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The Vestibular System

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The Vestibular System 40 The Vestibular System The Vestibular Apparatus in the Inner Ear Contains Five irplanes and submarines navigate in three Receptor Organs dimensions using sophisticated guidance sys- Hair Cells Transduce Mechanical Stimuli into tems that register every acceleration and turn. Receptor Potentials A Laser gyroscopes and computers make these navi- The Semicircular Canals Sense Head Rotation gational aids extremely precise. Yet the principles of The Otolith Organs Sense Linear Accelerations inertial guidance are ancient: Vertebrates have used analogous systems for 500 million years and inverte- Most Movements Elicit Complex Patterns of brates for still longer. Vestibular Stimulation In vertebrates the inertial guidance system is the Vestibulo-Ocular Refexes Stabilize the Eyes and Body When vestibular system, comprising fve sensory organs in the the Head Moves internal ear that measure linear and angular acceleration The Rotational Vestibulo-Ocular Refex Compensates for of the head. Acceleration of the head defects hair bun- Head Rotation dles protruding from the hair cells in the inner ear; this The Otolithic Refexes Compensate for Linear Motion distortion changes the cells’ membrane potential, altering and Head Deviations the synaptic transmission between the cells and the sen- Vestibulo-Ocular Refexes Are Supplemented by sory neurons that innervate them. The signals from these Optokinetic Responses vestibular neurons convey information on head velocity Central Connections of the Vestibular Apparatus Integrate and acceleration to vestibular nuclei in the brain stem. Vestibular, Visual, and Motor Signals This information keeps the eyes still when the head The Vestibular Nerve Carries Information on Head moves, helps to maintain upright posture, and infu- Velocity to the Vestibular Nuclei ences how we perceive our own movement and the A Brain Stem Network Connects the Vestibular System space around us by providing a measure of the gravita- with the Oculomotor System tional feld in which we live. In this chapter we describe Two Visual Pathways Drive the Optokinetic Refexes how the hair cells of the inner ear generate the signals for head acceleration and how these signals are inte- The Cerebral Cortex Integrates Vestibular, Visual, and Somatosensory Inputs grated with other sensory information in the brain. The Cerebellum Adjusts the Vestibulo-Ocular Refex Clinical Syndromes Elucidate Normal Vestibular Function The Vestibular Apparatus in the Inner Ear Unilateral Vestibular Hypofunction Causes Pathological Contains Five Receptor Organs Nystagmus Bilateral Vestibular Hypofunction Interferes with Vestibular signals originate in the labyrinths of the inter- Normal Vision nal ear (Figure 40–1). The bony labyrinth is a hollow An Overall View structure within the petrous portion of the temporal 918 Part VI / Movement A Direction of view Superior and inferior vestibular Endolymphatic ganglia Superior sac of Scarpa saccular ramus Greater Superior and inferior Sensory saccular vestibular nerves receptor organs: Figure 40–1 The vestibular apparatus nerve of the inner ear. Anterior A. The orientations of the vestibular and vertical Facial nerve Semicircular canals cochlear divisions of the inner ear are Posterior (head rotation) shown with respect to the head. Cochlear nerve vertical B. The inner ear is divided into bony and Spiral Horizontal ganglion membranous labyrinths. The bony laby- of cochlea rinth is bounded by the petrous portion of Utricle Otolith organs (linear motion) the temporal bone. Lying within this struc- Cochlea Saccule ture is the membranous labyrinth, which contains the receptor organs for hearing (the cochlea) and equilibrium (the utricle, saccule, and semicircular canals). The B space between bone and membrane is Ductus Saccule Endolymphatic Membranous reuniens sac labyrinth flled with perilymph, whereas the mem- Dura branous labyrinth is flled with endolymph. Endolymph Sensory cells in the utricle, saccule, Perilymph and ampullae of the semicircular canals Helicotrema Bone respond to motion of the head. (Adapted, Scala with permission, from Iurato 1967.) vestibuli Scala tympani Semicircular canals: Cochlear duct Anterior vertical Horizontal Perilymphatic Posterior duct vertical Round window Stapes in Utricle Ampulla oval window bone. Within it lies the membranous labyrinth, which are kept separate by a junctional complex that girdles contains sensors for both the vestibular and auditory the apex of each cell. systems. During development the labyrinth progresses The membranous labyrinth is flled with endo- from a simple sac to a complex of interconnected lymph, a Na+-poor, K+-rich fuid whose composition is sensory organs but retains the same fundamental maintained by the action of ion pumps in specialized topological organization. Each organ originates as an cells. Surrounding the membranous labyrinth, in the epithelium-lined pouch that buds from the otic cyst, space between the membranous labyrinth and the wall and the endolymphatic spaces within the several of the bony labyrinth, is perilymph. Perilymph is a high- organs remain continuous in the adult. The endo- Na+, low-K+ fuid similar in composition to cerebrospi- lymphatic spaces of the vestibular labyrinth are also nal fuid, with which it is in communication through connected to the cochlear duct through the ductus the cochlear aqueduct. The endolymph and perilymph reuniens (Figure 40–1B). Chapter 40 / The Vestibular System 919 The vestibular portion of the labyrinth, or vestibu- extensively studied by recording from hair cells in situ, lar apparatus, lies posterior to the cochlea and consists stimulation of the fbers from the brain stem changes of fve sensory structures. Three semicircular canals (hor- the sensitivity of the afferent axons from the hair cells. izontal, also called lateral; anterior, also called superior; Stimulation decreases the excitability of some hair and posterior) sense head rotations, whereas two otolith cells, as would be expected if activation of the effer- organs (utricle and saccule) sense linear motion (also ent fbers elicited inhibitory postsynaptic potentials in called translation). Because gravity is a linear accelera- hair cells. In other hair cells, however, activation of the tion, the otolith organs also sense the orientation, or efferent fbers increases excitability. tilt, of the head relative to gravity. Given that hair cells are essentially strain gauges (see Chapter 30), the key to grasping how the vestibu- Hair Cells Transduce Mechanical Stimuli into lar organs operate is to understand how mechanical Receptor Potentials stimuli are delivered to the constituent hair cells. Dis- tinctive mechanical linkages in the otolith organs and Each of the fve receptor organs has a cluster of hair semicircular canals account for the contrasting sensi- cells responsible for transducing head motion into ves- tivities of the two types of vestibular organs. tibular signals. Angular or linear acceleration of the head leads to a defection of the hair bundles in a par- The Semicircular Canals Sense Head Rotation ticular group of hair cells of the appropriate receptor organ (Figure 40–2). An object undergoes angular acceleration when its Vestibular signals are carried from the hair cells rate of rotation about an axis changes. The head there- to the brain stem by branches of the vestibulocochlear fore undergoes angular acceleration when it turns or nerve (cranial nerve VIII). Cell bodies of the vestibular tilts, when the body rotates, and during active or pas- nerve are located in the vestibular ganglia of Scarpa sive locomotion. The three semicircular canals of each within the internal auditory canal (Figure 40–1A). The vestibular labyrinth detect these angular accelera- superior vestibular nerve innervates the horizontal and tions and report their magnitudes and orientations to anterior canals and the utricle, whereas the inferior the brain. vestibular nerve innervates the posterior canal and the Each semicircular canal is a roughly semicircular saccule. The labyrinth’s vascular supply, which arises tube of membranous labyrinth extending from the from the anterior inferior cerebellar artery, mirrors its utricle. One end of each canal is open to the utricle innervation: The anterior vestibular artery supplies the whereas at the other end, the ampulla, the entire lumen structures innervated by the superior vestibular nerve, of the canal is traversed by a gelatinous diaphragm, and the posterior vestibular artery supplies the struc- the cupula. The cupula is attached to the epithelium tures innervated by the inferior vestibular nerve. along the perimeter and numerous hair bundles insert Like most other hair cells, those of the human ves- into the cupula (Figure 40–3). tibular system receive efferent inputs from the brain The vestibular organs detect accelerations of the stem. Although the effect of these inputs has not been head because the inertia of their internal contents Excitation Inhibition Figure 40–2 Hair cells in the vestibular labyrinth transduce mechanical stimuli into neural signals. At the apex of each cell is a hair bundle, the stereocilia of which increase in length toward a single kinocilium. The membrane potential of the receptor Hyperpolarization cell depends on the direction in which the hair bundle is bent. Defection toward the Receptor potential Depolarization kinocilium causes the cell to depolarize and thus increases the rate of fring in the affer- ent fber. Bending away from the kinocil- ium causes the cell to hyperpolarize,
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