We investigated how vestibular information is used to maintain posture and control movement by studying vestibularly deficient mice (IsK-/- mutant). In these mutants, microscopy showed degeneration of the cristae of the semicircular canals and of the maculae of the utriculi and sacculi, while behavioural and vestibulo-ocular reflex testing showed that vestibular function was completely absent. However, the histology of Scarpa's ganglia and the vestibular nerves was normal in mutant mice, indicating the presence of intact central pathways. Using X-ray and high-speed cineradiography, we compared resting postures and locomotion patterns between these vestibularly deficient mice and vestibularly normal mice (wild-type and IsK+/-). The absence of vestibular function did not affect resting posture but had profound effects on locomotion. At rest, the S-shaped, sagittal posture of the vertebral column was the same for wild-type and mutant mice. Both held the head with the atlanto-occipital joint fully flexed, the cervico-thoracic junction fully flexed, and the cervical column upright. Wild-type mice extended the head and vertebral column and could walk in a straight line. In marked contrast, locomotion in vestibularly deficient mice was characterized by circling episodes, during which the vertebral column maintained an S-shaped posture. Thus, vestibular information is not required to control resting posture but is mandatory for normal locomotion. We propose that vestibular inputs are required to signal the completion of a planned trajectory because mutant mice continued rotating after changing heading direction. Our findings support the hypothesis that vertebrates limit the number of degrees of freedom to be controlled by adopting just a few of the possible skeletal configurations.
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