The semicircular canals are found within the vestibular apparatus in the inner ear and are at right angles to each other. There are two sets of semicircular canals, one in the left inner ear and one in the right. The function of the semicircular canals is to sense head rotations in the X-, Y-, and Z-axes. These directions are called the yaw, pitch, and roll axes in flight. There are three semicircular canals, one for each axis, and each is a tiny fluid-filled tube attached to a larger, bony region containing the utricle and saccule. The canals are named for their orientation within the vestibular apparatus: horizontal, superior, and posterior semicircular canals. Together, these canals help animals maintain their balance by detecting acceleration in three perpendicular planes. The hair cells in the semicircular canals are similar to those in the organ of Corti (used for hearing); however, they detect movements of the fluid in the canals due to angular acceleration. The semicircular canals are connected to the vestibulocochlear nerve (the eighth of 12 paired cranial nerves) in the inner ear.
Anatomy and Physiology
The semicircular canals are part of the bony labyrinth in the skull and are different sizes. The horizontal semicircular canal (also known as the lateral or external semicircular canal) is the shortest canal (about 12 to 15 millimeters in length) and senses movement and rotation on a transverse plane or around a vertical basis—rotating the head to the left and then to the right, as if shaking your head “no.” The superior semicircular canal (also known as the anterior semicircular canal) is usually 15 to 20 millimeters in length and detects rotations on a sagittal plane—as if nodding your head up and down to answer “yes.” The posterior semicircular canal is the longest of the three canals (about 18 to 22 millimeters in length) and detects head rotations on the rostral-caudal (anterior-posterior) axis—bending your head to touch your ear to your shoulder.
Filled with endolymph fluid, each semicircular canal contains motion senses within the fluid. The utricle is an opening at the end of the canal that contains a dilated sac (osseous ampullae). In the sac, there are hair cells that contain many cytoplasmic projections called stereocilia, which are embedded within the cupula (a gelatinous structure). When rotating or moving your head, the duct moves; however, the endolymph lags behind, which in turn causes the stereocilia to bend. This mechanical movement of the stereocilia causes a change in their activation, resulting in a signal to the central nervous system about the new position of the head. After about 10 seconds of moving at a constant motion, the endolymph catches up and the sense of acceleration is diminished. This adjustment period is known as “the leans” and is often experienced by pilots as they enter a turn, causing the hair cells to be stimulated and alerting the brain that the aircraft and the pilot are no longer moving in a straight line.
Injury to the Semicircular Canals
Damage to the semicircular canals could be twofold. If any part of the canal does not work, then a person may seem to have lost his or her sense of balance or may feel dizzy. Damage to the vestibulocochlear nerve could also make a person lose the sense of balance and/or diminish the ability to hear. Injuries related to damage of the semicircular canals include, but are not limited to the sensation of spinning or the feeling that the room is spinning around (vertigo), the sensation of being off balance (disequilibrium), or the feeling of lightheadedness or feeling faint (presyncope).
A study published by Fitzpatrick and colleagues (2006) showed that upon applying electrical currents across the heads of people while they walked, the researchers were able to better understand how the vestibular system helps maintain upright posture in bipedal animals including humans.
See also: Balance; Dizziness; Otoliths; Saccule; Utricle; Vestibular System; Vestibulocochlear Nerve
Fitzpatrick Richard C., Jane E. Butler, & Brian L. Day. (2006). Resolving head rotation for human bipedalism. Current Biology, 16(15), 1509—1514.
Kandel, Eric R., et al. (Eds.). (2012). Principles of neural science (5th ed.). New York, NY: McGraw-Hill.
Purves, Dale, et al. (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.