The hearing system, also called also the auditory system, consists of the outer ear, middle ear, ... more The hearing system, also called also the auditory system, consists of the outer ear, middle ear, inner ear, and central auditory nervous system. The overall function of the hearing system is to sense the acoustic environment thus allowing us to detect and perceive sound. The anatomy of this system has been described in Chapter 8, Basic Anatomy of the Hearing System. The current chapter describes the function and physiology of the main parts of the hearing system in the process of converting acoustic events into perceived sound. In order to facilitate perception of sound, the hearing system needs to sense sound energy and to convert the received acoustic signals into the electro-chemical signals that are used by the nervous system. A schematic view of the processing chain from the physical sound wave striking the outer ear to the auditory percept in the brain is shown in Figure 9-1. Figure 9-1. A schematic view of the hearing system. The hearing system shown in Figure 9-1 has two functions: sound processing and hearing protection. Sound processing by the hearing system starts when the sound wave arrives at the head of a person. The head forms a baffle that reflects, absorbs, and diffracts sound prior to its processing by the hearing system. The first two sound processing elements of the hearing system are the outer and middle ears that form together a complex mechanical system that is sensitive to changes in intensity, frequency, and direction of incoming sound. Acoustic waves propagating in the environment are diffracted, absorbed, and reflected by the listener's body, head, and the pinnae and arrive through the ear canal at the tympanic membrane of the middle ear. After the acoustic wave strikes the eardrum, its acoustic energy is converted into mechanical energy and carried across the middle ear. At the junction of the middle ear and the inner ear, the mechanical energy of the stapes is transformed into the motion of the fluids of the inner ear and thence into the vibrations of the basilar membrane. The motion of the basilar membrane affects electro-chemical processes in the organ of Corti and results in generation of electric impulses by the array of the hair cells distributed along this membrane. The electrical impulses generated by the hair cells affect the inputs to the nerve endings of the auditory nerve and are transmitted via a network of nerves to the auditory cortex of the brain where the impulses are converted into meaningful perception. A secondary function of the hearing system is to provide some protection for the organ of Corti and the physical structures of the middle ear from excessive energy inputs and subsequent damage by modulating the 9
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