The utricle is one of the two otolithic organs within the inner ear. The other otolithic organ is the saccule. Both of the 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 utricle are small stones (called otoconia, consisting of calcium carbonate and a matrix protein) and a viscous fluid that are used to stimulate the sensory hair cells that line the utricle’s epithelial layer. It is the bending of the stereocilia on the sensory hair cells that detects motion and orientation of the head. The utricle specifically detects linear accelerations such as coming to an abrupt stop in a car, or tilting the head on the horizontal plane.
Comparatively, the utricle is larger than the saccule and is an oblong shape. It is compressed and occupies the upper and posterior part of the vestibule. The utricle makes a true contact with the recessus ellipticus labyrinthi ossei (also known as the elliptical recess of the bony labyrinth), which helps maintain its structure. Within the utricle is the macula utricle, which is a thickening on the wall and the epithelium that lines the utricle. The epithelium contains the sensory hair cells that detect changes in acceleration. Superior to the epithelium 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 utricle has mechanoreceptors that can distinguish between the different degrees that the head tilts. 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 from the vestibular portion of the vestibulocochlear nerve (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 ocular (vision) 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. More research is needed to improve this technique as well as to determine how to test the integrity of the utricle.
See also: Saccule; Semicircular Canals; Stereocilia; 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
Purves, Dale, et al. (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.