Maculae and Dynamic Equilibrium

Along with , the inner is responsible for encoding information about equilibrium (the of balance), which it does in the vestibular apparatus.

Similar to the , the vestibular structures use hair cells with to detect movement of fluid, in this case, in response to changes in head position or acceleration. Detection of head position when the body is stationary is termed static equilibrium. The information for static equilibrium comes from the and . Dynamic equilibrium is the perception of acceleration. Information for dynamic equilibrium can come from the utricle and saccule, which detect linear acceleration, and/or the , which detect angular acceleration. The neural signals generated from the vestibule are transmitted to the brainstem and cerebellum from sensory neurons in the vestibular ganglion. The saccule and utricle each contain a sense organ, called the macula, where stereocilia and their supporting cells are found. These maculae (plural) are oriented 90 degrees to one another so that they respond to positions in different planes.

The organs can respond to changes in position and acceleration because the tips of their stereocilia project into a dense made up of a mixture containing granules of calcium and protein, called , which gives rise to their common name, the otolithic organs. When the maculae (otolithic organs) move, gravity causes the dense otolithic membrane to move relative to the less dense cell layer beneath the stereocilia. This causes the stereocilia to bend, initiating action potentials in the fibers that innervate them. Bundles of stereocilia are arranged in various directions, so that any direction of inclination will depolarize a subset of the hair cells and hyperpolarize a corresponding subset of sensory neurons. How the body head position and the linear (horizontal or vertical) direction of acceleration is determined by the specific pattern of hair-cell activity across the maculae.