*5.2.1 Canals of cochlea*

Cochlea consists of three canal systems (**Figure 3C**); the scala vestibuli, the scala media and the scala tympani which envelop the modiolus. These three scala wind around the bony axis in a spiral stairway.

#### **Figure 3.**

*Cochlear anatomy. A. Cochlea Structure. B. Cross-section of cochlear duct showing fluid-filled cavities around the modiolus. C. Three main canals: Scala vestibuli, scala tympani, and scala media along with Reissner's membrane, stria vascularis and the organ of Corti in middle. D. Magnified view of the organ of Corti, containing outer and inner hair cells, stereocilia and supporting cells with tectorial and basilar membranes (taken from [27, 28] with permission).*

#### *5.2.1.1 Scala vestibuli*

Scala vestibuli is the exterior lymph-filled canal and it is connected to the vestibules of the inner ear. The oval window is present at the base of the scala vestibuli. It is the part of cochlea that receives vibrations from the middle ear (stapes). Scala vestibuli and scala tympani sense the change in pressure that is caused by the different frequencies of sound.

#### *5.2.1.2 Scala tympani*

Scala tympani is the inferior canal and it connects to the tympanic membrane forming the two-and-half coiled structure of cochlea. Its superior end is connected to the scala vestibuli, while its inferior end separates the cochlea from the round window. The point at cochlear apex where scala vestibuli and scala tympani meets is known as the helicotrema.

#### *5.2.1.3 Scala media*

Scala media is present between the scala vestibuli and the scala tympani and has the organ of Corti and the basilar membrane. A basilar membrane is present between the scala media and the scala tympani, thus separating them. Scala media also contains the spiral ganglions that are extended neurons from the hair cells. The stria vascularis of scala media is involved in the regulation of K+ into scala media, thus maintaining the potential of endo-cochlea [32].

#### *5.2.2 Fluids of cochlea*

The chambers of the cochlea are filled with three types of fluids: perilymph, endolymph, and intrastrial fluid. These fluids maintain the endo-cochlear potential which is important for sensory transduction. The intrastrial fluid only fills the cavities present in stria vascularis.

#### *5.2.2.1 Perilymph*

Perilymph is present in scala vestibuli and scala tympani, and its fluid composition is similar to the extracellular fluid of the body. It has a high sodium concentration (140 mM) and low concentration of calcium (1.2 mM) and potassium (5 mM). The perilymph present in scala media is continuation from CSF while that in scala media is from plasma of blood.

#### *5.2.2.2 Endolymph*

Endolymph is present only in the scala media and has a unique ionic composition i.e., high K<sup>+</sup> concentration (150 mM), which is not found anywhere in the body. This high concentration of K+ helps to maintain the endo-cochlear potential. Hence, endolymph is a noteworthy characteristic of the cochlea. It has considerably low concentration of sodium (1 mM) and calcium (0.002 mM) [32–34].

#### *5.2.3 Reissner's membrane*

Reissner's membrane is present between the scala vestibuli and scala media and is involved in the regulation of ions. The membrane along with the basilar membrane

#### *Structure and Physiology of Human Ear Involved in Hearing DOI: http://dx.doi.org/10.5772/intechopen.105466*

creates a cavity in the cochlear duct that is filled with endolymph. It is an avascular membrane that is made up of two types of cells. The part of Reissner's membrane cells that lines the scala vestibuli are fibroblasts, while the cells that line scala media are epithelial cells. This cavity also contains the sensory organ i.e., the organ of Corti. Two types of ion channels are present on Reissner's membrane: potassium ion channel and non-selective cation channel. These channels maintain the pressure between endolymph and perilymph [35, 36].

### *5.2.4 Organ of Corti*

The organ of Corti is the organ for audition and is present on the basilar membrane. It consists of outer and inner hair cells (mechanosensory cells) and supporting cells (**Figure 3D**). The organ of Corti hair cells also has stereocilia that attach it to the tectorial membrane (soft ribbon-like structure on the top of organ of Corti). Alterations in basilar and tectorial membrane help in the movement of stereocilia that stimulates the hair cell receptors [37–39].

#### *5.2.4.1 Tectorial membrane*

The tectorial membrane covers the mechanosensory and supporting cells of organ of Corti. It has a viscous structure consisting of collagen and non-collagen proteins (glycoproteins and proteoglycans). The membrane helps in storing the calcium ions for the sensory organs of the inner ear. The stereocilia present in the organ of Corti are embedded in the tectorial membrane [40].

### *5.2.4.2 Mechanosensory cells*

The hair cells are erect and contain micro-projections at their apical ends, known as stereocilia, that are filled with F-actin. The arrangement, size, and toughness of the hair cells in the cochlea are responsible for responding to different ranges (low to high) of sound frequencies. The cochlea shows a fundamental effect of tonotopy. Tonotopy refers to the orderly coding of sound based on high to low frequencies by hair cells and their afferents (spiral ganglion neurons). The hair cells residing at the apex of cochlea reciprocate to lower frequencies while the ones at the base, near to oval window reciprocate to a higher range of frequencies, thus creating a tonotopic gradient all over the cochlea. The hair cells convert the sound energy into neural signals.

#### *5.2.4.2.1 Outer hair cells*

Outer hair cells are oblong cells containing myosin and actin protein, which help these cells contract in rhythmic movement in response to sound stimuli from the middle ear. There are about 12,000 outer hair cells that are arranged in three rows. At the top of these cells are stereocilia that are embedded in the tectorial membrane. These cells are present on the basilar membrane area where the largest frequencies would be received [41]. These cells play a role in mechanoelectrical stimulation as well as in the feedback mechanism for low-frequency sounds for its amplification. They can amplify the faint sound by the inversion transduction through the positive feedback mechanism i.e., conversion of electrical signals to mechanical (sound) signals. The outer membrane of outer hair cells has a unique motor protein known as prestin, which is involved in the generation of movements that couple back to the

wave produced in a fluid membrane. In this way, weak sounds are amplified by the 'active amplifier' mechanism [42, 43].

#### *5.2.4.2.2 Inner hair cells*

The primary organ for the audition is the bundle of inner hair cells. These cells have pear-shaped morphology, and their stereocilia make weak connections with the tectorial membrane. There are about 3500 inner hair cells arranged in just a single row that is surrounded by supporting cells. These hair cells transmit the electrical signal to the auditory cortex of the brain through the nerve fibers. About 95% of auditory nerve projection to the brain is through inner hair cells. The outer hair cells help inner cells in the generation of synaptic nerve conduction to cochlear nerve fibers [37].

#### *5.2.4.2.3 Supporting cells*

Supporting cells are rigid sensory epithelial cells, organized in a mosaic manner that during the head movement and stimulation of sound maintain the integrity of the sensory hair cells. These cells play a vital role in maintaining the microenvironment for the proper functioning of hair cells. There are different types of these cells that are arranged in a row on the basilar membrane. They are Hensen's cells, Deiters' cells (phalangeal cells), pillar cells, Claudius cells, Boettcher cells, and border cells. These are arranged from the outer edge to the inner edge of an organ of Corti. These cells maintain the structure of the organ of Corti as well as the composition of the endolymph in the scala media. Supporting cells have negative resting potential so these cells tend to transport Na<sup>+</sup> out and K+ into the scala media through the channels present in these cells [44, 45].

#### **Figure 4.**

*Structure of stereocilia on hair cells. The stereocilia are made of F-actin protein and these stereocilia are linked to each other through the tip links. When the largest stereocilium moves due to the pressure at the tectorial membrane, the shorter ones move as well and mechanotransduction channels (MET channels) open and influx of ions takes place.*

*Structure and Physiology of Human Ear Involved in Hearing DOI: http://dx.doi.org/10.5772/intechopen.105466*

#### *5.2.5 Mechanotransduction in cochlea*

A small number of transduction channels present in the stereocilia are open at the resting state. The stereocilia consist of shafts of F-actin protein, which has upper and lower tip link densities that help in linking the long and short stereocilia through tip links (**Figure 4**) [46]. These stereocilia are arranged in ascending order of their height. When the largest stereocilium embedded in the tectorial membrane is displaced, the ones with shorter lengths also move. These movements of stereocilia open the mechanotransduction channels present at the tips of the stereocilia, leading to an influx of K<sup>+</sup> . As a result, the voltage-gated calcium ion channel opens and uptake of Ca2+ into the cells takes place. This depolarization of cells excites the cochlear nerves and in turn, results in the release of glutamate from the hair cells into the auditory nerves. The sound wave signal is then conveyed to the brain. Both the apical and basal regions of the cochlea are separated by membranes and their extracellular ionic environments are tightly regulated. These regulations of ions are important for converting the sound signals into electric impulses that are sent to the brain [47].

### **6. Conclusion**

The human ear, one of the most developed sensing organs, has structurally and functionally divided into three parts. Firstly, the outer part, containing pinna and auditory canal, passes the sound vibrations to the tympanic membrane which separates the outer and middle ear. The middle ear containing the three ossicles (malleus, incus, and stapes) receives these vibrations and amplifies them. Traveling through middle ear, the vibrations are passed to inner ear which contains the spiral-shaped cochlea as the sense organ. In cochlea, the hair cells present in the organ of Corti contain stereocilia whose rhythmic movements open the mechanotransduction channels, which send the nerve signal to the brain. In this way, these vibrations are converted into understandable sound.

## **Acknowledgements**

The authors thank Inna A. Belyantseva and NIDCD, NIH, USA, for permission to use figures in this chapter.

*Auditory System - Function and Disorders*
