The human ear is composed of both the auditory system that we use for hearing, and the vestibular system that is found within the inner ear and plays a significant role in maintaining balance, helping us with spatial orientation, and maintaining a sense of where we are within a three-dimensional space (proprioception). The vestibular system is composed of two primary components that include the semicircular canal system, which provides information about rotational movements of our heads, and the otoliths, which are small stones of calcium carbonate within the semicircular canals and also play a role in providing information about rotation as well as linear acceleration. The vestibular system is also integrated very closely with the muscles that control eye movement and the general body muscles that help us maintain an upright position.
Anatomy and Physiology of the Otolithic System
The vestibular system is composed of several structures including three semicircular canals, which are filled with a viscous fluid that moves within the canals as our heads move. These three semicircular canals known as the horizontal, anterior, and posterior canals connect with two other structures critical to balance known as the utricle and the saccule. In humans, the utricle is oriented in a horizontal direction and detects motion in a horizontal plane, while the saccule is oriented in a vertical direction and detects motion in a vertical plane. Both of these structures contain special “hairs” whose tips are embedded in the otolithic membrane of the inner ear. In addition to these hairs or “stereocilia,” there are small stones made of calcium carbonate called otoliths that are embedded in the membrane of the utricle and saccule. As individuals move their heads from one position to another, the otoliths move in response to positional change and/or acceleration and in doing so, create a sheering force across the stereocilia, stimulating the vestibular system. This information is sent to the brain via the vestibulocochlear nerve for further integration with information from the eyes and muscles of the neck.
There are a number of reflexes in the body that are associated with the vestibular system and otoliths in particular. These include, first, the otolith-ocular reflexes, which include (1) linear nystagmus, (2) a condition known as ocular counter-roll where the eyes rotate around a specific line of sight in the opposite direction when the head is tilted to the side, and (3) off-vertical axis nystagmus, which is induced when the head is moved in a lateral direction. Second, the otolith-body reflexes include the righting reflex, which allows us to respond to temporary loss of balance and maintain an upright position; and third are the otolith-canal interactions.
Pathologies of the Vestibular System
There are a number of different pathologies associated with the vestibular system that involve the various structures within this system. Virtually all of them are capable of producing vertigo (defined as a sensation of motion in which the individual’s surroundings seem to move even when the person is not moving), difficulty maintaining balance, and significant nausea and vomiting. The three most common diseases of the vestibular system include labyrinthitis, an infection of the inner ear, Meniere’s disease, and benign paroxysmal positional vertigo or BPPV. Benign paroxysmal positional vertigo is most likely caused by the abnormal movement or dislodging of the otoliths within the utricle and saccule of the inner ear. It is thought that as these stones become dislodged or break into smaller pieces, some of those pieces migrate over time into the semicircular canals and disrupt the normal flow of fluid within the canals. This creates a sense of vertigo that patients often describe. The most common location where these stones collect is within the posterior canal. Vertigo can be brought on by rapid movement of the head such as looking up or down, rolling over in bed, or rapidly tilting the head from side to side. BPPV is most commonly treated using a number of different repositioning maneuvers such as the Epley maneuver, the Semont maneuver, or the Brandt-Daroff exercises. The most common of these is the Epley maneuver, which uses deliberate movement of the head and gravity to reposition the otoliths within the utricle and saccule. In addition to these procedures, BPPV can be treated with more traditional medications such as antihistamines to reduce the fluid within the ear, and anticholinergic drugs such as meclizine or scopolamine, which help reduce the sense of dizziness and nausea. There are some surgical procedures that can be done to treat BPPV, but they are reserved as a last-choice option and not frequently performed.
Charles A. Ferguson
See also: Saccule; Semicircular Canals; Utricle; Vestibular System; Vestibulocochlear Nerve
Hain, Timothy C. (2010). Otoliths. Retrieved from http://www.dizziness-and-balance.com/disorders/bppv/otoliths.html
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