Intersegmental coordination of rhythmic motor patterns.

The study of the neural basis of motor patterns that underlie wavelike behaviors such as undulatory swimming has contributed to our general understanding of coordination in the nervous system. Numerous studies have shown that the isolated nervous system is capable of producing rhythmic motor output in the absence of sensory feedback (Marder and Calabrese 1996). In addition, in many cases these rhythmic motor patterns are similar to those required for specific behaviors in the intact animal. For example, in the lamprey forward swimming is accomplished by means of side-to-side undulations that travel from anterior to posterior along the length of the body (Wallén and Williams 1984). In the lamprey, as in many other animals, wavelike motor patterns arise from a network of neurons that is distributed longitudinally along the neural axis. This type of neural network consists of a chain of coupled segmental oscillators, local neural networks that are capable of independently generating rhythmic output (Skinner and Mulloney 1998a). The appropriate phase relationships between these segmental oscillators arise as an emergent property of the segmental oscillators and the coupling between them. Although this distributed organization is found in many different animals, there are large differences in terms of the properties of the segmental oscillators, the strength and symmetry of coupling, and the importance of sensory feedback. In this review, we will discuss some of the variations in design found in three animals: the lamprey, leech, and crayfish.