Neurobiology of the crustacean swimmeret system

The crustacean swimmeret system includes a distributed set of local circuits that individually control movements of one jointed limb. These modular local circuits occur in pairs in each segmental ganglion, and normally operate synchronously to produce smoothly coordinated cycles of limb movements on different body segments. The system presents exceptional opportunities for computational and experimental investigation of neural mechanisms of coordination because: (a) The system will express in vitro the periodic motor pattern that normally drives cycles of swimmeret movements during forward swimming. (b) The intersegmental neurons which encode information that is necessary and sufficient for normal coordination have been identified, and their activity can be recorded. (c) The local commissural neurons that integrate this coordinating information and tune the phase of each swimmeret are known. (d) The complete set of synaptic connections between coordinating neurons and these commissural neurons have been described. (e). The synaptic connections onto each local pattern-generating circuit through which coordinating information tunes the circuit's phase have been discovered. These factors make possible for the first time a detailed, comprehensive cellular and synaptic explanation of how this neural circuit produces an effective, behaviorally significant output. This paper is the first comprehensive review of the system's neuroanatomy and neurophysiology, its local and intersegmental circuitry, its transmitter pharmacology, its neuromodulatory control mechanisms, and its interactions with other motor systems. Each of these topics is covered in detail in an attempt to provide a complete review of the literature as a foundation for new research. The series of hypotheses that have been proposed to account for the system's properties are reviewed critically in the context of experimental tests of their validity.