Ear, temporal bone, and facial nerve

Question 1

Temporal bone

Answer 1

• Contains the external, middle, and internal ears
• Mastoid process: attachment of sternocleidomastoid muscle; filled w/ mastoid air cells
• Petrous part of the temporal bone: ridge of bone btwn middle and posterior cranial fossas that contains the middle and internal ear
• External auditory meatus: ends @ tympanic membrane
• Internal auditory meatus: exit for facial and vestibulocochlear nerves from cranial cavity
• Stylomastoid foramen: end of facial canal thru temporal bone transmitting facial nerve

Question 2

External ear

Answer 2

• Auricle (pinna): skin of the ear (covering cartilage)

- External auditory meatus: half cartilage and half bone)

Question 3

Middle ear 1

Answer 3

• Tympanic membrane (eardrum): oval fibrous membrane attached to the manubrium of the malleus (separates external and middle ear cavities)
• Ear ossicles: 3 small bones (malleus, incus, and stapes) bridge from the eardrum (laterally) to the oval window (medially)
• Oval window: filled by the footplate of the stapes w/ air on the middle ear side and perilymph on the internal ear side (moves slightly in and out of oval window)

Question 4

Middle ear 2

Answer 4

• Round window: at other end of the cochlea, closed off to the air in the middle ear by the secondary tympanic membrane (moves synchronously w/ movement of stapes due to displacement of perilymph)
• Auditory (eustachian) tube: middle ear connection anteriorly w/ the nasopharynx to allow for pressure equilibration
• Mastoid air cells: middle ear communicates posteriorly w/ mastoid air cells (epithelial-lined spaces filling mastoid process)

Question 5

Internal ear

Answer 5

• Cochlea: shell-like spiral turns of cochlea lies anterior to the course of the facial nerve thru internal auditory meatus (controls hearing)
• CN VIII will divide after entering inner ear to give one branch to cochlea and one branch to vestibule (thru vestibular ganglion)
• Semicircular canals (vestibule): lie posterior to course of facial nerve thru internal auditory meatus (controls balance)

Question 6

Overview of innervation of external, middle, and internal ear

Answer 6

• External auditory meatus only has somatosensory and is innervated by V3, VII, IX, and X
• Middle ear motor: both V3 (to tensor tympani) and VII (to stapedius)
• Middle ear somatosensory: IX to auditory tube, tympanic cavity and mastoid antrum

Question 7

Postnatal growth of temporal bone

Answer 7

• Mastoid process + air cells: develops after birth as infant uses the SCM to hold up its head
• External auditory meatus: only a narrow ring at birth, thus little protection for the eardrum and middle ear
• Auditory tube: relatively horizontal at birth, thus does not drain middle ear effectively (increases frequency of ear infections). Eventually becomes steeper in orientation to drain tympanic cavity into pharynx

Question 8

Innervation of external ear

Answer 8

• The cutaneous part of the external ear is innervated by 2 different sets of nerves (somatosensory)
• 3/4 of it are innervated by cutaneous branches of the cervical plexus
• The anteriosuperior quadrant is innervated by the auriculotemporal nerve (V3)
• The external auditory meatus somatosensory conveyed by V3, VII, IX and X

Question 9

Tympanic membrane

Answer 9

• Manubrium of the malleus is attached to the inner surface above the center of the eardrum
• Chorda tympani courses across the upper part of the inner surface of the eardrum
• Sensory innervation to the outside of the eardrum is continuous w/ innervation of the external auditory meatus (V3, VII, IX and X)
• The sensory to the internal mucosa of the eardrum is innervated by IX

Question 10

Ear ossicles 1

Answer 10

• Malleus (hammer): manubrium of the malleus is attached to and moves w/ vibrations of tympanic membrane
• Articulation of the head of malleus w/ incus at a small synovial joint
• Incus (anvil): body of the incus articulates w/ head of malleus. Has a long process that descends to articulate w/ stapes at another synovial joint

Question 11

Ear ossicles 2

Answer 11

• Stapes (stirrup): stapedius muscle attaches to the neck of the stapes. The footplate of the stapes sits in the oval window (bound by annular ligament) and can move slightly in and out of the oval window
• Movements: slight rotation of the incus and malleus around the anterioposterior axis
• Inward movement of the eardrum pushes the manubrium medially causing the incus’s long process to move medially which pushes the stapes in and out of the oval window (moves the perilymph)

Question 12

Muscles of the middle ear

Answer 12

• Tensor tympani (V3): belly lies in a bony canal in the medial wall of the tympanic cavity. The tendon passes from medial to lateral across the cavity to attach to the manubrium of the malleus
• Stapedius muscle (CN VII): arises w/in bone on the posterior wall of the tympanic cavity. Enters the cavity and passes directly anterior to attach to the neck of the stapes
• Actions: both muscle contract together as a protective reflex response to high intensity sounds
• Tensor tympani pulls on the manubrium to dampen vibrations from the eardrum and stapedius pulls on stapes to dampen its movement into the oval window

Question 13

Nerves of the middle ear 1

Answer 13

• CN VII: courses within petrous part of temporal bone
• Course: internal auditory meatus-> geniculate ganglion (splits, some synapses)-> walls of middle ear cavity (now CN VII proper)-> stylomastoid foramen
• Chorda tympani branch: arises from facial nerve near stylomastoid foramen-> enters middle ear cavity-> crosses just inside of eardrum-> exits middle ear cavity into infra temporal fossa thru bony fissure on medial side of mandibular fossa

Question 14

Nerves of the middle ear 2

Answer 14

-Greater petrosal nerve (branch of VII): arises at the geniculate ganglion (splits off from VII proper)-> passes forward thru fissure to enter middle cranial fossa on way to pterygoid canal-> enters pterygoid canal (name changes to nerve of pterygoid canal or vidian’s)-> pterygopalatine ganglion

Question 15

Nerves of the middle ear 3

Answer 15

• Tympanic nerve (branch of IX): provides sensory innervation to mucosa of middle ear, mastoid air cells and eustachian tube
• This branch also conducts the para/pre axons destined to the otic ganglion (eventual target parotid gland via auriculotemporal branch of V3). Another name for this branch: lesser petrosal nerve (this branch only contains para/pre)
• The tympanic nerve creates the tympanic plexus over the promontory on the medial wall of the middle ear

Question 16

Relationships of middle ear

Answer 16

• Lateral (tympanic) wall: tympanic membrane and surrounding bone, related to external auditory meatus
• Superior (roof): middle cranial fossa and temporal lobe of the brain
• Inferior (floor): beginning of the internal jugular vein
• Anterior: internal carotid artery and eustachian tube
• Posterior: mastoid air cells
• Medial (labyrinthian wall): inner ear is just medial to middle ear (cochlea and vestibule)

Question 17

CN VII components

Answer 17

• Sensory: from skin of external auditory meatus (shared w/ V3, IX and X)
• Taste: from anterior 2/3rds of tongue coming from lingual nerve/chorda tympani
• Motor: to stapedius, stylohyoid, posterior belly of diagastric, muscles of facial expression
• Para: to lacrimal, submandibular, and sublingual glands

Question 18

Course of sensory and taste of CN VII

Answer 18

• Cell bodies in the geniculate ganglion
• Sensory begins in external auditory meatus, runs along VII thru stylomastoid foramen and petrous part of temporal bone to reach geniculate ganglion (cell bodies)
• Continues thru ganglion w/o synapsing, enters the internal auditory meatus and synapses on the VII nucleus in the brainstem (pons)
• Taste begins in the tongue (anterior 2/3rds) and hitchhikes on lingual nerve (V3) to reach chord tympani
• Chorda tympani enters the middle ear and crosses just under eardrum to join CN VII
• Continues thru geniculate ganglion (cell bodies) w/o synapsing and enters internal auditory meatus to reach brainstem (pons)

Question 19

Course of para and motor of VII 1

Answer 19

• Motor cell bodies in the motor nucleus of VII exit the pons and enter the internal auditory meatus, continue thru geniculate ganglion (no synapse) and run w/ CN VII proper thru stylomastoid foramen to innervate the muscles of facial expression (runs thru parotid), posterior diagastric and stylohyoid
• Branch that controls muscle of facial expression we need to know: buccal branch (obicularis oris and buccinator)
• Para/pre cell bodies in superior salvatory nucleus of VII (pons) enter the internal auditory meatus and at the geniculate ganglion some remain w/ VII proper and some run w/ greater petrosal

Question 20

Course of para and motor of VII 2

Answer 20

• The para/pre on VII proper branch w/ chord tympani to reach lingual nerve and synapse @ submandibular ganglion (from there either right to submandibular gland or back on lingual for sublingual gland/tongue)
• The other branch runs w/ greater petrosal to vidian’s (nerve of pterygoid canal) to pterygopalatine ganglion where para/pre synapses on para/post
• From the pterygopalatine ganglion the para/post jump onto infraorbital nerve (V2) then onto lacrimal nerve (V1) to reach lacrimal gland
• They could also course through the palatine nerves (greater and lesser) to reach the nasal cavity (branches of greater) and palates (greater palatine nerve-> hard palate, lesser palatine nerves-> soft palate)