The vestibular system is found bilaterally within the inner ears and performs tasks to help maintain the orientation of the body with respect to gravity and forces exerted from the outside world. It provides the leading contribution about the sense of balance and spatial orientation to help with the coordination of movement with balance. The vestibular system consists of the semicircular canal system (to detect rotational movements of the head) and the otolithic organs (to detect linear movement and acceleration of the head).
The word “otolith” means “ear stone,” and the saccule is one of the two otolithic organs within the inner ear. The other otolithic organ is the utricle. Both otolithic organs are part of the balancing apparatus located in the vestibule of the bony labyrinth, which is a small oval chamber that consists of the vestibule (a swelling next to the semicircular canals), three separate semicircular canals, and the cochlea (necessary for the sense of hearing). Within the vestibule is the utricle, which is located between the semicircular canals and the cochlea. The saccule is closer to the cochlea compared to the utricle.
Within the saccule are small stones (called otoconia that consist of calcium carbonate and a matrix protein) and a viscous fluid that are used to stimulate the sensory hair cells that line the saccule’s epithelial layer. It is the bending of the stereocilia on the sensory hair cells that detect motion and orientation of the head. The saccule specifically detects linear movement in the vertical direction. It also detects the effect of gravity on the head and body.
Comparatively, the saccule is the smaller of the two otolithic organs and is globular in shape. The saccule lies in the recessus sphaericus (the spherical recess) of the bony labyrinth, near the opening of the scala vestibule of the cochlea. Within the anterior portion of the saccule is a thickening called the macula acustica sacculi, which contains filaments of the vestibular portion of the vestibulocochlear nerve (cranial nerve VIII) and the sensory hair cells that detect changes in acceleration and gravitational force. Superior to the macula acustica sacculi is the otolithic membrane, which contains a gelatinous layer, and superior to that the statoconia layer. The statoconia layer is a bed of otoconia. Each sensory hair cell has stereocilia (mechanoreceptors that respond to changes in pressure) and a true kinocilium (the only sensory cilium that can depolarize and produce an action potential) at its apical end. The tips of the stereocilia and kinocilium are embedded within the otolithic membrane.
The saccule has mechanoreceptors that can distinguish between the different degrees that the head tilts, particularly in the vertical direction. Because of gravity, the otolithic organ pulls on the embedded stereocilia and causes them to tilt. This shift in the direction of the stereocilia stimulates the kinocilium and induces an action potential that is sent to cranial nerve VIII, to the brainstem, and then to the brain. The brain interprets all head movements by comparing inputs from the direction of the tilt, detected by the kinocilium, to inputs from the eyes and stretch receptors in the neck. By doing so, the brain can determine if just the head is tilted or if the entire body is tilted.
Disorders of the Otolithic Organs
Damage to the otolithic organs can have an impact on the ocular (vision) function and body stabilization. Until recently, there has not been a way to measure how severely these organs could be damaged. Recent studies have developed the Vestibular Evoked Myogenic Potential (VEMP) test, which can determine the health of the saccule as it is inferior to the utricle and more proximal to the cochlea. To date, VEMP tests are employed to quantify otolithic organ input to the right and left vestibular systems. The purpose of VEMP is to determine if the saccule and the inferior vestibular nerve are functioning properly and are intact. The output of the saccule can be recorded using a sound generator. In addition, surface electrodes can be used to detect neck muscle activation or other muscles of interest.
See also: Otoliths; Semicircular Canals; Stereocilia; Utricle; Vestibular System; Vestibulocochlear Nerve
Drummond, Meghan C., et al. (2015). Live-cell imaging of actin dynamics reveals mechanisms of stereocilia length regulation in the inner ear. Nature Communications, 6, 6873. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4411292/
Hain, Timothy C. (2014). Otoliths. Dizziness-and-balance.com. Retrieved from http://www.dizziness-and-balance.com/anatomy/ear/otoliths.html
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