A sensory epithelium makes up a small portion of the wall of the membranous chamber. It is made up of the sensory cells together with "supporting" cells that help to support the function of the sensory cells both structurally and functionally. Some of the supporting cells may pump potassium into the endolymph and others will contribute to making substances that are connected to the ends of the sensory cells that are exposed to the endolymph. The inner ear sensory epithelium are among the smallest organs in the body, containing less than 20,000 sensory cells. Compare this with the million photoreceptors in the eye. The inner ear organs must be small because any increase in their size would increase their mass. An increase in mass would increase the mechanical force that would be required to make them vibrate. Any increase in the driving force would represent a decrease in the sensitivity of the system (a hearing loss). The small number of cells in the hearing organ means that the loss of even a small number affects hearing. While millions of photoreceptors in the retina improve visual resolution, millions of hair cells would decrease the sensitivity of the hair cell organs. Loss of a 1000 photoreceptors is hardly noticed - loss of a 1000 sensory cells in the ear leads to hearing loss. The mass of the cochlear sensory epithelium is further reduced because it has only a small number of blood vessels. The number of blood vessels required by the organ of Corti is reduced by a unique system for converting the metabolic energy from sugar and oxygen in the blood into an electrical potential. The indirect method for supplying the metabolic demands of the organ of Corti is adaptive because the auditory system is sensitive enough to "hear" the vibrations associated with blood moving through blood vessels. It is fortunate they are located away from the organ of Corti.
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