Our sense of balance

Is as good as it gets. All vertebrates, from mice to whales, have a set of three mutually perpendicular fluid-filled tubes—the vestibular semicircular canals (SCCs)—housed in each inner ear. The SCCs mechanically sense rotation, a function that is imperative not only for maintaining balance but also for vision: The SCCs’ neural output causes a reflexive, compensating motion of the eyes, which allows a fixed gaze to be maintained even while the head is moving. Knowing how the canals work is also important for understanding various forms of dizziness. Roughly, an SCC is donut-shaped, with a major radius of a few millimeters and minor radius of a few tenths of a millimeter. Each torus is interrupted by a membrane called a cupula that is impregnated with tiny hairs for sensing the sloshing of the fluid through the canal. Those hairs trigger the neural signal. Incredibly, although vertebrates span nearly three orders of magnitude in length, the size of SCCs varies by only about a factor of five across vertebrate species. Todd Squires, a physicist at Caltech, has now investigated the question of why the SCCs should have a roughly universal size, and suggested that evolutionary pressures had converged on an optimal solution. Squires derived expressions for the SCCs’ structure, mechanical operation, and signal transduction. He then optimized the system’s sensitivity by varying four size parameters—major radius, minor radius, cupula thickness, and cupula height—while enforcing physical and physiological constraints. The greatest canal sensitivity occurred for those parameter values manifested in actual vertebrates. (T. M. Squires, Phys. Rev. Lett. 93, 198106, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.198106 .)