The semicircular canals are fluid-filled, roughly semicircular tubes that are part of the structure of the inner ear. The canals sit at right angles to each other, are all attached and detect horizontal and vertical movement as well as acceleration. Movement of the fluid, called endolymph, inside the canals causes tiny hairs, or cilia, inside the tubes to shift and move. Tiny particles within the endolymph also function to stimulate the cilia. The cilia act as motion sensors, sending information to the brain that is interpreted to help maintain balance.
There are three semicircular canals: the horizontal, also known as the lateral canal; the superior canal, also called the anterior canal; and the posterior semicircular canal. The horizontal semicircular canal responds to horizontal head movement, and the other two semicircular ones react to vertical movements. Moving fluid within the canals is one cause of dizziness. After a person spins around, the fluid within the canals continues to move over the cilia after the spinning stops, giving the sensation that the spinning is still happening.
These important sensory organs are considered part of the bony labyrinth of the inner ear. This structure includes the semicircular canals, the cochlea and the vestibule of the ear. One form of ear infection, called labyrinthitis, involves infection of portions of the labyrinth. Symptoms can include vertigo, dizziness, nausea and an inability to maintain balance. With no single specific cause, labyrinthitis can occur as the result of bacterial or viral infections, or because of the side effects of decompression after deep-sea diving. Blockage of the inner ear also can cause labyrinthitis.
Balance information that is gathered by the semicircular canals is transmitted to the brain via the auditory nerve. Because of the close connection between the hearing and balance functions of the ear, some balance problems often occur in conjunction with hearing loss. Doctors who specialize in treating hearing loss often also help patients deal with problems involving balance and orientation perception. Many doctors have received special training to diagnose these issues and to recommend treatments.
The semicircular canals are found in most vertebrates and are different sizes, depending on a particular animal's typical style of movement. An animal that tends to move quietly and in small increments will have smaller canals, and larger animals capable of more rapid and agile movement tend to have larger canals. Although the vestibular system, which includes the semicircular canals, is similar in most vertebrates, the cochlear system varies greatly between mammals, birds, reptiles and amphibians.