A Geometric Analysis of Semicircular Canals and Induced Activity in Their Peripheral Afferents in the Rhesus Monkey

Abstract

Recent theoretical multidimensional analyses of the vestibulo-ocular have described the transformations thought to be necessary to effect a specific eye motion in response to a given head motion in order to maintain gaze. Accordingly, components of the angular head velocity vector in the coordinate system representing the geometry of the semicircular canals are transformed into components of an angular eye velocity vector in the coordinate system representing the geometry of the extraocular muscles. The transformations themselves are thought to manifest connectivity and signal processing of the central nervous system. These theoretical analyses were based in part upon anatomical measurements of the geometry of the semicircular canals and extraocular muscles made in the human>5 rabbit:' and cat.'.'Blanks and co-workers assessed the orientation of semicircular canals in cats by both anatomical' and physiological9 means. An objective of the physiological study was to ascertain the "sensitivity vector" for individual semicircular canal afferents. The direction of the sensitivity vector for a particular afferent is the direction of the axis about which a given head angular rotation will maximally modulate that afferent's activity. In the physiological study9 data obtained from right semicircular canal afferents of many cats were averaged and compared with the anatomical data of the earlier study,' in which the bony portions of the semicircular canals on both sides of the head were measured in different animals and averaged. The two methods of

Planar Relationships of the Semicircular Canals in Two Strains of Mice

“…Differences between the anatomic planes of the canals and their planes of maximum sensitivity may arise from variance in the shape and size of the bony canal and the membranous duct, the departure of the canals from a strict toroidal shape, and the fluid mechanics of the endolymph system (Ghanem et al 1998;Rabbitt 1999). Divergences in the two planes (anatomic and physiologic) have been reported for several species: 4-for the horizontal canal and 5-for the posterior canal in rhesus monkey (Reisine et al 1988); 10-for the horizontal canal and 8-for the posterior canal in pigeons (Dickman 1996); and 6.5-for the horizontal canal and 10.2-for the posterior canal in cats (Blanks et al 1975b). In situations where canals are relatively nonplanar, angles between anatomic and physiologic canal planes may be higher.…”

Orientation of Human Semicircular Canals Measured by Three-Dimensional Multiplanar CT Reconstruction

“…In toadfish (Opsanus tau), the difference between anatomic and maximal sensitivity directions is õ8.8-, õ16.8-, and õ3.8-for the horizontal, anterior, and posterior SCC, respectively (Rabbitt 1999). Similarly, in rhesus monkeys (Macaca mulatta), the difference between anatomic and maximal sensitivity directions is õ3.1-, õ9.9-, and õ5.3-for the horizontal, anterior, and posterior SCC, respectively (Reisine et al 1988). In humans, the difference between anatomic and maximal response planes is smaller, estimated to be 0.7-, 1.1-, and 5.7-for the horizontal, anterior, and posterior canals, respectively ).…”

Geometry of the semicircular canals of the chinchilla (Chinchilla laniger)

“…Variability in the orientation of canal planes among animals may make reliable positioning of an animal for physiologic experimentation difficult (Reisine et al, 1988). Each labyrinth may develop in a slightly different orientation in the skull and the canals forming that labyrinth may develop slightly differently relative to each other.…”

“…Physiologic study of semicircular canal function requires accurate positioning of the head to bring a particular canal into the plane of rotation. The orientation of the plane in which each canal lies has been determined geometrically from the shape of the labyrinth in many species, including toadfish (Ghanem et al, 1998), turtle (Brichta et al, 1988), pigeon (Dickman, 1996), guinea pig (Curthoys et al, 1975), rabbit (Ezure et al, 1984;Mazza et al, 1984), cat (Blanks et al, 1972;Ezure et al, 1984), monkey (Blanks et al, 1985;Reisine et al, 1988), and human (Blanks et al, 1975a;Spoor et al, 1998;Takagi et al, 1988;Takagi et al, 1989).…”

“…The use of this coordinate frame by the oculomotor system is suggested by the finding that the pulling directions of the extraocular muscles of rhesus monkeys align more closely with their canals' prime directions than with their maximal afferent response directions (Haque et al, 2003). However, because the two reference frames align closely in this species due to the relative orthogonality of its semicircular canals (Blanks et al, 1985;Reisine et al, 1988), this hypothesis is difficult to test. If chinchilla semicircular canals are less orthogonal than rhesus monkeys, they may offer a better opportunity to test the hypothesis that the central vestibular and oculomotor systems use prime directions to encode vestibular signals.…”