chapter 36 - Vestibular Mechanics - KEMT FEI TUKE

Each SCC has a bulge called the ampulla near the one end, and inside the ampulla is the cupula, which
is formed of saccharide gel. The capula forms a complete hermetic seal with the ampulla, and the cupula
sits on top of the crista, which contains the sensory receptor cells called hair cells. These hair cells have
small stereocilia (hairs) which extend into the cupula and sense its deformation. When the head is rotated
the endolymph fluid, which fills the canal, tends to remain at rest due to its inertia, the relative flow of
fluid in the canal deforms the cupula like a diaphragm, and the hair cells transduce the deformation into
nerve signals.
The otolithic organs are flat layered structures covered above with endolymph. The top layer consists
of calcium carbonate crystals with otoconia which are bound together by a saccharide gel. The middle
layer consists of pure saccharide gel, and the bottom layer consists of receptor hair cells which have
stereocilia that extend into the gel layer. When the head is accelerated, the dense otoconial crystals tend
to remain at rest due to their inertia as the sensory layer tends to move away from the otoconial layer.
This relative motion between the octoconial layer deforms the gel layer. The hair cell stereocilia sense
this deformation, and the receptor cells transduce this deformation into nerve signals. When the head is
tilted, weight acting on the otoconial layer also will deform the gel layer. The hair cell stereocilia also
have directional sensitivity which allows them to determine the direction of the acceleration acting in
the plane of the otolith and saccule. The planes of the two organs are arranged perpendicular to each
other so that linear acceleration in any direction can be sensed. The vestibular nerve, which forms half
of the VIII cranial nerve, innervates all the receptor cells of the vestibular apparatus.
36.2 Otolith Distributed Parameter Model
The otoliths are an overdamped second-order system whose structure is shown in Fig. 36.1. In this model
the otoconial layer is assumed to be a rigid and nondeformable, the gel layer is a deformable layer of
isotropic viscoelastic material, and the fluid endolymph is assumed to be a newtonian fluid. A small
element of the layered structure with surface area dA is cut from the surface, and a vertical view of this
surface element, of width dx, is shown in Fig. 36.2. To evaluate the forces that are present, free-body
diagrams are constructed of each elemental layer of the small differential strip. See the nomenclature
table for a description of all variables used in the following formulas, and for derivation details see Grant
and colleagues [1984, 1991].
FIGURE 36.1 Schematic of the otolith organ: (a) Top view showing the peripheral region with differential area dA
where the model is developed. (b) Cross-section showing the layered structure where dx is the width of the differential
area dA shown in the top view at the left.
© 2000 by CRC Press LLC