Spatial arrangement of the vestibular and the oculomotor system in the rat

The treatment of the spatial aspects of vestibular sensation, ocular movement near primary position, and their neural processing requires numerical information about the directions of maximal sensitivity of the semicircular canals (SCC), the direction of gaze at primary position, and the directions of eye rotation generated by each individual extraocular muscle (EOM). A good approximation of this information can be gained from stereotaxic measurements of the geometrical arrangement of the canals' bony structure, from measurements of the pupil's orientation, and from measurements of the directions of muscle pull as well as of the center of eye rotation. The results of measurements in pigmented rats are given as unit sensitivity vectors and unit action vectors in head-fixed coordinate systems and compared with data from rabbit, cat, monkey, and human. The misalignment of 'coplanar' SCC with 2.5-15.6 degrees is second only to humans, while the misalignment of the vectors of 'antagonistic' EOM with 27.2-39 degrees is even more oblique than in humans and thereby even more so than in the other mammals. Misalignment of SCC and 'corresponding' EOM with 15.5-34.2 degrees again is largest, followed by that in humans and then the other mammals. The rat may therefore be useful in studying those mechanisms by which the central nervous system deals with the obliqueness of systems that play such an important role in humans, too.