A principle that has emerged from studies of central pattern generators is that rhythmic patterns of firing elicit complex motor responses without need of ongoing sensory stimulation. A good example is the behavior mediated by a small group of nerve cells, called the stomatogastric ganglion (STG), that controls the muscles of the gut in lobsters and other crustaceans (Figure 1A). This ensemble of 30 motor neurons and interneurons in the lobster is perhaps the most completely characterized neural circuit known. Of the 30 cells, defined subsets are essential for two distinct rhythmic movements: gastric mill movements that mediate grinding of food by “teeth” in the lobster’s foregut, and pyloric movements that propel food into the hindgut. Phasic firing patterns of the motor neurons and interneurons of the STG are directly correlated with these two rhythmic movements. Each of the relevant cells has now been identified based on its position in the ganglion, and its electrophysiological and neuropharmacological properties characterized (Figures 1B and C).

Figure 1 (A) Location of the lobster stomatogastric ganglion in relation to the gut. (B) Subset of identified neurons in the stomatogastric ganglion that generates gastric mill and pyloric activity. The abbreviations indicate individual identified neurons, all of which project to different pyloric muscles (except the AB neuron, which is an interneuron). (C) Recording from one of the neurons—the lateral pyloric (LP) neuron in this circuit—showing the different patterns of activity elicited by several neuromodulators known to be involved in the typical synaptic interactions in this ganglion.

Patterned activity in the motor neurons and interneurons of the ganglion begins only if the appropriate neuromodulatory input is provided by sensory axons that originate in other ganglia. Depending on the activity of the sensory axons, neuronal ensembles in the STG produce one of several characteristic rhythmic firing patterns. Once activated, however, the intrinsic membrane properties of identified cells within the ensemble sustain the rhythmicity of the circuit in the absence of further sensory input.

Another key fact that has emerged from this work is that the same neurons can participate in different programmed motor activities, as circumstances demand. For example, the subset of neurons producing gastric mill activity overlaps the subset that generates pyloric activity. This economic use of neuronal subsets has not yet been described in the central pattern generators of mammals, but seems likely to be a feature of all such circuits.

References

Hartline, D. K. and D. M. Maynard (1975) Motor patterns in the stomatogastric ganglion of the lobster, Panulirus argus. J. Exp. Biol. 62: 405–420.

Marder, E. and R. M. Calabrese (1996) Principles of rhythmic motor pattern generation. Physiol. Rev. 76: 687–717.

Selverston, A. I. (2005) A neural infrastructure for rhythmic motor patterns. Cell. Mol. Neurobiol. 25: 223–244.