The semicircular canals or semicircular ducts are three

semicircular In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, radians, or a half-turn). It has only one line o ...

, interconnected tubes located in the innermost part of each ear, the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in th ...

. The three canals are the horizontal, superior and posterior semicircular canals.

Structure

The semicircular canals are a component of the bony labyrinth that are at right angles from each other. At one end of each of the semicircular canals is a dilated sac called an osseous ampulla, which is more than twice the diameter of the canal. Each ampulla contains an ampullary crest, the crista ampullaris which consists of a thick gelatinous cap called a cupula and many

hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...

s. The superior and posterior semicircular canals are oriented vertically at right angles to each other. The lateral semicircular canal is about a 30-degree angle from the horizontal plane. The orientations of the canals cause a different canal to be stimulated by movement of the head in different planes, and more than one canal is stimulated at once if the movement is off those planes. The lateral canal detects angular acceleration of the head when the head is turned and the superior and posterior canals detect vertical head movements when the head is moved up or down. When the head changes position, the endolymph in the canals lags behind due to inertia and this acts on the cupula which bends the cilia of the hair cells. The stimulation of the hair cells sends the message to the brain that acceleration is taking place. The ampullae open into the vestibule by five orifices, one of the apertures being common to two of the canals. 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.

Horizontal semicircular canal

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 The transverse plane (also known as the horizontal plane, axial plane and transaxial plane) is an anatomical plane that divides the body into superior and inferior sections. It is perpendicular to the coronal and sagittal planes. List of cli ...

. This occurs, for example, when one turns the head from side to side. It measures from , and its arch is directed horizontally backward and laterally; 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 Vestibule or Vestibulum can have the following meanings, each primarily based upon a common origin, from early 17th century French, derived from Latin ''vestibulum, -i n.'' "entrance court". Anatomy In general, vestibule is a small space or cavity ...

, just above the

oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...

, where it opens close to the ampullated end of the

superior canal The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...

; 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.

Superior semicircular canal

The ''superior or anterior semicircular canal'' is a part of the

vestibular system The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitut ...

and detects rotations of the head in around the lateral axis, that is, rotation in the

sagittal plane The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...

. This occurs, for example, when nodding one's head. It is in length, is vertical in direction, and is placed transversely to the long axis of the

petrous part of the temporal bone The petrous part of the temporal bone is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and occipital bones. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three an ...

, on the anterior surface of which its arch forms a round projection. 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.

Posterior semicircular canal

The ''posterior semicircular canal'' is a part of the vestibular system that detects rotation of the head around the antero-posterior (sagittal) axis, or in other words, rotation in the

coronal plane The coronal plane (also known as the frontal plane) is an anatomical plane that divides the body into dorsal and ventral sections. It is perpendicular to the sagittal and transverse planes. Details The coronal plane is an example of a longitud ...

. This occurs, for example, when one moves the head to touch the shoulders, or when doing a cartwheel. It is directed superiorly, as per its nomenclature, and posteriorly, nearly parallel to the posterior surface of the petrous bone. The

vestibular aqueduct At the hinder part of the medial wall of the vestibule is the orifice of the vestibular aqueduct, which extends to the posterior surface of the petrous portion of the temporal bone. It transmits a small vein, and contains a tubular prolongation ...

is immediately medial to it. The posterior canal is part of the bony labyrinth and is used by the vestibular system to detect rotations of the head in the coronal plane. It is the longest of the three canals, measuring from . Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the

crus commune Crus can refer to: *''Crus'', a subgenus of the fly genus '' Metopochetus'' *Crus (lower leg) *Crus, a plural of Cru (wine) *CRUs, an abbreviation of Civil Resettlement Units * Rektorenkonferenz der Schweizer Universitäten (CRUS; English: Recto ...

Development

Findings from a 2009 study demonstrated a critical late role for BMP-2b in the morphogenesis of semicircular canals in the

zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...

inner ear. It is suspected that the role of BMP-2 in semicircular canal duct outgrowth is likely to be conserved between different

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

species. Additionally, it has been found that the two semicircular canals found in the

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

inner ear are developmentally similar to the superior and posterior canals found in humans, as the canals of both organisms arise from two depressions in the

otic vesicle Otic vesicle, or auditory vesicle, consists of either of the two sac-like invaginations formed and subsequently closed off during embryonic development. It is part of the neural ectoderm, which will develop into the membranous labyrinth of the in ...

during early development. These depressions first form in lampreys between the 11 and 42 millimeter larval stages and form in zebrafish 57 hours post-fertilization

Function

The semicircular ducts provide sensory input for experiences of rotary movements. They are oriented along the pitch, roll, and yaw axes. 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 have many cytoplasmic projections on the apical surface called

stereocilia Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...

which are embedded in a gelatinous structure called the cupula. As the head rotates, the duct moves, but the endolymph lags behind owing to

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...

. This deflects the cupula and bends the stereocilia within. The bending of these stereocilia alters an electric signal that is transmitted to the brain. Within approximately 10 seconds of achieving constant motion, the endolymph catches up with the movement of the duct and the cupula is no longer affected, stopping the sensation of acceleration. The specific gravity of the cupula is comparable to that of the surrounding endolymph. Consequently, the cupula is not displaced by gravity, unlike the otolithic membranes of the utricle and

saccule The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves verticall ...

. As with macular hair cells, hair cells of the crista ampullaris will depolarise when the stereocilia deflect towards the kinocilium. Deflection in the opposite direction results in hyperpolarisation and inhibition. In the horizontal canal, ampullopetal flow is necessary for hair-cell stimulation, whereas ampullofugal flow is necessary for the anterior and posterior canals. This adjustment period is in part the cause of an illusion known as "

the leans The leans is the most common type of spatial disorientation for aviators. Through stabilization of the fluid in the semicircular canals, a pilot may perceive straight and level flight while actually in a banked turn. This is caused by a quick ret ...

" 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 with 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 In aviation, a graveyard spiral is a type of dangerous spiral dive entered into accidentally by a pilot who is not trained or not proficient in flying in instrument meteorological conditions (IMC).Federal Aviation Administration. 2016. Aeromedica ...

. 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 impact with the ground.

History

Jean Pierre Flourens, by destroying the horizontal semicircular canal of

pigeon Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...

s, noted that they continue to fly in a circle, showing the purpose of the semicircular canals.

References

Additional images

File:Gray913.png, Coronal section of right temporal bone. File:Gray923.png, The cochlea and vestibule, viewed from above. File:Gray902.png, Transverse section through head of fetal sheep, in the region of the labyrinth. X 30. File:Gray922.png, Position of the right bony labyrinth of the ear in the skull, viewed from above.

External links

* https://web.archive.org/web/20070616133210/http://www.bio.psu.edu/people/faculty/strauss/anatomy/nerv/innerear2.htm * https://www.ncbi.nlm.nih.gov/books/NBK10863/ * http://www.britannica.com/EBchecked/topic/175622/human-ear/65037/Vestibular-system?anchor=ref531828 {{Authority control Ear Sensory organs Vestibular system Human head and neck