Histology of the Ear- Heinbockel

Question 1

Outer Ear

Answer 1

Outer ear is the pinna with elastic cartilage covering by skin and the external

auditory canal that consists of elastic cartilage in the outer 1/3 and temporal bone in the inner 2/3 and
covered by stratified squamous epithelium and wax/cerumaneous glands that modify sweat glands in the skin secreting cerumen

ear canal has:

• hair follicles
• sebaceous glands
• cerumen is there to waterproof the skin and protects the canal from whatever goes in there (finger, insects)

TM:
-covered by skin on the external side
and simple squamous or cuboidal epithelium on the inner side
-the covered by simple squamous or cuboidal on
-in the center there is fibroelastic CT
-when sound strikes TM it starts to vibrate and brings us to the middle ear

middle ear:

• air filled space in between the tympanic membrane and structures of the inner ear
• transmits sounds from TM to the fluid structures of the inner ear
• ear ossicles: malleus, incus, stapes touching the cochlear

-Eustachian tube links the middle ear to the nasopharynx

Question 2

What is the main organ in the inner ear?

Answer 2

Cochlear is the main organ in the inner ear.

• organ in which hearing takes place
• consists of the scala vestibuli and scali tympani

-we have bony structures that cover membranous structures
and thus we talk about the bony AND membranous labyrinth which are both fluid filled

if you have fluid and there’s continuous production of fluid then it needs to go somewhere

Question 3

Describe the membranous labyrinth.

Answer 3

• it is surrounded by bone
• consists of semicircular ducts, utricle, saccule, and cochlear duct

-fluid in the membranous labyrinth ends up in the narrow endolymphatic sac and the sac projects through the dura mater into the subarachnoid space by capillary network into general circulation

the endolymphatic sac is in the subdural space

Question 4

What is the function of the vestibular system?

Answer 4

senses motion and the position of our heads

• utricle (horizontal) and saccule (vertical) senses gravity and linear acceleration (whether body is moving or stationary wrt gravity)
• they can be stimulated by sudden acceleration
• semicircular ducts sense head rotation or angular rotation

Question 5

Where is the endolymphatic sac located?

Answer 5

in the subdural space

Question 6

Describe the bony labyrinth.

Answer 6

• scala vestibuli and scala tympani of the cochlea
• semicircular canals (bony) with semicircular ducts (membranous)
• vestibule (bony structure): opening or entry hall houses the utricle and saccule (membranous structures)
• all these structures are filled with fluid called perilymph that comes from the perilymphatic duct which connects to the subarachnoid space

Question 7

What fluid is found in the bony and membranous labyrinth?

Answer 7

endolymph in membranous labyrinth

perilymph in bony labyrinth

Question 8

What is the difference between endolymph and perilymph?

Answer 8

Endolymph:
-High K+, low Na+

Perilymph:
-High Na+, low K+

Question 9

What condition is associated with the increase in the volume of the endolymph? What are symptoms related to this condition?

Answer 9

Ménière’s disease

• cause is unknown
• typically involves only one ear
• symptoms include episodic attacks of :
• vertigo
• nausea and vomiting
• nystagmus
• tinnitus

Question 10

Cochlea

Answer 10

• shape is like that of a snail; has an apex and base
• cochlear duct is the membranous structure surrounded by bone

cross section reveals 3 ducts:
-scala media or the cochlear duct which is lined by vestibular (Reissner’s membrane) on one side and the basilar membrane on the other side

• has bony structures called scala vestibuli and scala tympani
• Scala vestibuli and scala tympani are chambers of the bony labyrinth. Scala media is part of the membranous labyrinth.

Question 11

Where does hearing occur in the ear?

Answer 11

in the Organ of Corti in the cochlea where hair cells are found and assist in hearing

Question 12

The medial portion of the cochlea is filled with bony modiolus (part of the petrous bone) through which a nerve runs. What is this nerve?

Answer 12

cochlear nerve

-nerve fibers have cell bodies (spiral ganglion) and run in the cochlea and go to the CNS

Question 13

What type of neurons are found in the spiral ganglion and what do they innervate to form the cochlear nerve?

Answer 13

• bipolar neurons (remember special senses use bipolar not pseudounipolar)
• innervates the organ of corti to form the cochlear nerve

Question 14

What are the function of the spiral ligament and stria vascularis?

Answer 14

• spiral ligament it forms the outer wall of the cochlear duct and is a thickening of the periosteum
• stria vascularis is rich in capillaries (epithelium DO NOT typically have BVs); it is the epithelium that makes the endolymph

Question 15

Stria vascularis

Answer 15

• vascular epithelium
• secretes endolymph
• proper function essential for normal hearing
• contains a few melanocytes, also needed for normal hearing

Question 16

The organ of Corti sits on the basilar membrane to connect what two structures?

Answer 16

Osseous spiral lamina and spiral ligament

Osseous spiral lamina allows the nerve fibers to fun to the spiral ganglion

Question 17

How does sound travel for us to hear?

Answer 17

Sound comes into the ear through the acoustic canal and hit the TM the vibration is communicated through the ossicles
the stapes is connected to the cochlea and connects to the oval window

the sound waves or vibration get into the cochlea into the scala vestibuli to the scala media and excite hair cells and then go to the membrane to the scala tympani to the round window

The oval footplate of the stapes contacts the oval window. Sound waves travel AROUND to the round window.

So the sound moves the basilar membrane the stereocilia in the hair cells bend and the tip links are stretched, calcium channels open up triggering glutamate release and and the inner hair cells respond.

Question 18

The scala vestibuli and scala tympani are connected by what? What separates them

Answer 18

connected by helicotrema

separated by scala media

Question 19

The organ of Corti

Answer 19

-has a tectorial membrane expands from the spiral limbus and sits on top or forms the top of the organ of Corti (sits on top of the hair cells)

inner tunnel is lined by pillar cells

-has outer and inner phalangeal cells

Question 20

There are two types of hair cells in the organ of Corti. What are they and which type is involved in sensing sounds?

Answer 20

outer and inner hair cells

inner hair cells are involved in sensing sounds (transduce sound)

both inner and outer hair cells have stereocilia which are long microvilli (which have actin filaments in them)

the inner cells are medial and the outer cells are lateral

Question 21

What is the function of the outer hair cells?

Answer 21

outer hair cells contract to increase the sensitivity to sound. They move the basilar membrane relative to the tectorial membrane.

They are getting more input from the CNS and can change the sensitivity to sound. They also can lengthen or shorten sound and are believed to more amplify the sound in the associated inner hair cells.

There are 3-4 times more outer hair cells than inner hair cells. There are 3 rows of outer hair cells and 1 row of inner hair cells.

Question 22

If the outer hair cells are damaged, can we still hear?

Answer 22

no we cannot

Question 23

Stereocilia of hair cells have several unconventional myosins. What can occur with mutations in some of these myosins?

Answer 23

deafness

Question 24

What can occur with mutations in ankle links of the stereocilia of hair cells ?

Answer 24

Usher syndrome

deafness as a result of cochlea origin (CNS defect of the nerve fibers which affects the hair cells)

Stereocilia has lateral tip links and ankle links.

Question 25

What connects supporting cells and hair cells?

Answer 25

tight junctions and gap junctions for potassium recycling in hair cells

Question 26

What fluid are the hair cells bathed in?

Answer 26

stereocilia of the hair cells are bathed in the endolymph BUT basolaterally, the hair cells are bathed in perilymph

Question 27

What allows for good signal to noise ratio in hair cells?

Answer 27

the large voltage drop across the hair cell which allows for good signal to noise ratio

remember that
Endolymph:
-High K+, low Na+

Perilymph:
-High Na+, low K+

Question 28

Why is potassium recycling important in hair cells?

Answer 28

a lot of K+ enters the hair cells through gap junctions goes into the supporting cells and back to stria vascularis

potassium entering hair cells leaves the cells again on their basolateral side (going down its concentration gradient), and is recycled back to the stria vascularis via gap junctions.

mutations in some connexins (form the channels of gap junctions) can cause DEAFNESS

Question 29

How does the structure of the basilar membrane of the cochlea change with different pitches?

Answer 29

wide, flexible end of basilar membrane near helicotrema allows for LOW pitches

narrow, stiff end of basilar membrane near oval window allow for HIGH pitches

High frequency sounds have a short wavelength so they can vibrate the basilar membrane near the oval window. the lower the frequency of the sound the longer the wavelength and the farther away from the oval window. So information about the frequency of the sounds is translated to where along the basilar membrane is the sound activating the basilar membrane.

Question 30

How is the frequency and volume of sound perceived?

Answer 30

Frequency of sound is perceived and determined by which part of the cochlear duct is stimulated. The volume or loudness is perceived by how many hair cells are stimulated at a given location (either at the base or helicotrema giving you a different impression of loudness).

Question 31

Describe the Vestibular Organs.

Answer 31

• 3 crista ampullares (crest of the ampulla) is where we find hair cells that are part of the 3 semicircular ducts
• 2 maculae in saccule and utricle

Question 32

Vestibular organs have what type of hair cells?

Answer 32

Type I hair cells (with complete synapse around them)

Type II hair cells (with broken up synapse around them)

Type I and Type II hair cells have a kinocilium (a true cilium) and stereocilia.

Question 33

The direction in which the kinocilium and stereocilia are bending is important. Explain.

Answer 33

movements of the stereocilia TOWARDS the kinocilium DEPOLARIZES the hair cell, while movement AWAY from the kinocilium HYPERPOLARIZES the hair cell

Question 34

Ampulla of the semicircular canal

Answer 34

you have supporting cells and hair cells with kinocilium and stereocilia which are surrounded by cupula which is a glycoprotein containing gelatinous substance material that

Question 35

What is the function of the crista ampullares of the vestibular complex?

Answer 35

• senses angular rotation of the head

- sloshing of the endolymph causes the stereocilia buried in the cupula to bend

Question 36

What is the function of the macula of the vestibular complex?

Answer 36

maculae are located in the saccule and utricle

• senses linear acceleration of the head
• they also sit in a gelatin layer with ear stones called otoliths
• inertia of the otoliths causes the otolithic membrane to lag behind, bending the stereocilia