Ear

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THE EAR

Dr. Mohamad S. Aziz

Otolaryngologist
CABMS (ORL-HNS)
ENT Dept, College of Medicine, University of Mosul

Undergraduate, The EAR, 2016/2017

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وَاللّهُ أَخْرَجَكُم مِّن بُطُونِ أُمَّهَاتِكُمْ لاَ تَعْلَمُونَ شَيْئاً وَجَعَلَ لَكُمُ الْسَّمْعَ وَالأَبْصَارَ وَالأَفْئِدَةَ لَعَلَّكُمْ تَشْكُرُونَ [النحل: 78]

Anatomy of Ear

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THE EXTERNAL EAR
The external ear consists of the (i) auricle or pinna, (ii) external acoustic canal and (iii) the tympanic membrane




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1. Auricle or Pinna
The pinna, except its lobule, and the outer part of external acoustic canal are made up of yellow elastic cartilage covered with skin.
The cartilage is closely adherent to the perichondrium on its lateral surface while it is slightly loose on the medial surface.
There is no cartilage between the tragus and crus of the helix, and this area is called the incisura terminalis .
Cartilage and perichondrium from the tragus or concha, and fat from the lobule are frequently used for reconstructive surgery of the middle ear.



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2. External Acoustic (Auditory) Canal
extends from the bottom of the concha to the TM (24 mm) . not a straight tube; outer part is directed upwards, backwards and medially , inner part is directed downwards, forwards and medially.
(a) Cartilaginous Part
It forms outer one-third (8 mm) of the canal, is a continuation of the cartilage of the pinna. The skin covering the cartilaginous canal is thick and contains ceruminous glands which secrete wax. Hair is only confined to the outer canal .
(b) Bony Part
It forms inner two-thirds (16 mm). Skin lining the bony canal is thin and continuous over the TM. It is devoid of hair and ceruminous glands. About 6 mm lateral to TM, the bony meatus presents a narrowing called the isthmus. (Foreign bodies).
Antero-inferior part of the deep meatus, beyond the isthmus, is called the anterior recess which acts as a cesspool for discharge and debris in cases of ear infections.

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3. Tympanic Membrane or the Drumhead
It forms partition between the EAC and the middle ear.
It is 9-10 mm tall, 8-9 mm wide and 0.1 mm thick (oval).


TM consists of three layers:
(i) Outer epithelial layer continuous with the skin lining the meatus.
(ii) Inner mucosal layer continuous with the mucosa of the middle ear.
(iii) Middle fibrous layer.

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• TM can be divided into two parts:
• (a) Pars Tensa
• It forms most of TM. Its periphery is thickened to form a fibrocartilaginous ring called the annulus tympanicus which fits in the tympanic sulcus. The central part of pars tensa is tented inwards at the level of the tip of malleus and is called the umbo. A bright cone of light can be seen radiating from the tip of malleus to the periphery in the anteroinferior quadrant.
• (b) Pars Flaccida (Shrapnell's Membrane)
• situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds . It is not so taut .
• Fibrous layer in the para flaccida is thin.

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Landmarks of a normal tympanic membrane of right side.

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Nerve Supply of the External Ear
(i) Greater auricular nerve (C2,3) supplies most of the medial surface of pinna and only posterior part of the lateral surface .
(ii) Lesser occipital (C2) supplies upper part of medial surface.
(iii) Auriculotemporal (V3) supplies tragus, crus of helix and the adjacent part of the helix. (with anterior wall of EAC)
(iv) Auricular branch of vagus (CN X), also called Arnold's nerve, supplies the concha and corresponding eminence on the medial surface.(with posterior wall of EAC)
(v) Facial nerve, which is distributed with fibres of auricular branch of vagus, supplies the concha and retroauricular groove.





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THE MIDDLE EAR
The middle ear together with the eustachian tube, aditus, antrum and mastoid air cells is called the middle ear cleft. It is lined by mucous membrane and filled with air.
Middle ear can be likened to a six-sided box with a roof, a floor, medial, lateral, anterior and posterior walls .




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The roof
is formed by a thin plate of bone called tegmen tympani. It also extends posteriorly to form the roof of the aditus and antrum. It separates tympanic cavity from the middle cranial fossa.
The floor
is also a thin plate of bone which separates tympanic cavity from the jugular bulb. Sometimes, it is congenitally dehiscent and the jugular bulb may then project into the middle ear; separated from the cavity only by the mucosa.
The anterior wall
has a thin plate of bone which separates the cavity from internal carotid artery. It also has two openings; the lower one for the eustachian tube and the upper one for the canal of tensor tympani muscle.





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The posterior wall
lies close to the mastoid air cells. It presents a bony projection called the pyramid through the summit of which appears the tendon of the stapedius muscle to get attachment to the neck of stapes. Aditus, an opening through which attic communicates with the antrum, lies above the pyramid. Facial nerve runs in the posterior wall just behind the pyramid.
The lateral wal
formed largely by the TM and to a lesser extent by the bony outer attic wall called the scutum.



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The medial wall
formed by the labyrinth.
a bulge called promontory which is due to the basal coil of cochlea;
oval window into which is fixed the footplate of stapes;
round window or the fenestra cochleae which is covered by the secondary tympanic membrane.
Above the oval window is facial nerve canal, may be congenitally dehiscent.
Above the facial nerve is prominence of lateral semicircular canal.
Just anterior to the oval window a hook-like projection called the processus cochleariformis. The tendon of tensor tympani takes a turn here to get attachment to the neck of malleus


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Mastoid Antrum
is a large, air-containing space in the upper part of mastoid and communicates with the attic through the aditus.
Its roof is formed by the tegmen antri which is a continuation of the tegmen tympani and separates it from the middle cranial fossa.
The lateral wall of antrum is formed by a plate of bone which is on an average 1.5 cm thick in the adult. It is marked externally on the surface of mastoid by suprameatal (MacEwen's) triangle .



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Ossicles of the Middle Ear

There are three ossicles in the middle ear-the malleus, incus and stapes.



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Intratympanic Muscles
Tensor tympani attaches to the neck of malleus and tenses the tympanic membrane ,develops from the 1st arch and is supplied by a branch of mandibular nerve (V3).

The stapedius attaches to the neck of stapes and helps to dampen very loud sounds thus preventing noise trauma to the inner ear, is a 2nd arch muscle and is supplied by a branch of CN VII
Lining of the Middle Ear Cleft
ET is lined by ciliated epithelium which is pseudostratified columnar with several mucous glands in the submucosa.


Tympanic cavity is lined by ciliated columnar epithelium in its anterior and inferior part which changes to cuboidal type in the posterior part.

Epitympanum and mastoid air cells are lined by flat, nonciliated epithelium.

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Tympanic Plexus
lies on the promontory
formed by (i) tympanic branch of glossopharyngeal and (ii) sympathetic fibres from the plexus round the internal carotid artery.
supplies innervation to the medial surface of the TM, tympanic cavity, mastoid air cells and the bony ET.
It also carries secretomotor fibres for the parotid gland.
Section of tympanic branch of glossopharyngeal nerve can be carried out in cases of Frey's syndrome.

Chorda Tympani Nerve

It is a branch of the facial nerve
runs on the medial surface of the TM between the handle of malleus and long process of incus.
It carries taste from anterior two-thirds of tongue and supplies secretomotor fibres to the submaxillary and sublingual salivary glands.

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Blood Supply of Middle Ear

Middle ear is supplied by six arteries,
two are the main:
• Anterior tympanic branch of maxillary artery which supplies TM
• Stylomastoid branch of posterior auricular artery which supplies middle ear and mastoid air cells.


Four minor vessels are:
• Petrosal branch of middle meningeal artery.
• Superior tympanic branch of middle meningeal artery.
• Branch of artery of pterygoid canal (runs along ET).
• Tympanic branch of internal carotid.

Veins drain into pterygoid venous plexus and superior petrosal sinus

Lymphatic Drainage of Ear
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THE INTERNAL EAR

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Bony Labyrinth
1. Vestibule
is the central chamber of the labyrinth. In its lateral wall lies the oval window. The inside of its medial wall presents two recesses, a spherical recess for the saccule and an elliptical recess for the utricle. Below the elliptical recess is the opening of aqueduct of vestibule through which passes the endolymphatic duct.
2. Semicircular canals
They are three in number, the lateral, posterior and superior, and lie in planes at right angles to one another. Each canal has an ampullated end which opens independently into the vestibule and a nonampullated end. The non-ampullated ends of posterior and superior canals unite to form a common channel called the crus commune. Thus, the three canals open into the vestibule by five openings.

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3. Cochlea
The bony cochlea is a coiled tube making 2.5 to 2.75 turns round a central pyramid of bone called the modiolus. The modiolus transmits vessels and nerves to the cochlea. the promontory, is due to the basal coil of the cochlea.
The bony cochlea contains three compartments:
•(a) scala vestibuli,
•(b) scala tympani,
•(c) scala media or the membranous cochlea .
The scala vestibuli and scala tympani communicate with each other at the apex of cochlea through an opening called helicotrema. Scala vestibuli is closed by the footplate of stapes (at oval window) which separates it from middle ear. The scala tympani is closed by secondary tympanic membrane (at round window) .


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Membranous Labyrinth
It consists of the cochlear duct, the utricle and saccule, the three semicircular ducts, and the endolymphatic duct and sac.
1. Cochlear duct
Also called membranous cochlea or the scala media. It is a blind coiled tube. It appears triangular on cross-section formed by: (a) the basilar membrane, which supports the organ of corti,
(b) the Reissner's membrane which separates it from the scala vestibuli, (c) the stria vascularis, which is concerned with secretion of endolymph.
Cochlear duct is connected to the saccule by ductus reuniens





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2. Utricle and saccule
The utricle lies in the posterior part of bony vestibule. It receives the five openings of the three semicircular ducts. It is also connected to the saccule through utriculosaccular duct. The sensory epithelium of the utricle is called the macula and is concerned with linear acceleration and deceleration. The saccule also lies in the bony vestibule, anterior to the utricle and opposite the stapes footplate. Its sensory epithelium is also called the macula. It probably also responds to linear acceleration and deceleration.





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3. Semicircular ducts

They are three in number and correspond exactly to the three bony canals. They open in the utricle. The ampullated end of each duct contains a thickened ridge of neuroepithelium called crista ampullaris.

4. Endolymphatic duct and sac

Endolymphatic duct is formed by the union of two ducts, one from each of the saccule and the utricle. It passes through the vestibular aqueduct. Its terminal part is dilated to form endolymphatic sac which lies between the two layers of dura on the posterior surface of the petrous bone.






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Inner Ear Fluids and their Circulation
Perilymph
resembles extracellular fluid and is rich in Na ions. It fills the space between the bony and the membranous labyrinth. It communicates with CSF through the aqueduct of cochlea which opens into the scala tympani near the round window.
Endolymph
fills the entire membranous labyrinth and resembles intracellular fluid, being rich in K ions. It is secreted by the secretory cells of the stria vascularis of the cochlea and by the dark cells of the vestibule.

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Blood Supply of Labyrinth
The entire labyrinth receives its arterial supply through labyrinthine artery which is a branch of anterior-inferior cerebellar artery .
Venous drainage is through three veins, namely internal auditory vein, vein of cochlear aqueduct and vein of vestibular aqueduct which ultimately drain into inferior petrosal sinus and lateral venous sinus.



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Eustachian Tube Anatomy
also called auditory or pharyngotympanic tube, connects nasopharynx with the tympanic cavity.
In an adult, it is about 36 mm long and runs downwards, forwards and medially from its tympanic end, forming an angle of 45° with the horizontal.
It is divided into two parts: The bony (12 mm) and fibrocartilaginous (24 mm). The two parts meet at isthmus which is the narrowest part of the tube .


The tympanic end of the tube is situated in the anterior wall of middle ear. The pharyngeal end of the tube is slit-like, vertically. The cartilage at this end raises an elevation called torus tubarius which is situated in the lateral wall of the nasopharynx, 1-1.25 cm behind the posterior end of inferior turbinate

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Muscles Related to Eustachian Tube
Three muscles are related to the tube: Tensor veli palatini, Levator veli palatini and the Salpingopharyngeus. The tensor when contract help to open the tubal lumen.
The exact role of the levator veli palatini and the Salpingopharyngeus muscles to open the tube is uncertain.



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Lining of the Eustachian Tube
Histologically, the mucosa shows pseudostratified ciliated columnar epithelium interspersed with mucous secreting goblet cells.
The cilia beat in the direction of nasopharynx and thus helps to drain secretions and fluid from the middle ear into the nasopharynx.
Nerve Supply
Tympanic branch of CN IX supplies sensory as well as parasympathetic secretomotor fibres to the tubal mucosa.
Tensor veli palatini is supplied by mandibular branch of trigeminal (V3) nerve.
Levator veli palatini and salpingopharyngeus receive motor nerve supply through pharyngeal plexus (Cranial part of CN XI through vagus).


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Differences Between the Infant and Adult ET
The ET of infants is wider, shorter and more horizontal; thus infections from the nasopharynx can easily reach the middle ear.
Even the milk may regurgitate into the middle ear if the infants are not fed in head-up position
Functions
Physiologically, ET performs three main functions:
•1. Ventilation and thus regulation of middle ear pressure.
•2. Protection against (a) Nasopharyngeal sound pressure and (b) Reflux of nasopharyngeal secretions.
•3. Middle ear clearance of secretions

DEVELOPMENT OF EAR (EMBRYOLOGY)

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Auricle
Around the sixth week of embryonic life, six tubercles from the first and second branchial arches ( hillocks of His) appear around the first branchial cleft. They progressively coalesce to form the auricle.
By the 20th week, pinna achieves adult shape.

External auditory meatus

develops from the first branchial cleft. By about the 16th embryonic week, the ectodermal cleft form a meatal plug.
Recanalisation begins from the deeper part near the TM and progresses outwards. External ear canal is fully formed by the 28th week of gestation.


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Tympanic membrane

develops from all the three germinal layers. Outer epithelial layer is formed by the ectoderm, inner mucosal layer by the endoderm and the middle fibrous layer by the mesoderm.
Middle ear cleft
The ET, tympanic cavity, attic, antrum and mastoid air cells develop from the endoderm of tubotympanic recess mainly from first pharyngeal pouch
Malleus and incus are derived from mesoderm of the first arch while the stapes develop from the second arch.


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Membranous inner ear
Development of the inner ear starts in the third week of foetal life and is complete by the 16th week(without end organs).
Ectoderm in the region of hind brain thickens to form an auditory placode which is invaginated to form auditory vesicle or the otocyst. The latter then differentiates into the endolymphatic duct and sac; the utricle, the semicircular ducts; and saccule and the cochlea.
The cochlea is developed sufficiently by 20 weeks of gestation and the foetus can hear in the womb of the mother.

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Peripheral Receptors and Physiology of Auditory and Vestibular Systems


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AUDITORY SYSTEM

Organ of Corti
is the sense organ of hearing and is situated on the basilar membrane
1. Tunnel of Corti which is formed by the inner and outer rods.
2. Hair cells important receptor cells of hearing & transduce sound into electrical energy.
3. Supporting cells
4. Tectorial membrane It consists of gelatinous matrix with delicate fibres. It overlies the organ of Corti. The shearing force between the hair cells and tectorial membrane produces the stimulus to hair cells




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Differences between inner and outer hair cells

Inner hair cells
Outer hair cells
Total no.
3500
12,000
Rows
One row
Three or four rows
Shape
Flask-shaped
Cylindrical
Nerve supply
Primarily afferent fibres and very few efferent
Mainly efferent fibres and very few afferent
Development
Develop earlier
Develop late
Function
Transmit auditory stimuli
Modulate function of inner hair cells
Vulnerability
More resistant
Easily damaged by ototoxic drugs and high intensity noise


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Mechanism of Hearing
A sound signal in the environment is collected by the pinna, passes through external auditory canal and strikes the TM.
Vibrations of the TM are transmitted to stapes footplate through a chain of ossicles .
Movements of stapes footplate cause pressure changes in the labyrinthine fluids which move the basilar membrane. This stimulates the hair cells of the organ of Corti.
It is these hair cells which act as transducers and convert the mechanical energy into electrical impulses which travel along the auditory nerve.

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Conduction of Sound
This function of the middle ear is called impedance matching mechanism or the transformer action.
(a) Lever action of the ossicles. Handle of malleus is 1.3 times longer than long process of the incus, providing a mechanical advantage of 1.3.
(b) Hydraulic action of TM. The area of TM is much larger than the area of stapes footplate, the average ratio between the two being 21:1. As the effective vibratory area of tympanic membrane is only two-thirds, the effective areal ratio is reduced to 14:1.




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Frequency localisation in the cochlea. Higher frequencies are localised in the basal turn and then progressively decrease towards the apex.


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Neural Pathways (ECOLI-MG-AC)

Hair cells get innervation from the bipolar cells of spiral ganglion. Central axons of these cells collect to form cochlear nerve which goes to cochlear nuclei. From there fibres travel to the superior olivary nucleus, lateral lemniscus, inferior colliculus, medial geniculate body and finally reach the auditory cortex of the temporal lobe.




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VESTIBULAR SYSTEM
consists of the three SCC, the saccule and the utricle. They are fluid filled and have cells with cilia which bend as the fluid moves relative to them. This excites or depresses the nerve cells .
The semicircular canals detect changes in angular acceleration.
The utricle and saccule detect linear acceleration.
The nerve impulses from the labyrinth go to the vestibular nuclei in the brain stem. Here they are integrated with vision and proprioception. The brain stem computerizes these three inputs and with the help of the cerebellum maintains the balance and co-ordination of the head and body.



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Peripheral Receptors
1. Cristae
They are located in the ampullated ends of the three semicircular ducts. These receptors respond to angular acceleration.


2. Maculae
They are located in otolith organs (i.e. utricle and saccule). They sense position of head in response to gravity and linear acceleration.





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