1. The labyrinth
Figure The membranous labyrinth, outlined in red, sits within the bony labyrinth (blue). The three semicircular canals form the most posterior portion of the inner ear. All three semicircular canals open up into the utriculus (U). The sensory epithelium for each canal is contained in a swelling called an ampulla. The ampullae of the anterior (A-A), posterior (P-A), and horizontal (H-A) semicircular canals are labeled. Anterior to the utriculus is the sacculus (S). The entire membranous labyrinth is bathed in endolymph, which is formed within the cochlea. Endolymph reaches the labyrinth through the narrow canal reuniens.

2. Linear and angular acceleration
Figure A: An example of an angular rotation in the horizontal or yaw plane is illustrated. The center of the angular rotation shown in A is the midpoint of the head. B: Angular rotation can also occur off-axis, as when a person rides a merry-go-round. C: Linear motion, or translation, follows a simple linear path. D: Gravity is a constant acceleration of about 10 m/s2.

3. Horizontal canal
Figure 19-3A: The horizontal canal opens into the utriculus at both its anterior and posterior ends. The ampulla of the horizontal canal is located anteriorly. Within the ampulla, the sensory epithelium, termed a crista ampullaris (blue), is located on top of the ridge of the crista. The stereocilia of the hair cells within the crista are embedded in a gelatinous structure called a cupula.

4. The crista ampullaris and cupula
Figure B: A cross-section through the ampulla is shown. The cupula covers the entire crista and stretches across the lumen of the canal. C: Hair cell stereocilia are embedded within the cupula.

5. Hair cell physiology
Figure Hair cell stereocilia are arranged from shortest to tallest. The tallest stereocilia is called a kinocilium. Deflection of the stereocilia toward the kinocilium is the preferred direction of stimulation and results in hair cell depolarization. Deflection of the stereocilia away from the kinocilium is the nonpreferred direction of stimulation and results in hair cell hyperpolarization. Hair cells do not respond to deflection of the stereocilia in the orthogonal direction (into or out of the screen).

6. Bidirectional vestibular responses
Figure A: All canal hair cells have the same orientation. In the resting state, hair cell stereocilia are in a neutral position. B: Endolymph flow in the preferred direction deflects the cupula, which in turn deflects the stereocilia toward the kinocilium. C: Endolymph flow in the nonpreferred direction deflects the cupula, which in turn deflects the stereocilia away from the kinocilium. D: Like cochlear hair cells, vestibular hair cells respond to stimulation with graded potentials. They do not fire action potentials. The hair cell releases glutamate from a specialized type of active zone called a ribbon synapse onto postsynaptic vestibular afferents. At rest (left), the stereocilia are in the neutral position (top row), the hair cell membrane potential is about -50 mV (middle row), and vestibular afferents have a resting discharge of 50–100 spikes per second (bottom row). In response to a stimulus that deflects the stereocilia in the preferred direction (middle), the hair cell depolarizes and releases more glutamate. Consequently, the vestibular afferent fires more rapidly. In contrast, in response to a stimulus that deflects the stereocilia in the nonpreferred direction (right), the hair cell hyperpolarizes and releases less glutamate than at rest. Consequently the vestibular afferent fires less rapidly. The depolarized rest potential in the hair cell and the resting discharge of the vestibular afferent enable the vestibular system to respond to stimulation in opposing directions.

7. Saccular and utricular responses
Figure A: In an upright person at rest, the otoconial mass of the sacculus is displaced downward due to the force of gravity. During linear accelerations in the vertical plane, the sacculus is displaced either farther downward during upward acceleration or upward during downward acceleration (B). When the sacculus floats up, as occurs during a downward acceleration, the resting effect of gravity on the sacculus is relieved momentarily, resulting in a feeling of “weightlessness.”
C: The resting state of the utricular otoconial mass in an upright individual involves no displacement of stereocilia. However, during static tilt (D) or linear accelerations (E), the otoconial mass shifts, and the stereocilia are deflected. The deflection of the utricular stereocilia can be the same during a static tilt and a linear acceleration, as is the case in the examples illustrated in D and E. Additional input from the semicircular canals, somatosensory afferents, and motor centers are used by central vestibular neurons to disambiguate these signals.
In both the sacculus and utriculus, there is a dividing line called the striola (red arrows in A and C). In the sacculus, the preferred direction of hair cells is always away from the striola, and in the utriculus, the preferred direction is toward the striola.

8. Canal planes pitch roll yaw
Figure Rotations moving the head forward and backward as when nodding yes, or nodding off, are contained in the pitch plane (left). The center of rotation in the pitch illustrated is within the head. An example of an off-axis pitch would be the rotation involved in doing a handstand or flip. Side-to-side rotations of the head are contained in the roll plane (middle), with the center of rotation located within the head. A cartwheel is an example of an off-axis roll rotation. Shaking the head to signal no is a movement in the yaw plane (right). The center of rotation in the yaw illustrated is within the head. An example of an off-axis pitch would be the rotation involved in riding a merry-go-round.
pitch
roll
yaw

9. Canals’ orientations and locations
Figure A: The horizontal semicircular canals on the left and right side (HSCl and HSCr) form a pair. This means that any rotation with a component in the yaw plane will have opposing effects on the two horizontal canals. The other two canal pairs are (1) the right anterior semicircular canal (ASCr) and the left posterior semicircular canal (PSCl) shown in red, and (2) the left anterior semicircular canal (ASCl) and the right posterior semicircular canal (PSCr) shown in blue. The arrowheads on the right side of the head indicate the location and orientation of the hair cells in the ampulla of each canal. The hair cells in the horizontal ampulla are oriented toward the utriculus, whereas the preferred direction of hair cells in the anterior and posterior canals is away from the utriculus.
B: The location of the ampullae (filled circles) and the preferred direction of hair cells in each side semicircular canal (arrowheads) are illustrated on a side view of the right canals. From this perspective, it is clear that the lowest point in the vestibular apparatus is the ampulla of the posterior canal. Because of this topography, debris within the membranous labyrinth is thought to preferentially accumulate in the posterior ampulla.

10. From acceleration to hair cell response
Figure A: The right horizontal semicircular canal, viewed in isolation from above, is diagrammed with anterior toward the top of the page and lateral to the right. During a clockwise head acceleration (A), the bony and membranous labyrinths rotate clockwise while the endolymph stays stationary. This means that endolymph located at a given point (red asterisk in A) is located in the same place in space (red asterisk in B), even after the surrounding structures have moved. As the crista and ampulla are anchored to the membranous labyrinth, endolymph moves relative to these structures. The relative motion (brown arrows) of the endolymph with respect to the crista and cupula deflects the cupula, which leads in turn to the deflection of the stereocilia of the hair cells. In the rotation illustrated, the stereocilia are deflected toward the utriculus, which is the preferred direction of hair cells in the horizontal canals. Thus, a clockwise or rightward acceleration results in the depolarization of right horizontal hair cells. The same logic used here can be employed to deduce the effect of any rotational acceleration.

11. Vestibular vector system
Figure The vestibular system operates by decomposing every movement into the components contained within the planes of the canal pairs, the utriculi and the sacculi. Three examples are illustrated.
A: A pitch forward (black arrow) is equal parts forward movement in the right anterior semicircular canal—left posterior semicircular canal plane (red) and forward movement in the left anterior semicircular canal—right posterior semicircular canal plane (blue).
B: A roll right (black arrow) is equal parts forward movement in the right anterior semicircular canal—left posterior semicircular canal plane (red) and backward movement in the left anterior semicircular canal—right posterior semicircular canal plane (blue).
C: Natural head movements typically involve both angular and linear acceleration. The same principle of decomposition applies here. The rightward pivot illustrated is composed of a rightward translation and a clockwise rotation. These movements evoke responses in hair cells in the utriculi and the horizontal semicircular canals.