Otoliths are small granules of solid material that are responsible for sensing acceleration and gravity in all organisms. They are usually composed of calcium carbonate or starch. In mammals, including humans, calcite granules are located in the inner ear and regulate balance.
What are otoliths?
Otoliths are responsible for the sense of balance in mammals. They are located in the inner ear in the vestibular organ (organ of balance) and enable the perception of acceleration and gravity. Otoliths belong to the statoliths, which are found in all living things and control the position of the organism. In mammals they serve the sense of balance and in fish also the sense of hearing. In a narrower sense, they are also called otoconia in mammals. Otoconia are inorganic biomaterials made of calcium carbonate, which in this case is in the form of calcite. These calcite grains are attached to the hairs of sensory cells, where they indicate the direction to the center of the earth. In doing so, they enable the perception of tensile, compressive and shear forces. The reflexes thus triggered by the organs of equilibrium cause the organism to maintain its normal position in space. In invertebrates, statoliths are free to move, occupying the lowest point in space and thereby irritating the sensory organs located at that point.
Anatomy and structure
In mammals, including humans, the utriculus and sacculus contain otoliths of calcite crystals that are contained in a gel-like matrix. As parts of the vestibular organ (organ of balance), the utriculus and sacculus are two outpouchings of the membranous labyrinth in the inner ear. The processes of the hair cells (stereocilia) adhere to these gel clumps. In this process, the relative movements of the otoliths are transmitted to the hairs and cause sensory stimuli. The essential noncellular components above the sensory cells in the organ of equilibrium (vestibular organ) are called otoconia. They are embedded in a gel-like mass of organic material. The gel represents a protein matrix. Here, the otoliths consist of 95 percent chemical modification of calcium carbonate (calcite) and 5 percent glycoproteins and calcium-binding proteins. On the nanoscale, a mosaic structure is recognizable in human otoliths (otoconia). Therefore, they are also characterized as mosaic-controlled nanocomposites. The internal structure of otoconia is volume dense and nanostructurally ordered in the form of a dumbbell, while the external structure appears less ordered.
Function and tasks
Otoliths have the function of keeping the organism in balance. In doing so, the calcite granules embedded in the gel matrix are subject to mechanical forces such as gravity, shear and tensile forces, and compressive forces. Under the influence of these forces, the fluid is moved in the arcuate ducts (sections of the vestibular organ). From there, the motion stimuli travel through the calcite granules to the sensory cilia. The sensory cilia transmit the stimulus to the sensory cells of the vestibular organ. The vestibular organ triggers reflexes that return the body to its normal position. The movement of the calcite granules is produced by the inertia of mass when the velocity is accelerated or decelerated. This accelerating or decelerating motion would result in loss of orientation and thus balance without the organ of equilibrium and otoliths. The otoliths in the inner ear thus provide coordination of complicated movements and steadiness of the body.
Diseases
A typical disease associated with otoliths is benign paroxysmal positional vertigo (BPLS). This is a very unpleasant but still harmless form of vertigo. The word “benign” means benign. The term “paroxysmal” indicates a seizure-like condition. Thus, the vertigo occurs suddenly. It is caused by the detachment of calcite stones and their displacement into the arcades. In the arcades, the otoliths move back and forth with appropriate head movements. The suction caused by this movement irritates the arcuate receptors. These then send movement signals to the brain. However, since there are no movement signals from other sensory organs, a contradictory signal situation occurs. The brain processes this signal situation as vertigo. The movements are no longer coordinated.The vertigo attack occurs during certain movements such as turning the head, looking up or down, or lying down. The spinning vertigo attacks usually last only about 30 seconds. Some patients suffer from nausea and vomiting at the same time. If the cause is not recognized and positional vertigo is not treated, some sufferers develop avoidance behavior of certain movements. In many cases, the seizure series ends spontaneously. This is when the calcite granules have cleared from the arcades. The causes for the detachment of the otoliths are usually unknown. Since positional vertigo occurs especially in older people, degenerative processes are suspected. However, accidents and trauma can also cause detachment of calcite granules. Why women suffer from positional vertigo more often than men is also not known. To diagnose benign paroxysmal positional vertigo, the physician applies the so-called Dix-Hallpike positional test to the patient. This involves performing specific movements on the patient that provoke a vertigo attack. In addition to the vertigo attack, a nystagmus (eye tremor) also occurs at the same time. If the nystagmus does not occur, the vertigo has other causes. Positional vertigo cannot be treated with medication. Treatment is exclusively through positional exercises. In this process, a seizure is provoked. The exercises are performed until the calcite granules have cleared from the arcades. Positional vertigo is an extremely unpleasant but also harmless condition that often disappears on its own. However, it can recur again and again due to re-detachment of otoliths.
