Suzanne de Bruijn

21 General introduction ANATOMY AND PHYSIOLOGY OF THE EAR The sense of hearing is an essential component of the human communication system. Well-functioning of the ear is critical for a person’s well-being and defects can be socially debilitating and even lead to anxiety, cognitive decline and depression. 2,23,24 The ear can be divided into three anatomical compartments: the outer, middle and the inner ear. Under normal circumstances, sound waves are captured by the outer ears, conducted through the middle ear in which they are amplified or attenuated (in case of excessive noise), and converted into neuronal activity by complex biochemical processes that take place in the inner ear. The inner ear also harbors the vestibular system that is essential for our sense of balance. The outer and middle ear The outer ear consists of the pinna (auricle) and the external ear canal (auditory meatus) ( Figure 3A ). Soundwaves are collected by the pinna, and travel via the external ear canal to the middle ear. The external ear canal directs the waves to the tympanic membrane (eardrum) that separates the outer and the middle ear and starts vibrating upon capture of sound waves. 25 The middle ear is an air-filled cavity and forms a bridge between the outer ear and the fluid-filled inner ear. It contains the tympanic membrane that is connected with the oval window via three interconnected auditory ossicles (malleus, incus and stapes) ( Figure 3A ). 25 The main function of the middle ear is to efficiently transmit the sound wave energy across the air-fluid boundary of the outer and inner ear. Vibrations of the tympanic membrane are transmitted by the ossicles, in a pistol-like movement, to the oval window. The resulting inwardmovement of the oval window leads to displacement of the fluids that are present in the inner ear. 25,26 The inner ear: Cochlea The inner ear contains the sensory organs that enable hearing (cochlea) and importantly contribute to the control of our balance and spatial orientation (vestibular system) ( Figure 3A ). The inner ear is embedded in the temporal bone, the strongest bone of the skull, which serves as protection for the delicate organ. The cochlea is the auditory part of the inner ear and is a small, coiled structure. The cochlea consists of three fluid-filled chambers that run from base to apex: the scala media, the scala tympani and the scala vestibuli ( Figure 3B ). The fluid movements induced by pulses of the oval window and consequently, bulging of the round window