The ear is divided into the external, middle and internal compartments. The external compartment consists of the pinna that directs sound waves via the external auditory meatus to the tympanic membrane. Vibrations of the tympanic membrane transmit sound energy to the middle ear that consists of the ossicles: malleus, incus and stapes. The latter stimulates the inner ear through the oval window. The middle ear is air-filled and is connected to the pharynx via the Eustachian tube, which allows equilibration of pressure to occur between the middle ear and the environment. The inner ear consists of the cochlea which is a bony coiled tube, which is divided lengthwise into three canals by two membranes: the scale vestibuli and scale media (or cochlear duct) is separated by Reissner's membrane whereas the scala media and scala tympani is separated by the basilar membrane (Figure NE.11). The scale tympani communicates with the middle ear via the round window. The inner ear is fluid-filled: the scale media is filled with endolymph, while the scale vestibuli and tympani, being joined at the helicotrema, are filled with perilymph. The sound receptors are found in the organ of Corti, which is located on the basilar membrane within the cochlear duct. The organ of Corti (Figure NE.12) is made up of an epithelium of hair cells and supporting cells. Each hair cell is anchored on the basilar membrane and has a bundle of hairs projecting from its tip into the scale media. Directly opposite to these hair cells is the tectorial membrane. The hair cells synapse with dendrites of the ganglion cells which in turn synapse with fibres from the cochlear nerve. The latter traverses the subarachnoid space and enters the brainstem at the pontomedullary junction.
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