| Rhythmic motor patterns, such as walking, are generated, in part, by rhythmically active neural networks called central pattern generators (CPG's; Marder and Calabrese, 1996). Typically, CPG's provide rhythmically patterned synaptic drive onto motor neurons in order to coordinate them, with appropriate phase differences, into a motor pattern appropriate for the behavior. These premotor patterns of drive contain both timing information and patterns of synaptic strengths. Invertebrate preparations, with their simple and accessible nervous systems, have been used to generate principles that underlie how premotor patterns of synaptic input interact with motor neurons to produce stereotyped motor outputs (Marder and Bucher, 2007). Here, I use the leech heartbeat CPG, a system in which patterns of synaptic drive onto motor neurons can be easily measured, to address how a CPG circuit coordinates its motor neurons to produce stereotyped motor patterns.;In the first of two studies, I show that, although the segmental input pattern is the primary determinant of motor neuron output, the intrinsic properties of the heart motor neurons play an important role in determining how they are coordinated by their segmental synaptic input pattern, particularly when receiving one of the two input patterns these motor neurons receive.;In the second study, I show, in both modeling and in follow-up experiments in the living system, that the generation of one motor pattern is a consequence of the nearly synchronous premotor timing information produced by the leech heartbeat CPG. For the other motor pattern, I show that premotor timing information determines the range over which motor neurons can fire while synaptic strength profiles define the actual motor progression.;These experiments provide a direct assessment of how motor neuron intrinsic properties interact with their premotor pattern of synaptic drive to produce rhythmic motor output. Furthermore, the data presented here may inform studies on motor pattern generation in other systems, including studies on recovery of locomotor control in patients with spinal cord injury. |
