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.

G. N. SIMKIN

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 ?
7) Hall et al suggested that ampullary stimulation by these loose particles causes vertigo and nystagmus when the head is moved in the same plane as the posterior semicircular canal.
The presence of a burst of rotary nystagmus, which is counterclockwise at the right side and clockwise at the left side in the Dix-Hallpike test and subsides within seconds, indicated a typical posterior semicircular canal BPPV.
They found that the inner ears of living cheetahs differ markedly from those of all other felids alive today, with a greater overall volume of the vestibular system and longer anterior and posterior semicircular canals.
Right posterior semicircular canal BPPV was diagnosed, and Epley maneuver was the treatment of choice as the canalith-repositioning maneuver.
It also enables the individual evaluation of each semicircular canal (5).
Radiological signs indicating cholesteatoma in the attic included erosion or destruction of scutum or spur (the lateral wall of the attic), widening of the aditus ad antrum (loss of figure of "8" appearance) , antral wall erosions and widening , ossicular erosions and destruction , medial attic wall erosion which may lead to facial nerve canal involvement, which may cause paresis or paralysis , erosion of the lateral semicircular canal which may result into fistula formation (Figure 1), dehiscence of tegmen tympani , dehiscence of sigmoid sinus plate, erosion of the external auditory canal (EAC), and automastoidectomy.
BPV can result from canalolithiasis where the otoconia are freely floating in the duct of the semicircular canal or cupulolithiasis where the otoconia are adherent to the cupula.
One inner ear entity has gained recognition as a cause of CHL--namely superior semicircular canal dehiscence (SSCD).
The cause of BPPV is mechanical, owing to the movement of loose otoconial particles within one semicircular canal, most often the posterior one.
It could be as simple as a puppy's ear canals or semicircular canals still being too small to handle the rapid shifts in direction or the acceleration of the car, or it could have to do with what the puppy was exposed to in his first eight weeks like rolling play with littermates or being handled upside down, which cultivates the still-developing vestibular centers in the brain.
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.
There are two main components of the vestibular system, the semicircular canals and the otolith organs.