One of two proprioceptive sensory systems that provide us with input about the positions of our own bodies.
The equilibrium sense, generally associated with balance, provides feedback about the positions and movements of our heads and bodies in space. The other system—the kinesthetic sense—tells us about the orientation of different parts of our bodies in relation to each other. While the kinesthetic information needed by the brain comes from joints and muscle fibers throughout the body, the receptors for equilibrium are located in the semicircular canals and vestibular sacs of the inner ear. (The equilibrium sense is also called the vestibular sense, and the relevant parts of the inner ear are sometimes called the vestibular system or apparatus).
The semicircular canals are three pretzel-like curved tubes arranged at angles roughly perpendicular to each other, with the two vestibular sacs located at their base. Both the canals and sacs contain fluid and tiny hair cells, which act as receptors. When a person's head moves, the fluid disturbs the hair cells, which stimulate a branch of the auditory nerve, signaling the brain to make adjustments in the eyes and body. A movement at any given angle will have its primary effect on one of the three canals. Overstimulation from extreme movements will produce dizziness and nausea. Our sense of body position when we are at rest is provided by the vestibular sacs, which contain small crystals called otoliths (literally, "ear stones") that exert pressure on the hair cells. In their normal position, the otoliths inform our brains that we are standing or sitting upright. When the head is tilted, the position of the otoliths changes, and the signal sent to the brain changes accordingly. The neural connections of the vestibular system lead to the cerebellum, the eye muscles, and a part of the autonomic nervous system involved in digestion (which accounts for the link between dizziness and nausea).
Burke, Shirley R. Human Anatomy and Physiology in Health and Disease. New York: Delmar, 1992.
Martini, Frederic. Fundamentals of Anatomy and Physiology. Englewood Cliffs, NJ: Prentice-Hall, 1995.