review of anatomy of the ear

8/10/2019 Review of Anatomy of the Ear

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REVISIT ANATOMY OF EAR

LEARNING OBJECTIVES

At the end of this class the student should be able to,

Discuss the division of ear in parts.

Understand the parts of external ear.

Understand the parts of middle ear.

Understand the parts of internal ear.

Consists of auricle or pinna and external acoustic meatus.

THE EXTERNAL EAR

PINNA: Crumpled plate of elastic cartilage covered by skin. LOBULE: Lower part and soft part of ear, consists ofconnective tissue covered by skin.

CONCHA: Is large depression which leads in to externalacoustic meatus.

INTRINSIC MUSCLES: Which can alter the shape of ear. EXTERNAL MUSCLES: Move ear (are rudumentary in human).

PINNA cont.

Blood supply: Posterior auricular artery and superior temporal artery. Lymphatic drainage: Preauricular and posterior auricular and superficial temporal lymph

nodes. Nerve supply:


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Upper two third of lateral surface auriculotemporal nerve and lowerone thirds by greater auricular nerve.

Medial surface upper two third by lesser occipital nerve and lower onethird by greater auricle nerve.

Root of auricle by is supplied by branches of facial nerve.

EXTERNAL ACOUSTIC MEATUS

Conducts sound waves. S shaped. About 3cm in length. It is straightened for examination by pulling

upwards backwards and slightly laterally. Anterior wall and floor are longer than posterior

wall and roof. Medial third is bony and lateral two thirds is cartilaginous.


BONY PART:

C shaped in cross section.

Lined by thin skin firmly adherent to periosteum. Anteriorly formed by tympanic part of temporal bone. Posterosuperior part is formed by squamous temporal

bone.

Cartilaginous part:

Also C shaped in section gap is filled by fibrous tissue.

Skin is adherent to the perichondrium. Contains sebaceous gland and ceruminous glands (modified sweatglands).


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Blood supply: Outer part by superficial temporal and posteriorauricular arteries. Inner part by deep auricular branch of maxillaryartery.

Lymphatic drainage: Preauricular, postauricular and superficial cervical lymph nodes. Nerve supply: Anterior half by auriculotemporal nerve and posterior half by auricular

branch of vagus.

THE MIDDLE EAR

Middle ear or Tympanic cavity is about 15 mm inanteroposterior and vertical diameter.

Shape of a biconcave lens. The lateral wall :Is largely occupied by the

tympanic membrane. Tympanic membrane extends upwards for 10 mm

from the floor and also bulges inwards. Above the membrane the temporal bone is hollowed out into the

epitympanic recess .

THE TYMPANIC MEMBRANE

Thin fibrous structure. Circular . 1cm in diameter. Covered externally: By thin layer of stratified

squamous epithelium (consists of collagen fibers).

Lies obliquely at 55 degree with the external acoustic meatus. Facing downward forward and laterally.

Concave towards the meatus. At the depth of the concavity is a small depression, the umbo.


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Cone of light (is at anteroinferior quadrant).


The handle of the malleus is firmly attached to the innersurface of membrane.

From the lateral process of the malleus two thickenedfibrous folds (mallear folds) diverge up to the margins ofthe tympanic bone between them is parsflaccidasharpnells membrane . (crossed internally by the chordatympani ).

Pars tensa . (It is held tensa by the inward pull of the tensor tympanicmuscles). The tympanic membrane is thickened at its circumference and

slotted into a groove in the tympanic plate.


BLOOD SUPPLY: Deep auricular artery from maxillary artery on themeatal side and stylomastoid artery from posterior auricular artery onthe mucosal surface forms a circular anastomosis with the anteriortympanic branch of the maxillary artery round the margin of themembrane

NERVE SUPPLY: on the meatal surface : auriculotemporal nervesupplemented by the vagus and on the mucosal surface: Tympanicbranch of the glossopharyngeal nerve via the tympanic plexus.

THE MEDIAL WALL OF THE TYMPANIC CAVITY

Is also lateral wall of the internal ear. The promontory (first turn of thecochlea and indented with fine grooves by the tympanic plexus.

Above it is a horizontal ridge for the canal for the facial nerve , and


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immediately above that is the (horizontal) bulge dueto the lateral semicircular canal.

Above and behind the promontory is the oval windowthe fenestra vestibuli (closed in life by the foot piece

of the stapes). Below and behind the promontory is the round

window the fenestra cochleae closed in life by thefibrous secondary tympanic membrane.

THE ROOF OF THE TYMPANIC CAVITY

Tegmen tympani (petrous bone that roofs also the

canal for the tensor tympanic and the tympanicantrum.

THE FLOOROF THE TYMPANIC CAVITY Is a thin plate of bone above the jugular fossa. At the anterior end is the internal opening of the tympanic canaliculus.

(for tympanic branch of the glossopharyngeal nerve).

THE ANTERIOR WALL OF THE TYMPANIC CAVITY

Shortend by approximation of roof and floor . It is perforated by the opening of two canals . Lower is the bony part of the auditory tube. Upper is the canal for the tensor tympani muscle. Lower part of this wall forms the posterior wall of the

carotid canal (perforated by tympanic branches of theinternal carotid artery and sympathetic fibers from the internal carotidplexus)

THE POSTERIOR WALL OF THE TYMPANIC CAVITY

Is deficient above , where there is an aperture, the aditus , which leadback in to the tympanic antrum.


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Below the aditus a hollow cone, the pyramid, projectsin to the tympanic cavity.

Apex is perforated by the tendon of stapedius.

TYMPANIC ANTRUM

Small, circular, air filled space, situated in the posterior part of thepetrous part of the temporal bone.1 cm in size, is of adult size at birth.

Superiorly . Tegmen tympani. Beyond it is temporal lobe of

cerebrum.

Inferiorly. Mastoid process with mastoid air cells. Anteriorly. Communicates with epitympanic recess through aditus. Posteriorly . Separated from sigmoid sinus by thin plate of bone. Medially . Petrous temporal bone. Laterally . Squamous temporal bone. Relation with suprameatal triangle).

CONTENTS OF MIDDLE EAR CAVITY 1. Ossicles malleus, incus, stapes 2. Muscles tensor tympani, stapedius 3. Vessels and nerves chorda tympani and

tympanic plexus of ear.

FUNCTIONS OF MIDDLE EAR CAVITY


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Narrow, oblique, slit like, air containing cavity in the petrous part oftemporal bone.

Transmit the vibrations of tympanic membrane to the perilymph ofinternal ear.

OSSICLES1. MALLEUS

The malleus or hammer is a hammer-shaped bone Connected with the incus and is attached to the inner surface of the

tympanic membrane

It transmits the sound vibrations from the eardrum to the incus.

2. INCUS The incus or anvil is the anvil-shaped small bone. It connects the malleus to the stapes The incus transmits sound vibrations from the malleus to the stapes.

MUSCLES

STAPEDIUS

The stapedius is the smallest skeletal muscle in thehuman body. At just over one millimeter in length, itspurpose is to stabilize the smallest bone in the body,the stapes.

The stapedius emerges from a pinpoint foramen inthe apex of the pyramidal eminence (a hollow, cone-shaped prominence in the posterior wall of thetympanic cavity), and inserts into the neck of the stapes.

Innervated by the nerve to stapedius, a branch of cranial nerve VII, thefacial nerve.

MUSCLES


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TENSOR TYMPANI

The larger of the two muscles of the tympanic cavity, iscontained in the bony canal above the osseous portionof the auditory tube.

Its role is to dampen sounds, such as those producedfrom chewing.

It arises from the cartilaginous portion of the auditorytube and the adjoining part of the great wing of the sphenoid

Inserted into the handle (manubrium) of the malleus, near its root.

AUDITORY TUBE

Is the channel through which the tympanic cavity communicates withthe nasal part of the pharynx.

35 mm long (in adults). Extends from the anterior wall of the middle ear to the lateral wall of

the nasopharynx.

Has two parts Osseous part Cartilagenous part

ARTERIES OF MIDDLE EAR

Anterior tympanic branch (maxillary artery)

Superior tympani (middle meningeal artery) and petrosal branch

Posterior tympani (posterior auricular artery) Inferior tympanic (ascending Ph artery) Tympanic branch from artery of pterygoid canal.Coticotympanic from

I.C artery

VEINS AND LYMPH NODES OF MIDDLE EAR CAVITY Pterygoid


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plexus of veins which drain into superior petrosal sinus.

LYMPH NODES: Periauricular and retro pharyngeal nodes

NERVE SUPPLY Tympanic branch of glossopharyngeal nerve(tympanic plexus). Superior and inferior corticotympanic nerves fromthe sympathetic plexus around internal carotid artery .

INTERNAL EAR

All these structures are in the petrous part of thetemporal bone between the middle ear laterallyand the internal acoustic meatus medially.

PARTS OF INTERNAL EAR

INTERNAL EAR CONSISTS OF: Bony labyrinth

Vestibule, Three semicircular canals Cochlea

Membranous labyrinth.

Semicircular ducts Cochlear duct Two sacs (utricle and saccule).

These bony cavities are lined with periosteum and contain a clear fluidthe perilymph .

The membranousspaces are filled with endolymph .


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FUNCTION OF INTERNAL EAR

The internal ear converts the mechanical signalsreceived from the middle ear, which start as soundcaptured by the external ear, into electrical signals totransfer information to the brain.

The internal ear also contains receptors that detect motion andposition.

INTERNAL EAR

The structures in the internal ear convey informationto the brain about balance and hearing:

Cochlear duct is the organ of hearing Semicircular ducts, utricle, and saccule are the

organs of balance.

The nerve responsible for these functions is the vestibulocochlearnerve [VIII], which divides into vestibular (balance) and cochlear(hearing) parts after entering the internal acoustic meatus.

BONY LABYRINTH

Vestibule : It is the central part of the bony labyrinth.

contains the oval window in its lateral wall It communicates anteriorly with the cochlea andposterosuperiorly with the semicircular canals.

A narrow canal (the vestibular aqueduct ) leaves the vestibule, andpasses through the temporal bone to open on the posterior surface ofthe petrous part of the temporal bone.


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BONY LABYRINTH

Semicircular Canals

Projecting in a posterosuperior direction from the vestibule are three innumber Anterior (superior) vertical Posterior (vertical) Lateral (horizontal)

Each of these canals forms two-thirds of a circleconnected at both ends to the vestibule and withone end dilated to form the ampulla .

The canals are oriented so that each canal is at right angles to theother two.

BONY LABYRINTH

COCHLEA

It is a bony structure that twists on itself two and one-half to two and three-quarter times around a centralcolumn of bone (the modiolus ).

This arrangement produces a cone-shaped structurewith a base of cochlea that faces posteromedially andan apex that faces anterolaterally.

This positions the wide base of the modiolus near the internal acousticmeatus, where it is entered by branches of the cochlear part of thevestibulocochlear nerve [VIII].

BONY LABYRINTH


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LAMINA OF MODIOLUS , OR SPIRAL LAMINA ).

Circling around the modiolus, and held in a central position by itsattachment to the lamina of modiolus, is the cochlear duct, which is acomponent of the membranous labyrinth.

Cochlear duct creates 2 canals Scalavestibuli Continuous with the vestibule Scala tympani Separated from the middle ear by the secondary

tympanic membrane covering the round window

They extend throughout the cochlea and are continuous with eachother at the apex through a narrow slit (the helicotrema ) .

COCHLEAR AQUEDUCT

Near the round window is a small channel (the cochlearcanaliculus ), which passes through the temporal boneand opens on its inferior surface into the posterior cranialfossa.

This provides a connection between the perilymph-containing cochlea

and the subarachnoid space.

MEMBRANOUS LABYRINTH It is a continuous system of ducts and sacs within the

bony labyrinth.

It is filled with endolymph and separated from theperiosteum that covers the walls of the bony labyrinthby perilymph.

Consisting of Two sacs (the utricle and the saccule) and Four ducts (the three semicircular ducts and the cochlear duct). The general organization of the parts of the membranous labyrinth

places:


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ORGANS OF BALANCE

5 of the 6 components of the membranous labyrinth are concernedwith balance. These are the: Two sacs (the utricle and the saccule)

Three ducts (anterior, posterior, and lateral semicircular ducts).

Utricle The utricle is the larger of the two sacs. It is oval, elongated and

irregular in shape and is in the posterosuperior part of the vestibule ofthe bony labyrinth.

Three semicircular ducts empty into the utricle.

Saccule The saccule is a smaller, rounded sac lying in the

anteroinferior part of the vestibule of the bonylabyrinth. The cochlear duct empties into it.

The utriculosaccular duct establishes continuitybetween all components of the membranous labyrinthand connects the utricle and saccule

ORGANS OF BALANCE Branching from utricosaccular duct is the endolymphatic duct , which

enters the vestibular aqueduct (a channel through thetemporal bone) to emerge onto the posterior surfaceof the petrous part of the temporal bone in the

posterior cranial fossa. Here the endolymphatic duct expands into the

endolymphatic sac , which is an extradural pouch.

Each semicircular duct is similar in shape, including a dilated endforming the ampulla, to its complementary bony semicircular canal,only much smaller.


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SENSORY RECEPTORS Functionally, sensory receptors for balance are organized into unique

structures and in each of the components of the vestibular apparatus. In the utricle and saccule this sense organ is the macula of utricle

and macula of saccule ,

Ampulla of each of the three semicircular ducts it is the crista. Utricle responds to centrifugal and vertical acceleration, Saccule responds to linear acceleration. Receptors in the three semicircular ducts respond to movement in any

direction.

ORGAN OF HEARING Triangular-shaped cochlear duct has: An outer wall against the bony cochlea consisting of

thickened, epithelial-lined periosteum (the spiralligament ).

Roof (vestibular surface membrane )

which separates the endolymph in the cochlear duct from the perilymph inthe scalavestibuli and consists of a membrane with a connective tissuecore lined on either side with epithelium;

Floor , which separates the endolymph in the cochlear duct from theperilymph in the scala tympani and consists of the free edge of the

lamina of modiolus, and a membrane ( basilar membrane ) extendingfrom this free edge of the lamina of modiolus to an extension of thespiral ligament covering the outer wall of the cochlea.

VESSELS


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Arterial supply to the internal ear is divided between vessels supplyingthe bony labyrinth and the membranous labyrinth.

Bony labyrinth is supplied by the same arteries that supply thesurrounding temporal bone-these include

Anterior tympanic branch from the maxillary artery stylomastoid branch from the posterior auricular artery Petrosal branch from the middle meningeal artery.

MEMBRANOUS LABYRINTH

LABYRINTHINE ARTERY , which either arises from the anteriorinferior cerebellar artery or is a direct branch of the basilar artery-whatever its origin, it enters the internal acoustic meatus with the facial[VII] and glossopharyngeal [IX] nerves and eventually divides into

INNERVATION The vestibulocochlear nerve [VIII] carries special afferent fibers for

hearing (the cochlear component) and balance (the vestibularcomponent).

It enters the lateral surface of the brainstem, between the pons andmedulla, after exiting the temporal bone through the internal acousticmeatus and crossing the posterior cranial fossa.

Inside the temporal bone, at the distal end of the internal acousticmeatus, the vestibulocochlear nerve divides to form: the cochlearnerve , and the vestibular nerve .

INNERVATION


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The vestibular nerve enlarges to form the vestibular ganglion , beforedividing into superior and inferior parts , which distribute to the threesemicircular ducts and the utricle and saccule.

The cochlear nerve enters the base of the cochlea and passesupwards through the modiolus.

The ganglion cells of the cochlear nerve are in the spiral ganglion atthe base of the lamina of modiolus as itwinds around the modiolus.

Branches of the cochlear nerve pass throughthe lamina of modiolus to innervate thereceptors in the spiral organ.

THE END

review of anatomy of the ear

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