chapter 36 - Vestibular Mechanics - KEMT FEI TUKE

More documents

Recommendations

Info

Wallace Grant Virginia Polytechnic Institute and State University © 2000 by CRC Press LLC 36 Vestibular Mechanics 36.1 Structure and Function 36.2 Otolith Distributed Parameter Model 36.3 Nondimensionalization of the Motion Equations 36.4 Otolith Transfer Function 36.5 Otolith Frequency Response 36.6 Semicircular Canal Distributed Parameter Model 36.7 Semicircular Canal Frequency Response The vestibular system is responsible for sensing motion and gravity and using this information for control of postural and body motion. This sense is also used to control eyes position during head movement, allowing for a clear visual image. Vestibular function is rather inconspicuous and for this reason is frequently not recognized for its vital roll in maintaining balance and equilibrium and in controlling eye movements. Vestibular function is truly a sixth sense, different from the five originally defined by Greek physicians. The vestibular system is named for its position within the vestibule of the temporal bone of the skull. It is located in the inner ear along with the auditory sense. The vestibular system has both central and peripheral components. This chapter deals with the mechanical sensory function of the peripheral end organ and its ability to measure linear and angular inertial motion of the skull over the frequency ranges encountered in normal activities. 36.1 Structure and Function The vestibular system in each ear consists of the utricle and saccule (collectively called the otolithic organs) which are the linear motion sensors, and the three semicircular canals (SCCs) which sense rotational motion. The SCC are oriented in three nearly mutually perpendicular planes so that angular motion about any axis may be sensed. The otoliths and SCC consist of membranous structures which are situated in hollowed out sections of the temporal bone. This hollowed out section of the temporal bone is called the bony labyrinth, and the membranous labyrinth lies within this bony structure. The membranous labyrinth is filled with a fluid called endolymph, which is high in potassium, and the volume between the membranous and bony labyrinths is filled with a fluid called perilymph, which is similar to blood plasma. The otoliths sit within the utricle and saccule. Each of these organs is rigidly attached to the temporal bone of the skull with connective tissue. The three semicircular canals terminate on the utricle forming a complete circular fluid path, and the membranous canals are also rigidly attached to the bony skull. This rigid attachment is vital to the roll of measuring inertial motion of the skull.
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
Page 1: Grant, W. “Vestibular Mechanics.
Page 5 and 6: The gel layer is treated as a Kelvi
Page 7 and 8: 36.4 Otolith Transfer Function A tr
Page 9 and 10: 36.5 Otolith Frequency Response ©
Page 11 and 12: FIGURE 36.5 Schematic structure of
Page 13 and 14: where λ n represents the roots of
Page 15: ρ o = density of the otoconial lay

chapter 36 - Vestibular Mechanics - KEMT FEI TUKE

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?