The hemisegmental locomotor network revisited.

The organization of the minimal neuronal substrate capable of generating locomotor rhythmicity in vertebrates is investigated in several species, with an emphasis on identifying evolutionary-conserved features. In lamprey, an eel-like lower vertebrate that swims by undulatory movements of the body, the network has been identified as a recurrent network of excitatory interneurons localized in each spinal hemisegment. This conclusion rested upon the observation that each side of the spinal cord is able to express rhythmic locomotor-related bursting after being surgically separated along the midline, even in the absence of inhibition. An important caveat, however, is that this rhythmicity must be an intrinsic capability of the hemisegmental networks and not a newly acquired property as a result of a plastic remodeling of the network occurring after the lesion. Here we examine this issue by recording the motor output expressed by the electrically activated hemicord in the first minutes after hemisection. We observed clear rhythmic bursting in the frequency range previously linked to the operation of the central pattern generator for swimming. Moreover, we recorded the output of the unilateral networks in the intact spinal cord (i.e. no midline section performed) by activating them with asymmetrical stimulation. We thus conclude that the lamprey hemicord does possess the intrinsic capability of generating the basic rhythmic drive of locomotion. The wider significance of these data stems from the lamprey being a model of axial locomotion, and from the many lesion studies previously performed in other animals.