10. Vestibular System: Structure and Function
Part 1 of 2

Lincoln Gray, Ph.D.
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All living organisms monitor their environment and one important aspect of that environment is gravity and the orientation of the body with respect to gravity. In addition, animals that locomote must be able to adjust their orientation with respect to self generated movements, as well as forces that are exerted upon them from the outside world. The vestibular system performs these essential tasks. It engages a number of reflex pathways that are responsible for making compensatory movements and adjustments in body position. It also engages pathways that project to the cortex to provide perceptions of gravity and movement. The first section of the Chapter begins with a description of the components of the peripheral sensory apparatus and describes the ways in which specialized receptors transduce mechanical signals into electrical events. The second section describes the projections of the vestibular afferents to the vestibular nuclei, and projection pathways from the vestibular nuclei to other brain structures such as the cerebellum.

The membranous labyrinth of the inner ear consists of three semicircular ducts (horizontal, anterior and posterior), two otolith organs (saccule and utricle), and the cochlea (which is discussed in the section on Auditory System: Structure and Function).

Figure 10.1 shows the main action of the semicircular ducts. These sensory organs respond to angular acceleration. In Figure 10.1, press the "expand" button to see progressively finer views of the horizontal semicircular duct. This expansion proceeds from the inner ear as it sits in the head, to a sketch of the horizontal semicircular duct, to a detail of the ampulla. (In the outline of the single horizontal semicircular duct the angle has changed, and what was initially horizontal is now seen as a vertically-oriented duct on the computer screen.) The ampulla is a localized dilatation at one end of the semicircular duct. A patch of innervated hair cells is found at the base of the ampulla in a structure termed a crista (meaning crest). The crista contains hair cells with stereocilia oriented in a consistent direction. The cupula, a thin vane, sits atop this crest, filling the lumen of the semicircular duct. The stereocilia of the hair cells are embedded in the gelatinous cupula.

By pressing the "play button" in Figure 10.1, the animation will show the effects of head rotation. As the head rotates in one direction, inertia of the fluid causes it to lag, and hence generate relative motion in the semicircular duct in the direction opposite that of the head movement. This moving fluid bends the broad vane of the cupula. The stereocilia of the hair cells are bent because they are embedded in the gelatinous cupula. Shearing of the hair cells opens potassium channels, as discussed at the beginning of the auditory section (See Figure 12.1).

Figure 10.1

The Semicircular Duct. Press to see drawings of the horizontal semicircular duct. Then, press to watch the reaction to head movement.

There are three pairs of semicircular ducts, which are oriented roughly 90 degrees to each other for maximum ability to detect angular rotation of the head. Each slender duct has one ampulla. When the head turns, fluid in one or more semicircular ducts pushes against the cupula and bends the cilia of the hair cells. Fluid in the corresponding semicircular duct on the opposite side of the head moves in the opposite direction.

The basic transduction mechanism is the same in the auditory and vestibular systems (See Figure 12.1). A mechanical stimulus bends the cilia of the hair cells. Fine thread-like tip links connect to trap doors in the adjacent cilium. Bending the hair cells stretches the tip link, causing an influx of K⁺ ions and the generation of neural impulses in the VIIIth cranial nerve.

Hair cells in the vestibular system are slightly different from those in the auditory system, in that vestibular hair cells have one tallest cilium, termed the kinocilium. Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity. Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity.

The semicircular ducts work in pairs to detect head movements (angular acceleration). A turn of the head excites the receptors in one ampulla and inhibits receptors in the ampulla on the other side.

Figure 10.2 is a diagram of the cranial nerves and their nuclei that mediate interactions between the vestibular system and eye muscles appears as an inset. By pressing the "play" button you will see an animation of this. A constant low level of spontaneous activity keeps all the muscles slightly and equally contracted, causing the eyes to look straight ahead. When the head turns, inertia causes the fluid to move more slowly than the head, generating relative fluid motion in the semicircular duct in the opposite direction of the head turn. This moving fluid, shown by arrows in the lumens of the semicircular duct, bends the hair cells on both sides of the head. Because the two sides are mirror images, the stereocilia are bent toward their kinocilium on one side and away from their kinocilium on the other side. Shearing of the stereocilia toward the kinocilium causes a depolarization of the receptor potential and an increase in afferent action potentials. There is an opposite effect on the other side – a decrease in afferent activity. These counteracting bilateral changes in afferent activity affect the vestibular and occulomotor nuclei. The ampullo fugal movement of fluid on the patient's right (reader's left) causes an increase in afferent activity (shown in green for "go" in the inset). This has a positive effect on the right medial and superior vestibular nuclei, which in turn stimulate the ipsilateral occulomotor and contralateral abducens nuclei. There are exactly opposite effects on the other side (shown in red for "stop" in the inset). The result of these combined counteracting effects is a smooth movement of the eyes toward the left, keeping the visual field stable as the head turns.

Figure 10.3 illustrates the otolithic organs, the saccule and utricle. Press "expand" to see the utricle at the top of Figure 10.3 and the saccule at the bottom. These two similar organs lie against the walls of the inner ear between the semicircular ducts and the cochlea. The receptors, called maculae (meaning "spot"), are patches of hair cells topped by small, calcium carbonate crystals called otoconia. The saccule and utricle lie at 90 degrees to each other. Thus, with any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer. This bending of the cilia produces afferent activity going through the VIIIth nerve to the brainstem.

Activate Figure 10.3 to view the actions of the utricle and saccule. The utricle is most sensitive to tilt when the head is upright. The saccule is most sensitive to tilt when the head is horizontal. Unlike the semicircular ducts, the kinocilia of hair cells in the maculae are NOT oriented in a consistent direction. The kinocilia point toward (in the utricle) or away from (in the saccule) a middle line called the striola. The striola is shown as a dashed line in Figure 10.3. Because hair cells are oriented in different directions, tilts in any direction will activate some afferents.

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