The semicircular canals are three semicircular 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 the ...

. The three canals are the lateral, anterior and posterior semicircular canals. They are the part of the

bony labyrinth The bony labyrinth (also osseous labyrinth or otic capsule) is the rigid, bony outer wall of the inner ear in the temporal bone. It consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the ...

, a

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. (At the joints of long bones the bone's outer surface is lined with "articular cartila ...

-lined cavity on the petrous part of the

temporal bone The temporal bone is a paired bone situated at the sides and base of the skull, lateral to the temporal lobe of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples where four of the cranial bone ...

filled with perilymph. Each semicircular canal contains its respective semicircular duct, i.e. the lateral, anterior and posterior semicircular ducts, which provide the sensation of

angular acceleration In physics, angular acceleration (symbol α, alpha) is the time rate of change of angular velocity. Following the two types of angular velocity, ''spin angular velocity'' and ''orbital angular velocity'', the respective types of angular accele ...

and are part of the membranous labyrinth—therefore filled with

endolymph Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. ...

Structure

The semicircular canals are a component of the

that are at right angles from each other and contain their respective semicircular duct. At one end of each of the semicircular ducts is a dilated sac called a membranous ampulla, which is more than twice the diameter of the ducts. 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 ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about a 30-degree angle from the horizontal plane. The orientations of the ducts cause a different duct to be stimulated by movement of the head in different planes, and more than one duct is stimulated at once if the movement is off those planes. The lateral semicircular duct detects angular acceleration of the head when the head is turned and the anterior and posterior semicircular ducts detect vertical head movements when the head is moved up or down. When the head changes position, the

in the ducts 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 semicircular canals open into the vestibule by five orifices, one of the apertures being common to two of them. 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.

Lateral semicircular canal

The lateral semicircular canal (also known as horizontal or external semicircular canal) is the shortest of the three canals. Movement of fluid within its duct corresponds to rotation of the head around a vertical axis (i.e. the neck), or in other words, rotation in the

transverse plane A transverse plane is a plane that is rotated 90° from two other planes. Anatomy The transverse plane is an anatomical plane that is perpendicular to the sagittal plane and the dorsal plane. It is also called the axial plane or horizonta ...

. This occurs, for example, when one turns the head from side to side (yaw axis). 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, just above the

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

, where it opens close to the ampullated end of the anterior semicircular 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.

Anterior semicircular canal

The anterior semicircular canal (also known as superior semicircular canal) contains the 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 motor coordination, movement with balance. Together with the cochlea, a part of the auditory sys ...

that 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 divi ...

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

, 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 semicircular canal to form the ''crus osseum commune'', which opens into the upper and medial part of the vestibule.

Posterior semicircular canal

The posterior semicircular canal contains the 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 dorsal plane (also known as the coronal plane or frontal plane, especially in human anatomy) is an anatomical plane that divides the body into Anatomical terms of location#Dorsal and ventral, dorsal and ventral sections. It is perpendicular t ...

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

vestibular aqueduct At the posterior lateral wall of the temporal bone is the vestibular aqueduct, which extends to the posterior surface of the petrous portion of the temporal bone. The vestibular aqueduct parallels the petrous apex, in contrast to the cochlear ...

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

Development

Findings from a 2009 study demonstrated a critical late role for

bone morphogenetic protein 2 Bone morphogenetic protein 2 or BMP-2 belongs to the TGF-β superfamily of proteins. Function BMP-2 like other bone morphogenetic proteins, plays an important role in the development of bone and cartilage. It is involved in the hedgehog path ...

(BMP-2) in the morphogenesis of semicircular canals in the

zebrafish The zebrafish (''Danio rerio'') is a species of freshwater ray-finned fish belonging to the family Danionidae of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (an ...

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 () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

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

lamprey Lampreys (sometimes inaccurately called lamprey eels) are a group of Agnatha, jawless fish comprising the order (biology), order Petromyzontiformes , sole order in the Class (biology), class Petromyzontida. The adult lamprey is characterize ...

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. The lateral semicircular canal is oriented in the yaw axis, the anterior semicircular canal is oriented in the pitch axis, and the posterior semicircular canal is oriented in the roll axis. Each duct is filled with a fluid called

and contains motion sensors within the fluids. The base of each duct is enlarged, opening into the utricle, and has a dilated sac at one end called the membranous ampulla. 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 cel ...

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 natural tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes the velocity to change. It is one of the fundamental principles in classical physics, and described by Isaac Newto ...

. 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 (Latin: sacculus) is a bed of sensory cells in the inner ear that detects linear acceleration and head tilting in the vertical plane, and converts these vibrations into electrical impulses to be interpreted by the brain. When the he ...

. As with macular hair cells, hair cells of the crista ampullaris will depolarise when the stereocilia deflect towards the

kinocilium A kinocilium is a special type of cilium on the apex of hair cells located in the sensory epithelium of the vertebrate inner ear. Contrasting with stereocilia, which are numerous, there is only one kinocilium on each hair cell. The kinocilium can ...

. Deflection in the opposite direction results in hyperpolarisation and inhibition. In the lateral semicircular duct, ampullopetal flow is necessary for hair-cell stimulation, whereas ampullofugal flow is necessary for the anterior and posterior semicircular ducts. 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 ducts 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 ducts 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 re-enter the turn. As the endolymph stabilizes, the semicircular ducts 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 small heads, relatively short necks and slender bills that in some species feature fleshy ceres. ...

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

* * * {{Authority control Ear Sensory organs in animals Vestibular system Human head and neck Otorhinolaryngology Otology Inner ear Bones of the head and neck