Anatomy of Ear and Physiology of Hearing

Hitesh  Verma

Section 1

1Ear

Section Outline

Anatomy of Ear and Physiology of Hearing
Hearing Evaluation
Diseases of External Ear
Eustachian Tube Function Test and Otitis Media
Tympanic Membrane Anatomy, Diseases, and Myringoplasty
Complications of Middle Ear Infection
Chronic Otitis Media, Cholesteatoma, and Tympanoplasty
Mastoid Surgeries
Otosclerosis
Facial Nerve
Sensorineural Hearing Loss: Causes, Evaluation, and Management
Anatomy and Tumor of Temporal Bone
Tinnitus
Differential Diagnosis for Ear Symptoms
Anatomy and Physiology of Vestibule and Vertigo
Examination of Ear
Ear Instruments2

Anatomy of Ear and Physiology of HearingCHAPTER 1

DEVELOPMENT OF EAR

The pinna development starts on the 38th day of the fatal intrauterine life (IUL) from the first and second branchial arches. Pinna development begins at the upper neck level, and the developing pinna migrates posterolaterally towards its final position, lateral to the eye, which is 1/3 above and 2/3 below Frankfurt's plane. Six hillocks derived from the 1st and 2nd arches begin fusion at the 7th week of IUL and develop the initial pinna. A fully-develop pinna is formed by the 20th day of IUL. The first arch develops the tragus and portion of the anterior crus of the helix (Figs. 1.1A to F). The remaining portion of the pinna is derived from the 2nd arch. The failure of the fusion of hillocks presented as periauricular tags and sinuses.

The external auditory canal develops from the dorsal part of the first branchial groove in the 8th week of life. The medial end of the developing external auditory canal (meatal plate) is approximated with the mesenchymal layer (generates the middle fibrous layer of the tympanic membrane) at the 9th week of IUL. The meatus is fully canalized by the 18th week of IUL (Fig. 1.2). The failure of the canalization of the canal is presented as atresia of the external auditory canal (EAC). The notch of Rivinus is formed by the nonunion of tympanic ring elements superiorly. The squamous part of the temporal bone and the cortical surface of the mastoid bone is formed from the posterior projection of the tympanic ring. The tubotympanic recess of the first pharyngeal pouch leads to the middle ear cleft (middle ear, mastoid cavity, and Eustachian tube). Failure of proper development or fusion of 1st (Meckel's) and 2nd (Reichert's) arch leads to the formation of the atresia and congenital cholesteatoma. Malleus, incus, and tensor tympani muscle arises from mandibular arch (1st arch). Stapedius muscle and stapes arise from the hyoid arch (2nd arch). Incomplete pneumatization of the middle ear cleft leads to congenital ossicular fixation. At birth, the mastoid process is not well-formed, so the facial nerve lies deep into cutaneous plans after the exiting stylomastoid foramen. The tympanic bone is a flat ring at birth, so the external auditory canal is horizontal. The inner ear develops from the otic placode (ectodermal).

Figs. 1.1A to F: Development of pinna.

Fig. 1.2: Development of the ear.

ANATOMY AND PHYSIOLOGY OF EXTERNAL EAR

Pinna directs sound toward EAC by its projections and depression. The parts of the pinna are shown in Figure 1.3. The curved outer edge of the pinna is a helix. Prominent anterior elevation and parallel to the helix is antihelix.4

Fig. 1.3: Parts of pinna.

Fig. 1.4: Parts of EAC.

(EAC: external auditory canal; ET: Eustachian tube; TM: tympanic membrane)

Small prominence on the posterosuperior part of the helix is known as Darwin's tubercle. The scaphoid fossa is above the superior division of the antihelix, and the triangular fossa is the space between divisions of the antihelix. Cymba concha is a small superior, and cavum concha is a more significant inferior portion of concha defined by the crux of the helix anteriorly. The tragus is elevation below the crux of the helix and lies anterior to the antitragus, which is the lowermost part of the crux of the helix behind EAC. A tragal perichondrium is a good tool for tympanic membrane grafting. The intertragus notch is devoid of cartilage. The intertragus notch is used for endaural surgical approaches. The lobule is an inferior soft portion of the pinna that consists of fibrous and adipose tissue. Lobular fat is used to seal small tympanic membrane defects (fat myringoplasty) and the fistula in stapedectomy. The perichondrium is firm on the lateral aspect and loosely attached to the medial surface. Perichondrium supplies nourishment to the pinna cartilage. Hematoma and abscess collection under the perichondrium lead to necrosis of cartilage. Pinna is attached to the temporal bone by two extrinsic ligaments. Pinna skin possesses hair and sebaceous glands.

The external auditory canal extends from the concha to the tympanic membrane. It is around 24 mm in length. The direction of the canal is downward, forward, and medially. The upward, backward, and outward pull makes EAC in a straight line. Outer one-third of the EAC is cartilaginous (8 mm in length), and attaches with bony EAC via fibrous tissue (isthmus). Cartilage is a deficit in the superior aspect of the cartilaginous EAC (incisura terminalis). Fissure of Santorini is the perforations present in the anterior cartilaginous canal responsible for the anterior spread of EAC infection into the periparotid and temporomandibular joint (TMJ) region. The cartilaginous canal continues with the pinna cartilage, and the lining skin contains sweat glands and hair follicles. The inner two-thirds of EAC is bony (16 mm) (Fig. 1.4). The bony EAC forms by tympanic bone with contributions from the squamous part of the temporal bone superiorly and mastoid bone posteriorly. The isthmus is the bony cartilaginous junction, the narrowest part of EAC. Another bony EAC narrowing forms by an anterior canal bulge close to the tympanic membrane. The tympanic sulcus is the medial end of the bony canal occupied by the annulus of the tympanic membrane. The superior deficit part of the tympanic sulcus is known as the Notch of Rivinus.

Table 1.1 Relationship of EAC with surroundings.

Superiorly	Middle cranial fossa
Inferiorly	Upper one-third of neck and parotid gland
Anteriorly	TMJ and superior part of parotid gland
Medially	Tympanic membrane
(EAC: external auditory canal; TMJ: temporomandibular joint)

The relationship of EAC with its surroundings is mentioned in Table 1.1.

The greater auricular, lesser occipital, auriculotemporal branch of the trigeminal nerve, and auricular branch of the vagus nerve (Arnold's nerve) are the sensory supply of the external ear. Branches of posterior auricular and superficial temporal arteries are the vascular supply. Periauricular nodes (preauricular, mastoid tip, and level II nodes) drain lymphatic from the external ear. The function of the external ear is localization and collection of sound.

MIDDLE EAR ANATOMY

The middle ear cleft consists of the middle ear space, mastoid air cells, and the Eustachian tube. It lies inside the temporal bone. The middle ear cleft's volume ranges from 2 to 20 cc. The middle ear (tympanic cavity) is a slit-like space, present medial to the tympanic membrane, and has 6 walls. The lateral dimension of the middle ear space is 6 mm, 2 mm, and 4 mm at the upper, middle, and lower parts, respectively. The anteroposterior and superoinferior dimension is 15 mm. Middle ear space is divided into three subspaces by imaginary line passes from the upper and lower limit of tympanic sulcus— epitympanum, mesotympanum, and hypotympanum (Fig. 1.5).5

Fig. 1.5: Middle ear space division, also depicting the lateral dimensions at different levels.

(LPI: lenticular process of the incus)

Epitympanum (attic) is the superior most part, mesotympanum is the middle part, and hypotympanum is the inferior most part. The facial canal defines the medial demarcation line for the attic and mesotympanum. The tympanic diaphragm forms the floor of the epitympanum.

Relationship with Surroundings

Middle ear has six walls surrounded by different structures (Table 1.2 and Fig. 1.6)

Contents of Middle Ear (Table 1.3) (Figs. 1.7 to 1.9)

The tympanic diaphragm, ossicles (malleus, incus, and stapes), middle ear muscles (tensor tympani and stapedius), tympanic plexus, and vessels are the contents of the middle ear.

The mastoid antrum is the space inside the mastoid bone. The relationship of mastoid antrum with the surroundings is mentioned in (Table 1.4 and Fig. 1.9).

Mastoid air cells are broadly divided into five regions by Allam—(1) middle ear, (2) mastoid, (3) perilabyrinthine (supra- and infralabyrinthine), (4) petrous apex (peritubal and apical) and (5) accessory cells. Mastoid antrum proper and peripheral cells (tegmen, sinodural, tip, facial, and sinus cells) are subdivisions of mastoid air cells. Zygomatic, squamous, occipital, and styloid air cells are the cells that come under the accessory division of air cells.

The middle ear cleft lined by ciliated columnar epithelium. Secretary cells are more abundant in promontory, hypotympanum, and anterior epitympanum areas, so pars tensa pathologies present with moderate to severe ear discharge. The anterior tympanic branch of the maxillary artery, the stylomastoid branch of the posterior auricular artery, tympanic branch of the ascending pharyngeal artery are the feeders from the external carotid system. Caroticotympanic branches from the internal carotid artery (ICA) also supply the middle ear cleft.

ANATOMY OF THE INNER EAR

Cochlea, vestibule, and semicircular canals are the parts of the bony labyrinth (Fig. 1.10). Cochlea is a 2½ turn along the central axis bony modiolus. The apex of the cochlea corresponds to the tensor tympani muscle medially. Basal turn generates a promontory bulge in the middle ear.

The bony cochlear space is divided into three areas the basilar membrane below and Reissner's membrane above. The basilar membrane is thick, stiff, and narrow at the base of the cochlea and thin and flexible at the apex level.

Table 1.2 Surrounding structures of the middle ear.

Floor (inferior wall)	It is formed by a bony plate over the jugular bulb and separates the middle ear space from the jugular vein.
Roof (superior wall)	The roof is separated from the middle cranial fossa by the tegmen tympani. Tegmen is the thin plate of the petrous temporal bone.
Lateral wall	Epitympanum (attic) outer bony wall, called scutum, forms by the temporal bone's squamous part. The lower part of the outer epitympanum wall forms by pars flaccida. The pars tensa part of the tympanic membrane forms the mesotympanum lateral wall. Tympanic bone forms the outer wall of the hypotympanum.
Posterior wall	Attic communicates posteriorly with mastoid antrum and mastoid air cells via aditus. The pyramidal process (medial) and chorda tympani (lateral) are structures related to the posterior mesotympanum wall. Facial nerve (mastoid segment) runs superolateral to pyramidal process
Anterior wall	It is a thin plate of bone separating the middle ear space from the internal carotid artery. It contains two openings: Tensor tympani canal superiorly and Eustachian tube inferiorly. The tensor tympani canal is the superior opening in the anterior wall of the mesotympanum. The eustachian tube orifice is present just inferior to the tensor tympani canal. Petrotympanic fissure is located at the anterior and lateral wall junctions. Anterior malleolar ligament, chorda tympani, and tympanic branch of maxillary artery occupies petrotympanic fissure.
Medial wall	Promontory is a prominent bulge in the medial wall of the mesotympanum. The basal turn of the cochlea forms it. Processus cochleariformis is the prominent bulge above the medial wall's promontory and lies just anteroinferior to the first genu of the facial nerve. It also provides attachment to the tensor tympani muscle. Facial nerve (tympanic segment) runs posteriorly from the processus cochleariformis above the oval window. Oval window and round window niche is located posterosuperior and posterior-inferiorly to the promontory. The oval window niche occupies a stapes footplate. Ponticulus and subiculum are the bony prominences that continue posteriorly from the promontory, and they define the upper and lower limit of sinus tympani, which is a deep recess in the posterior wall. Sinus tympani is the common site for cholesteatoma recidivism.

Fig. 1.6: Relationship of the middle ear with surrounding structures.

Table 1.3 Contents of middle ear.

Tympanic diaphragm	It is the mucosal fold between the epitympanum and mesotympanum. The aeration of the epitympanum occurs via two openings in the tympanic diaphragm.
Malleus (Figs. 1.7 and 1.8)	Malleus is the lateral most ossicle and has the head, neck, handle (manubrium), anterior, and lateral processes. Pars propria (middle layer of the tympanic membrane) splits and occupies the distal end of the handle of the malleus (umbo). The lateral process provides attachment to the malleolar folds. The part of the malleus above the lateral process is in the attic, and the part below is in the mesotympanum. It units with incus posteriorly
Incus (Figs. 1.7 and 1.8)	Incus is the largest ossicle and has a body, short, long, and lenticular process. It lies in between the malleus and stapes bones. The socket in the incus's body attaches to the malleus's head. Short process lies in fossa insidious. It is the landmark for the identification of the facial nerve Lenticular process units with stapes head. The incus's long and lenticular process is most vulnerable to necrosis because of its tenuous blood supply.
Stapes (Figs. 1.7 and 1.8)	The stapes is the smallest and lightest (3–5 mg) bone of the body. It has a head, neck, footplate, anterior crura, and posterior crus. Posterior crura is thicker and longer than the anterior crura. The dimensions of the foot plate are 3 × 1.4 mm
Tensor tympani muscle	The tensor tympani muscle originates from the tensor tympani canal, the greater wing of the sphenoid and cartilaginous portion of the Eustachian tube. The muscle attaches to the malleus neck's medial aspect and the malleus handle's upper part after swinging around the processus cochleariformis. Tensor tympani open the Eustachian tube by increasing the middle ear pressure by the inward pulling effect of the tympanic membrane. The muscle receives motor supply from the nerve to the medial pterygoid muscle, a branch of the mandibular division of the trigeminal nerve.
Stapedius muscle (Fig. 1.7)	Stapedius muscle originated from the pyramidal process and is inserted on the posterior crura and neck's posterior aspect. The contraction of the middle ear muscles decreases the amount of sound transmission to the inner ear (stapedial reflex). The 2nd branch of the facial nerve supplies the muscle.
Chorda tympani	It is the third branch of the facial nerve. It arises 3–4 mm above the stylomastoid foramen and runs anteriorly just deep to the mucosal lining of the tympanic membrane. It comes out from the middle ear via a petrotympanic fissure.
Tympanic plexus	Tympanic branch of the glossopharyngeal nerve (Jacobson's nerve) and the sympathetic around the internal carotid artery (ICA) form it. It is present on the medial wall of the middle ear over promontory.

Fig. 1.7: Left temporal bone after canal wall removal is showing 1—tegmen, 2—tympanic membrane, 3—malleus, 4—incus, 5—stapes, 6—promontory, 7—chorda tympani, 8—stapedius muscle, 9—facial nerve, 10—lateral semicircular canal bulge, and 11—hypotympanic cells.Courtesy: Dr Anup Singh, Assistant Professor, AIIMS, New Delhi, India

Fig. 1.8: Ossicular assembly of the right side.

Fig. 1.9: Right temporal bone after canal wall removal is showing 1—tegmen, 2—tympanic membrane, 3—malleus, 4—incus, 5—oval window, 6—round window, 7—promontory, 8—ponticulus, 9—subiculum, 10—sinus tympani, 11—facial nerve, 12—sinus plate, and 13—lateral semicircular canal bulge.Courtesy: Dr Anup Singh, Assistant Professor, AIIMS, New Delhi, India

Fig. 1.10: Inner ear.

Scala vestibuli is the superior space, scala media is the middle space, and scala tympani is the inferior space. The oval window is between the stapes footplate and superior space (scala vestibuli). The round window connects the middle ear space with scala tympani. The organ of Corti is located within scala media on the basilar membrane.

Helicotrema (at cochlea apex) is the merge site for superior and inferior spaces. Perilymph is the name of the fluid in the scala vestibule and scala tympani, whereas endolymph is the fluid of scala media. High sodium and low potassium ions concentrations, similar to extracellular fluid, are present in the perilymph. The aqueduct of the cochlea drains perilymph into subarachnoid space. Endolymph fluid contains low sodium and high potassium ions, similar to intracellular fluid. Endolymphatic sac and stria vascularis drain endolymph.

The organ of Corti consists of two types of sensory cells—(1) inner and (2) outer hair cells. The tunnel of the Corti is the space between both hair cells.

Table 1.4 Relationship of the mastoid antrum.

Roof	Tegmen antri separates antrum from middle cranial fossa.
Medial wall	The petrous temporal bone forms it. Bony labyrinth, endolymphatic sac, and posterior cranial fossa lie deep to the medial wall Facial canal runs anteroinferior to the bulge of lateral semicircular canal Second genu lies deep to by fossa incudis (occupies short process of incus)
Anteriorly	Aditus is present in superior part Bony canal wall separates it from bony EAC
Posteriorly	Sigmoid plate separates antrum from sigmoid sinus
Floor	The origin of the sternocleidomastoid muscle and posterior belly of the digastric muscle lies on the outer surface of the floor. Digastric ridge is the bony project present on the floor.
Lateral wall	Mastoid antrum is 1.5–2 cm deep to the lateral wall. The persistence of petrosquamous suture line forms Korner's septum. It creates a surgical challenge in the identification of mastoid antrum.

Fig. 1.11: Organ of Corti.

Inner hair cells are 3,500 in number, flask shape, arranged in a single row along the length of the cochlea. Outer hair cells are arranged in three rows, 10,000 in number, cylindrical-shaped and located lateral to inner hair cells. Stereocilia of both hair cells are arranged in descending order of length from lateral to medial direction. Deiters’, inner marginal, Hensen's, Claudius, and Boettcher's cells are the supporting cells for both hair cells. The tectorial membrane is the acellular covering over the entire length of the organ of Corti. Stria vascularis is a layered vascular tissue of the lateral wall of scala media (Fig. 1.11).

PHYSIOLOGY

The environmental sound transmits (air conduction) to the inner ear via the external and middle ear by creating a relative movement of the middle ear and inner ear structures and by direct compression of inner ear fluid. The vibration of the skull induces vibration of the basilar membrane, which is the mechanism of sound perception in bone conduction tests (Weber, absolute bone conduction, pure tone audiometry, etc.).

The cochlea is the peripheral sensory organ in the bony labyrinth. The cochlea converts mechanical energy into electric energy. The phase difference between the middle and inner ear (air to fluid) induces 30 dB sound energy loss. The tympano-ossicular system allows similar gain by reducing the effect of energy loss by the phase difference. The external ear significantly influences hearing gain at the middle ear level, which can be moderate by changing the ear's position. The maximum gain is 20 dB at a 2,500 Hz sound frequency when the sound source is in the line of EAC. The external ear is helpful in sound localization by inducing a difference in the sound pressure at the level of the tympanic membrane in both ears (head shadow effect). The middle ear increases sound pressure by two coupling mechanisms by 28 dB at the footplate level.

Area ratio: Footplate surface area is 14 times smaller than the tympanic membrane, which increases the sound pressure by 26 dB at the footplate level.
Ossicular lever mechanism: The difference in the rotating length of the malleus and incus (1.3) provides a hearing gain of 2 dB.
Acoustic coupling is the direct sound transmission from the tympanic membrane to the oval window. The contraction of the tensor tympani and stapedius muscle is the protective mechanism to reduce the impact of loud noise. It allows sound gain by 2 dB.

The vibration of the footplate generates by the ossicular chain creates motion in the inner ear fluid (scala vestibule → scala media → scala tympani), which induces the bulging of a round window into the middle ear space (Fig. 1.12). Asymmetry in the vibration pattern of the basilar membrane allows the perception of complex sounds. High-frequency sounds displace the thicker proximal part of the basilar membrane, and the thinner distal portion is by low-frequency sounds.

Human models showed a gain of 20 dB at a 1,000 Hz frequency due to variations in the vibration level in different parts of the tympanic membrane. Vibratory physics does not include the forces needed to stretch the tympanic membrane, ligaments of ossicles, etc.

Air conduction tests (Rinne's tuning fork test, audiometry, etc.) based on the sound transmission mechanism of the external and middle ear.
Frequency of sound: The vibration of particles generates sound through particle forward and backward movement. The frequency of sound is the number of vibratory cycles per second. Hertz is the unit of frequency (1 Hz = 1 cycle/s). The audible sound frequency is ranged from 20 to 20,000 Hz. The human ear is sensitive most to 500–8,000 Hz sound frequencies.
Hearing loss in tympanic membrane perforation is because of loss of pressure difference across the tympanic membrane, reduction in the vibrating area, and less energy transmission into the ossicular chain, so the amount of hearing loss varies from 0 to 50 dB conductive hearing loss (CHL).
The ossicular coupling is lost in the presence of ossicular chain discontinuity. The expected hearing loss is 60 dB conductive hearing loss (CHL) with an intact tympanic membrane.
Acoustic coupling increases in tympanic membrane perforation, so the hearing loss is less than ossicular discontinuity. Acoustic coupling is increased by 10–20 dB in tympanic membrane perforation because of the loss of protective effect of the tympanic membrane, so hearing loss is around 40–50 dB.
The simultaneous presence of ossicular discontinuity further decreases the amount of sound transmission into the inner ear. Myringosclerosis, ossicular sclerosis or fixation, and middle ear effusion decrease sound transmission by affecting one or both coupling mechanisms, and CHL range from negligible to 60 dB.

Physiology of Inner Ear

Sound energy is transmitted from the ossicular chain to the perilymph of scala vestibuli via an oval window. The incompressibility of inner ear fluids (perilymph and endolymph) induces the movement of the basilar membrane. The transmitted wave causes movement in the organ of Corti against the tectorial membrane. The sound wave deterioration occurs after it reaches its demarcated place on the basilar membrane as per the sound frequency. This topotonic arrangement (frequency specific) is identical in an organ of Corti and afferent nervous system. The basal part of the cochlea recognizes higher frequencies, and the apical part recognizes lower frequencies (the hearing range is 20–20,000 Hz).9

Fig. 1.12: Physiology of hearing.

Stereocilia are responsible for the conversion of mechanical energy into electric energy. The motion of stereocilia by sound wave allows potassium and calcium ions to enter the hair cells, generating electric energy. By providing feedback, outer hair cells act as modifiers, especially for low-threshold sounds. Afferent neutrons receive input from an organ of Corti and relay information into the cochlear nucleus. Thick, myelinated type 1 ganglionic fibers receive information primarily from inner hair cells, while thin, unmyelinated type II ganglionic fibers receive input from outer hair cells. Single inner hair cells innervate by multiple fibers, whereas outer hair cells receive a single fiber.

Cochlear Nerve and Central Connection

The cochlear nerve consists of 30,000 nerve fibers. 95% of cochlear nerve fibers innervate inner hair cells. The nerve forms the peripheral synaptic connection with the hair cells and the central connection with the brainstem. The nerve runs in the internal auditory meatus. Dorsal and ventral cochlear nuclei are the first relay station of cochlear fibers. The superior olivary complex is the second relay station for the cochlear nerve. The cochlear function is regulated by the efferent fibers from the superior olivary complex, which synapse with afferent fibers of the cochlear nerve. Medial fibers of the cochlear nerve decussate at the level of the superior olivary complex. Postganglionic fibers from the superior olivary complex and lateral fibers of the cochlear nerve follow the pathway of hearing: lateral lemniscus, medial geniculate body, and auditory cortex (Fig. 1.13).

The internal auditory meatus is 7–9 mm long and 2–6 mm wide. The porous is the wide medial opening, and the fundus is the narrow lateral end. Fundus lies medial to the basal turn of the cochlea. The falciform crest divides the fundus end into small superior and large inferior parts. The superior part is divided by a vertical crest (Bill's bar) into anterior space for the facial nerve and posterior space for the superior vestibular nerve.
Bill's bar is used as an anatomical landmark for the facial nerve in the translabyrinthine approach.
Cochlear nerve traverses in the anterior part, and the inferior vestibular nerve traverses in the posterior part of the inferior space.

Fig. 1.13: Central auditory pathway.

(RM: Reissner's membrane; SM: scala media; ST: scala tympani; SV: scala vestibuli)

SUMMARY

The middle ear cleft consists of the middle ear space, mastoid air cells, and the Eustachian tube.
The lateral dimension of the middle ear space is 6 mm, 2 mm, and 4 mm at the upper, middle, and lower parts, respectively. The anteroposterior and superoinferior dimension is 15 mm.
Middle ear space is divided into three subspaces by imaginary line passing from the upper and lower limit of tympanic sulcus—epitympanum, mesotympanum, and hypotympanum.
A bony plate forms the floor over the jugular bulb.
The roof is separated from the middle cranial fossa by tegmen.
The scutum, tympanic membrane, and part of the tympanic bone form the lateral wall.
The posterior wall contains aditus (superiorly), pyramidal process, and opening for the chorda tympani nerve.
The tensor tympani canal and Eustachian tube orifice are present in the anterior wall. It also separates the middle ear from the internal carotid artery.
The promontory is the medial wall bulge created by the basal turn of the cochlea. Oval and round windows are present posterosuperior and posteroinferior to the promontory.
Processus cochleariformis is inferior to the geniculate ganglion in the medial wall. It provides attachment to the tensor tympani muscle.
The facial canal forms the medial limit of the epitympanum.
The contents of the middle ear are ossicles (malleus, incus, and stapes), muscles (tensor tympani, stapedius), tympanic diaphragm, and chorda tympani nerve.
The cochlea is divided into three spaces (scala vestibule, scala media, and scala tympani) by two membranes (Reissner's membrane above and basilar membrane below).
Organ of Corti is present in scala media over the basilar membrane.
The organ of Corti consists of two types of sensory cells—(1) inner and (2) outer hair cells. Inner hair cells are 3,500 in number, flask shape, arranged in a single row along the length of the cochlea. Outer hair cells are arranged in three rows, 10,000 in number, cylindrical-shaped and located lateral to inner hair cells.
Deiter's, inner marginal, Hensen's, Claudius, and Boettcher cells are the supporting cells for both hair cells.
The tectorial membrane is the acellular covering over the organ of Corti.
The sound reaches the inner ear through three mechanisms: compression distortion, the skull's vibration, and the bone's inertia.
The sound transmits via EAC to the tympanic membrane. The vibration of the tympanic membrane transmits to the inner ear via an ossicular chain (ossicular coupling) and by direct transmission to an oval window (acoustic coupling). The organ of the Corti and cochlear nerve assesses the inner ear fluid movement topologically.
Inner hair cells input modifies by outer hair cells.
The induced electric signals traverse via the cochlear nerve, cochlear nucleus, superior olivary complex, lateral lemniscus, medial geniculate body, and auditory cortex.

SELF-ASSESSMENT

REVIEW QUESTIONS

What is middle ear cleft? Explain anatomy in detail and its relationship with surroundings?
What are the contents of middle ear? Explain in detail.
Explain the anatomy of middle ear with line diagram.
What is the hearing physiology?
Explain the sound transmission mechanism of inner ear.
How sound transmitted to auditory cortex?
Prepare line diagram of the following:
1. Organ of Corti
2. Auditory cortex
3. Medial wall of middle ear
4. Middle ear anatomy

MULTIPLE CHOICE QUESTIONS

1. Which structure not belongs to auditory pathway?

Superior olivary complex
Lateral lemniscus
Inferior colliculus
Lateral geniculate body

2. Primary auditory cortex is known as:

Brodmann area 41
Brodmann area 6
Brodmann area 16
Brodmann area 4

3. Which nerve supplies stapedius muscle?

7th cranial nerve
5th cranial nerve
9th cranial nerve
12th cranial nerve

4. Humans can detect sounds in a frequency range from about:

200–20,000 Hz
20–20,000 Hz
20–200 Hz
20–2,000 Hz11

5. The picture showing cortical mastoidectomy. The thick yellowish part (black arrow) is representing which structure?

Labyrinth
Endolymphatic sac
Facial nerve
Dorello's canal

6. Which is not a content of the middle ear?

Stapes muscle
Tegmen tympani muscle
Facial nerve
Tympanic diaphragm

7. Which structure is not come in surrounding relationship with middle ear?

Korner's septum
Tegman tympani
Sigmoid plate
Dural plate

8. Which is not a supporting cell for organ of Corti?

Deiter's cell
Hensen's cell
Claudius cell
Mikulicz cell

ANSWERS

1. d 2. b 3. a 4. b 5. a 6. c 7. a 8. d

TRUE AND FALSE

State whether the following statements are true or false.

For the impedance matching of the middle ear, one mechanism is called the lever ratio. It refers to the difference in length between the malleus's manubrium and the incus's long process.
Sinus tympani is the recess in the superior wall of the middle ear.

ANSWER KEY:

True
False

Chapter Notes

Anatomy of Ear and Physiology of HearingCHAPTER 1

SUMMARY