Balance and Equilibrium

The human ear is divided into three compartments:  the external ear, middle ear, and inner ear. The inner ear contains the spiral shaped cochlea, where sound waves are transduced into neural signals, and the vestibular complex, which contains the receptors for our sense of equilibrium.

The central, egg-shaped cavity is the vestibule, which contains a pair of membranous sacs, the saccule and the utricle.  Inside the utricle and saccule are hair cells similar to those in the organ of Corti. The hair cells are clustered in the macula, where their processes are embedded in a gelatinous mass and lie under a thin layer of crystals, called otoconia.  When the head tilts, gravity moves the crystal mass and distorts the stereocilia of the hair cells. This is how the saccule and utricle provide information about position, with respect to gravity.

Behind the vestibule is the third portion of the bony labyrinth known as the semicircular canals. The canals project from the posterior region of the vestibule and are responsible for the detection of head motion in three spatial planes. The anterior duct sense forward and backward motion. The posterior duct detects moving up and down. The lateral duct senses moving left to right.

Each canal contains a membranous semicircular duct where angular momentum is sensed.  At the base of each duct is an expansion called the ampulla. Within the ampulla, long stereocilia of hair cells are embedded in the cupula, which sticks out into the endolymph.  When your head moves, the endolymph moves the cupula and stimulates the stereocilia.