How does the ear function to maintain equilibrium? 

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The ear maintains both static and dynamic equilibrium. Static equilibrium is maintenance of the proper head position in response to changes in linear motion such as walking. Dynamic equilibrium is the maintenance of proper head position in response to rotational movement such as turning. The vestibule lies between the semicircular canals and the cochlea (eardrum). The vestibule is responsible for maintaining static equilibrium while the semicircular canals maintain dynamic equilibrium. The vestibular system consists of two bulblike sacs, the saccule and the utricle - both of which contain a sensory receptor in their walls called the macula. Receptor hair cells extend from the macula and even more hairs (stereocilia and cilia) project from these hair cells. The stereocilia extend into a gel like membrane called the otholitic membrane whose density is increased by calcium carbonate crystals. When the body moves, the fluid in this membrane moves up and down in the saccule and back and forward in the utricle. The movements are detected by the hair cells which initiate action potentials. The semicircular canals consist of three canals at right angles to each other. The base of the semicircular canals has receptors called cristae ampullaris which contain numerous hair cells which project into a gel-like matrix called the cupula. Movements cause changes in the gel which are detected by the hair cells which in turn create graded potentials which are passed on to the brain.

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The ear maintains equilibrium by detecting the position and the motion of the body.  

The sacculus and utriculus are found just above the cochlea. They are interconnecting chambers that are each filled with fluid. There are hair cells inside each chamber. Attached to the hair cells are tiny spheres of calcium carbonate called ear stones. As the head moves, the ear stones move. The movement of the ear stones initiate an action potential that is sent to the brain. In this way, we are able to detect the orientation of our bodies.

Motion is detected by the semicircular canals of the ear. These canals are found on the top of the inner ear. There are hair cells at each end of the canals. The movement of the fluid in the canals lags compared to the movement of our bodies. This results in a relative motion between the walls of the canals and the fluid (endolymph). Thus, the hair cells move and send messages to the brain. The brain is then able to detect movement.  

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