**4. Function of outer and middle ear**

To understand the role of the outer and middle ear in hearing physiology, it is important to first study the conducting mechanism of sound. The audible sound range is about 10 octaves from somewhere between 16 and 32 Hz to somewhere between ~16,000 and 20,000 Hz. The sensitivity of sound is above 128 Hz to ~4000 Hz and this range of maximum audibility and sensitivity decreases with age. As mentioned earlier, the head itself forms a natural barrier between the two ears. This plays a role in sound localization based both on the intensity and the difference in time of arrival of sound. Moving to the pinna, its crinkle shape catches and funnels the high frequency sounds to the auditory canal, which acts as a resonating tube since it amplifies the sounds falling between 3000 and 4000 Hz to increase sensitivity of the ear at these respective frequencies. The ear responds to very lowintensity sounds owing to its sensitivity. The equal pressure of air on both sides of the tympanic membrane also enables this sensitivity. The Eustachian tube provides this equalized pressure by opening for short intervals with every 3rd or 4th swallow. If it remained open all the time, one could even hear the sound of his or her own breath. If the Eustachian tube is closed for too long; it can absorb oxygen and carbon dioxide from the air in the middle ear. As the middle ear comprises lining of a respiratory membrane that can absorb gases, this process produces negative pressure. This may cause pain as is the case during descent of an airplane if the Eustachian tube is not unblocked. The middle ear cavity is quite small, containing mastoid air cells which act as air reservoirs to provide the cushion effects to pressure change. If the negative pressure remains for too long then the fluid is secreted by the middle ear cavity, which can cause conductive hearing loss [12, 15].

The outer and middle ears amplify the sound signal since the pinna has a relatively large surface area and funnels the sound to smaller tympanic membrane which has in turn large surface area as compared to the stapes footplate. This results in hydraulic amplification [16] i.e., a smaller movement over a large area is being converted into larger movement to a smaller area. The ossicular chain acting as a lever system amplifies the sound. Overall, both the outer and middle ears amplify the sound by about 30 dB on its passage from outside to the inner ear.

### **5. Inner ear**

The human inner ear is present between the middle ear and acoustic meatus and is labeled as a labyrinth of ear that can be a bony or membranous labyrinth that each is further divided into three portions. Bony labyrinth comprises of semicircular canals, the vestibule, and the cochlea whereas the membranous labyrinth comprises of the semicircular duct, two sac-like structures of the vestibule; namely the saccule and the utricle and the cochlear duct (**Figure 1**). The space between the membranous and bony labyrinth is filled with watery fluid named perilymph that is obtained from

the lymphatic system, and it is similar but not identical to the aqueous humor of eyes and cerebrospinal fluid. It is poor in potassium and rich in sodium ions [17, 18]. Membranous labyrinth also has enclosed fluid named endolymph, which has a high potassium concentration, and its composition is different from that of perilymph. Endolymph is produced by vestibular dark cells that have a resemblance with stria vascularis, which is part of the cochlea [18]. Endolymph within the membranous labyrinth of inner ear interacts with hair cells and causes depolarization of hair cells by providing high potassium gradient, resulting in afferent nerve transmission [19]. These structures form two systems of inner ear, a vestibular system involved in maintaining equilibrium and the cochlear system only part of the ear that participates in hearing. The vestibular system is proprioceptive (feedback loop between sensory organs and nervous system, external stimuli is not involved in it), whereas the cochlear system is exteroceptive (sensation in cochlea is caused by external stimuli e.g., sound).

#### **5.1 Vestibular system**

Many hearing loss disorders are accompanied by vestibular defects, which necessitate some description when considering the auditory system. The vestibular system is a sensory system of inner ear that is important for postural equilibrium maintenance and helps develop coordination between the position of the head and eye movements. It comprises of five organs; three semicircular canals that are present at right angles to each other and control angular (Head) rotation and two otolith organs that play a vital role in linear acceleration (straight line movement) [20]. Semicircular canals based on their position are designated as superior, posterior, and horizontal. Each canal opens into the vestibule through its expanded end known as Ampulla. Sensory neuroepithelium in ampulla is known as crista ampullaris consisting of ridge of tissues. From cristae arises a gelatinous protein-polysaccharide structure, cupula that divides the ampulla into equal parts and is important to keep hair cells in place [21]. Rotational acceleration causes endolymph to displace cupula which results in bending of hair cells in direction opposite to acceleration [22]. It is the middle part of bony labyrinth that is connected posteriorly with the semicircular canal and anteriorly with the cochlea and separated through the oval window from the middle ear.

Two membranous structures of the vestibule are the utricle and saccule are designated as otolith organs [22]. A single patch of sensory neuroepithelium in the vestibular system is called macula, which is present on the inner surface of a membranous sac. It lies in the utricle in the horizontal plane and originates from the anterior wall of the tubular sac. Whereas in saccule it is in the vertical plane and covers the bone of the vestibular inner wall. Gelatinous otolithic membrane (macula) of utricle and saccule contains thousands of otoconia (calcium carbonate crystals) embedded in a protein matrix [23]. In mammals, these otoconia are arranged to form various layers that help the hair cells to respond to endolymph drag. The sensory cells in vestibular region are hair-like cilia that project out from the apical end that are flexible motile kinocilia and stiff non-motile stereocilia. The stereocilia are arranged according to curvilinear line called striola [24], the area of thickening of saccule and thinning of utricle [25]. If the endolymph pressure is toward the kinocilium; it causes opening of cation channel and potassium influx resulting in depolarization of hair cells. This depolarization of hair cells results in release of glutamate to afferent nerve receptors and neurotransmission. Various signals from the vestibular nucleus are sent to the cortex, thalamus, and cerebellum that in return send efferent signals to the ocular and postural muscles [26].
