A quantitative analysis of the spatial organization of the vestibulo-ocular reflexes in lateral- and frontal-eyed animals—I. Orientation of semicircular canals and extraocular muscles

Abstract

The spatial relationship between extraocular muscles and semicircular canals was evaluated in cat (frontal-eyed animal) and rabbit (lateral-eyed animal). Semicircular canal orientations in the rabbit were determined by a principal components analysis of data points obtained from the exposed osseous canals using a three-axis micromanipulator. Canal orientations were presented in terms of unit sensitivity vectors. Orientation of cxtraocular muscles in rabbits and cats was derived from measurements of the insertion and origin of each muscle with respect to a reference point on the skull and a calculated estimate of the center of the eye. Muscle orientations were presented in terms of unit action vectors. Semicircular canal planes of the rabbit labyrinth were not orthogonal, having deviations up to 14. Pairs of antagonistically acting vertical semicircular canals, left (right) anterior right (left) posterior deviated from coplanarity by 16, while the deviation for the horizontal canals was 9. In both animals, muscles of an antagonistic pair were coplanar to within 8, with the exception of the oblique muscles in the rabbit for which the deviation was 19. The three pairs of antagonistic muscles were almost orthogonal to each other, the maximum deviation between any of the pairs being 8 in the cat and 18 in the rabbit. Comparing extraocular muscle planes and semicircular canal planes reveals that they are roughly aligned. However, there were slight but consistent differences between a given semicircular canal plane and the planes of the muscles to which this canal is connected by the classical three-neuron-arc (principal vestibulo-ocular reflex circuits). This incongruence should be reflected in the connectivity of vestibuloocular reflex pathways, other than the principal connections.