Timothy C. Hain,
Last edited: 7-02. Please read our disclaimer.
Orientation of the otolithic organs Function Turnover of otoconia Education Index
The utricle is one of two "otolithic organs" in the human ear, the utricle and saccule. On the diagram above, the utricle are located in the vestibule, between the semicircular canals (5), and the cochlea (9).
This figure shows a closeup of the inner ear. The utricle is contained within a swelling adjacent to the semicicircular canals, and the saccule is close to the cochlea. The black dots surrounding the utricle and saccule are the dark cells.
The otolith organs sense gravity and linear acceleration such as from due to initiation of movement in a straight line. Persons or animals without otolith organs are imbalanced. The schematic diagram above illustrates how they work. A set of hair cells are coupled to a mass of stones. When the stones accelerate, with respect to the hairs, they exert a shearing force on the hairs. This force is detected by the hair cells and sent to the brain via branches of the vestibular nerve. The utricle sends input to the brain via the superior division of the nerve, and the saccule, via the inferior division. There is considerably more complexity to the organization of the utricle and saccule, including different types of hair cells and detail to the sensory macule (patch of sensory cells) that we have omitted.
The otolithic organs sense motion according to their orientation. The utricle is largely horizontal in the head, and largely registers accelerations acting in the horizontal plane of the head (called the axial plane by radiologists). The saccule is largely vertical, actually parasagittal, in the head, and registers accelerations in the vertical plane (called parasaggital or coronal plane).
The motion sensation from the otoliths is used for a large number of reflexes:
Damage to the otoliths or their central connections can impair ocular and body stabilzation (Lempert et al, 1997).
Turnover: The otoconia are made of calcium carbonate (CaCo3) combined with a protein matrix protein. Calcium carbonate is also a constituent of "limestone", so otoconia are literally small stones. They have a hexagonal symmetry. In birds and mammals, the "calcite" form of calcium carbonate is used. The matrix protein in birds in mammals is called otoconin-90. Otoconia of humans are very small -- ranging in size from roughly 3 to 30 microns. Without a high-resolution microscope, one would not be able to see the crystalline structure. Otoconia of fish may be much larger.
Otoconia are initially formed early in life during embryogenesis and their formation is completed in early postembryonic development. The outer, calcium containing part of the otoconia clearly turns over. It is not clear if the inner part can be replaced in humans. This is important because the outer layer of otoconia are only loosely attached and can fall off into the inner ear. When they do this, they are probably dissolved and reabsorbed by the "dark cells" of the labyrinth (Lim, 1973, 1984), which are found adjacent to the utricle and the crista, although this idea is not accepted by all (see Zucca, 1998, and Buckingham, 1999). Progressive degenerative changes of aging otoconia and also damage to otoconia mediated through ototoxic drugs such as streptomycin have been well documented.
In Benign Paroxysmal Positional Vertigo (BPPV) , a common vertigo condition, dizziness is thought to be due to debris, probably otoconial, which has collected within a part of the inner ear. There is no particular reason why otoconial matrix, or fragments of other parts of the inner ear might not accumulate, but it would seem likely that such debris would be rapidly disposed, while small stones (otoconia), might persist. Some controversy exists about this as some authors (e.g. Zucca et al), feel that stones might dissolve rapidly. Some authors (e.g. Buckingham) reject the entire idea that otoconia are the cause of BPPV, but nevertheless, the present consensus opinion is that the loose otoconia are the cause of BPPV.
The illustrations in this document were funded by a grant to the department of Otolaryngology at Northwestern University, from the National Institutes of Health. These images are used with their permission. If others wish to use them, please contact Northwestern University.
The text is presently due entirely to Dr. Hain's work. We welcome participation of other clinicians or scientists in updating this document.
(c) 2002 Timothy C. Hain, firstname.lastname@example.org