Structure and Physiology of Human Ear Involved in Hearing

*Alishbah Sheikh, Bint-e-Zainab, Kanwal Shabbir and Ayesha Imtiaz*

### **Abstract**

Hearing is the fundamental sense based on the normal functioning of the hearing organ "the ear," which plays a vital role in social interaction and the ability of learning. The human ear is divided into three parts: the outer, middle, and inner ear. Defects in outer and middle ear can cause conductive hearing loss, while the defective inner ear may lead to sensorineural hearing loss. So, it is important to study the structure and physiology of the human ear. When a sound of particular frequency enters the outer ear, it passes through the auditory canal and strikes the tympanic membrane. It vibrates and passes these vibrations to three ossicles present in the middle ear. The ossicles amplify the vibrations of sound and send them to the cochlea in the inner ear. Cochlea contains organ of Corti, which converts these vibrations into electrical signals by its hair cells. The neural signals in turn are interpreted by the brain, which one can hear and understand. The aim of this chapter is to review the basic structure and physiology of different parts of the human ear that are involved in the hearing process.

**Keywords:** hearing, human ear, organ of Corti, auditory system

### **1. Introduction**

Sound is a mechanical energy wave which can travel through in air or any other physical medium (gas, liquid and solid). These longitudinal waves consist of alternating compressions and refractions. When sound waves travel through a medium, the particles of that medium vibrate parallel to the direction of sound wave that explains the longitudinal wave nature of sound. A human speaker or any other sound source produces the specific vibration patterns that are converted into appropriate auditory signals by the ear [1].

Hearing is the fundamental sense that allows one to perceive the sound. It also helps the person to communicate and detect different environmental signals. Human ear converts the physical vibration (sound) into a nerve impulse which is further processed by central auditory pathway of the brain. This mechanism of the sound interpretation is complex [2]. This chapter will mainly discuss the structure of different parts of the ear and their physiological interplay in hearing.

#### **Figure 1.**

*Structure of the mammalian ear. The ear is partitioned into three parts: Outer, middle, and inner which are shown here. The outer ear contains the pinna, ear/auditory canal, and the tympanic membrane which separates it from the middle ear cavity. The middle ear is linked to the back of the nose by the Eustachian tube and contains ossicles known as malleus, stapes, and incus. The inner ear is divided into vestibular labyrinth and cochlea. The vestibular labyrinth further contains semicircular canals and the vestibule (source NIDCD, with permission).*

The hearing organ "the ear" is a paired organ, located one on each side of the head. Each ear contains the cochlea, a snail-shaped coiled moiety, as the sense organ. A human ear has hearing range of 20–20,000 Hz through the air conduction while this range is greater for much higher frequencies in case of bone conduction. The former part of the ear deals with conducting the sound to the sense organ cochlea and then the cochlea is responsible for the transduction of vibrations, which is performed by delicate hair cells. The ear is structurally and functionally partitioned into three parts that are required for normal hearing: the outer inner, middle ear, and the inner ear—the latter is further divided into the vestibular labyrinth and cochlea (**Figure 1**). These are discussed in detail below.

### **2. Structure and physiology of outer ear**

The outer ear comprises the pinna and the auditory canal both of which transmit the focused sound signal on the tympanic membrane which separates the outer ear and middle ear. For functional hearing, proper development of the outer ear is essential. As outer ear defects are involved in a number of syndromic and non-syndromic conditions of conductive deafness, it is very crucial to understand the structure of the outer ear [3].

#### **2.1 Pinna**

The pinna protruding from the side of the skull is comprised of cartilage and is completely covered with skin. It is responsible for collecting the sound vibrations and funneling them to the auditory canal. The pinna is helpful in localizing the sound as it catches the sounds, which are more efficiently coming from the front than those coming from behind because of its angle. But this effect is applicable only in the case of high frequencies because of the wavelength of audible sound vibrations and also the relative size of the head. The head itself has a role in localizing the sound as it casts a shadow of sound in the case of middle frequencies, and in lower frequencies, the phases of sound arrival between the ears are responsible for localizing the sound.

#### **2.2 Auditory Canal**

The auditory canal is about 4 cm in length and the outer part with hairy skin and the inner thinner part (**Figure 1**). The outer hairy part has sebaceous and sweat glands [4, 5], both of which together with keratin form ear wax. The ear wax and the hair growth in the outer part of the canal serve as a disinfectant and provide a protective barrier for the ear. Moving inward, the skin of the auditory canal is thin and it is firmly attached to the deeper ear canal bone, which is a hard cavity that absorbs faint sound and then directs it to the tympanic membrane at its base.

#### **2.3 Tympanic membrane**

The tympanic membrane has an outer layer of skin that is continuous with the auditory canal and an inner layer called the endoderm [6]. The outer ectodermal layer is made of stratified and squamous epithelium, which displays lateral unique migration of cells from the center to the edges of the tympanic membrane where these epidermal cells can then exfoliate [7]. This process is referred to as the self-cleaning property of the outer ear. The inner layer of the tympanic membrane comprises of a simple squamous epithelium [8].

The tympanic membrane is divided into two main regions based on its morphology (**Figure 2**); firstly, the tense structure appropriate for the vibration known as the ventral pars tensa, and second the more elastic one the dorsal pars flaccida.

#### **Figure 2.**

*The structural morphology of the tympanic membrane. Schematic of tympanic membrane, which is partitioned into two parts based on morphology: One the pars flaccida and second the pars tensa.*

The pars flaccida also called as Shrapnell's membrane is in the upper part of the tympanic membrane above the malleolar fold and is a relatively more fragile region than the other larger part of the tympanic membrane, the pars tensa [8, 9]. Both of these regions are tri-layered structures containing an inner layer of neural crest cells, which are in the arrangement of loose connective tissue, and this middle layer is sandwiched between two epithelium layers. The inner layer of pars tensa is the lamina propia consisting of further two collagen-rich connective tissue layers [10]; the outer radiative layer and an inner circular layer. While in pars flaccida there is no regular arrangement of extracellular matrix in the inner layer. So, both these regions of the tympanic membrane, the pars tensa and pars flaccid, are different from each other at a cellular and gross level. These structural and functional differences explain why retraction pocket (a condition in which the tympanic membrane is pulled more deeply into the middle ear cavity and may cause pain) more commonly occurs in pars flaccida [11].

The whole tympanic membrane structure has a thickness of about 0.1 millimeters and in the middle ear cavity, it covers an opening (round) of about 1 cm in diameter. Although the tympanic membrane is generally referred to as the eardrum, technically the middle ear cavity is the eardrum with the tympanic membrane acting as the drum skin [12].

### **3. Structure and physiology of middle ear**

The middle ear, an air-filled cavity is associated with the back of the nose by a long and thin tube known as the Eustachian tube (**Figure 1**). In the middle ear, the outer wall is the tympanic membrane while the cochlea is the inner wall. The middle ear floor is a thin bony plate that covers the beginning of the jugular bulb, a great vein that drains the blood from the head. At the upper limit of the middle ear, it forms the bone beneath middle lobe of the brain. At the middle ear front end, there is the opening of the Eustachian tube, and at its posterior end lies a passageway to mastoid cells, which are a group of air cells present within temporal bone [12]. The middle ear, lined with the respiratory membrane, is basically an extension of respiratory air spaces of the sinuses and the nose. This respiratory membrane, thick at the Eustachian tube and thin when passing through mastoid, can produce mucus [13]. The Eustachian tube is a bony structure as it leaves the middle ear but, in the nasopharynx, comprises of cartilage and muscle. The tube is opened by active contraction of muscles which also allows to equalize the air pressure in both the middle ear and nose.

#### **3.1 Auditory ossicles**

The middle ear contains three small bones: malleus, incus, and stapes (also commonly known as hammer, anvil, and the stirrup, respectively, (**Figure 1**)). These ossicles conduct the sound from the ear drum to the inner ear. The malleus is club-shaped with its handle buried in the tympanic membrane, running along its center to upward, and its head lying in the middle ear cavity above the tympanic membrane where it is suspended through a ligament from a bone that forms brain covering. Here, the head of a club articulates with a cone-shaped incus. The base of cone articulates with the malleus head, above the tympanic membrane. The incus, present between two other ossicles, has a thin projection protruding out from it called as its long process. It freely hangs in the middle ear and is connected to stapes at its tip which has a bend of right-angle. The third ossicle stapes is an arch-shaped bone comprising a footplate and an arch. The footplate is articulated by the joint of the stapedio-vestibular as it covers the oval window which is an opening into the vestibular system of inner ear or cochlea [14].
