Intro | Anvil | Ear Canal | Semicircular Canals | Cochlea | Eardrum | Hammer | Auditory Nerve | Stirrup
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
The Semicircular Canals of the inner ear compose the largest part of the vestibular system. The vestibular system is responsive to gravity. The three canals are shaped like tubes, each oriented in a unique plane in space. The canals share a common base. Any movement of the head results in a unique combination of fluid movement throughout each of the canals. These shifts in the fluid are detected by hair cells, similar to those found in the cochlea. Vestibular information is integrated with information from the other senses higher in the brain. This multi-sensory information is then used by the system to monitor body position in space and to maintain balance or equilibrium. The vestibular system helps us to maintain perceptual constancy, even as we skip or jog through our day.
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The vestibular system has two components, the vestibular sacs and the semicircular canals of the inner ear. The two vestibular sacs, the utricle and saccule, respond to gravity as do the semicircular canals, and encode information about the head's orientation. Low frequency stimulation of the vestibular sacs can produce dizziness and rhythmic eye movements called nystagmus; the underlying cause of the disorienting motion sickness experience by some while on a cruise. The utricle and saccule are circular in shape and each contains a patch of receptive tissue. When the head is in the upright position, this tissue is located on the floor of the utricle and on the wall of the saccule. The receptive tissue contains hair cells with cilia projections embedded in a gelatinous mass that lies overhead. The highly viscous mass contains small crystals of calcium carbonate called otoconia. When orientation of the head changes, the weight of the crystals causes the gelatinous mass to shift, thereby triggering the cilia of the receptor hair cells.
The semicircular canals consist of membranous floating canals within a bony encasement. The membranous canals contain a fluid called endolymph and floats within a fluid balled perilymph. The receptor cells of each canal are contained within an enlargement called the ampulla. The cilia of the ampulla's hair cells are also embedded in a gelatinous substance called cupula. When the head begins to rotate, the endolymph within the canals resists movement. This inertial resistance pushes the endolymph against the cupula, causing it to bend until the endolymph begins to move at the same speed of the head. If head rotation is stopped, the endolymph pushes the cupula the other way. Any angular acceleration is translated into bending of the cupula, which in turn exerts great force on the cilia of hair cells.
Most of the axons carrying vestibular information away from the inner ear synapse in vestibular nuclei within the medulla. Neurons of these nuclei project to the cerebellum, spinal cord, pons, and other regions of the medulla. There may also be a vestibular projection to temporal lobe. Most investigators believe this cortical projection is responsible for the feeling of dizziness. The lower brain stem projections can produce nausea, vomiting, and control neck muscles that maintain the upright position of the head. Vestibular projections to cranial nerves that control eye muscles control the vestibulo-ocular reflex. This reflex maintains a relatively steady retinal image as we move. As the head is jarred, eye movements are automatically made to compensate. Can you imagine the role of this system in the life of an athlete?