Equilibrioception, or the sense of balance, is a sense by which animals and humans receive information about their external or internal environments. In humans, the sense of balance is mostly maintained by the vestibular system, the sensory system that detects acceleration or changes in movement. The basic function of this system is to help prevent people from falling over while walking or standing still.
A significant component of the vestibular system is the labyrinth of the inner ear, where equilibrioception is determined by levels of fluid, which is called endolymph. There are two components of this system in the inner ear: the otoliths and the semicircular canal system. The otoliths indicate linear acceleration, and the semicircular canal system indicates rotational movement. Based on input from these components, the vestibular system maintains balance by sending signals to the muscles that keep the body upright and to the muscles that control eye movement.
The importance of the auditory system in the greater vestibular system is evident by the fact that auditory problems, such as inner ear infections, can result in balance problems. Other components of the vestibular system include vision, the hands and fingers, and the pressors on the soles of the feet — all of which provide feedback to the vestibular system regarding spatial orientation, postural positions, and the presence of horizontal or slanted surfaces, among others. Additionally, the trigeminal nerve, which lies along the surface of the face and eyes and functions by carrying sensations from the face to the brain, is part of the vestibular system.
Disturbances of the vestibular system usually cause dizziness and nausea. Some of the common causes of balance disruption include inner ear infections, bad head colds, and certain medications or medical conditions. The rapid and sudden movements that may be experienced during certain activities, such as riding a merry-go-round, may also cause temporary dizziness. Also, alcohol can cause dizziness due to changes that occur in the viscosity of the blood and the endolymph in the inner ear labyrinth with alcohol consumption.
Other causes of vestibular dysfunction include inner ear disorders that cause swelling and irritation, such as vestibular neuritis and labyrinthitis. Additionally, dislodged movement-sensing crystals in the otoliths cause the sudden onset of dizziness associated with changes in head position, which is known as benign paroxysmal positional vertigo (BPPV). Serious medical disorders, such as brain stem injuries and brain tumors, can also cause vertigo and other associated symptoms.