Modulation of circuits underlying rhythmic behaviors

Summary

What have we learned about behavior from neuromodulatory studies of the crustacean stomatogastric system? The emphasis of this paper has been on the analysis of one single class of behaviors (rhythmic) in terms of microcircuitry (synaptic connections between identified neurons). But in the general case, all behaviors result from the generation of spatio-temporal patterns by the central nervous system. How individual nerve cells interact with each other to produce such patterns is of fundamental interest. We know from work on simple networks that it is possible to link the circuitry of the nervous system with behavior in a precise way, and that instead of a large number of dedicated circuits, behaviors can be altered by chemically adjusting the functional properties of the neuronal elements. One circuit can be configured to perform a variety of different behaviors by activating neurons which contain neuromodulatory substances or in response to neurohormones circulating in the hemolymph. At present we know only a few of the ways neuromodulatory neurons are triggered to release their contents onto the neurons making up CPGs.

The findings described here raise many questions. What are the parameters which control the distribution of neuromodulatory substances throughout the nervous system? What happens when more than one neuromodulator is present? At the cellular level, what mechanisms are involved in transforming each neuron from one functional state to another, and then how does the entire constellation of changes give rise to a new output? It is important to answer such questions in reduced networks, because there are presently no techniques available to answer them in the more complex networks of the brain. While there is no question that modulatory activity occurs in the brain, whether or not the principles which have been discovered by using “simple” invertebrate circuits scale up to vertebrate circuits remains an intriguing question.