Dynamic Networks of Neurons

Animals choose constantly among a wide range of behavioral capabili? ties: walking, running, fighting, court? ing and so on. An animal's nervous system generates each of these activi? ties by turning on a specific network of neurons, which produces a characteris? tic sequence of electrical signals, called motor output, that instructs muscles to perform particular movements. Never? theless, a single network of neurons can produce different motor outputs to tailor movement to the prevailing cir? cumstances. In other words, a single neuronal circuit might direct an animal to step gingerly while hunting or to run wildly when being pursued. How does one network of neurons produce more than one motor output? In large part, our understanding of the neural basis of behavior has arisen from the study of identifiable motor acts, including feeding, locomotion and respiration. Each of these actions recurs rhythmically, making it relatively pre? dictable and thus easier to study. More? over, many neurobiologists have exam? ined these activities in invertebrates, because they often exhibit less compli? cated forms of behavior that are con? trolled by a simpler nervous system. The neural basis of flight in locusts and swimming in leeches, for example, has taught investigators much about the neural programs behind many other more complex types of behavior. Rhyth? mic behavior often depends on so called central pattern generators?net? works of neurons, entirely inside an animal's central nervous system, that generate a rhythmically patterned mo? tor output, without requiring sensory feedback from the periphery. In this article, we shall focus on a well-described model for central pattern generation, the stomatogastric nervous system that controls coordinated Inges? tion and food-processing movements in the foregut (esophagus and stomach) of decapod crustaceans, including the Eu? ropean lobster. Results obtained from the stomatogastric and other rhythmic circuits have revealed how a given net? work of neurons can express different functional capabilities?for instance, how a horse's nervous system can di? rect a walk, trot and gallop. In addition, we shall show that individual neurons