semicircular canals

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Semicircular Canals


canals of the inner ear of vertebrate animals and man that help regulate equilibrium and body orientation. Fish, terrestrial vertebrates, and man have three semicircular canals arranged in three mutually perpendicular planes. There is a horizontal (in man, lateral) semicircular canal and two vertical ones—the anterior and the posterior. In fossil Agnatha and extant cyclostomes and petromyzons there are only two vertical semicircular canals, anterior and posterior. In Myxini there is one posterior canal.

The membranous labyrinth of the ear is filled with fluid endolymph and is located within osseous sheaths; the space between the membranous and the osseous labyrinth is filled with perilymph. Each semicircular canal ends in two branches, one of which is dilated to form an ampulla. The two adjacent branches of the vertical semicircular canals are united into a single common branch in most vertebrates; the semicircular canals open into the vestibule by five openings.

Each ampulla of the semicircular canals has sensory hair cells, grouped in a crista, or crest. Over the crista is a gelatinous cupula, into which these hair cells enter. The bases of the sensory cells are entwined with fibers of the vestibular nerve, which transmit excitation to the brain. Changes in the position of the head or body, accompanied by shifting of the cupulae and endolymph, and by displacement of the otoliths suspended in the endolymph, cause stimulation of the hairs and of the sensory cells of the semicircular canals. This leads to the transmission of an electrical discharge along the nerve and into the brain. From the brain the nerve impulses are directed to the muscles and the other organs that regulate the body’s orientation.


semicircular canals

semicircular canalsclick for a larger image
The two sets of three canals in the human ear, which are at right angles to each other. The set of semicircular canals acts to sense angular rotation about each of the three axes. Each of the canals is filled with fluid, and there is a flaplike structure called a cupula at the end of each canal.
These canals are the main vestibular organs that give us a sense of spatial orientation in the absence of visual clues. The system is unable to sense small angular velocities and thus is nearly useless in flight, often sending false signals to the brain.
References in periodicals archive ?
One inner ear entity has gained recognition as a cause of CHL--namely superior semicircular canal dehiscence (SSCD).
Rotational vertigo is seen in patients with diseases of the semicircular canals or their central connections, or damage to the otoliths resulting in linear vertiginous sensations or illusions of body tilt.
MRI demonstrated T1-weighted hyperintensity and a complete loss of CISS signal in all three turns of the cochlea (figure 2, B), in the vestibule, and in the semicircular canals on the left.
When you move your head, fluid in the semicircular canals moves around.
Almost in every case where there is a fossilized skull, the semicircular canals are present and well preserved," Timothy Ryan, assistant professor of anthropology, geosciences and information sciences and technology, Penn State, said.
This illusion is the result of a misperception of the magnitude and direction of rotation due to stimulation of the sensors in the semicircular canals of the middle ear.
The semicircular canals and the visual and skeletal systems have specific functions that determine an individual's orientation.
This vestibulo-ocular reflex does not depend on the visual system (which has a "slow" response time of 1/10th of a second), but on the semicircular canals of the inner ear (which respond in 1/100th of a second).
According to the researchers, the nauseating spinning sensation of vertigo is caused by free-floating particles in the inner ear that move within the ear's semicircular canals when a person moves his head.
Bilateral vestibulopathy (BVH) occurs when the balance system associated with the semicircular canals of the inner ear are damaged or destroyed.
The independent development of the individual semicircular canals in relation to the cochlea and vestibule is a complex embryologic process rather than merely a case of arrested development.