Organization of higher-order brain areas that initiate locomotor activity in larval lamprey

In the lamprey, spinal locomotor activity can be initiated by pharmacological microstimulation in several brain areas: rostrolateral rhombencephalon (RLR); dorsolateral mesencephalon (DLM); ventromedial diencephalon (VMD); and reticular nuclei. During DLM- or VMD-initiated locomotor activity in in vitro brain/spinal cord preparations, application of a solution that focally depressed neuronal activity in reticular nuclei often attenuated or abolished the locomotor rhythm. Electrical microstimulation in the DLM or VMD elicited synaptic responses in reticulospinal (RS) neurons, and close temporal stimulation in both areas evoked responses that summated and could elicit action potentials when neither input alone was sufficient. During RLR-initiated locomotor activity, focal application of a solution that depressed neuronal activity in the DLM or VMD abolished or attenuated the rhythm. These new results suggest that neurons in the RLR project rostrally to locomotor areas in the DLM and VMD. These latter areas then appear to project caudally to RS neurons, which probably integrate the synaptic inputs from both areas and activate the spinal locomotor networks. These pathways are likely to be important components of the brain neural networks for the initiation of locomotion and have parallels to locomotor command systems in higher vertebrates.