Gastric Central Pattern Generator Research Articles

Multiple effects of an identified proprioceptor upon gastric pattern generation in spiny lobsters.

1. Using deafferented preparations of the stomatogastric nervous system of spiny lobsters (Panulirus interruptus), we stimulated the central soma of the Anterior Gastric Receptor neuron (AGR) and analyzed sensorimotor integration in the gastric central pattern generator during rhythm production. 2. Driving AGR to spike tonically at lower frequencies (10-20/s) accelerated the gastric rhythm, while higher frequencies (> or = 30/s) suppressed it. 3. Shorter spike trains in AGR evoked phase-dependent resetting of the gastric rhythm. Repetitive trains could entrain rhythms to both longer and shorter cycle periods. Some pattern-generating effects are consistent with effects upon the lateral gastric neuron, an influential member of the gastric mill network. 4. AGR affected the burst intensity of many of the gastric neurons in specific, complex ways. Some power-stroke motor neurons were excited because AGR activated excitatory, premotor interneurons (E cells). However, AGR also activated parallel, seemingly inhibitory inputs, whose mechanism remains unclear. Still other effects on motor neurons may be mediated partly by synaptic interactions within the network.

Modeling the gastric mill central pattern generator of the lobster with a relaxation-oscillator network

1. The gastric mill central pattern generator (CPG) controls the chewing movements of teeth in the gastric mill of the lobster. This CPG has been extensively studied, but the precise mechanism underlying pattern generation is not well understood. The goal of this research was to develop a simplified model that captures the principle, biologically significant features of this CPG. We introduce a simplified neuron model that embodies approximations of well-known membrane currents, and is able to reproduce several global characteristics of gastric mill neurons. A network built with these neurons, using graded synaptic transmission and having the synaptic connections of the biological circuit, is sufficient to explain much of the network's behavior. 2. The cell model is a generalization and extension of the Van der Pol relaxation oscillator equations. It is described by two differential equations, one for current conservation and one for slow current activation. The model has a fast current that may, by adjusting one parameter, have a region of negative resistance in its current-voltage (I-V) curve. It also has a slow current with a single gain parameter that can be regarded as the combination of slow inward and outward currents. 3. For suitable values of the fast current parameter and the slow current parameter, the isolated model neuron exhibits several different behaviors: plateau potentials, postinhibitory rebound, postburst hyperpolarization, and endogenous oscillations. When the slow current is separated into inward and outward fractions with separately adjustable gain parameters, the model neuron can fire tonically, be quiescent, or generate spontaneous voltage oscillations with varying amounts of depolarization or hyperpolarization. 4. The most common form of synaptic interaction in the gastric CPG is reciprocal inhibition. A pair of identical model cells, connected with reciprocal inhibition, oscillates in antiphase if either the isolated cells are endogenous oscillators, or they are quiescent without plateau potentials, or they have plateau potentials but the synaptic strengths are below a critical level. If the isolated cells have widely differing frequencies (or would have if the cells were made to oscillate by adjusting the fast currents), reciprocal inhibition entrains the cells to oscillate with the same frequency but with phases that are advanced or retarded relative to the phases seen when the cells have the same frequency. The frequency of the entrained pair of cells lies between the frequencies of the original cells. The relative phases can also be modified by using very unequal synaptic strengths.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of gastric rhythm generation in the isolated stomatogastric ganglion of spiny lobsters: bursting pacemaker potentials, synaptic interactions, and muscarinic modulation.

1. The gastric central pattern generator (CPG), located in the stomatogastric ganglion (STG) of the spiny lobster (Panulirus interruptus), is nonrhythmic when deprived of neuromodulatory inputs from anterior ganglia. Leaving these inputs intact in vitro can sustain a gastric rhythm but also introduces numerous, uncontrolled and largely unknown modulatory and synaptic influences that greatly complicate analysis of this CPG. 2. Here we induced gastric rhythms in the isolated STG, by superfusing a specific modulator, the muscarinic agonist, pilocarpine. Muscarinic agents sustain vigorous gastric rhythms in the isolated STG. Our aim was to analyze the pattern-generating functions of the restricted gastric circuit, free of complicating influences from other ganglia, and under specific (muscarinic) modulation. 3. We used combinations of multiple cell hyperpolarizations, photodeletions, and synaptic blockade by picrotoxin to assess the pattern-generating role of individual gastric neurons and to study the activity of subcircuits. 4. Four identified gastric neurons [lateral gastric (LG), dorsal gastric (DG), 2 electrically coupled lateral posterior gastric (2LPGs)] acted as pattern-generating cells. They showed bursting pacemaker potentials (BPPs), i.e., plateau (or driver) potentials that underlay bursts of axonal spikes and slow, interburst depolarizing potentials that underlay repetitive burst activity. LG and DG, at least, became conditional bursters, able to burst repetitively because of intrinsic oscillations. The other gastric neurons behaved mainly as follower cells and derived their rhythmic bursting from synaptic coupling to the pattern-generator cells and from their own intrinsic (but nonoscillatory) properties. 5. The pattern-generating neurons form a novel "kernel" circuit that works by the cooperative interaction of cellular properties and synaptic connectivity. 6. This study constitutes the first complete and fully consistent analysis of pattern generation in the gastric network of the isolated STG. These mechanisms pertain to muscarinic rhythms in particular but also, we suggest, to gastric rhythm generation and CPG function in general. We suggest that 1) rhythmicity normally depends on the induction of bursty membrane properties in at least some component neurons; 2) different subcircuits can produce rhythmic patterns and may be activated by different modulators; and 3) the gastric network shares several important "building blocks" with CPGs that have been analyzed in other systems. 7. Muscarinic inputs are implicated as an important gastric regulator. We compare these responses with the reported modulatory actions of the anterior pyloric modulator (AMP), an identified, putatively cholinergic input interneuron that may act via muscarinic mechanisms.

Gastric mill activity in the lobster. I. Spontaneous modes of chewing.

1. The gastric central pattern generator (CPG) driving the three teeth of the gastric mill inside the lobster stomach has often been used as a model for the study of central nervous systems, but the actual functioning of the mill has never been observed directly. By using a small endoscope inserted through the esophagus a video analysis of the tooth movements was performed with restrained, but otherwise intact lobsters. 2. The teeth show spontaneous periodic chewing (cycle duration from 4 to 70 s) in two different basic modes. In the squeeze mode only the cusps of the three teeth move together simultaneously. In the cut-and-grind mode the lateral teeth close first with not only their cusps, but also their serrated edges. After this cut phase the lateral teeth grind backward along the file of the medial tooth, which simultaneously moves forward. 3. Simultaneous endoscope recordings of the teeth, filming of stomach muscles and ossicles combined with electrical stimulation of selected muscles reveal that muscle gm3c is responsible for this hitherto unknown backward grinding of the lateral teeth. 4. The complete behavioral repertoire includes the following modifications of the two basic modes. 1) The lateral teeth can perform chewing movements while the medial tooth stays still and vice versa, forms of chewing regarded even weaker than the squeeze. 2) There do not appear to be intermediates between the squeeze and cut-and-grind movements, with the latter as the strongest form of chewing. Transitions only occur as switching on a cycle-by-cycle basis. 3) A gradual change of the cut-and-grind chewing was observed as the gradual development of an additional opening over the time course of several periods. 4) After their grind phase, the lateral teeth can even move further back beyond the medial tooth. This can serve to push food into the pyloric filter apparatus. 5. Inflation of the cardiac sac can elicit single bites in a resting gastric mill. 6. The behavioral repertoire is compared with the in vivo activity of the gastric oscillator represented by simultaneous intracellular recording from 7 representative cells of the 11 CPG neurons.