonging to the output elements of the withdrawal network (Arshavsky et al 1994b). Considering the above observations, we suggest that the PRM is innervated by neurons from different loci along the CNS, and that the different segments of the PRM are also innervated by different loci.…”

“…In Planorbis, the whole body withdrawal response, when the PRM produces sustained contraction, may be mediated by a CPG network triggered by sensory inputs rather than generated by an ongoing processing of the sensory inputs. Additionally, the CPG is able to respond in a graded fashion depending on the strength of the stimulus (Arshavsky et al 1994b). During feeding, when the PRM produces rhythmic phasic contractions, cyclic feeding movements in gastropods are patterned mainly by the buccal CPG (for reviews, see Elliott and Susswein 2002;Benjamin and Elliott 1989).…”

“…Our present findings that FMRFa markedly increases the amplitude of the electrically evoked twitch contraction of the PRM and that FMRFa injected into the hemocoel evokes the long-lasting shortening of the PRM support this suggestion. In Planorbis, a pair of identified pleural CM neurons, which are electrically connected to cerebral CM neurons and receive excitatory peripheral inputs have proved to be intrinsic elements of the withdrawal CPG network and are able to activate the pool of withdrawal motoneurons located in various ganglia (Arshavsky et al 1994b). These observations suggest that the FMRFa-containing pleural interneurons that suppress the activity of the buccal feeding CPG (for a review, see Alania et al 2004) belong to the elements of the withdrawal CPG network.…”

Central Pattern Generators: Mechanisms of Operation and Their Role in Controlling Automatic Movements

“…Although CPG control not only rhythmic, but also single movements (for example, standard defense reactions [Arshavsky et al, 1994] or swallowing movements which, depending on the nature of the food, can be either single or rhythmic), the mechanism of activity has been studied mainly for CPG controlling rhythmic movements. An understanding of the neuronal mechanisms of CPG operation presupposes answering to two basic questions:…”