Synonyms
Definition
Small calcium carbonate particles that compose the otolithic membrane.
The Saccule
The otolithic membrane covers the utricular macula which is a sensory organ endowed of hair cells and cilia (Seikel et al. 2010). The saccule has a globular-shaped sac which is located near the scala vestibuli in the vestibule (Lee et al. 2016). It is connected anteriorly to the cochlear duct by the ductus reuniens and posteriorly to the endolymphatic duct through the utriculosaccular duct (Lee et al. 2016). The saccule is connected with the cochlea through the miniscule ductus reuniens (Seikel et al. 2010).
The Utricle
The utricle is larger than the saccule, and is located posterosuperiorly to the saccule in the elliptical recess of the medial wall of the vestibule. It is connected anteriorly through the utriculosaccular duct to the endolymphatic duct. The three semicircular canals are opened into it through five openings. The posterior and the superior semicircular canals have a single opening at the crus commune. The macula of the utricle lies in the horizontal plane and located in the utricular recess, which is the anterior part of the utricle (Lee et al. 2016).
Both the saccule and utricle are connected through the endolymphatic duct, which is implanted in the dura mater. Since otoliths are denser than endolymph, gravitational forces help to deflect the stereocilia of the hair cells when the head is not moving (Hain and Helminsky 2007). The linear motion or the tilting of the head creates inertial drag and cropping force between the otolithic membrane and the macular surface, having as a result the bending of the hair cells (Oghalai and Brownell 2012) which helps to keep a person balanced while moving or staying constant. In other words, the saccule senses up and down movement and the utricle senses forward and backward movement, left and right movement, or a combination of the two (Lee et al. 2016).
Functions of the Otoliths
The functions of the otolith organs vary. Clarke et al. (2003) have listed in their study a list with citations of researchers who have studied the functions of the otoliths. Firstly, they state that the otolith signals have a vestibulo-ocular function. Specifically, there is an ocular counterrolling when the head tilts; a linear vestibulo-ocular reflex (to stabilize the images in the eyes while the head moves) during the translation of the otolithic signals; a dynamic ocular-counterrolling; inertial processing of vestibulo-ocular signals; vergence control during linear movements; and a modulation of velocity storage that leads to interaction between the semicircular canals and the otolith organs.
Secondly, the otolithic organs serve a vestibulo-spinal function. Specifically, they help to the maintenance of postural tonus, and there is a vestibulo-spinal readaptation after spaceflight (Clarke et al. 2003).
Thirdly, the otolithic organs serve a vestibulo-autonomic function. Specifically, they help with vestibular autonomic regulation, post-spaceflight orthostatic intolerance, and vestibular control of the sympathetic activity (Clarke et al. 2003).
Fourthly, the otoliths serve a gravitational function, where they help with oculogravic illusion and determination of subjective vertical and horizontal axis. They, also, serve to the perception of spatial orientation and the cognitive aspects of it (Clarke et al. 2003).
Conclusion
Otoliths are the small calcium carbonate particles that compose the otolithic membrane. They are known as the saccule and utricle, and they serve a variety of functions that help with balance and posture.
Cross-References
References
Chang, R., & Khana, S. (2013). Anatomy of the vestibular system: A review. NeuroRehabilitation, 32, 437–443. https://doi.org/10.3233/NRE-130866.
Clarke, A. H., Schönfeld, U., & Helling, K. (2003). Unilateral examination of utricle and saccule function. Journal of Vestibular Research, 13(4–6), 215–225. Retrieved from https://www.researchgate.net/profile/Kai_Helling/publication/228727499_Unilateral_examination_of_utricle_and_saccule_function/links/54fb01ea0cf2859b88579a17/Unilateral-examination-of-utricle-and-saccule-function.pdf.
Hain, T. C., & Helminsky, J. O. (2007). Anatomy and physiology of the normal vestibular system. In Vestibular rehabilitation (3rd ed., p. 214). Philadelphia: FA Davis Company.
Lee, S. C., Razek, O. A., & Dorfman, B. E. (2016). Vestibular system anatomy. In A. Meyers, F. Talavera, & P. Roland (Eds.), Medscape. Retrieved from https://emedicine.medscape.com/article/883956-overview.
Oghalai, J. S., & Brownell, W. E. (2012). Chapter 44. Anatomy & physiology of the ear. In A. Lalwani (Ed.), Current diagnosis & treatment in otolaryngology – Head & neck surgery (3rd ed.). New York: McGraw-Hill Global Education Holdings, LLC.
Seikel, A. J., King, D. W., & Drumright, D. G. (2010). Chapter 9: Anatomy of hearing. In Anatomy & physiology for speech, language, and hearing (4th ed., pp. 447–478). New York: Delmar Cengage Learning.
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Khalil, M. (2018). Otolith. In: Shackelford, T., Weekes-Shackelford, V. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-16999-6_988-1
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DOI: https://doi.org/10.1007/978-3-319-16999-6_988-1
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