Functions of Semicircular Canals

The semicircular canals are three fluid-filled tubes in the inner ear that help you maintain balance.  These are about two-thirds of a circle and are located in the temporal bone. They are arranged at right angles to each other and are superoposterior to the vestibule. Each canal has a bulbous expansion at its base called the ampulla. The ampulla contains the sensory epithelium, or crista, which contains hair cells. These hair bundles extend out of the crista into a gelatinous mass called the cupula. The canals are filled with endolymph. 

1.0Semicircular Canals Location

• The anterior semicircular canal, also called the “superior” canal, is vertically positioned in a manner dividing the right and left parts of the body.
• The lateral semicircular canal is the shortest of the three and is angled at about 30 degrees to the horizontal plane, which is why it’s sometimes called the “horizontal” canal.
• The posterior semicircular canal is oriented on the frontal plane, which vertically divides the front and back sides of the body.
•  It is also known as the “inferior” semicircular canal.

2.0Functions of Semicircular Canals

• The semicircular canals work together to sense rotational and angular movements of the head. They sense motion via the ampulla. 
• The ampulla contains a fluid known as endolymph, which is also present in the crista, (a cone-shaped structure lined with hair cells and supporting cells). 
• The hair cells form stereocilia, protrusions that extend from the crista. 
• The longest of the stereocilia are the kinocilia, which point in a single direction and are sensitive to movement. 
• The kinocilia extend from the crista into the cupula,( a jellylike mass that surrounds the hair cells completely,) separating them from the endolymph.

• The ampulla of the posterior semicircular canal showing the crista, hair bundles, and cupula. The cupula is distorted by the fluid in the membranous canal when the head rotates.
• The crista is flexible. When the head moves, endolymph pushes the cupula one way or the other, which in turn displaces the hairs and alters the electrical potential of the hair cells. 
• A change in potential is linked to the flow of potassium ions through mechanically gated ion channels in stereocilia. 
• When movement causes stereocilia to bend toward the kinocilium, ion channels open, allowing for the flow of potassium ions into the cell and resulting in depolarization; when movement causes stereocilia to bend away from the kinocilium, ion channels close, preventing potassium ions from entering and resulting in hyperpolarization. 
• The change in polarization causes the corresponding neurons to send impulses to the central nervous system. The information transmitted by the neural impulses enables agile mobility of the body and head and stability of vision while moving. 
• Each semicircular canal in one ear functions in tandem with a canal in the opposite ear. The left anterior canal pairs with the right posterior canal; likewise, the right anterior canal pairs with the left posterior canal. The lateral canals also form a pair. 
• These relationships allow the vestibular apparatus to perceive the orientation of the head in three-dimensional space and enable the visual system to fixate on a point while the head is moving.

3.0Functional organization of the semicircular canals

• The position of the cupula without angular acceleration. 
• Distortion of the cupula during angular acceleration. When the head is rotated in the plane of the canal (arrow outside canal), the inertia of the endolymph creates a force (arrow inside the canal) that displaces the cupula. 
• Arrangement of the canals in pairs. The two horizontal canals form a pair; the right anterior canal (AC) and the left posterior canal (PC) form a pair; the left AC and the right PC form a pair.
• Each semicircular canal works in concert with a partner located on the other side of the head, which has its hair cells aligned oppositely. 
• There are three such pairs: the two pairs of horizontal canals, and the superior canal on each side working with the posterior canal on the other side . 
• Head rotation deforms the cupula in opposing directions for the two partners, resulting in opposite changes in their firing rates . For example, the orientation of the horizontal canals makes them selectively sensitive to rotation in the horizontal plane. 
• More specifically, the hair cells in the canal towards which the head is turning are depolarized, while those on the other side are hyperpolarized. 
• For example, when the head turns to the left, the cupula is pushed toward the kinocilium in the left horizontal canal, and the firing rate of the relevant axons in the left vestibular nerve increases. 
• In contrast, the cupula in the right horizontal canal is pushed away from the kinocilium, with a concomitant decrease in the firing rate of the related neurons. If the head movement is to the right, the result is just the opposite. 
• This push-pull arrangement operates for all three pairs of canals; the pair whose activity is modulated is in the rotational plane, and the member of the pair whose activity is increased is on the side toward which the head is turning. 
• The net result is a system that provides information about the rotation of the head in any direction.

4.0Disorders

• There are several disorders of the semicircular canals, with varying impacts on hearing and balance. 
• Superior semicircular canal dehiscence (SSCD), for example, is caused by thinning or lack of the otic bone that makes up the exterior of the semicircular canal. Patients with SSCD may experience amplified body sounds, such as those caused by eye movement and heartbeat. 
• Another disorder that can affect the semicircular canals is Ménière disease, in which abnormal buildup of endolymph in the labyrinth causes symptoms such as a feeling of congestion in the ear, hearing loss, and tinnitus. 
• Affected individuals often experience muffled hearing or tinnitus followed by dizziness; the sense of vertigo may be so strong as to trigger falls.