Vestibular system


The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the function of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear in most mammals.
As movements consist of rotations and translations, the vestibular system comprises two components: the semicircular canals, which indicate rotational movements; and the otoliths, which indicate linear accelerations. The vestibular system sends signals primarily to the neural structures that control eye movement; these provide the anatomical basis of the vestibulo-ocular reflex, which is required for clear vision. Signals are also sent to the muscles that keep an animal upright and in general control posture; these provide the anatomical means required to enable an animal to maintain its desired position in space.
The brain uses information from the vestibular system in the head, and from proprioception throughout the body to enable an understanding of the body's dynamics and kinematics from moment to moment. How these two perceptive sources are integrated to provide the underlying structure of the sensorium is unknown.

Semicircular canal system

The semicircular canal system detects rotational movements. Semicircular canals are its main tools to achieve this detection.

Structure

Since the world is three-dimensional, the vestibular system contains three semicircular canals in each labyrinth. They are approximately orthogonal to each other, and are the horizontal, anterior, and posterior semicircular canals. Anterior and posterior canals may collectively be called vertical semicircular canals.
  1. Movement of fluid within the horizontal semicircular canal corresponds to rotation of the head around a vertical axis, as when doing a pirouette.
  2. The anterior and posterior semicircular canals detect rotations of the head in the sagittal plane, and in the frontal plane, as when cartwheeling. Both anterior and posterior canals are oriented at approximately 45° between frontal and sagittal planes.
The movement of fluid pushes on a structure called the cupula which contains hair cells that transduce the mechanical movement to electrical signals.

Push-pull systems

The canals are arranged in such a way that each canal on the left side has an almost parallel counterpart on the right side. Each of these three pairs works in a push-pull fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa.
This push-pull system makes it possible to sense all directions of rotation: while the right horizontal canal gets stimulated during head rotations to the right, the left horizontal canal gets stimulated by head rotations to the left.
Vertical canals are coupled in a crossed fashion, i.e. stimulations that are excitatory for an anterior canal are also inhibitory for the contralateral posterior, and vice versa.

Vestibulo-ocular reflex (VOR)

The vestibular-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed.
This reflex, combined with the push-pull principle described above, forms the physiological basis of the Rapid head impulse test or Halmagyi-Curthoys-test, in which the head is rapidly and forcefully moved to the side while observing whether the eyes keep looking in the same direction.

Mechanics

The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with, and the head velocity with, the cupula deflection is approximately
α is a proportionality factor, and s corresponds to the frequency. For fluid simulations, the endolymph has roughly the same density and viscosity as water. The cupula has the same density as endolymph, and it is a jelly mostly made of polysaccharides with Young's modulus.
T1 is the characteristic time required for the cupula to accelerate until it reaches terminal velocity, and T2 is the characteristic time required for the cupula to relax back to neutral position. The cupula has a small inertia compared to the elastic force and the viscous force, so T1 is very small compared to T2. For humans, the time constants T1 and T2 are approximately 5 ms and 20 s, respectively. As a result, for typical head movements, which cover the frequency range of 0.1 Hz and 10 Hz, the deflection of the cupula is approximately proportional to the head velocity. This is very useful since the velocity of the eyes must be opposite to the velocity of the head to maintain clear vision.

Central processing

Signals from the vestibular system also project to the cerebellum and to different areas in the cortex. The projections to the cortex are spread out over different areas, and their implications are not clearly understood.

Projection pathways

The vestibular nuclei on either side of the brainstem exchange signals regarding movement and body position. These signals are sent down the following projection pathways.
  1. To the cerebellum. Signals sent to the cerebellum are relayed back as muscle movements of the head, eyes, and posture.
  2. To nuclei of cranial nerves III, IV, and VI. Signals sent to these nerves cause the vestibular-ocular reflex. They allow the eyes to fix on a moving object while staying in focus.
  3. To the reticular formation. Signals sent to the reticular formation signal the new posture the body has taken on, and how to adjust circulation and breathing due to body position.
  4. To the spinal cord. Signals sent to the spinal cord allow quick reflex reactions to both the limbs and trunk to regain balance.
  5. To the thalamus. Signals sent to the thalamus allow for head and body motor control as well as being conscious of body position.

    Otolithic organs

While the semicircular canals respond to rotations, the otolithic organs sense linear accelerations. Humans have two otolithic organs on each side, one called the utricle, the other called the saccule. The utricle contains a patch of hair cells and supporting cells called a macula. Similarly, the saccule contains a patch of hair cells and a macula. Each hair cell of a macula has forty to seventy stereocilia and one true cilium called a kinocilium. The tips of these cilia are embedded in an otolithic membrane. This membrane is weighted down with protein-calcium carbonate granules called otoconia. These otoconia add to the weight and inertia of the membrane and enhance the sense of gravity and motion. With the head erect, the otolithic membrane bears directly down on the hair cells and stimulation is minimal. However, when the head is tilted, the otolithic membrane sags and bends the stereocilia, stimulating the hair cells. Any orientation of the head causes a combination of stimulation to the utricles and saccules of the two ears. The brain interprets head orientation by comparing these inputs to each other and other input from the eyes and stretch receptors in the neck, thereby detecting whether the head is tilted or the entire body is tipping. Essentially, these otolithic organs sense how quickly you are accelerating forward or backward, left or right, or up or down. Most of the utricular signals elicit eye movements, while the majority of the saccular signals projects to muscles that control our posture.
While the interpretation of the rotation signals from the semicircular canals is straightforward, the interpretation of otolith signals is more difficult: since gravity is equivalent to constant linear acceleration, one somehow has to distinguish otolith signals that are caused by linear movements from those caused by gravity. Humans can do that quite well, but the neural mechanisms underlying this separation are not yet fully understood.
Humans can sense head tilting and linear acceleration even in dark environments because of the orientation of two groups of hair cell bundles on either side of the striola. Hair cells on opposite sides move with mirror symmetry, so when one side is moved, the other is inhibited. The opposing effects caused by a tilt of the head cause differential sensory inputs from the hair cell bundles allowing humans to tell which way the head is tilting. Sensory information is then sent to the brain, which can respond with appropriate corrective actions to the nervous and muscular systems to ensure that balance and awareness are maintained.

Experience from the vestibular system

Experience from the vestibular system is called equilibrioception. It is mainly used for the sense of balance and for spatial orientation. When the vestibular system is stimulated without any other inputs, one experiences a sense of self-motion. For example, a person in complete darkness and sitting in a chair will sense that they have turned to the left if the chair is turned to the left. A person in an elevator, with essentially constant visual input, will sense they are descending as the elevator starts to descend. There are a variety of direct and indirect vestibular stimuli which can make people sense they are moving when they are not, not moving when they are, tilted when they are not, or not tilted when they are. Although the vestibular system is a very fast sense used to generate reflexes, including the righting reflex, to maintain perceptual and postural stability, compared to the other senses of vision, touch and audition, vestibular input is perceived with delay.