Anatomy of the ear

Provides a detailed description of the gross anatomy of the ear for undergraduate medical students; i.e. parts of the ear, structures found, their blood supply, their innervation, developmental origins & their functions. It also includes examples of common disorders associated with those parts.

1. Anatomy of the Ear Card-5 Item-14

2. Introduction The ear is the organ of hearing and balance. It has three parts : External Ear Middle Ear Inner Ear

3. External Ear Parts of the External Ear: Auricle: A wrinkly musculocutaneous tissue that is attached to the skull and functions to capture sound. External Acoustic Meatus: A bony-cartilaginous canal that projects from the auricle to the middle ear, from which it is separated by the tympanic membrane. The tympanic membrane: It separates the external acoustic meatus from the middle ear

4. Auricle: Parts & Muscles Parts of the Auricle: Helix: The large outside rim of the auricle . It ends inferiorly at the fleshy lobule , the only part of the auricle not supported by cartilage . Antihelix: A smaller curved rim, parallel and anterior to the helix. Concha of the auricle: The fossa that is continuous with the external acoustic meatus at the center of the auricle. Tragus & Anitragus : The tragus is an elevation anterior to the opening of the external acoustic meatus , in front of the concha. Opposite the tragus , and above the fleshy lobule , is another elevation the antitragus. Muscles: help in positioning auricle (innervated by CN VII) Intrinsic Muscles Extrinsic Muscles: the anterior, superior, and posterior auricular muscles

5. Auricle: Innervation, Vessels Innervation: Sensory: S uperficial surfaces : are supplied by the great auricular nerve, lesser occipital nerve, Auriculotemporal branch of Mandibular nerve Deeper parts : supplied by the vagus nerve [X] (the auricular branch) and the facial nerve [VII] Blood Supply: Arterial supply: posterior auricular branch, superior auricular artery from External Carotid Artery , anterior auricular branches from superficial temporal artery & a branch of O ccipital Artery Venous drainage : follows arterial supply Lymphatic drainage : Parotid nodes anteriorly, posteriorly into mastoid nodes and deep cervical lymph nodes

6. External acoustic meatus The external acoustic meatus extends from the deepest part of the concha to the tympanic membrane (eardrum), a distance of approximately 1 inch . It consists of: Membranous External Acoustic M eatus : makes up the lateral 1/3 of this canal . It is directed posteriorly and superiorly Bony External Acoustic M eatus : makes up the medial 2/3 of the canal . It is directed anteriorly and inferiorly . Throughout its length the external acoustic meatus is covered with skin , some of which contains hair and modified sweat glands producing cerumen (earwax) . Its diameter varies, being wider laterally and narrow medially.

7. Tympanic Membrane The tympanic membrane is a thin, oval shaped translucent partition between the external acoustic meatus & middle ear. It measures 9×10mm and is positioned at an acute angle of 55 o with the floor of external acoustic meatus. Surfaces: It has 2 surfaces: Outer Surface : Lined by skin and faces the external ear . I nner Surface : Lined by mucous membrane . At its center, a concavity is produced by the attachment of the lower end of the handle of the malleus . This point of attachment is the umbo of the tympanic membrane . Anteroinferior to the umbo is a bright reflection of light (seen in otoscopic examinations), referred to as the cone of light . Subdivisions: Extending away from the elevation of the lateral process of the malleus are the anterior and posterior malleolar folds . These divide the membrane into: Pars flaccida (thin)- Between folds Pars tensa (thick)- rest of the membrane

8. Tympanic Membrane Histological Structure and Embryological origins : The tympanic membrane is composed of 3 layers: Outer Cuticular layer (Ectoderm of 1 st pharyngeal cleft): Lined by hairless, keratinized stratified squamous epithelium . Intermediate fibrous layer (Surrounding Mesenchyme): made up of superficial radiating fibers and deep circulating fibers Inner mucous layer (Endoderm of 1 st pharyngeal pouch) : lined by simple columnar or squamous epithelium. Blood Supply: Arterial supply: Outer Surface : deep auricular branch of maxillary artery Inner Surface: Anterior tympanic branch of maxillary artery and posterior tympanic branch of stylomastoid branch of posterior auricular artery Venous drainage : Outer Surface: External Jugular vein Inner Surface: Transverse sinus and into venous plexus around auditory tube Lymphatic drainage : Pre auricular & retro pharyngeal lymph nodes Innervation: Outer surface: Auriculotemporal nerve and Auricular branch of vagus Inner Surface : Tympanic branch of Glossopharyngeal nerve

9. External Ear: Clinical Anatomy + Examination of the External ear The otoscopic examination begins by grasping the posterosuperior aspect of the ear and gently retracting it to straighten the external auditory meatus . The normal tympanic membrane is relatively translucent and has a gray – reddish tinge. + Perforation of the Tympanic Membrane The tympanic membrane is a relatively thin and is susceptible to perforation  (usually by trauma or infection). An infection of the middle ear (otitis media) causes pus and fluid to build up behind the tympanic membrane . This causes an increase in pressure within the middle ear, and eventually the eardrum can rupture . In some cases the tympanic membrane heals itself, but in larger perforations surgical grafting may be required . I t may be necessary to enter the middle ear through the tympanic membrane. Because the chorda tympani runs in the upper 1/3 of the tympanic membrane , incisions are always below this level . The richer blood supply to the posterior aspect of the tympanic membrane determines the standard surgical approach in the posteroinferior aspect .

10. Middle Ear Parts of the middle ear: Tympanic cavity immediately adjacent to the tympanic membrane, and Epitympanic recess superiorly . Ear ossicles Boundaries of the middle ear Roof   – formed by a thin bone from the petrous part of the  temporal bone . It separates the middle ear from the middle cranial fossa. Floor   – known as the jugular wall , it consists of a thin layer of bone , which  separates  the middle ear from the internal jugular vein Lateral wall   – made up of the tympanic membrane and the lateral wall of the epitympanic recess . Medial wall   – formed by the lateral wall of the internal ear . It contains a prominent bulge , produced by the facial nerve as it travels nearby. Anterior   wall   – a thin bony plate with 2 openings ; for the auditory tube and the tensor tympani muscle . It separates the middle ear from the internal carotid artery. Posterior wall  (mastoid wall) – it consists of a bony partition between the tympanic cavity and the mastoid air cells .

11. Middle Ear: Boundaries

12. Boundaries: Medial Wall The labyrinthine (medial) wall of the middle ear is also the lateral wall of the internal ear. A prominent structure on this wall is a rounded bulge (the promontory) produced by the basal coil of the cochlea . Associated with the mucous membrane covering the promontory is a plexus of nerves (the tympanic plexus ) Tympanic Plexus primarily consists of: tympanic branch of the Glossopharyngeal nerve CN (IX) branches from the internal carotid plexus Functions: supplies the mucous membrane of the middle ear , the mastoid area , and the pharyngotympanic tube. Sends a branch (the lesser petrosal nerve ) to otic ganglion. Windows & structures passing through medial wall: The oval window is posterosuperior to the promontory , is the point of attachment for the base of the stapes (footplate) The round window is posteroinferior to the promontory Posterior and superior to the oval window on the medial wall is the prominence of the facial canal , which is a ridge of bone produced by the facial nerve [VII] Just above and posterior to the prominence of the facial canal is a broader ridge of bone ( prominence of the lateral semicircular canal) produced by the lateral semicircular canal

13. Branches of the Facial Nerve Course: The facial nerve (CN VII) enters the internal acoustic meatus along with CN VIII. CN VII enters the facial canal and continues laterally between the internal and middle ear. It is at this point that the sensory geniculate ganglion forms a bulge in CN VII and gives rise to the following branches: Greater petrosal nerve . Provides visceral motor innervation to the lacrimal, nasal , and palatal glands . Nerve to the stapedius muscle (branchial motor ). Provides innervation to the stapedius muscle . Chorda tympani nerve .

14. Chorda Tympani Nerve Course: Arises from CN VII before it exits the skull , courses through the middle layer of the tympanic membrane, continues between the malleus and stapes , and exits the skull at the petro-tympanic fissure. Innervation: The chorda tympani innervates the submandibular and sublingual salivary glands Conveys taste sensation (special sensory) from the anterior 2/3s of the tongue.

15. Mastoid Area Location: Posterior to the epitympanic recess of the middle ear is the aditus to the mastoid antrum , which is the opening to the mastoid antrum Description: The mastoid antrum is a cavity continuous with collections of air-filled spaces (the mastoid cells) , throughout the mastoid part of the temporal bone, including the mastoid process. Function: The mastoid air cells act as a ‘ buffer system ’ of air –   releasing air into the tympanic cavity when the pressure is too low. + Mastoiditis (Clinical) Infection within the mastoid antrum and mastoid cells is usually secondary to infection in the middle ear because the mucous membrane lining the mastoid air cells is continuous with that of the middle ear. This is called Mastoiditis

16. Pharyngotympanic Tube Description : The pharyngotympanic tube connects the middle ear with the nasopharynx Formation : Its opening in the middle ear is on the anterior wall , and from here it extends forward, medially, and downward to enter the nasopharynx just posterior to the inferior meatus of the nasal cavity Parts: It consists of: bony part (the 1/3 nearest the middle ear) cartilaginous part (the remaining 2/3 s). Function : Equalizes pressure on both sides of the tympanic membrane. Blood Supply: Arterial supply: Branches arise from the ascending pharyngeal artery (a branch of the external carotid artery) and from 2 branches of the maxillary artery Venous drainage : pterygoid plexus of veins in the infratemporal fossa Innervation: Primarily from the Tympanic plexus

17. Auditory ossicles Malleus The malleus is the largest and is attached to the tympanic membrane . Its posterior surface articulates with the incus . (Saddle- type joint) The anterior process is attached to the anterior wall of the middle ear by a ligament. The lateral process is attached to the anterior and posterior malleolar folds of the tympanic membrane. It is the smallest long bone in the body! Incus The 2 nd bone in the series of auditory ossicles is the incus. The body of the incus articulates with the head of the malleus and is in the epitympanic recess. The long limb bends medially to articulate with the stapes . The short limb is attached by a ligament to the upper posterior wall of the middle ear. Stapes The stapes is the most medial bone in the osseous chain and is attached to the oval window The head of the stapes articulates with the long process of the incus. (Ball & Socket Joint) The 2 limbs separate from each other and attach to the oval base . It’s the smallest bone in the body!

18. Muscles associated with Auditory Ossicles

19. Muscles associated with Auditory Ossicles Functions: Tensor tympani : Tenses the tympanic membrane, reducing the force of vibrations in response to loud noises Stapedius : Contraction of the stapedius muscle, usually in response to loud noises , pulls the stapes posteriorly and prevents excessive oscillation . It is the smallest muscle in the body!

20. Middle Ear: Blood Supply Arterial supply : Numerous arteries supply the structures in the middle ear: The 2 largest branches are the tympanic branch of the maxillary artery and the mastoid branch of the occipital or posterior auricular arteries ; Smaller branches come from the middle meningeal artery, the ascending pharyngeal artery, the artery of the pterygoid canal, and tympanic branches from the internal carotid artery . Venous drainage: By the pterygoid plexus of veins and the superior petrosal sinus .

21. Middle Ear: Innervation & Origins Innervation: General Sensory: mucosal side of the tympanic membrane via the  tympanic branches  of the  Glossopharyngeal nerve (CN IX). tympanic cavity is also innervated by CN IX via the tympanic plexus . Sympathetic: caroticotympanic nerves  from the carotid sympathetic plexus. Motor: (to muscles) Stapedius from a branch of the facial nerve (CN VII) Tensor tympani is supplied via the  medial pterygoid branch  of CN V3 (Mandibular nerve) . Embryological Origins: Tympanic cavity – Distal part Endoderm of the 1st pharyngeal pouch Pharyngotympanic tube – Proximal part Endoderm of the 1st pharyngeal pouch Ear ossicles - Mesenchyme of the 1 st pharyngeal arch (malleus & incus ) and 2 nd pharyngeal arch (stapes)

22. Middle Ear: Clinical Anatomy + Otitis media Otitis media is a viral or bacterial infection of the middle ear observed commonly in children . The infection may migrate along the pharyngotympanic tube .  Therefore, it is common to see patients presenting with both otitis media and pharyngitis or tonsillitis. This is because the position of the pharyngotympanic tube in children is more horizontal and also the length of the pharyngotympanic tube is smaller than that of adults . These features may facilitate the spread of pathogenic agents to and from nasopharynx . + Hyperacusis A lesion of CN VII may cause paralysis of the stapedius muscle , resulting in wider oscillation of stapes. Consequentially, there is a heightened reaction of the auditory ossicles to sound vibration . This condition is known as hyperacusis and results in an increased sensitivity to loud sounds .

23. Internal Ear

24. Internal Ear: Location & Components Location: The inner ear is located within the  petrous part of the Temporal bone. It lies between the middle ear and the internal acoustic meatus , which lie laterally and medially respectively. Components: The inner ear has 2 main components – the bony labyrinth and membranous labyrinth. Bony labyrinth  – consists of a series of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and 3 semi-circular canals. All these structures are lined internally with periosteum and contain a fluid called perilymph . Membranous labyrinth  –  lies within the bony labyrinth. It consists of the cochlear duct, 3 semi-circular ducts, utricle and the saccule. The membranous labyrinth is filled with fluid called endolymph . Openings: The inner ear has 2 openings into the middle ear, both covered by membranes. Oval window  lies between the middle ear  and the vestibule , whilst Round window   separates the middle ear  from the scala tympani (part of the cochlear duct).

25. Bony labyrinth

26. Bony labyrinth: Vestibule The vestibule , which contains the oval window in its lateral wall, is the central part of the bony labyrinth . It communicates anteriorly with the cochlea and posterosuperiorly with the 3 semicircular canals . A narrow canal (the vestibular aqueduct ) leaves the vestibule, and passes through the temporal bone to open on the posterior surface of the petrous part of the temporal bone. Two parts of the membranous labyrinth ; the  saccule  and  utricle , are located within the vestibule .

27. Bony labyrinth: Semi Circular Canals Projecting in a posterosuperior direction from the vestibule are the anterior, posterior, and lateral semicircular canals Each of these canals forms 2/3s of a circle connected at both ends to the vestibule and with one end dilated to form the ampulla. The canals are oriented so that each canal is at right angles to the other two .

28. Bony labyrinth: Cochlea Structure : The cochlea houses the cochlea duct of the membranous labyrinth – the auditory part of the inner ear. It  twists upon itself 2.5 times around a central portion of bone called the  modiolus,  producing a cone shape which points in an  anterolateral direction . Branches from the cochlear portion of the  vestibulocochlear (CN VIII) nerve  are found at the base of the modiolus . Extending outwards from the modiolus is a ledge of bone known as  spiral lamina ,  which attaches to the cochlear duct, holding it in position. The presence of the cochlear duct creates two perilymph-filled chambers above and below: Scala vestibuli : Located superiorly to the cochlear duct . As its name suggests, it is continuous with the vestibule. Scala tympani : Located inferiorly to the cochlear duct . It terminates at the round window. The scala vestibuli and the scala tympani are separated completely, except at the narrow apex of the cochlea called the helicotrema , where they are continuous.

29. Membranous labyrinth

30. Membranous labyrinth Description: The membranous labyrinth is a continuous system of ducts filled with  endolymph.   It lies within the bony labyrinth , surrounded by perilymph. Arrangement: The general organization of the parts of the membranous labyrinth places: cochlear duct within the cochlea of the bony labyrinth , anteriorly, saccule and utricle within the vestibule of the bony labyrinth , in the middle. 3 semicircular ducts within the 3 semicircular canals of the bony labyrinth, posteriorly Organs of balance : 5 of the 6 components of the membranous labyrinth are concerned with balance. Utricle Saccule 3 Semicircular ducts Organ of hearing : Cochlear duct

31. Membranous labyrinth: Organs of balance Utricle, saccule, and endolymphatic duct 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 of the bony labyrinth. The 3 semicircular ducts empty into the utricle . Saccule - A smaller, rounded sac lying in the anteroinferior part of the vestibule of the bony labyrinth. The cochlear duct empties into it. Endolymphatic   duct- Duct via which Endolymph from the saccule and utricle drains into. Function: In the utricle and saccule the sense organ of balance is the macula of the utricle and the macula of the saccule , respectively. They respond to linear acceleration : Utricle responds to linear acceleration in the horizontal plane and sideways head tilts Saccule responds to linear acceleration in the vertical plane , Semicircular Ducts: The semi-circular ducts are located within the semi-circular canals , and share their orientation. Upon movement of the head, the flow of endolymph  within the ducts changes speed and/or direction . Function : Sense organ of balance is in the ampulla of each of the 3 semicircular ducts is the crista. Sensory receptors in the crista respond to rotational movement in any direction

32. Membranous labyrinth: Organ of hearing Cochlear duct: Description: The cochlear duct is located within the bony scaffolding of the cochlea. It is held in place by the spiral lamina. The presence of the duct creates 2 canals above and below it – the  scala vestibuli  and  scala tympani  respectively. Arrangement : The cochlear duct can be described as having a triangular shape : Lateral wall  – Formed by thickened periosteum , known as the spiral ligament . Roof  – Formed by a membrane which separates the cochlear duct from the scala vestibuli , known as the vestibular membrane /Reissner’s membrane . Floor  – Formed by a membrane which separates the cochlear duct from the scala tympani , known as the basilar membrane . The basilar membrane houses the epithelial cells of hearing – the  Spiral Organ/ Organ of Corti. It projects into the enclosed, endolymph-filled cochlear duct

33. Internal Ear: Blood Supply Blood supply: Arterial Supply: Bony Labyrinth: 3 arteries , which also supply the surrounding temporal bone: Anterior tympanic branch  (from maxillary artery). Petrosal branch  (from middle meningeal artery). Stylomastoid branch  (from posterior auricular artery). Membranous Labyrinth: supplied by the  labyrinthine artery , a branch of the inferior cerebellar artery has 3 branches: Cochlear branch  – supplies the cochlear duct. Vestibular branches (x2)  – supply the vestibular apparatus. Venous drainage: Venous drainage of the inner ear is through the  labyrinthine vein , which empties into the sigmoid sinus or inferior petrosal sinus.

34. Internal Ear: Innervation Innervation: The inner ear is innervated by the  vestibulocochlear (CN VIII) nerve . It enters the inner ear via the internal acoustic meatus, where it divides into the  vestibular nerve  (responsible for balance) and the  cochlear nerve  (responsible for hearing): Vestibular nerve  – enlarges to form the  vestibular ganglion , which then splits into superior and inferior parts to supply the utricle, saccule and 3 semi-circular ducts. Cochlear nerve   – enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti.

35. Internal Ear: Clinical Anatomy + Motion sickness   Motion sickness is a condition characterized by nausea and vomiting due to travelling. It is due to fluctuations in maculae . + Vertigo Vertigo is a condition in which a person has a false sensation that either him or surroundings are in motion . It can cause nausea, dizziness, sweating and vomiting . This condition is associated with vestibular malfunction.  + Meniere’s disease Meniere’s disease is a disorder of the inner ear , characterised by episodes of vertigo, low-pitched tinnitus and hearing loss . The symptoms are thought to be caused by an excess accumulation of   endolymph   within the membranous labyrinth , causing progressive distension of the ducts. The resulting pressure fluctuations damage the thin membranes of the ear that detect balance and sound .

36. Internal Ear: Embryological Origins Membranous Labyrinth: At about 22 days , a thickening of ectoderm on either side of the hindbrain develops; this is the otic placode. The placode invaginates forming a pit that later becomes separated from the ectoderm , forming the otic vesicle. The otic vesicle is surrounded by mesoderm that will become the otic capsule, the cartilaginous precursor of the bony labyrinth. Bony Labyrinth: The cartilage that surrounds the membranous labyrinth (otic capsule) is ossified (weeks 16–24) and creates a perilymph‐filled protective space for the inner ear.

37. Transmission of Sound

38. Transmission of Sound A sound wave enters the external acoustic meatus and strikes the tympanic membrane . The sound wave transfers its energy into the vibration of the tympanic membrane . As the tympanic membrane vibrates , it causes the malleus to move medially , which in turn causes the incus and stapes to move sequentially , amplifying the sound wave. The stapes is attached to the oval window/vestibular window ; thus, the oval window also moves , resulting in a wave forming in the perilymph within the scala vestibuli of the cochlea. The fluid wave in the perilymph progresses from the scala vestibuli of the cochlea , resulting in an outward bulging of the round window/cochlear window at the end of the scala tympani. This bulging causes the basilar membrane in the cochlea to vibrate, which in turn results in stimulation of the receptor hair cells in the spiral organ (of Corti). The receptor hair cells conduct impulses to the Brain through the cochlear division of CN VIII , where the Brain interprets the wave as sound .

39. Central Auditory Pathway The 1 st order axons entering the brainstem from the cochlea terminate ipsilaterally on the dorsal and ventral cochlear nuclei . From here, 2 nd order axons forming the ascending auditory pathway ascend to the pons and project bilaterally on the superior olivary nucleus forming the trapezoid body . (Bilateral projections are important for auditory acuity and localizing the sound origin).

40. Central Auditory Pathway From superior olivary nucleus and the posterior nucleus of trapezoid body, fibers continue to ascend as the lateral lemniscus and terminate in the inferior colliculus of the midbrain. Axons leaving the inferior colliculus then have a synaptic relay in the medial geniculate nucleus of the thalamus . Those axons then reach the final synaptic target: the primary auditory cortex The sound is then interpreted by the secondary auditory cortex ( Wernicke’s Area)

41. Summary of Central Auditory Pathway

42. Summary of Central Auditory Pathway 1 st order axons from cochlea enter dorsal and ventral cochlear nuclei (in brainstem) ipsilaterally (same side as they are situated) via cochlear nerve . From here, 2 nd order axons ascend and project bilaterally on the superior olivary nucleus and forming the trapezoid body . (in Pons ) Axons continue to ascend as the lateral lemniscus (3 rd order) They terminate in the inferior colliculus of the midbrain . 4 th order axons (leaving inferior colliculus ) then have a synaptic relay in the medial geniculate nucleus of the thalamus Axons then reach the Primary auditory cortex in the superior temporal gyrus of the cerebrum Primary auditory cortex then send signals to Secondary auditory cortex to interpret sound .