TBL29 - Ear

Question 1

What three parts does the auricle of the external ear contain? What forms the external auditory meatus? What is the fate of the 2nd, 3rd, and 4th pharyngeal clefts?

Answer 1

1) The auricle of the external ear contains its helix, lobule (aka lobe) and tragus
2) The external auditory meatus is formed by ectodermal invagination of the 1st pharyngeal cleft (groove)
3) The 2nd, 3rd, and 4th pharyngeal clefts are normally obliterated

Question 2

Where does the external acoustic meatus terminate and what does this structure separate?

Answer 2

The external acoustic meatus terminates at the tympanic membrane, which separates the external ear from the middle ear

Question 3

What does the auditory tube derive from? Where is the proximal opening of the auditory tube? What does expansion of the distal portion of the auditory tube form? What do the three bony ossicles form?

Answer 3

1) Endoderm of the 1st pharyngeal pouch forms the auditory tube
2) Its proximal opening is in the nasopharynx
3) Expansion of the distal portion of the auditory tube forms the tympanic cavity of the middle ear
4) Three bony ossicles form a chain that spans across the cavity

Question 4

Where is the tympanic cavity located? What does the tympanic membrane occupy? What nerve innervates the mucosa of the external acoustic meatus including the external surface of the tympanic membrane?

Answer 4

1) The tympanic cavity is located in the petrous temporal bone
2) The tympanic membrane occupies most of the lateral wall of the cavity
3) The auriculotemporal nerve, a branch of CN V3, innervates the mucosa of the external acoustic meatus including the external surface of the tympanic membrane

Question 5

What three parts make up the air-filled tympanic cavity? What is the tympanic nerve a branch of, what does it pierce, and what does it provide somatic sensory innervation to?

Answer 5

1) The roof, walls, and floor make up the air-filled tympanic cavity
2) The tympanic nerve, a branch of CN IX, pierces the floor of the cavity to provide somatic sensory innervation to the mucosa and internal surface of the tympanic membrane

Question 6

Where does CN IX exit the posterior cranial fossa via? What does the tympanic nerve pierce to enter the mucosa of the tympanic cavity?

Answer 6

1) CN IX exits the posterior cranial fossa via the jugular foramen
2) The tympanic nerve pierces a fissure in the external surface of the petrous temporal bone to enter the mucosa of the tympanic cavity

Question 7

What does the lesser petrosal nerve branch from, pierce, and enter? Where does the lesser petrosal nerve exit the middle cranial fossa via, what does it enter next, and what does it synpase with?

Answer 7

1) The lesser petrosal nerve branches from the tympanic plexus and pierces a fissure in the roof of the tympanic cavity to enter the middle cranial fossa
2) The lesser petrosal nerve exits the middle cranial fossa via the foramen ovale and enters the infratemporal fossa to synapse in the otic ganglion

Question 8

En route to the stylomastoid foramen, where does CN VII course? After arising in the facial canal, where does the chorda tympani enter, cross, and enter next?

Answer 8

1) En route to the stylomastoid foramen, CN VII courses in the facial canal within the medial and posterior walls of the tympanic cavity
2) After arising in the facial canal, the chorda tympani enters the tympanic cavity and crosses the medial surface of the malleus bone en route to a fissure in the floor of the cavity for entrance into the infratemporal fossa

Question 9

Where does the malleus insert into? What does the malleus join to form a moveable bony chain and where does this chain span across?

Answer 9

1) The malleus inserts into the superior portion of the tympanic membrane
2) The malleus joins the incus and stapes to form a moveable bony chain that spans across the tympanic cavity

Question 10

What does the stapes insert into? What does the window cover and what does this create a boundary between?

Answer 10

1) The stapes inserts into the membranous oval window
2) The window covers an oval opening in the medial wall of the tympanic cavity to create a boundary between the tympanic cavity and vestibule of the bony labyrinth

Question 11

What does the auditory (pharyngotympanic tube) open into the tympanic cavity through? Describe how the levator palatine and tensor palatine muscles contribute to the function of the auditory tube.

Answer 11

1) The auditory (pharyngotympanic) tube opens into the tympanic cavity through its anterior wall
2) The tube is opened by the expanding girth of the belly of the levator veli palatini as it contracts longitudinally, pushing against one wall while the tensor veli palatini pulls on the other
3) Because these are muscles of the soft palate, equalizing pressure (“popping the eardrums”) is commonly associated with activities such as yawning and swallowing

Question 12

What are the causes and symptoms of otitis media? How is myringotomy performed to relieve the symptoms and what functional losses can occur if it is improperly performed?

Answer 12

1) An earache and a bulging red tympanic membrane may indicate pus or fluid in the middle ear, a sign of otitis media
2) Infection of the middle ear is often secondary to upper respiratory infections
3) Inflammation and swelling of the mucous membrane lining the tympanic cavity may cause partial or complete blockage of the pharyngotympanic tube
4) Because the superior half of the tympanic membrane is much more vascular than the inferior half, incisions to release pus from a middle ear abscess (myringotomy), for example, are made postero-inferiorly through the membrane
5) This incision also avoids injury to the chorda tympani nerve and auditory ossicles

Question 13

How is hearing affected by blockage of the auditory tube?

Answer 13

When the pharyngotympanic tube is occluded, residual air in the tympanic cavity is usually absorbed into the mucosal blood vessels, resulting in lower pressure in the tympanic cavity, retraction of the tympanic membrane, and interference with its free movement

Question 14

Where is the otic (auditory) vesicle derived from? After invagination, what does the vesicle separate from? What does the ectoderm-derived vesicle subsequently form?

Answer 14

1) The otic (auditory) vesicle is in the surface ectoderm at the level of the hindbrain
2) After invagination, the vesicle separates from the surface ectoderm
3) The ectoderm-derived vesicle subsequently forms the membranous labyrinth of the inner ear

Question 15

The bony labyrinth consists of a series of cavities in the petrous temporal bone called what? What are components of the membranous labyrinth embedded in? What are both labyrinths filled with?

Answer 15

1) The bony labyrinth consists of a series of cavities in the petrous temporal bone called the vestibule, semicircular canals, and cochlea
2) Components of the membranous labyrinth are embedded in the bony labyrinth
3) Both labyrinths are filled with CSF-like fluid

Question 16

What occupies the vestible of the bony labyrinth? What do outpocketings from the utricle form? Where does the cochlear duct extend from?

Answer 16

1) The utricle and saccule of the membranous labyrinth occupy the vestibule of the bony labyrinth
2) Outpocketings from the utricle form the three semicircular ducts that fill the semicircular canals of the bony labyrinth
3) The cochlear duct extends from the saccule into the cochlea of the bony labyrinth where it spirals to form 2.5 turns

Question 17

Where is the cochlear duct found within? Where are the three semicircular ducts found within? What does one end of each semicircular duct contain? Sensory epithelia also resides in what two structures?

Answer 17

1) The cochlear duct is found within the shell-shaped cochlea
2) The three semicircular ducts are found within the semicircular canals
3) One end of each semicircular duct contains sensory epithelium (purple dots)
4) The red dots denote sensory epithelia residing in the utricle and saccule

Question 18

Where do peripheral projecting fibers from bipolar neurons of the vestibular ganglion synapse?

Answer 18

Peripheral projecting fibers from bipolar neurons of the vestibular ganglion synapse with sensory epithelia of the semicircular ducts, utricle and saccule

Question 19

What does angular acceleration (spinning motion) activate and what does linear acceleration activate? What do these activations induce? What do central axons from the ganglion form?

Answer 19

1) Angular acceleration (spinning motion) activates sensory cells of the semicircular ducts and linear acceleration activates the sensory epithelia of the utricle and saccule
2) This activation induces conduction of action potentials to the vestibular ganglion
3) Central axons from the ganglion form the vestibular division of CN VIII

Question 20

What does the spiral cochlear duct partition the cochlea into? At the apex of the cochlear spiral, how do the scalae communicate?

Answer 20

1) The spiral cochlear duct partitions the cochlea into the scala vestibuli and scala tympani
2) At the apex of the cochlear spiral, the scalae communicate via the helicotrema

Question 21

What does the cochlear duct contain? What structure has peripheral fibers of its bipolar neurons that synapse with the spiral organ? What do central projecting fibers from this structure form?

Answer 21

1) The cochlear duct contains the spiral organ with sensory epithelium
2) The spiral ganglion has peripheral fibers of its bipolar neurons that synapse with sensory cells of the spiral organ
3) Central projecting fibers from the spiral ganglion form the cochlear division of CN VIII

Question 22

What does CN VIII traverse and what does it synapse with?

Answer 22

CN VIII traverses the internal acoustic meatus of the petrous temporal bone to synapse with sensory neurons in the brainstem

Question 23

What initiates vibrations of the tympanic membrane and where are these vibrations tranismited by the ossicles to? Where does vibration of the oval window creates pressure waves and where do these waves continue into?

Answer 23

1) Sound waves initiate vibrations of the tympanic membrane that the ossicles transmit to the oval window
2) Vibration of the oval window creates pressure waves in the fluid-filled scala vestibuli that continue into the scala tympani at the helicotrema

Question 24

What does movement of the spiral organ by the pressure waves activate? What does the round window disspiate hydraulic pressure waves in the scala tympani into?

Answer 24

1) Movement of the spiral organ by the pressure waves activates its sensory cells to induce conduction of action potentials to the spiral ganglion and onto CN VIII
2) The round window dissipates hydraulic pressure waves in the scala tympani into the air of the tympanic cavity

Question 25

What do contractions of the tensor tympani and stapedius muscles do and where do these muscles insert?

Answer 25

Contractions of the tensor tympani and stapedius muscles, which insert into the malleus and stapes, dampen the oscillatory range of the tympanic membrane and oval window

Question 26

Cite respective innervations of the tensor tympani and stapedius muscles. What do their contractions serve to prevent?

Answer 26

1) Tensor tympani is innervated by CN V3 of the trigeminal ganglion
2) Stapedius muscle is innervated by CN VII
3) Their contractions serve to prevent damage to the inner ear during exposure to loud sounds

Question 27

Why do the tensor tympani and stapedius muscles have different innervations?

Answer 27

They have different innervations because the tensor sympani derives from the 1st pharyngeal arch and the stapedius muscle derives from the 2nd pharyngeal arch

Question 28

How do histologic changes characteristic of Meniere’s disease predict its symptoms?

Answer 28

1) Characteristic distention and distortion of the membranous labyrinth may lead to degeneration of receptor hair cells in both vestibule and cochlea
2) Patients show malfunction in both parts of the inner ear with recurrent episodes of vertigo (dizziness), tinnitus (ringing), and low frequency deafness