3.26 – Vestibular System

The semicircular canals of the labyrinth of vertebrates provide one way of motion detection in three-dimensional The fully developed form of the vertebrate labyrinth consists of six semicircular canals, three on each side of the head, whose spatial arrangement (vertical canals are placed diagonally in the head, horizontal canals are oriented earth horizontally) follows three interconnected principles: (1) bilateral symmetry, (2) mutual orthogonality, and (3) push-pull operational mode. Motor systems related to the vestibular reflexes such as the extraocular muscles, or the neck muscles, share the same geometrical framework. This framework is also reflected in the anatomical networks mediating compensatory eye and head movements, linking each of the semicircular canals to a particular set of extraocular muscles (so-called principal vestibulo-ocular reflex connections to yoke muscles) and to particular head and neck muscles. These connections are identical across species throughout evolution. The particular spatial arrangement of the vertical semicircular canals is already present in fossil ostracoderms, who, however, lacked horizontal canals. The fully developed vertebrate labyrinth with its six semicircular canals displays distinct differences that are obvious when comparing different taxa (e.g., elasmobranchs versus other vertebrates). Whereas the common crus of the semicircular canals in teleosts through mammals is formed between the anterior and the posterior semicircular canal, it occurs between the anterior and the horizontal canal in elasmobranchs. However, despite this morphological difference, these two vertebrate labyrinth prototypes constitute a functionally identical solution. A similar analysis holds for certain invertebrate species (squid, octopus, crab), which display an even wider variety in the physical expression of movement detection systems when compared to vertebrates. Although the physical expressions of motion detection systems differ in the animal kingdom, the functional solutions (providing the best signal-to-noise ratio) with adherence to bilateral symmetry, mutual orthogonality, and push-pull operational mode are identical. Furthermore, this functional principle is reflected in the intrinsic organization of related motor systems. The extraocular muscle system displays at least three manifestations, notably a lamprey pattern, an elasmobranch pattern, and a bony fish/tetrapod pattern (bony fish through mammals). Lampreys lack a medial rectus muscle. In elasmobranchs, the medial rectus is innervated by contralateral motoneurons, in other vertebrates by ipsilateral motoneurons. This innervation pattern seems to reflect the surmised evolutionary history of the medial rectus muscle as derived from respective agnathan ancestors. The abducens is thought not to be of an original branchiomotor category, but of a group of spinal-motor nerves that migrated into a previously foreign domain. In the vestibulocollic system subserving head movements, we find the same vestibular reference frame underlying reflex and orienting movements as in the vestibulo-ocular system. Originally, the function of the vestibulo-oculomotor system was to stabilize the visual world by keeping the eyes still, not to move them.