Organization of brain and spinal cord locomotor networks in larval lamprey

In vertebrates, rhythmic locomotor behaviors, such as walking, flying, and swimming, are initiated by "command" systems in the brain that activate locomotor networks in the spinal cord to initiate rhythmic motor activity and locomotor movements (Grillner, 1981). The spinal locomotor networks are distributed along the spinal cord and coupled by a spinal coordinating system to form a central pattern generator (CPG). Spinal CPGs are capable of producing the basic pattern of rhythmic activity in the absence of sensory feedback, although sensory feedback is critical for "fine tuning" the centrally generated locomotor pattern (Grillner, 1975). In in vitro brain/spinal cord preparations from larval lamprey, spinal locomotor activity that is similar to swimming activity in whole animals can be initiated by pharmacological microstimulation in several brain areas: rostrolateral rhombencephalon (RLR); dorsolateral mesencephalon (DLM); ventromedial diencephalon (VMD); and reticular nuclei. However, the actual movements that would result from this in vitro burst activity have not been investigated in detail. In the present study, pharmacological stimulation was applied to the above brain locomotor areas in semi-intact preparations from larval lamprey, and swimming movements and muscle burst activity in the freely moving caudal body were analyzed (Jackson and McClellan, 2001; Jackson et al., 2006). First, bilateral stimulation with pharmacological agents in the VMD, DLM, or RLR initiated symmetrical swimming and well-coordinated muscle burst activity that was not significantly different than those during swimming in whole animals. Unilateral stimulation in these areas usually did not produce symmetrical swimming. Second, with synaptic transmission blocked in the brain, bilateral stimulation in reticular nuclei also