Vestibular System Anatomy

Vestibular Anatomy Overview:


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    The inner ear has two primary roles. The first and most familiar role is hearing. The organ of hearing is called the cochlea. For more information regarding the anatomy of hearing please refer to the following link: The second and lesser known role of the inner ear is balance, which is controlled by what is called the vestibular system. The vestibular system plays a large role in the following: 1) maintaining balance while still, 2) maintaining balance while in motion, 3) maintaining focused vision during head and body movements. There are two types of sensory structures that makeup the vestibular system:


Semicircular Canals


There are three semicircular canals, six total if you count both ears. Each is at a 90⁰ angle to the other two canals and is named after the spatial plane in which it is oriented: anterior (front), posterior (back) and lateral (side). Each semicircular canal is filled with fluid called endolymph. When the head moves, endolymph within the semicircular canals flows, bending sensory hair cells located at the base of each canal to begin the process of transduction. The sensory organ that houses the sensory hair cells is called the cupula. The cupula is the same mass as endolymph. This means the semicircular canals are not reactive to gravity. Each semicircular canal is most sensitive to head movements within the spatial plane it is named for: the anterior canals are most sensitive to head movements forward, the posterior canals are most sensitive to head movements backwards and the lateral canals are most sensitive to rotary head movements.


For example: if you tilt your head backwards, endolymph flows in all semicircular canals; however, endolymph flow is greatest in the posterior canals, letting you know your head is moving backwards in space.


The semicircular canals use this information to not only let you know where your head is moving in     space but also to reflexively control ocular muscles in order to keep your vision focused during head     movements.


For example: if you are looking at yourself in the mirror and turn your head left, your eyes will move right at an equal but opposite velocity and degree in order to maintain focus on your reflection. This reflexive process is known as the vestibulo-ocular reflex (VOR).


Otolithic Organs


There are two otolithic organs: the utricle and the saccule. Otoliths are responsible for the detection of linear acceleration. The utricle detects horizontal acceleration and the saccule detects vertical acceleration. The otolithic organs are stimulated not by the inertial flow of endolymph, but by a shift in mass relative to gravity. Otoconia, or calcium carbonate crystals, are attached to the end of each sensory hair cell and add mass to the system. The mass provided by the otoconia makes the otolithic organs sensitive to gravity. When there is linear acceleration, otoconia will shift relative to gravity and bend the attached sensory hair cells to begin the process of transduction. The otolithic organs use this information to reflexively control neck, core and leg muscles in order to maintain upright posture. This reflexive process is known as the vestibulospinal reflex (VSR).