The sense of balance is given by a very complex series of organs in the inner ear called the vestibular labyrinth. Although anatomically the vestibular labyrinth is intertwined with the auditory cochlea, they are two different and separate systems. The vestibular system has two mayor components, one made by three semicircular canals filled with endolimph, a special type of extracellular fluid, that measure angular velocity of the head (the speed with which we turn our head on its axis), and another component is made up by the saccule and utricle, which are two sac like bulges, and are responsible for detecting linear velocity.
The three semicircular canals are all perpendicular to each others plane, forming a three dimensional representation of all possible head movements. The ability to detect angular velocity derives from the fact that when we rotate our heads in any direction, the liquid inside the corresponding canal to the plane of movement tends to stay put, due to inertia. At the base of each semicircular canal lies a dilatation of the canal called ampulla, and inside the ampulla there is a thickening of the epithelium that contains the specialized receptor cells, called vestibular hair cells. On this thickening there is a diaphragm-like gelatinous mass that covers the lumen of the canal, called the ampullary crest. The crest is deformed by the endolimph when some angular force is exerted, and itself deforms the cilia (hair like protuberances) of the vestibular hair cells. The cilia of these cells are not symmetrically arranged on the surface and have a conformation such that allows them to detect if the movement is in one direction or the opposite, by having all the same unidirectional organization under the crest, and depolarizing on one direction, and hyperpolarizing in the other.
Linear velocity is also detected by inertia. On a portion of the floor of the saccule and utricle there is a gelatinous material called the macula. This material is embedded with a large amount of calcium carbonate crystals called otholiths, and the epithelium underneath it is filled with receptors. The macula of the utricle lies roughly horizontal to the head, and when the head moves in any linear direction of the horizontal plane, the macula will deform due to the weight of the otholiths and in turn deform the cilia of the receptors. As in the case of the ampulla, the receptors are each unsymmetrical, but all are organized in the same direction, so as to detect the direction of the head movement. The macula of the saccule is roughly vertical to the head, as to detect up and down movement.
Each vestibular hair cell receptor has not only an efferent neuron that detects its dopolarization, but also has a afferent axon that comes from the vestibular nucleus of the brainstem. This afferent neuron ensures that there is a constant rate of firing when in repose. If the receptor depolarizes in response to movement in one direction, then the firing rate of the efferent neuron increases, if it hyperpolarizes due to movement in the opposite direction, then firing decreases. This allows one receptor to detect two directions, plus a resting state. The neurons that innervate the vestibular receptors are in the superior and inferior vestibular ganglia of Scarpa, and from there project form the vestibular section of the vestibulocochlear cranial nerve. The balance signals relay bilaterally in the vestibular nuclei of the brain stem. The lateral vestibular nucleus projects into the nodule and uvula of the cerebellum, and is in charge of the control of posture. The medial and superior vestibular nuclei project balance information to the oculomotor and trochlear nuclei, and are involved in the oculomotor reflexes to vestibular input.
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