Ear and Auditory Pathways PDF

The Ear and CN VIII Dr. A. Aktar 1 3 parts: The Ear • External ear • Auricle • Ext. acoustic meatus 2 air filled spaces • Middle Ear • Tympanic cavity • Inner ear • Labyrinth Fluid filled space 2 The Ear External Ear • Auricle, a.k.a. pinna • Ext. acoustic meatus 3 The Ear Auricle, a...

The Ear and CN VIII Dr. A. Aktar 1 3 parts: The Ear • External ear • Auricle • Ext. acoustic meatus 2 air filled spaces • Middle Ear • Tympanic cavity • Inner ear • Labyrinth Fluid filled space 2 The Ear External Ear • Auricle, a.k.a. pinna • Ext. acoustic meatus 3 The Ear Auricle, a.k.a. Pinna • Tragus • Antitragus • Helix • Antihelix • Concha • Lobule 4 5 The Ear Cartilage of the Auricle 6 The Ear External Ear Muscles 7 The Ear Auricle Innervation Sensory: • Greater occipital n. & Lesser occipital n. (cervical plexus) Cranial surface of auricle (i.e. medial), post part (helix, anti-helix, lobule) • Auriculotemporal n. (CN V 3 ) Skin anterior to ext. acoustic meatus Motor: • Post auricular branch of facial n. Intrinsic & extrinsic muscles • Vagus n. auricular branch (Arnold n.) Minor contribution 8 The Ear Auricle Vascular Supply • Post auricular a. (Ext carotid a.) • Ant auricular a. (Superficial temporal a.) • Auricular a. (Occipital a.) 9 The Ear Lymphatics • Parotid • Deep cervical • Mastoid 10 The Ear External Acoustic Meatus • 2.5 – 3 cm • Cartilaginous part (outer 1/3rd) • Bony part 11 The Ear External Acoustic Meatus cartilage • Ceruminous glands: Specialized sudoriferous (sweat) glands Located subcutaneously In ext auditory canal, in outer 1/3rd • Sebaceous Glands • Cerumen • Tragi 12 The Ear External Acoustic Meatus Innervation • Auriculotemporal n. (CN V3) • Vagus n. auricular branch a.k.a. Arnold n. Vascular Supply • Post auricular a. (Ext carotid a.) • Ant auricular a. (Superficial temporal a.) • Deep auricular a. (Maxillary a.) 13 14 The Ear Tympanic Membrane • Covered w/ skin on outside & mucous membrane on inside • Membrane connected to surrounding temporal bone by a fibrocartilaginous ring • Translucent. Allows structures within middle ear to be observed w/otoscopy • On inner surface of membrane, handle of malleus attaches to tympanic membrane, at point called umbo 15 The Ear Tympanic Membrane • Central concavity • Umbo 16 The Ear Tympanic Membrane • Separates ext acoustic meatus from middle ear • Connective tissue core • Skin on outside, mucosa on inside • Lies at an angle: • Slopes medially from top to bottom • Slopes posteriorly to anteriorly ➔ Lateral surface faces anteriorly & inferiorly 17 The Ear Tympanic Membrane • Around its periphery a fibrocartilaginous ring attaches it to tympanic part of temporal bone • Lower end of handle of malleus attached to internal surface → central concavity • Point of attachment: umbo • Deepest part of concavity 18 The Ear Tympanic Membrane • Line made by malleolus: Stria mallearis 19 20 The Ear Tympanic Membrane Sensory Innervation Skin on External surface: • Auriculotemporal n. (CN V 3 ) • Auricular branch of vagus • Small contribution of facial n. to auricular branch of vagus Mucosa on Internal Surface • Glossopharyngeal n. 21 Middle Ear The Ear Lies in temporal bone Picture: Medial Wall – labryinthine part of tympanic cavity shown Vertical section along longitudinal axis of petrous part of temporal bone Anterolateral view 22 Middle Ear The Ear Picture: Medial Wall – labryinthine part of tympanic cavity shown After removal of lateral wall & adjacent parts of ant & sup wall Facial canal & carotid canal have been opened up Anterolateral view 23 The Ear Tympanic Cavity • Irregular cavity • Filled w/air from nasopharynx • Contains ossicles • Lined w/mucosa • Tympanic cavity proper • Epitympanic recess Function: • Transmit vibrations from ext. acoustic meatus to inner ear 24 The Ear General scheme and orientation of components • Ext acoustic meatus runs lateral to medial • Axis of tympanic membrane runs inferiorly & anteriorly as it proceeds laterally • Same as above for axis of cochlea • The long axes of the bony & membranous labyrinths & of the pharyngotympanic tube & the parallel tensor tympani & levator palatini m.m. lie perpendicular to those of tympanic membrane & cochlea (i.e., they run inferiorly & anteriorly as they proceed medially). 25 The Ear General scheme and orientation of components . 26 The Ear Auditory Ossicles • • • • • • • Chain of interonnected, moveable ossicles 1st bones to be fully ossified during fetal development Mature at birth Exceptionally dense bone Covered w/mucosa lining tympanic cavity Do not have surrounding layer of osteogenic periosteum Make synovial joints w/each other 27 Incus – Anvil A metalworking tool consisting of a large block of metal (usually forged or cast steel), with a flattened top surface, upon which another object is struck (or "worked"). Örs Malleus - Mallet Tokmak • A kind of hammer, often made of rubber or sometimes wood • Usually has a relatively large head • Most mallets have striking faces that are softer than steel Stapes – Stirrup üzengi The Ear Auditory Ossicles – In Situ 31 The Ear Auditory Ossicles - Isolated manubrium 32 The Ear Auditory Ossicles – Joints & Ligaments Note chorda tympani crossing medial surface of neck of malleus 33 The Ear Tympanic Cavity • Body of incus & head of malleus are in epitympanic recess • Neck of malleus lies against flaccid part of tympanic membrane • Handle of malleus embedded in tympanic membrane w/its tip at the umbo → malleus moves w/membrane • Base (footplate) of stapes fits into oval window on medial wall of tympanic cavity • Chorda tympani crosses medial surface of neck of malleus (see prior slide picture) 34 Cadeveric Image: Medial view of lateral wall - looking towards outer ear 35 The Ear Muscles Within Tympanic Cavity Tensor tympani muscle • Inserts into manubrium of malleus • Lies in bony canal above pharyngotympnaic tube • Ends in rounded tendon, inserts into upper part of manubrium • Innervation: Branch from mandibular n. Stapedius muscle • Originates from inside pyramidal eminence • Inserts into post surface of neck of stapes • Innervation: Branch from facial n. • Pyramidal eminence (pyramid): Hollow, cone-shaped prominence on post. wall of tympanic cavity 36 Middle Ear Eustachian Tube, a.k.a. Pharyngotympanic Tube a.k.a. • Ostium pharyngeum tubae auditivae • Ostium tympanicum tubae auditivae • Allows air pressure to be equalized on both sides Tuba Auditoria of tympanic membrane • 3.5-4 cm length • Opened during swallowing by contraction of tensor veli palatini & levator veli palatini m.m of soft palate • Motor innervation: Pharyngeal plexus of vagus n. & mandibular n. 37 Tuba Auditoria • Middle Ear Cartilaginous part: More transverse oriented vs. bony part → angle formed btwn 2 parts - looks upwards & outwards • Inner upper part of outer wall made of a rolled up cartilagineous plate • Lamina membranacea: Lower wall & remainder of outer wall is made of fibrous connective tissue • Narrowest part: Isthmus. Mtg point of bony & cartilaginous parts • Mucosa : Medial part continous w/pahryngeal mucosa lateral part continous w/mucosa of tympanic cavity 38 Middle Ear Tuba Auditoria Bony part: • 1/3rd nearest middle ear • Starts at front wall • Gradually narrows & ends where squamous & petrous parts of temporal bone meet 39 Tuba Auditoria Cartilagenous Part 40 Tuba Auditoria Middle Ear Pars Cartilagenea • Lamina membranacea: Lower wall & remainder of outer wall is made of fibrous connective tissue • Narrowest part: Isthmus. Mtg point of bony & cartilaginous parts 41 Middle Ear Eustachian Tube - Children • Course of tube more horizontal than in adults • App. 10 degrees to horizontal axis in children vs in adults 35 degrees to horizontal axis • Believed to be contributing factor in development of acute otitis media in children due to impaired middle ear drainage & reflus of nasopaharyngeal contents into middle ear 42 Tympanic Cavity The Ear Shaped like a narrow box w/concave sides Roof: Tegmental Wall Floor: Jugular Wall Lat. wall: Membranous Medial Wall: Labyrinthine Front Wall: Carotid Back Wall: Mastoid 43 1) Jugular Wall (Floor) Tympanic Cavity A.k.a. Fundus tympani • Thin layer of bone, which separates middle ear from jugular fossa • Superior bulb of int jugular v. sits underneath • Int. aperture of tympanic canal : Entrance for tympanic n. (Glossopharyngeal n.) On medial part 44 Tympanic Cavity 2) Tegmental Wall (Roof) A.k.a. Tegmen tympani • Formed by thin layer of bone from petrous part of temporal bone • Separates middle ear from middle cranial fossa • Forms part of floor of lat. part of middle cranial fossa • Apertures for veins passing into sup. petrosal sinus → Clinical Correlate: Middle ear infections may spread to meninges 45 Tympanic Cavity 3) Medial Wall (Labyrinthine) • Separates tympanic cavity from int. ear • Promontory of labyrinthine wall : Formed by initial part (basal turn) of cochlea • Oval & round windows: In dry cranium, communicate w/ int. ear Oval window: Fenestra vestibuli Round window: Fenestra cochlea • Tympanic plexus runs over promontorium • Prominence of facial canal : Post & sup to promontory • Prominence of lateral semicircular canal Post & sup to prominece of facial canal • Cochlear canaliculus: Near round window Passes thru temporal bone & opens into post. cranial fossa Connection btwn perilymph & subarachnoid space 46 Round window: sealed by secondary tympanic membrane (round window membrane), which vibrates w/ opposite phase to vibrations entering inner ear thru the oval window. Allows fluid in cochlea to move, which in turn ensures that hair cells of basilar membrane will be stimulated & audition will occur. 47 Tympanic Cavity 4) Lateral (External) Wall (Membranous) • Formed almost entirely by peaked convexity of tympanic membrane • Superiorly formed by lateral bony wall of epitympanic recess • Petrotympanic fissure A.k.a. Glasserian fissure, a.k.a. Squamotympanic fissure Runs from TMJ to tympanic cavity Divides mandibular fossa into 2 • Chorda tympani passes thru • Ant tympanic a. (branch of maxillary a.) passes thru 48 The Ear Reminder - Petrotympanic Fissure • Ant. tympanic a. (branch of maxillary a.) & veins • Chorda tympani (run thru to infratemporal fossa) 49 Tympanic Cavity 5) Posterior (Mastoid) Wall • Superior part wider than inf part • Entrance to mastoid air cells: Aditus (L. Access) • Mastoid Antrum: (L. Cave) common cavity into which mastoid air cells open • Pyramidal eminence: Contains stapedius m. • Behind oval window 50 Tympanic Cavity - Innervation The Ear Tympanic Plexus • Innervates mucosa lining walls & contents of tympanic cavity • Forms on mucosa covering promontory Components: • Tympanic n: Branch of glossopharyngeal n. • Carotidotympanic n.n.: Branches of int. carotid plexus • Tympanic plexus gives off lesser petrosal n. • Supplies preganglionic parasympathetics to otic ganglion 51 The Ear Tympanic Cavity - Alternative Perspective Back Front 52 The Ear Tympanic Cavity 53 Inner Ear Vestibulocochlear organ Consists of: • Bony labyrinth – a series of bony cavities • Membranous ducts & sacs – within the bony cavities • Conveys info to brain about balance & hearing 54 Inner Ear - Orientation 55 Inner Ear Bony Labyrinth • • Series of bony cavities within petrous part of temporal bone Composed of: • • • Cochlea Vestibule 3 Semicircular canals • All lined internally with periosteum • Contain perilymph • Perilymph similar to CSF Vestibule: Central part of bony labyrinth Separated from middle ear by the oval window Communicates anteriorly w/ cochlea & posteriorly w/ semi-circular canals 56 Inner Ear Membranous Labyrinth • 2 parts of membranous labyrinth; saccule & utricle - located within vestibule • Membranous labyrinth lies within bony labyrinth Continuous system of ducts Filled w/endolymph • • • Consists of: • Cochlear duct organ of hearing • Semicircular ducts • Utricle • Saccule - Cochlear duct occupies cochlea • Endolymphatic sac - 3 semicircular ducts occupy semicircular canals organs of balance - Saccule & utricle occupy vestibule 57 Inner Ear Labyrinth 58 Inner Ear Membranous Labyrinth – Left Ear 59 Cochlea Inner Ear • Looks like a snail shell • Has a canal – cochlear duct • Canal winds spirally around central axis for 23/4 turns, from the base to the apex • • Modiolus: Conical shaped central axis of the cochlea It has a delicate lamina - Osseous spiral lamina • Basal lamina : A membrane • Stretches from the free border of osseous spiral lamina to the outer wall of the bony cochlea and completely separates the canal into two passage • The 2 passages communicate with each other at the apex of the modiolus by a small opening named the helicotrema • The 2 passages/canals: Scala vestibuli & scala tympani 60 Cochlea Inner Ear • Cochlear duct • Modiolus- Osseous spiral lamina • Basal lamina : helicotrema • Scala vestibuli & scala tympani 61 Inner Ear Cochlea • Scala vestibuli continous w/ vestibule • Scala tympani separated from middle ear by membrane covering round window (secondary tympanic membrane) • Cochlear duct outer wall sits against bony cochlea w/epithelial lined perisoteum called spiral ligament in btwn • Cochlear duct ‘roof’ is the vestibular membrane – separates contents of cochlear duct (endolymph) from contents of scala vestibuli (perilymph) • Cochlear duct ‘floor’ is the free edge of the bony lamina of modiolus + basilar membrane • Spiral organ sits on basilar membrane 62 Inner Ear Section Through Turn of Cochlea • Scala vestibuli continous w/ vestibule • Scala tympani separated from middle ear by membrane covering round window (secondary tympanic membrane) • Cochlear duct outer wall sits against bony cochlea w/epithelial lined perisoteum called spiral ligament in btwn • Cochlear duct ‘roof’ is the vestibular membrane separates contents of cochlear duct (endolymph) from contents of scala vestibuli (perilymph) • Cochlear duct ‘floor’ is the free edge of the bony lamina of modiolus + basilar membrane • Spiral organ sits on basilar membrane 63 Inner Ear Cochlea 64 Ductus Cochlearis 1. 2. 3. 4. Basilar membrane Reissner's membrane, Tectorial membrane, Organ of Corti 65 66 67 Cochlea Inner Ear Cochlear canaliculus: • Small chanel near round window • Passes thru temporal bone & opens into post. cranial fossa • Creates connection btwn perilymph & subarachnoid space 68 Inner Ear Semicircular Canals (Bony) • Project posterosuperiorly from vestibule • Anterior, posterior, lateral • Each forms 2/3rds of a circle • Each connected at both ends to vestibule • One end of each dilated to form ampulla • Each at right angle to other 2 • Each contains a semicirclar duct (membranous) Vestibular aqueduct: • Narrow canal • Leaves vestibule, passes thru temporal bone, opens onto post. surface of petrous part 69 Saccule • • Inner Ear Smaller of the 2 sacs Cochlear duct empties into saccule Utricle • • • Larger of the 2 sacs Oval, elongated, irregular shaped In posterosuperior part of vestibule Semicircular Ducts • Each empties into the utricle Utriculosaccular Duct • Provides continuity btwn all these spaces Endolymphatic Duct • • Branches from utriculosaccular duct Enters vestibular aqueduct (channel thru temporal bone thru which it passes) Endolymphatic Sac • • Expansion of endolymphatic duct An extradural pouch. Functions in resorption of endolymph 70 Vestibulocochlear Nerve CN VIII • Inner ear innervated by the vestibulocochlear nerve (CN VIII) • Enters inner ear via int. acoustic meatus • Divides into vestibular n. (responsible for balance) & cochlear n. (responsible for hearing) • Internal acoustic meatus closed laterally by a thin, perforated plate of bone that separates it from internal ear • Passing thru: Facial nerve (CN VII), the vestibulocochlear nerve (CN VIII) & its divisions, & blood vessels. 71 Vestibulocochlear Nerve The Auditory System Sound waves travel thru ext. auditory canal → Cause tympanic membrane to vibrate → Movement of tympanic membrane causes vibrations of osscicles in middle ear → Vibrations of ossicles transferred thru oval window & into inner ear • Auditory ossicles amplify vibrations received by tympanic membrane → Transmit them to fluid of inner ear w/minimal energy loss → Mechanical signals from middle ear are thus convereted into electrical signals, which transfer information to the auditory pathway in the brain. 72 Vestibulocochlear Nerve Sound Transmission Thru Ear Cochlea depicted schematically as if consisting of a single coil to demonstrate transmission of sound stimuli 73 Inner Ear Section Through Turn of Cochlea 74 Inner Ear Hair Cells • High pitched sounds cause maximum displacement of hair cells at the base of the cochlea • Low frequency sounds stimulate hair cells at the apex of the cochlea ➔ Hair cells are frequency specific 75 Inner Ear • Inner ear innervated by CN VIII • Enters inner ear thru internal acoustic meatus • Divides into the vestibular n. – responsible for balance - & cochlear n. – responsible for hearing 76 Vestibular Nerve Divides into the cochlear & vestibular nerves 77 Auditory Pathway Spiral Ganglion • Contains cell bodies of neurons which send peripheral axons to innervate the auditory hair cells in the organ of Corti • Axons from hair cells form the fibers of CN VIII • Auditory info is sent to MGB 78 Cochlear Nerve Base of cochlea corresponds with bottom of internal acoustic meatus It is perforated by numerous apertures for passage of the cochlear division of the acoustic nerve. 79 Auditory Pathway ➔ 1st neuron in spiral ganglion → 2nd neuron in cochlear nuclei in medulla ➔ Medial Geniculate Body (nucleus) (MGB) in thalamus – 3rd neuron ➔ Primary auditory cortex on temporal lobe: Brodmann Areas 41, 42 • Adjacent auditory association area: Post sup temporal gyrus, Brodmann area 22 language processing 80 Cochlear Nerve Other Connections - Relay • Pathways that do not lead to primary auditory cortex • Multisensory integration • Reflexes • Attention • Emotional responses 81 Vestibular Nerve Vestibular System • Contains 2 kinds of sensory receptors: - One kind in utricle & saccule - One kind in semicircular ducts - macula crista • 3 semicircular ducts: Anterior, posterior, horizontal • Each lying in a bony semi-circular canal • Oriented such that they lie in the 3 planes of space • Each semi-circular duct contains an ampullar crest of hair cells that detect changes in angular acceleration & deceleration – resulting from circular motions of the head • Circular movements of the head in any plane will depolarize hair cells in a semicircular duct & hyperpolarize cells in the corresponding ducts of the opposite • Utricle & saccule: 2 sacs, each containing a patch of hair cells • Sensory receptors in utricle & saccule : Linear acceleration & pull of gravity 82 Vestibular System • Medulla & Pons: 4 vestibular nuclei • Vestibular nuclei receive & send info to flocculonodular lobe of cerebellum • Cerebral cortex vestibular area: Postcentral gyrus just above lateral fissure • Probably serves to orient the individual consciously in space 83 Vestibular Nerve Vestibular Ganglion • 1st order neuron • Located inside int. acoustic meatus 84 85 Vestibular System Vestibular System • Efferent fibers (connections) to: Spinal cord Nuclei of oculomotor m. Nuclei of trochlear n. Nucleir of abducens n. • From lateral vestibular nucleus, efferent fibers descend uncrossed to spinal cord & form vestibulospinal tract • Enable movements of head & eyes to be coordinated so that visual fixation on an object can be maintained • Info received from int. ear can assist in maintaining balance by influencing muscle tone of limbs & trunk 86 Internal Acoustic Meatus • Int. acoustic opening (porus acusticus internus) • Subdivided by 2 thin crests of bone to form 3 separate canals • Transverse falciform crest divides meatus into sup. & inf. sections • A vertical crest, Bill's bar (named by William F. House) divides upper passage into ant. & post. sections VI : Facial n. NI : Nervus intermedius VIII Vs: Vestibular n. sup. division VIII Vı: Vestibular n. inf. division VIIIc: Cochlear n. 87 Internal Acoustic Meatus fundus of int. acoustic meatus • After partial removal of posterior wall • Medial view 88 Clinical Correlates Hearing Loss Conductive • Passage of sound waves thru external or middle ear is interrupted • Causes: Obstruction, otosclerosis, otitis media Sensorineural • Damage to • Cochlea • CN VIII • Central auditory connections Meniere’s Disease • Inner ear disease • Vertigo, hearing loss, tinnitus • Mostly starts > 40 years old • Females more commonly affected than males 89 Facial Nerve Canal • Z-shaped canal • Running thru temporal bone from int. acoustic meatus to stylomastoid foramen • App. 3 cm → longest human osseous canal of a nerve Intermediate nerve, nervus intermedius: • Part of the facial n. located btwn motor component of facial n. & CN VIII) • Contains sensory & parasympathetic fibers of facial n. • Joins w/motor root of facial n. at the geniculate ganglion 90 Nerves in Petrous Part of Temporal Bone • Petrous part of temporal bone has been partly sectioned. Anterior view • Nerves illustrated translucently 91 Facial Nerve in Temporal Bone . 92