Week 7 Ear, Temporal Bone, and Facial Nerve

Question 1

What are features of the temporal bone?

Answer 1

1. Mastoid process: attachment of SCM, filled with mastoid air cells.
2. Petrous part of temporal bone: ridge of bone between middle and posterior cranial fossas, contains middle and internal ear
3. External auditory meatus: ends at tympanic membrane
4. Internal acoustic meatus: exit for facial and vestibulocochlear nerves from cranial cavity
5. Stylomastoid foramen: end of facial canal through temporal bone transmitting facial nerve

Question 2

What are the parts of the external ear?

Answer 2

1. Auricle (pinna); skin covered cartilage

2. external auditory meatus: canal that is half cartilage, half bone

Question 3

What are the components of the middle ear?

Answer 3

1. tympanic membrane (eardrum): slightly oval fibrous membrane attached to manubrium of malleus.
2. Ear ossicles: malleus, incus, stapes-bridge the tympanic cavity from eardrum to oval window
3. Oval window: filled by footplate of stapes with air on middle ear side and perilymph on internal ear side. Footplate held in oval window by annular ligament
4. Round window: closed by secondary tympanic membrane, when footplate of stapes pushes into oval window, perilymph is displaced through scala vestibule, the scala tympani of cochlea causes round window to bulge outward into tympanic cavity
5. Auditory tube (eustachian): middle ear cavity communites anteriorly with nasopharynx via auditory tube, has bonnie and cartilage part (later 2/3)
6. Mastoid air cells: middle ear cavity communicates posteriorly with mastoid air cells

Question 4

What are the components of the inner ear?

Answer 4

1. cochlea: shell-like, spiral turns. Anterior to course of facial nerve through the internal auditory meatus.
2. semicircular canals: like posterior to course of facial nerve through internal auditory meatus. Look like small holes surrounded by compact bone where the canals have been cut in cross section.

Question 5

What is the embryological origin of the external ear? What are its innervations?

Answer 5

1. from 1st pharyngeal cleft: external auditory meatus (ectoderm=skin)
2. innervation: cervical plexus, V3, VII, IX, X (sensory to skin)

Question 6

What is the embryological origin of structures in the middle ear? What are its innervations?

Answer 6

1. origins
   - 1st pharyngeal arch: malleus, incus, tensor tympani muscle
   - 1st pharyngeal pouch (inside): auditory tube, tympanic cavity, mastoid antrum (endoderm-mucus)
   - 2nd pharyngeal arch: stapes, stapedius muscle
2. Innervation
   - malleus, incus, tensor tympani muscle: V3 (motor)
   - auditory tube, tympanic cavity, mastoid antrum: IX, sensory to mucosa
   - stapes, stapedius: VII (motor)

Question 7

Describe the embryonic development of the inner ear.

Answer 7

• develops from ectoderm, which invaginated to form otic pit, then pinches off to form otic vesicle
• vesicle differentiates into:
• utricular part: endolymphatic duct, utricle, semicircular canals
• saccular part: saccule, cochlear duct, spiral organ of corti

Question 8

Describe the important developmental changes in the temporal bone from birth to adulthood.

Answer 8

1. Mastoid process and air cells: develops when infant begins to use SCM to hold up its head. Air cells invade from mastoid antrum as bone enlarges.
2. External auditory meatus: only a narrow ring at birth with little protection for eardrum and middle ear. Easily injured at birth.
3. Auditory tube: tube is relatively horizontal at birth, as face grows downwards and forwards, the Eustachian tube becomes progressively steeper in orientation to allow better drainage from tympanic cavity into pharynx
   - poor drainage of middle ear reason for frequent ear infections in children

Question 9

Describe the innervation of the external ear.

Answer 9

• posterior 3/4 of auricle innervated by cutaneous branches of cervical plexus
• anterior superior quadrant by auriculotemporal nerve (V3)
• skin of external auditory meatus and external surface of eardrum: mainly by CN V, also by CN VII, XI, and X
• referred pain from ear due to toothache or throat cancer due to CN V innervation
• stimulation of external auditory meatus may cause coughing, sneezing b/c of other 3 cranial nerves

Question 10

What are the relationships with the tympanic membrane?

Answer 10

• manubrium of malleus is attached to inner surface above center of eardrum
• chorda tympani named for its course across the upper part of the inner surface of the eardrum

Question 11

Describe the skin and mucosa of the tympanic eardrum.

Answer 11

• outer surface: connective tissue of eardrum covered by modified hairless ectodermally derived skin
• inner surface: mucous membrane continuous with endodermally derived mucosa that lines middle ear cavity, mastoid antrum, mastoid air cells, and Eustachian tube
• mucosa envelops all bones, tendons, and nerves within middle ear cavity

Question 12

What is the innervation of the tympanic membrane?

Answer 12

• outside: same as walls of meatus- CN V3, VII, IX, X

- internal mucosa: CN IX

Question 13

Describe features of the 3 ear ossicles.

Answer 13

1. Malleus (hammer): manubrium is attached to and moves with the vibrations of the tympanic membrane. Tendon of tensor tympani muscle attaches to upper part of manubrium. Head of malleus articulates with incus via a tiny synovial joint
2. Incus (anvil): body, long process that descends to articulate with stapes at a synovial joint
3. Stapes (stirrup): stapedius muscle attaches to neck of stapes. footplate sites in oval window and is bound there by annular ligament, allows footplate to move in and out and seals the oval window

Question 14

Describe the movement of the ear ossicles.

Answer 14

• malleus and incus tied to roof, anterior and posterior walls by small ligaments
• slight rotation around anteroposterior axis that passes through malleus and incus
• medial movement due to inward movement of eardrum that pushes manubrium and causes incus’s long process to push stapes into oval window

Question 15

Describe the muscles of the middle ear. List their innervations.

Answer 15

1. Tensor tympani muscle (V3): lies in bony canal in medial wall of tympanic cavity. tendon passes medial to lateral across cavity to attach to manubrium of malleus
   -Action: pulls on manubrium to dampen vibrations of eardrum
2. Stapedius muscle (CN VII): arises within bone on posterior wall of tympanic cavity. Tendon enters cavity and passes directly anteriorly to attach to neck of stapes
   -Action: pull on stapes to dampen its movement in oval window
   Both muscles contract go ether as protective reflex. Paralysis results in sensitivity to noise

Question 16

Describe the nerves of the middle ear.

Answer 16

1. Facial nerve: courses from internal auditory meatus–>geniculate ganglion–>walls of middle ear cavity–>stylomastoid foramen
   - chorda tympani: arises from facial nerve near stylomastoid foramen–>enters middle ear cavity–>crosses inside of eardrum–>exits middle ear cavity into infra temporal fossa through bony fissure on medial side on mandibular fossa
   - greater petrosal nerve: course within petrous part of temporal bone, arises from geniculate ganglion and passes forward through fissure to enter middle cranial fossa on way to pterygoid canal and pterygopalatine ganglion
2. Tympanic nerve (branch of CN IX)
   - CN IX sends tiny branch into middle ear cavity to provide sensory to mucosa of middle ear, mastoid air cells, and Eustachian tube. Also has para/pre axons of CN IX to otic ganglion (which go to parotid gland)

Question 17

Describe the walls of the middle ear and the relationships with structures within the middle ear.

Answer 17

1. Lateral (tympanic wall): tympanic membrane, related to external auditory meatus
2. Superior wall (roof): temporal lobe above, separated by thin, translucent bone.
3. Inferior (jugular) wall (floor): beginning of internal jugular vein lies inferior to middle ear
4. Anterior (carotid) wall: internal carotid artery turns 90 degrees within the carotid canal just anterior to the middle ear. Opening of auditory canal, pointed forward, downward, and medially, in in anterior wall.
5. Posterior (mastoid) wall: tympanic cavity communicates with mastoid air cells through opening in posterior wall.
6. Medial (labyrinth) wall: inner ear is medial to tympanic cavity. can see promontory, oval window, and round window through external auditory meatus

Question 18

What are the components of the Facial Nerve, and what do those components innervate?

Answer 18

1. Sensory: from skin of external auditory meatus (overlaps with CN V3, IX, X)
2. Taste: from taste buds on anterior 2/3 of tongue
3. Motor: to stapedius, stylohyoid, posterior belly of digastric, and muscles of facial expression
4. Parasympathetic: to lacrimal, submandibular, and sublingual gland

Question 19

What cell bodies are in the geniculate ganglion?

Answer 19

From facial nerve

1. Taste from anterior 2/3 of tongue
2. somatosensory: small area of cutaneous sensation from external auditory meatus

Question 20

Describe the course of the greater petrosal nerve and what it innervates.

Answer 20

• para/pre arises from geniculate ganglion, courses anteriorly through temporal bone to enter and leave middle cranial fossa
• enters pterygoid canal through sphenoid bone, name changes to Nerve of Pterygoid Canal
• nerve enters pterygopalatine fossa and goes to pterygopalatine ganglion (para/post cell bodies)
• para/post axons travel on maxillary and ophthalmic nerves to reach lacrimal gland

Question 21

Trace the course of the facial nerve (proper).

Answer 21

• from geniculate ganglion, facial nerve turns posteriorly to enter facial canal
• takes facial nerve posteriorly through medial tympanic wall and turns 90 degrees inferiorly to pass down through posterior wall of middle ear towards stylomastoid foramen
• has taste, SS, SM, para/pre

Question 22

Describe the course of the chorda tympani and what it innervates.

Answer 22

Has taste and para/pre

• arises before facial nerve exits stylomastoid foramen
• recurves upwards and enters middle ear cavity
• courses across tympanic membrane, exits middle ear through fissure that takes nerve to medial side of mandibular fossa
• descends in infratemporal fossa and joins lingual nerve to be distributed to submandibular ganglion (para/post for submandibular and sublingual ganglia) and taste buses on anterior 2/3 of tongue

Question 23

What are the minor, undirected branches of the facial nerve?

Answer 23

1. branch to stapedius muscle (SM)
2. branches to stylohyoid and posterior belly of digastric muscles (SM)
3. branch to external auditory meatus (SS)