The Vestibular System

Figure 22-1. A cross section of the outer, middle, and inner ear.

Figure 22-2. The membranous labyrinth and associated vessels and nerves. The approximate configuration of the receptor sites in the ampulla, utricle, and saccule are shown in green. The detail shows the relationship between bony and membranous labyrinths.

Between the membranous labyrinth and bony labyrinth is a space containing fluid called perilymph, which is similar to cerebrospinal fluid. Perilymph has a high sodium content (150 mM) and a low potassium content (7 mM), and it bathes the vestibular portion of the eighth cranial nerve.

The membranous labyrinth is filled with a different type of fluid, called endolymph, which covers the specialized sensory receptors of both the vestibular and the auditory systems. Endolymph has a high concentration of potassium (150 mM) and a low concentration of sodium (16 mM). It is important to note the differences in these two fluids because both are involved in the normal functioning of the vestibular system. Disturbances in the distribution or ionic content of endolymph often lead to vestibular disease.

Vestibular Receptor Organs

The five vestibular receptor organs in the inner ear complement each other in function. The semicircular canals (horizontal, anterior, and posterior) transduce rotational head movements (angular accelerations). The otolith organs (utricle and saccule) respond to translational head movements (linear accelerations) or to the orientation of the head relative to gravity. Each semicircular canal and otolith organ is spatially aligned to be most sensitive to movements in specific planes in three-dimensional space.

In humans, the horizontal semicircular canal and the utricle both lie in a plane that is slightly tilted anterodorsally relative to the nasooccipital plane (Fig. 22-3). When a person walks or runs, the head is normally declined (pitched downward) by approximately 30 degrees, so that the line of sight is directed a few meters in front of the feet. This orientation causes the plane of the horizontal canal and utricle to be parallel with the earth and perpendicular to gravity. The anterior and posterior semicircular canals and the saccule are arranged vertically in the head, orthogonal to the horizontal semicircular canal and utricle (Fig. 22-3). The two vertical canals in each ear are positioned orthogonal to each other, whereas the plane of the anterior canal on one side of the head is coplanar with the plane of the contralateral posterior canal (Fig. 22-3).

Figure 22-3. Orientation of the vestibular receptors. In the lateral view (A), the horizontal semicircular canal and the utricle lie in a plane that is tilted relative to the nasooccipital plane. In the axial view (B), the vertical semicircular canals lie at right angles to each other.

The receptor cells in each vestibular organ are innervated by primary afferent fibers that join with those from the cochlea to comprise the vestibulocochlear (eighth) cranial nerve. The cell bodies of these bipolar vestibular afferent neurons are in the vestibular ganglion (Scarpa ganglion), which lies in the internal acoustic meatus (Fig. 22-4). The central processes of these bipolar cells enter the brainstem and terminate in the ipsilateral vestibular nuclei and cerebellum.

Figure 22-4. Computed tomography scans of the human temporal bone. The horizontal (A, arrowhead) and anterior and posterior (B, arrowheads) semicircular canals, utricle (A, small arrow), and internal acoustic canal (A, large arrow) are visible.

The blood supply to the labyrinth is primarily via the labyrinthine artery, usually a branch of the anterior inferior cerebellar artery. This vessel enters the temporal bone through the internal auditory meatus. Although it is not as important as the labyrinthine artery, the stylomastoid artery also provides branches to the labyrinth, mainly to the semicircular canals. An interruption of blood supply to the labyrinth will compromise vestibular (and cochlear) function, resulting in labyrinth-associated symptoms, such as vertigo or oscillopsia, and clinical signs, such as nystagmus or unstable gait.

Membranous Labyrinth

The membranous labyrinth is supported inside the bony labyrinth by connective tissue. The three ducts of the semicircular canals connect to the utricle, and each duct ends with a single prominent enlargement, the ampulla (Fig. 22-2). Sensory receptors for the semicircular canals reside in a neuroepithelium at the base of each ampulla. The receptors in the utricle are oriented longitudinally along its base, and in the saccule they are oriented vertically along the medial wall (Fig. 22-2). Endolymph in the labyrinth is drained into the endolymphatic sinus via small ducts. In turn, this sinus communicates through the endolymphatic duct with the endolymphatic sac, which is located adjacent to the dura mater (Fig. 22-2). The saccule is also connected to the cochlea by the ductus reuniens.

Meniere Disease

The balance between the ionic contents of endolymph and perilymph is maintained by specialized secretory cells in the membranous labyrinth and the endolymphatic sac. In cases of advanced Meniere disease, there is disruption of normal endolymph volume, resulting in endolymphatic hydrops (an abnormal distention of the membranous labyrinth). Symptoms of Meniere disease include severe vertigo (a sense of spinning in space), positional nystagmus, and nausea. Affected persons often have unpredictable attacks of auditory and vestibular symptoms, including vomiting, tinnitus (ringing in the ears), and a complete inability to make head movements or even to stand passively. For patients with frequent debilitating attacks, the first course of treatment is often administration of a diuretic (e.g., hydrochlorothiazide) and a salt-restricted diet to reduce the hydrops. If persistent symptoms of Meniere disease continue, second treatment options include either the implantation of a small shunt into the abnormally swollen endolymphatic sac or the delivery of a vestibulotoxic agent such as gentamicin into the perilymph.

Semicircular Canal Dehiscence

On occasion, a condition may develop in which a portion of the temporal bone overlying either the anterior or the posterior semicircular canal thins so much that an opening (dehiscence) is created next to the dura (Fig. 22-5). In affected patients, the canal dehiscence exposes the normally closed bony labyrinth to the extradural space. Symptoms can include vertigo and oscillopsia (a sense that objects are moving to and fro, oscillating, in the visual fields) in response to loud sounds (the Tullio phenomenon) or in response to maneuvers that change middle ear or intracranial pressure. The eye movements evoked by these stimuli (nystagmus) align with the plane of the dehiscent superior canal. Surgical closure of the defect by bone replacement is often performed.

Figure 22-5. Computed tomography scan of the temporal bone projected into the plane of the left superior canal in a patient with superior canal dehiscence syndrome. The patient had vertigo, oscillopsia, and eye movements in the plane of the left superior canal in response to loud noises and pressure in the left ear. A dehiscence is noted overlying the left superior canal (arrowhead).

VESTIBULAR SENSORY RECEPTORS

Hair Cell Morphology

The sensory receptor cells in the vestibular system, like those in the auditory system, are called hair cells because of the stereocilia that project from the apical surface of the cell (Fig. 22-6A). Each hair cell contains 60 to 100 hexagonally arranged stereocilia and a single longer kinocilium. The stereocilia are oriented in rows of ascending height, with the tallest lying next to the lone kinocilium. The stereocilia arise from a region of dense actin, the cuticular plate, located at the apical end of the hair cell. The cuticular plate acts as an elastic spring to return the stereocilia to the normal upright position after bending. Each stereocilium is connected to its neighbor by small filaments.

Figure 22-6. The receptor cells (A, type I and type II hair cells) of the vestibular system. The relation of these cells to the crista and cupula (B) in the ampullae and to the macula and otolith membrane (C) of the otolith organs is shown.

There are two types of hair cells, and they differ in their pattern of innervation by fibers of the eighth cranial nerve (Fig. 22-6A). Type I hair cells are chalice shaped and typically are surrounded by an afferent terminal that forms a nerve calyx. Type II hair cells are cylindric and are innervated by simple synaptic boutons. Excitatory amino acids such as aspartate and glutamate are the neurotransmitters at the receptor cell–afferent fiber synapses. Both types of hair cells, or their afferents, receive synapses from vestibular efferent fibers that control the sensitivity of the receptor. These efferent fibers contain acetylcholine and calcitonin gene–related peptide as neurotransmitters. Efferent cell bodies are located in the brainstem just rostral to the vestibular nuclei and lateral to the abducens nucleus. They are activated by behaviorally arousing stimuli or by trigeminal stimulation.

Within each ampulla, the hair cells and their supporting cells lie embedded in a saddle-shaped neuroepithelial ridge, the crista, which extends across the base of the ampulla (Fig. 22-6B). Type I hair cells are concentrated in central regions of the crista, and type II hair cells are more numerous in peripheral areas. Arising from the crista and completely enveloping the stereocilia of the hair cells is a gelatinous structure, the cupula. The cupula attaches to the roof and walls of the ampulla, forming a fluid-tight partition that has the same specific density as that of endolymph. Rotational head movements produce angular accelerations that cause the endolymph in the membranous ducts to be displaced so that the cupula is pushed to one side or the other like the skin of a drum. These cupular movements displace the stereocilia (and kinocilium) of the hair cells in the same direction.

For the otolith organs, a structure analogous to the crista, the macula, contains the receptor hair cells (Fig. 22-6C). The hair cell stereocilia of otolith organs extend into a gelatinous coating called the otolith membrane, which is covered by calcium carbonate crystals called otoconia (from the Greek, meaning “ear stones”). Otoconia are about three times as dense as the surrounding endolymph, and they are not displaced by normal endolymph movements. Instead, changes in head position relative to gravity or linear accelerations (forward-backward, upward-downward) produce displacements of the otoconia, resulting in bending of the underlying hair cell stereocilia.

Hair Cell Transduction

The response of hair cells to deflection of their stereocilia is highly polarized (Figs. 22-7 and 22-8A). Movements of the stereocilia toward the kinocilium cause the hair cell membranes to depolarize, which results in an increased rate of firing in the vestibular afferent fibers. If the stereocilia are deflected away from the kinocilium, however, the hair cell is hyperpolarized and the afferent firing rate decreases.

Figure 22-7. Physiologic responses of vestibular hair cells and their vestibular afferent fibers. Asp, aspartate; Glu, glutamate.

Figure 22-8. Morphologic polarization of vestibular receptor cells showing polarity of stereocilia and kinocilia (A) and the orientation of receptors in the ampullae (B) and maculae (C).

The mechanisms underlying the depolarization and hyperpolarization of vestibular hair cells depend, respectively, on the potassium-rich character of endolymph and the potassium-poor character of the perilymph that bathes the basal and lateral portions of the hair cells. Deflection of the stereocilia toward the kinocilium causes potassium channels in the apical portions of the stereocilia to open. Potassium flows into the cell from the endolymph, depolarizing the cell membrane (Fig. 22-7). This depolarization in turn causes voltage-gated calcium channels at the base of the hair cells to open, allowing calcium to enter the cell. The influx of calcium causes synaptic vesicles to release their transmitter (aspartate or glutamate) into the synaptic clefts, and the afferent fibers respond by undergoing depolarization and increasing their rate of firing. When the stimulus subsides, the stereocilia and kinocilium return to their resting position, allowing most calcium channels to close and voltage-gated potassium channels at the base of the cell to open. Potassium efflux returns the hair cell membrane to its resting potential (Fig. 22-7).

Deflection of the stereocilia away from the kinocilium causes potassium channels in the basolateral portions of the hair cell to open, allowing potassium to flow out from the cell into the interstitial space. The resulting hyperpolarization of the cell membrane decreases the rate at which the neurotransmitter is released by the hair cells and consequently decreases the firing rate of afferent fibers.

Almost all vestibular primary afferent fibers have a moderate spontaneous firing rate at rest (approximately 90 spikes per second). Therefore it is likely that some hair cell calcium channels are open at all times, causing a slow, constant release of neurotransmitter. The ototoxic effects of some aminoglycoside antibiotics (e.g., streptomycin, gentamicin) may be due to direct reduction of the transduction currents of hair cells.

Morphologic Polarization of Hair Cells

Given that deflections of the stereocilia toward and away from the kinocilium cause opposing physiologic responses, it is clear that the directional orientation of the hair cells in the vestibular organs will play an essential role in signaling the direction of movement. On the cristae of the horizontal semicircular canal, the hair cells are all arranged with their kinocilium on the side closer to the utricle (Fig. 22-8B). Thus movement of endolymph toward the ampulla in the horizontal canal causes the stereocilia to be deflected toward the kinocilium, resulting in depolarization of the hair cell. In the vertical semicircular canals, the hair cells are arranged with their kinocilium on the side farther from the utricle (closest to the endolymphatic duct). Thus the hair cells of the vertical canals are hyperpolarized by movement of endolymph toward the ampulla (ampullipetal movement) and are depolarized by movement away from the ampulla (ampullifugal movement).

In both the utricle and the saccule, the otolith membrane overlying the hair cells contains a small, curving depression, the striola, that roughly bisects the underlying macula (Fig. 22-8C). Hair cells on the utricular macula are polarized so that the kinocilium is always on the side toward the striola (Figs. 22-6C and 22-8C), which effectively splits the receptors into two morphologically opposed groups. In contrast, the kinocilia of saccular hair cells are oriented on the side away from the striola. Because the striola curves through the macula, otolith hair cells are polarized in many different directions (Fig. 22-8C). In this way, utricular and saccular hair cells are directionally sensitive to a wide variety of head positions and linear movements.

SEMICIRCULAR CANALS AND OTOLITH ORGANS

As stated previously, the vestibular receptors transduce movement and position stimuli into neural signals that are sent to the brain. The semicircular canals are responsive to rotational acceleration resulting from turns of the head or body. The otolith organs are responsive to linear accelerations. The most prominent linear acceleration on earth is the constant force of gravity. Linear motion, such as experienced during swinging on a swing or flying in an airplane through turbulence, couples with gravity to change the direction and amplitude of the resultant gravitoinertial acceleration

Only gold members can continue reading. Log In or Register to continue