MODERN VEHICULAR TRAVEL ON EARTH and through extraterrestrial space relies upon sophisticated guidance systems that integrate acceleration, velocity, and positional information through transducers, computational algorithms, and satellite triangulation. Yet the principles of inertial guidance are ancient: Vertebrates have used analogous systems for 500 million years and invertebrates for even longer. In these animals, the inertial guidance system, termed the vestibular system, serves to detect and interpret motion through space as well as orientation relative to gravity.
Through extensive research over many decades, it is apparent that most, if not all, organisms on Earth have evolved to sense one of the most prevalent “forces” in our universe, gravity. The mechanisms for the sensory transduction are as diverse as nature could devise. Gravity is most precisely referenced as gravito-inertial acceleration (GIA), a distinct form of linear acceleration directed toward the core of our planet. In truth, gravity varies systematically by as much as 0.5% between the equator and the poles; it increases over mineral-dense regions and decreases over mineral-light regions of the Earth’s surface. Yet every single behavior that animals perform is referenced to the GIA, and all of our actions and cognitive directives depend upon knowledge of our motion and orientation relative to it. The first developments of what we refer to as a vestibular system were actually gravity sensors; as behavior became increasingly mobile, sensory organs evolved to process rotational accelerations as well.
In this chapter we will concentrate on the vestibular system of vertebrates, which has remained highly conserved across many species. Vestibular signals originate in the labyrinths of the internal ear (Figure 27–1B). The bony labyrinth is a hollow structure within the petrous portion of the temporal bone. Within it lies the membranous labyrinth, which contains sensors for both the vestibular and auditory systems.
The vestibular apparatus of the inner ear.
A. The orientations of the vestibular and cochlear divisions of the inner ear are shown with respect to the head.
B. The inner ear is divided into bony and membranous labyrinths. The bony labyrinth is bounded by the petrosal portion of the temporal bone. Lying within this structure is the membranous labyrinth, which contains the receptor organs for hearing (the cochlea) and equilibrium (the utricle, saccule, and semicircular canals). The space between bone and membrane is filled with perilymph, whereas the membranous labyrinth is filled with endolymph. Sensory cells in the utricle, saccule, and ampullae of the semicircular canals respond to motion of the head. (Adapted from Iurato 1967.)
The vestibular receptors consist of two parts: two otolith organs, the utricle and saccule, which measure linear accelerations, and three semicircular canals, which measure angular accelerations. Rotational motion (angular acceleration) is experienced during head turns, whereas linear acceleration occurs during walking, falling, vehicular travel (ie, translations), or head tilts relative to gravity. ...