The rhythmic, alternating movements of locomotion have a sizeable local spinal control element, and can be generated even in the absence of stimulus from the brain, but how these patterns are organized and controlled on a cellular level has been unclear. In the March 21 Science, Klas Kullander and colleagues at Gothenburg University, Sweden, show that neurons bearing the EphA4 receptor play a pivotal role, and that the absence of either EphA4 or its ligand disrupts normal locomotor development and activity (Science, 299:1889-1892, March 21, 2003).

Kullander et al. examined neurophysiologic activity in isolated spinal cords from mice null for EphA4 or its ligand, ephrinB3. Opposing ventral nerve roots displayed synchronous activity in both mouse types, versus alternating activity in wild-type mice, giving rise to a rabbit-like gait pattern. Heterozygotes displayed a mixed or drifting pattern. In mice homozygous null for either gene, neurons whose...

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