The vestibular system is a sensory system that detects the position and movement of the head. By monitoring the position and movement of the head, the vestibular system contributes to the sense of balance and equilibrium. The main organs of the vestibular system are located within the inner ear on both sides of the head, just posterior to the cochlea of the auditory system. There are two components in the vestibular system: the semicircular canal system and the otoliths. Each component is responsible for detecting different types of movement. The semicircular canals detect rotational movement, and the otoliths detect gravity and linear acceleration. The vestibular system is closely tied to the visual centers of the brain that control eye muscle movement as well as areas of the autonomic nervous system within the cerebellum that maintain subconscious muscle tension. When the vestibular system is malfunctioning and the subconscious muscle tension is abnormal, it may cause symptoms like motion sickness, vertigo, or uncontrolled eye movements.
Anatomy and Physiology
The vestibular system has two portions: the otolith organs and the semicircular canals. The otolith organs are two fluid-filled, round structures that detect the force of gravity, tilts of the head, and linear acceleration. The semicircular canals are made up of three half-circle, fluid-filled tubes that detect rotational and head tilt movements. Both of these organs are found in the inner ear just posterior to the cochlea. The otolith organs are found centrally between the cochlea and the semicircular canals, and the semicircular canals loop out posteriorly from the otolith. There is a set of vestibular organs located on each side of the head within the temporal bone.
The three semicircular canals detect head rotations such as nodding vertically, shaking horizontally, or movement from shoulder to shoulder. The semicircular canals also detect angular acceleration that is created by a sudden rotation, like spinning in a circle. Each canal is located in a plane that is 90 degrees to the other two semicircular canals. The hair cells are located within an enlargement at the base of each canal called the ampulla. The hair cells within the ampulla are attached to the crista, which is analogous to the organ of Corti in the cochlea. The cilia of the hair cells extend into a membrane called the cupula. When there is a head rotation, the walls of the canal and the cupula move, but the fluid movement lags behind due to inertia. The opposing movement between the fluid and the cupula bends the cilia of the hair cells in the opposite direction of the head movement. The bending motion in one direction causes an excitation of the hair cells and a release of neurotransmitters. Moving in the opposite direction inhibits neurotransmitter release. The released neurotransmitters stimulate action potentials within the vestibular nerve. The on/off arrangement of the hair cells allows the brain to detect the orientation of the head at all times. The function of the semicircular canals is easily demonstrated by spinning rapidly in a circle for 15 to 30 seconds and then stopping. The sustained motion within the semicircular canals eventually stops bending the cupula and the sensation of spinning subsides. However, once the spinning motion ceases, the fluid causes the cupula to bend in the opposite direction, which gives the sensation of motion in the opposite direction.
The otolith organs are located between the semicircular canals and the cochlea; they contain a pair of large chambers called the saccule and the utricle. These two structures within the otolith detect changes in the angle of the head and linear acceleration, which are all responses to gravity. The utricle and saccule have a sensory epithelium called the macula that is vertical in the saccule and horizontal in the utricle when the head is upright. The vestibular macula contains hair cells with their cilia projecting into a gelatinous membrane called the otolith membrane. Otolith means “ear stone” in Greek, and within the otolith membrane are many tiny calcium carbonate stones (1—5 micrometers in diameter), the otoliths. These stones are heavier than the surrounding fluid and membrane, so gravity pulls them down. The weight of the otoliths being pulled down by gravity also causes the membrane to be pulled down and thus bends the cilia of the hair cells. Bending in one direction stimulates an action potential in the vestibular nerves, and bending in the opposite direction inhibits an action potential. The hair cells within the macula can detect head movement in any direction due to their respective orientation within the utricle and saccule. In addition, the otolith organs on each side of the head are mirror images of each other. This means that a head tilt will result in the activation of hair cells on one side of the head, while the corresponding location on the opposite side of the head will be inactivated. Any head tilt or acceleration will result in the activation of certain hair cells and the inactivation of others. Collectively the brain can interpret these activation/inactivation patterns for all forms of head orientations unambiguously. A great example of the otolith organs at work is being on a moving sidewalk. A person will experience a sudden acceleration stepping onto the walkway and then feel a deceleration stepping off.
See also: Auditory System; Autonomic Nervous System; Balance; Cochlea; Dizziness; Meniere’s Disease; Nystagmus; Vestibulocochlear Nerve
Bear, Mark F., Barry W. Connors, & Michael A. Paradiso. (2007). Neuroscience exploring the brain (3rd ed.). Baltimore, MD: Lippincott Williams & Wilkins.
Gray, Lincoln. (2013).Vestibular system: Structure and function (Chap. 10). Retrieved from http://neuroscience.uth.tmc.edu/s2/chapter10.html
Watson, Mary Ann, & F. Owen Black. (2013). The human balance system. Retrieved from http://vestibular.org/understanding-vestibular-disorder/human-balance-system