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The internal ear, with "semicircular ducts" at left
The internal ear, with "semicircular ducts" at left
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The anterior and posterior semicircular ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about 30-degree angle from the horizontal plane. The orientations of the ducts cause a different duct to be stimulated by rotation of the head in different planes. Thus, the horizontal canal detects horizontal head movements (such as when you spin in a rotating chair), while the superior and posterior canals detect vertical head movements (such as when you bend forward to pick something up from the floor).
The semicircular canals are a component of the bony labyrinth. Among species of mammals, the size of the semicircular canals is correlated with their type of locomotion. Specifically, species that are agile and have fast, jerky locomotion have larger canals relative to their body size than those that move more cautiously.
The lateral or horizontal canal (external semicircular canal) is the shortest of the three canals. Movement of fluid within this canal corresponds to rotation of the head around a vertical axis (i.e. the neck), or in other words rotation in the transverse plane. This occurs, for example, when you turn your head to the left and right hand sides before crossing a road
It measures from 12 to 15 mm., and its arch is directed horizontally backward and lateralward; thus each semicircular canal stands at right angles to the other two. Its ampullated end corresponds to the upper and lateral angle of the vestibule, just above the fenestra vestibuli, where it opens close to the ampullated end of the superior canal; its opposite end opens at the upper and back part of the vestibule. The lateral canal of one ear is very nearly in the same plane as that of the other; while the superior canal of one ear is nearly parallel to the posterior canal of the other.
The superior semicircular canal (anterior semicircular canal) is a part of the vestibular system and detects rotation of the head around a rostral-caudal (anterior-posterior) axis, or in other words rotation in the coronal plane. This occurs, for example, when you move your head to touch your shoulders, or when doing a cartwheel.
It is 15 to 20 mm in length, is vertical in direction, and is placed transversely to the long axis of the petrous portion of the temporal bone, on the anterior surface of which its arch forms a round projection. As part of the vestibular system it detects rotation of the head around a rostral-caudal (anterior-posterior) axis. It describes about two-thirds of a circle. Its lateral extremity is ampullated, and opens into the upper part of the vestibule; the opposite end joins with the upper part of the posterior canal to form the crus commune, which opens into the upper and medial part of the vestibule.
The posterior semicircular canal is a part of the vestibular system and detects rotations of the head in around the lateral axis, or in other words rotation in the sagittal plane. This occurs, for example, when nodding your head.
It is directed superiorly, as per its nomenclature, and posteriorly, nearly parallel to the posterior surface of the petrous bone. The vestibular canaliculus is immediately medial to it. It is part of the bony labyrinth and is used by the vestibular system to detect rotations of the head in the sagittal plane. It is the longest of the three canals, measuring from 18 to 22 mm. Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus commune.
Findings from a 2009 study demonstrated a critical late role for bmp2b in the morphogenesis of semicircular canals in the zebrafish inner ear. It is suspected that the role of bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species.
Each canal is filled with a fluid called endolymph and contains motion sensors within the fluids. At the base of each canal, the bony region of the canal is enlarged which opens into the utricle and has a dilated sac at one end called the osseous ampullae. Within the ampulla is a mound of hair cells and supporting cells called crista ampullaris. These hair cells are composed of many cilia and embedded in a gelatinous structure called the cupula. As the head rotates the duct moves but the endolymph lags behind. This deflects the cupula and bends the cilia within. The bending of these cilia alters an electric signal that is transmitted to the brain. Within approximately 25–30 seconds of constant motion, the endolymph catches up to the movement of the duct and the cupula is no longer affected, stopping the sensation of acceleration.
This adjustment period is in part the cause of an illusion known as "the leans" often experienced by pilots. As a pilot enters a turn, hair cells in the semicircular canals are stimulated, telling the brain that the aircraft, and the pilot, are no longer moving in a straight line but rather making a banked turn. If the pilot were to sustain a constant rate turn, the endolymph would eventually catch up to the ducts and cease to deflect the cupula. The pilot would no longer feel as if the aircraft was in a turn. As the pilot exits the turn, the semicircular canals are stimulated to make the pilot think that they are now turning in the opposite direction rather than flying straight and level. In response to this, the pilot will often lean in the direction of the original turn in an attempt to compensate for this illusion. A more serious form of this is called a graveyard spiral. Rather than the pilot leaning in the direction of the original turn, they may actually reenter the turn. As the endolymph stabilizes, the semicircular canals stop registering the gradual turn and the aircraft slowly loses altitude until impacting with the ground.
The semicircular ducts provide sensory input for experiences of rotary movements. They are oriented along the pitch, roll, and yaw axes.
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