VII. THE INNER EAR: THE VESTIBULAR APPARATUS

1. It is the primary organ of equilibrium and thus plays a major role in the subjective sensation of motion and spatial orientation.

2. Vestibular input to areas of the nervous system involved in motor control elicits adjustments of muscle activity and body position to allow for upright posture.

3. Vestibular input to regions of the nervous system controlling eye movements helps stabilize the eyes in space during head movements. This reduces the movement of the image of a fixed object on the retina.

The block diagram below illustrates the role of the vestibular system in control of posture, eye movements and perception of orientation.

Sense organs of the vestibular system are mechanoreceptors. The vestibular apparatus or vestibular labyrinth contains the three semicircular canals, the utricle and the saccule. The semicircular canals are so arranged that they lie in planes orthogonal to one another (Figure VII-1).

Semicircular canals sense angular acceleration

Sense cells within each organ are hair cells, which are specialized epithelial cells, hair cells, having ciliary tufts protruding from their apical surface.  The three semicircular canals have swellings, called ampullae and within each ampulla is the sense organ, called the crista. In the cristae the hairs of the hair cells are embedded in a gelatinous mass, called the cupula, which extends across the ampulla (Figure VII-2).

Fluid inertia during angular acceleration results in displacement of the cupula and bending of the sensory hairs. This is the adequate stimulus for exciting the hair cell. Figure VII-3 illustrates movement of the cupula and its embedded hairs during rotation first in one direction and then in the opposite direction.

Canal functions can be tested clinically. This may be done by rotating a patient in a special chair (creating angular acceleration) or by irrigating the ear with cold or warm water (caloric test). In order to test the function of each canal receptor organ, it is necessary to place the canal in its most effective position. For example, the plane of the horizontal or lateral canal is 30 degrees off the horizontal during normal upright posture. Thus, if one wishes to test the lateral semicircular canal, the head of the seated patient is tilted to bring the canal to a horizontal position (Figure VII-4).

The utricle and saccule sense linear acceleration or head tilt (gravity)

The sense organs within the saccule and utricle are called maculae (Figure VII-5). Both the saccular macula and utricular macula are covered by a gelatinous mass called the otolithic membrane containing concretions of calcium carbonate called otoconia or otoliths.

This loading of the cilia by inertial mass makes the organs sensitive to linear acceleration and changes of position of the head in the gravitational field. Figure VII-6 is a schematic representation of the macula and its relationship to the otolithic membrane.

In the vestibular organs of birds and mammals, two types of hair cells are usually distinguished although they may in fact represent the extremes of a spectrum of morphological types. Along with the most common type, called type II, are found flask-shaped cells, called type I enclosed up to their neck by a large nerve chalice which may enclose more than one cell. Between the hair cell and membrane of the chalice is a complicated pattern of contacts where the synaptic space is reduced to less than 100A. Synaptic terminals densely packed with vesicles are regularly observed in contact with the base of the chalice. These are interpreted as being presynaptic terminals. Type II hair cells are innervated by several thin nerve branches forming synaptic contact with the bottom of the cell. Efferent endings are presynaptic to the hair cell and are filled with vesicles. 

Afferent endings are formed by the distal branches of bipolar cells of the vestibular (Scarpa's) ganglion. Terminals may be transitional between chalice and bouton-type endings. Information is transmitted between hair cell and eighth nerve terminal by normal chemical transmission.  

Adequate Stimulus of the Hair Cell

The vestibular hair cells, like those in the cochlea, are directionally sensitive displacement detector. During head tilt or head rotation, lateral force is transmitted to the sensory hair bundle via the overlying auxiliary structure (cupula or otolithic membrane). As in all hair cells, regardless of their differences in morphology, the resultant displacement of the stereocilia opens ion channels resulting in inward ionic current. This leads to a receptor potential, release of neurotransmitter and the generation of action potentials in the distal processes of afferent nerve fibers.

The output from the crista ampullaris is proportional to angular displacement of the cupula. The output of the maculae of the utricle and saccule are excited by very small linear movement of the otolithic membrane. Thus, while the adequate stimulus for the different sense organs may differ, the adequate stimulus for the sensory cell appears to be the same, shearing displacement of the sensory hairs.

Directional Sensitivity of the cristae - Functional polarization of the receptor organ

Recall that hair cells are functionally polarized:  displacement of the sensory hair bundle in the direction in of the kinocilium is excitatory, resulting in depolarization of the hair cell and increased firing of vestibular nerve fibers; displacement in the opposite direction is inhibitory and results in hyperpolarization of the hair cell and reduced firing in the vestibular nerve. 

The cristae of the semicircular canals, as receptor organs, are also functionally polarized. In each crista ampullaris all cells are oriented with their kinocilia pointing in the same direction. Angular acceleration causes deflection of the cupula in only one direction and thus affects simultaneously all the hair cells oriented in that direction (Figure VII-8). Thus, all afferent fibers innervating each cristae fire together depending on the direction of angular motion.

The orientation of the hair cells is such that the receptors of the horizontal canal, for example, are excited by deflection of the cupula towards the utricle (utriculo-petal endolymphatic flow) whereas the two vertical canals are excited by deflection of the cupula away from the utricle (utriculo-fugal endolymphatic flow), as illustrated in Figure VII-9.

Otolithic organs are omnidirectional

The utricle and saccule serve in the maintenance of body posture by responding to linear acceleration and changes in head position. The adequate stimulus is displacement of the otolithic membrane which is free to move in any direction determined by the direction of acceleration. Again, as in other inner ear receptors, the hair cells are functionally polarized. The spatial arrangement of hair cells in the utricle is very elaborate, however, and hair cells are not oriented in the same direction. Instead, the hair cells are oriented in a "fanning" fashion on either side of a line called the strioli of Werner, dividing the utricle into medial and lateral portions. Thus, for essentially any direction of otolithic motion a pair of orthogonal hair cells can be found to supply information about stimulus direction. Figure VII-10 is a schematic drawing of directions of polarization of sensory cells in the maculae of the utricle. A similar situation obtains for the saccule although the orientation of hair cells there is not identical to that of the utricle.

A microelectrode inserted into the ampullar nerves (or cells of Scarpa ganglion) innervating the canals records a resting discharge from individual nerve fibers. The average resting discharge with the head motionless is about 90 impulses per second in canal fibers and about 60 per second in fibers innervating the utricular macula. If we assume that there are about 20,000 fibers in the vestibular nerve then the central nervous system (and the vestibular nuclei in particular) with the head motionless receives a tonic input amounting to 1.5 x 106 impulses per second. This high resting discharge is necessary for the expression of functional polarization of the hair cells; when the cupula moves towards the utricle in the horizontal canal there is an increase in discharge rate; movement in the opposite direction results in a decrease in discharge rate. In the vertical canals excitation occurs when the cupula moves away from the utricle and inhibition occurs when the cupula moves toward the utricle.  

The high resting discharge also makes the hair cell very sensitive since very small movements of the cupula in either direction affect the discharge of the fiber. This may account for the low human perceptual threshold to angular acceleration (0.1 deg/sec2).

Differential sensitivity to sound, head position and head movement

How can a hair cell respond differentially to sound, to head position and to head movement? The answer lies not in the function of the hair cell per se, but in its relationship to other elements of the receptor organ. A hair cell responds only to the deformation of its cilia. Thus, the various receptor organs of the inner ear, each of which is equipped with hair cells, supporting cells and an auxiliary structure, are specialized for the kind of mechanical distortion they can detect. The organ of Corti, but not the vestibular organs, is set in motion by small pressure waves set up in the cochlear fluid; this movement results in a shearing motion between the auxiliary structure (tectorial membrane) and the sensory hairs on hair cells. Neither angular or linear acceleration of the head, nor head tilt can create such a shearing motion. Angular acceleration, on the other hand, is a most effective way to create in the ampullae of the semicircular canals a shearing motion between the hair cell cilia of the cristae and the overlying auxiliary structure, the cupula. While refractory to angular acceleration and to sound, the otolithic organs in the saccule and utricle are especially sensitive to linear acceleration and to head tilt (gravity). Again, the action is between the auxiliary structure (the otolithic membrane) and the cilia of the hair cells in the maculae.

Projections of primary vestibular nerve fibers