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What Is The Vestibular System?

The vestibular system, in vertebrates, is part of the inner ear. In most mammals, the vestibular system is the sensory system that provides the leading contribution to the sense of balance and spatial orientation for the purpose 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
  • the otoliths which indicate linear accelerations

The vestibular system sends signals primarily to the neural structures that control eye movements, and to the muscles that keep an animal upright and in general control posture.

The projections to the former provide the anatomical basis of the vestibulo-ocular reflex, which is required for clear vision; while the projections to the latter 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 the animal to understand its body’s dynamics and kinematics (including its position and acceleration) from moment to moment. How these two perceptive sources are integrated to provide the underlying structure of the sensorium is unknown.

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 feel that he or she has turned to the left if the chair is turned to the left. A person in an elevator, with essentially constant visual input, will feel she is descending as the elevator starts to descend. There are a variety of direct and indirect vestibular stimuli which can make people feel they are moving when they are not, not moving when they are, tilted when they are not, or not tilted when they are.[1]

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 hearing, vestibular input is perceived with delay.[2]

Vestibular System Diseases

The most common vestibular diseases in humans are vestibular neuritis, a related condition called labyrinthitis, Ménière’s disease, and BPPV. In addition, the function of the vestibular system can be affected by tumors on the vestibulocochlear nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy.

When the vestibular system and the visual system deliver incongruous results, nausea often occurs. When the vestibular system reports no movement but the visual system reports movement, the motion disorientation is often called motion sickness (or seasickness, car sickness, simulation sickness, or airsickness).

In the opposite case, such as when a person is in a zero-gravity environment or during a virtual reality session, the disoriented sensation is often called space sickness or space adaptation syndrome. Either of these “sicknesses” usually ceases once the congruity between the two systems is restored.

Benign paroxysmal positional vertigo (BPPV) is a condition resulting in acute symptoms of vertigo. It is probably caused when pieces that have broken off otoliths have slipped into one of the semicircular canals. In most cases, it is the posterior canal that is affected. In certain head positions, these particles shift and create a fluid wave which displaces the cupula of the canal affected, which leads to dizziness, vertigo and nystagmus.

Vestibular dysfunction has also been found to correlate with cognitive and emotional disorders, including depersonalization and derealization.[3]

Structure

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

Since the world is three-dimensional, the vestibular system contains three semicircular canals in each labyrinth. They are approximately orthogonal (at right angles) to each other, and are the horizontal (or lateral), the anterior semicircular canal (or superior), and the posterior (or inferior) semicircular canal. Anterior and posterior canals may collectively be called vertical semicircular canals.

Cochlea of the inner ear
Cochlea of the inner ear.
Credit: Dr David Furness. CC BY-NC

Movement of fluid within the horizontal semicircular canal corresponds to rotation of the head around a vertical axis (i.e. the neck), as when doing a pirouette.

The anterior and posterior semicircular canals detect rotations of the head in the sagittal plane (as when nodding), and in the frontal plane, as when cartwheeling. Both anterior and posterior canals are orientated 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 (see below), the left horizontal canal gets stimulated (and thus predominantly signals) by head rotations to the left.

Push-pull system of the semicircular canals, for a horizontal head movement to the right.
Push-pull system of the semicircular canals, for a horizontal head movement to the right.Credit: Thomas Haslwanter CC BY-SA 3.0

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.

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 40-70 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. When the head is tilted, however, 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 to 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 a 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 allow humans to tell which way the head is tilting,[4] 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.

[1] Lawson, B. D., & Riecke, B. E. (2014). The Perception of Body Motion. Handbook of Virtual Environments, CRC Press, 163-196

[2] Barnett-Cowan, M. (2013), Vestibular perception is slow: a review. Multisensory Research, 26: 387-403. Barnett-Cowan, Michael (2013). “doi: 10.1163/22134808-00002421”. Multisensory Research. 26 (4): 387–403

[3] Smith, Paul F; Darlington, Cynthia L (2013). Personality changes in patients with vestibular dysfunction. Frontiers in Human Neuroscience. 7: 678. doi:10.3389/fnhum.2013.00678