Anatomy of Hearing

Question 1

Transduction

Answer 1

a process by which one kind of signal, energy, or stimulus is converted into another form
- transduction is not perfect; parts of the signal can be lost due to the medium, for example

Our hearing works through transduction:
1) air pressure to mechanical pressure
2) mechanical movements to neural signals
3) neural signals into an experience of our world

Question 2

Outer Ear

Answer 2

The outer ear consists of the:
- pinna (auricle)
- ear canal
- tympanic membrane (ear drum)

Question 3

Pinna/Auricle

Answer 3

the outer part of the ear that provides protection, prevents pressure seals, and can amplify sounds
- provides localization: helps us understand where sound is coming from because different areas of the pinna have different resonances
- amplification at around 3000 Hz; pinna distorts the sound somewhat
- shape of the pinna “directs sound”
- piercings can modify the resonances of the pinna, but not typically to the point that it has a noticeable effect on hearing
- we can exist without the pinna

Question 4

Ear Canal

Answer 4

The pinna and the tympanic membrane that contains hair and cerumen
- cartilaginous portion: hair and cerumen glands
- bony portion: no hair, might have wax, covered with skin
- around 1 inch long and 0.25 inches in diameter for adults
- mostly for protection of the middle/inner ear

Question 5

Cerumen

Answer 5

aka ear wax
- lubricates, cleans, and protects the ear canal
- prevents infections and fungal growth by keeping bacteria and fungi from touching the skin
- earwax type is determined by a gene (ABCC11)
—wet wax AA: European & African people
—dry-flaky wax GA/GG: East Asian and Indigenous people
- earwax impaction is a common cause of hearing loss that is easy to treat

Question 6

Tympanic Membrane

Answer 6

A transparent membrane that separates the ear canal from the middle ear, and transduces changes in air pressure (sound waves) into mechanical movements
- aka eardrum
- protection: prevents water & debris from entering the middle ear; separates ear canal and middle ear
- transmits sound waves: captures vibrations and transmits them to the ossicular chain
- the seal that the TM provides is beneficial for a few reasons, but is not necessarily for hearing
- Attached to the malleus via the manubrium of the malleus

Question 7

Pars Tensa

Answer 7

The fibrous tissue layer of the tympanic membrane that is between the skin of the ear canal and the mucous tissue of the middle ear
—provides strength to the TM

Question 8

Pars Flaccida

Answer 8

A small, thin area of the tympanic membrane that does not have fibrous tissue
—aka Shrapnell’s membrane

Question 9

Middle Ear

Answer 9

The middle ear consists of the:
- eustachian tubes
- ossicular chain
- chorda tympani nerve
- stapedius muscle

Question 10

Malleus

Answer 10

the first bone in the ossicular chain which is attached to the tympanic membrane and the incus
- latin for “hammer”

Question 11

Incus

Answer 11

the second bone in the ossicular chain which is attached to the malleus and the stapes
- latin for “anvil”
- the incus-stapes joint is most likely to break

Question 12

Stapes

Answer 12

• latin for “stirrup”
• smallest bone in the human body

Question 13

Ossicular chain

Answer 13

The chain of three bones (ossicles)—the malleus, incus, and stapes—which connects the tympanic membrane to the cochlea
- supported by ligaments and muscles
- “provides coupling between the outside world and pressure movements there, and the oval window”
- important for the acoustic reflex, amplification, impedance matching, protecting the cochlea
—the stapedius and tensor tympani muscles dampen the vibration of the ossicles in response to loud sounds & our own speech
—ossicular chain can break (think of a crumple zone)
—match impedance between air (outside world) and fluid (inside ear)
- prosthetic ossicles are possible

Question 14

Eustachian Tubes

Answer 14

aka auditory tube or pharyngotympanic tube
- connects the nasopharyngeal cavity and the middle ear
- regulates pressure inside the middle ear
- opens and closes with the tensor and levator veli palatini during chewing, yawning, and swallowing
- has a bony and cartilaginous portion
- located adjacent to the carotid artery (supplies blood to the brain)
- allows for potential drainage if there is something stuck in the middle ear

Question 15

Chorda tympani nerve

Answer 15

A nerve which carries tongue sensations
- passes through the middle ear, between the malleus and incus bones

Question 16

Stapedius Muscle

Answer 16

A muscle attached to the stapes bone which is involved in the acoustic reflex
- tightens in response to loud noises & our own voice, dampening the vibrations of the ossicles
- works together with the tensor tympani muscle
- smallest skeletal muscle in the human body

Question 17

Inner Ear

Answer 17

The inner ear is located in the temporal bone, and is composed of the bony labyrinth:
- cochlea
- vestibular system

The inner ear is a series of canals inside of the temporal bone; it cannot be removed.

Question 18

Bony Labyrinth

Answer 18

The cavity which is “carved out” of the temporal bone and contains the structures of the inner ear.

Question 19

Cochlea

Answer 19

the coiled, spiral structure in the inner ear
- surrounded by bone
- the “magic snail”
- 9 mm long and 3 mm high; 33 mm long when unraveled
- consists of three cavities: scala vestibuli, scala media (cochlear duct), and scala tympani
- filled with fluids that are crucial to our hearing (endocochlear potential)

Question 20

Vestibular System

Answer 20

part of the cochlea that helps us with balance and orientation
- looks like multiple big loops (semicircular canals)
- detects movement of fluid inside of the semicircular canals
- consists of the semicircular canals and the vestibule (which contains the saccule and utricle)

Question 21

Semicircular Canals

Question 22

Scala Media

Answer 22

The cavity of the cochlea between the scala tympani and the scala vestibuli
- filled with endolymph fluid
—”endo” as in “inside” (the middle/inside cavity is filled with endolymph)

Question 23

Scala Tympani

Answer 23

The cavity at the base of the cochlea
- filled with perilymph fluid
—”peri” as in “peripheral” (the two peripheral cavities are filled with perilymph)
- connected to the scala vestibuli via the helicotrema (a small hole located at the apex of the cochlea)

Question 24

Scala Vestibuli

Answer 24

The cavity towards the apex of the cochlea
- filled with perilymph fluid
- connected to the scala tympani via the helicotrema (a small hole located at the apex of the cochlea)
- the scala tympani and the vestibuli are continuous

Question 25

Round Window

Answer 25

A membrane at the base of the cochlea anteriorly that pushes outwards when the oval window is pushed inwards via the stapes, and that is sucked inwards when the oval window is pulled outwards via the stapes

Question 26

Oval Window

Answer 26

A membrane where the stapes presses, connecting the ossicular chain to the base of the cochlea. - connected to the round window - when the oval window pushes in, the round window pushes out (because of fluid)

Question 27

Basilar Membrane

Answer 27

The membrane which separates the scala media from the scala tympani, and is where the **organ of corti** is located - extremely essential to sound perception - stiffer at the base of the cochlea and more flexible at the apex of the cochlea, which allows different areas of it to respond to different frequencies —**when the oval window pushes in, it causes deflection of the basilar membrane** —the base vibrates at higher frequencies, whereas the apex of the BM vibrates at lower frequencies; this is why we're not as good as detecting sounds 7000 Hz - 20,000 Hz

Question 28

Reissner’s Membrane

Answer 28

The membrane which separates the scala vestibuli from the scala media

Question 29

The Organ of Corti

Answer 29

A part of the cochlea that sits on top of the basilar membrane inside of the scala media The organ of corti contains the: - Deiter cells - Hensen cells - Outer hair cells - Inner hair cells - stria vascularis - tectorial membrane And the organ of corti

Question 30

Tectorial Membrane

Question 31

Outer Hair Cells

Question 32

Inner Hair Cells

Question 33

The Stria Vascularis

Question 34

The Spiral Ganglion

Answer 34

The spiral ganglion collect all of the signals from the inner hair cells and "bundle into" the cochlear nerve

Question 35

The Cochlear Nerve

Answer 35

Delivers transmissions on the nerves that indicate the presence of sound to the brain - also known as the **8th cranial nerve** or **auditory nerve** —splits off from the vestibulocochlear nerve —if you cut the cochlear nerve, the result is instant and profound deafness

Question 36

The 8th Cranial Nerve/Vestibulocochlear Nerve

Answer 36

—if you cut the vestibulocochlear nerve, the result is deafness and a loss of balance

Question 37

Tonotopic Organization

Answer 37

Tonotopic organization is another way of describing how neurons/cells are organized according to how they respond to sound. The basilar membrane in the cochlea is tonotopically organized because it vibrates in response to different frequencies: the basilar membrane in the base of the cochlea responds to higher frequencies, whereas the basilar membrane in the apex of the cochlea responds to lower frequencies.

Question 38

What functions does the Pinna serve? Put differently, why don’t we just have two holes in our heads?

Answer 38

The pinna is part of the outer ear, and it serves to amplify & locate sound, to protect the ear from debris, and to prevent pressure seals. Technically, we do not need the pinna to survive. However, without the pinna, we'd be at greater risk for debris/water getting into the ear (causing infection or injury), and we could experience ear damage if our ear were right up against a flat surface due to pressure seals. Also, different parts of the pinna have different resonances, which helps us locate sound in the space around us.

Question 39

What is the difference between the two sections of the ear canal, physiologically?

Answer 39

There are two sections of the ear canal: the bony and cartilaginous portion. The cartilaginous portion has cerumen (earwax) glands and hair, and the bony portion does not have hair nor cerumen glands, but could have earwax stuck in it.

Question 40

Why do we excrete cerumen?

Answer 40

Cerumen helps clean & lubricate the ear canal, and it prevents bacterial and fungal infections. It keeps bacteria & other debris from touching the skin.

Question 41

What structure prevents water (e.g.) from making its way through the ear canal and into the middle ear?

Answer 41

The tympanic membrane separates the ear canal and the middle ear, and it prevents water from entering the middle ear.

Question 42

What functions does the ossicular chain serve?

Answer 42

The ossicular chain is involved in the transduction of sound pressure waves into mechanical waves. It connects the tympanic membrane (which vibrates from air pressure variations) to the oval window that is part of the cochlea. The ossicular chain vibrates and pulls/pushes on the oval window, moving the fluid inside of the cochlea. The ossicular chain is involved in: - **the acoustic reflex** - **protecting the cochlea from injury** (it can break) - **sound amplification** - **impedance matching** between the air of the outside world and the fluid inside of the cochlea

Question 43

What is the difference between the bony labyrinth and the cochlea?

Answer 43

The bony labyrinth is the structure that makes up the inner ear; the cochlea is part of the bony labyrinth. The bony labyrinth contains the cochlea, vestibules, and the semicircular canals.

Question 44

Where does the Eustachian tube start and end?

Answer 44

The Eustachian tube connects the nasopharynx to the middle ear. It opens and closes via the tensor and levator veli palatini when we swallow, chew, or yawn. It is important that the Eustachian tubes be able to open and close because if they were perpetually open, we would hear a lot of nasopharyngeal sounds & we'd be at risk for debris getting into the middle ear. Also, the Eustachian tubes allow for the possibility of drainage from the middle ear.

Question 45

Why do we have Eustachian tubes at all?

Answer 45

Eustachian tubes help use regulate pressure between the air in the middle ear & the air in our environment. In other words, it helps us maintain the air pressure in our middle ear as equivalent to atmospheric pressure. This is helpful for proper sound transduction/transmission. - Young children have differences in their eustachian tubes that make them more susceptible to ear infection

Question 46

What, roughly, do the cochlea and vestibular system do?

Answer 46

Cochlea: helps us with hearing Vestibular system: aids us with balance and spatial orientation The vestibular system consists of the semicircular canals, which are filled with fluid. We use the movement of this fluid to help orient ourselves. The cochlea helps us with transmitting mechanical movement (i.e. basilar membrane movement) into electrical (neural) signals.

Question 47

How, if at all, are the scala media, scala vestibuli, and scala tympani connected?

Answer 47

The scala media is a separate cavity from the scala vestibuli and the scala tympani. The scala vestibuli and the scala tympani are connected at the apex of the cochlea through a hole called the helicotrema. If the scala vestibuli+scala tympani, and the scala media were connected, the two fluids in these cavities would mix and you would not be able to hear, since the potential between the two fluids is essential for our hearing.

Question 48

Where is endolymph found? What about perilymph?

Answer 48

Endolymph is found in the scala media (middle cavity of the cochlea). Perilymph is found in the scala vestibuli (upper cavity of the cochlea) and in the scala tympani (lower cavity of the cochlea). Reissner's membrane separates the scala media and the scala vestibuli, and the basilar membrane separates the scala media and the scala tympani. The stria vascularis maintains the electrochemical gradient of the endolymph and perilymph by giving positive K+ ions in the endolymph.

Question 49

What is the difference in function, roughly, between outer and inner hair cells?

Answer 49

**Outer hair cells**: embedded in the tectorial membrane and supported by the Deiter's cells; pull on the tectorial membrane in response to basilar membrane movement in order to amplify the movement of the basilar membrane and thus the fluid inside of the cochlea **Inner hair cells**: are innervated (connect to the spiral ganglion, which then amass into the auditory nerve and then the vestibulocochlear nerve) —transduce movement of the tips of the inner hair cells into neural signals; tips of inner hair cells move due to movement of the basilar membrane —NOT embedded in the tectorial membrane

Question 50

What structures in the organ of corti connect to the Spiral Ganglion?

Answer 50

The inner hair cells.

Question 51

What’s the difference between the cochlear nerve and the 8th Nerve?

Answer 51

The cochlear nerve is only part of the 8th cranial nerve. The cochlear nerve is made up on spiral ganglion that are connected to inner hair cells, and it transmits neural signals to the brain to help us hear. The 8th cranial nerve is also known as the vestibulocochlear nerve, and it consists of the cochlear nerve and the vestibular nerve—essentially, it has the function of transmitting signals for sound and balance.

Question 52

How large, roughly, is an adult human’s cochlea?

Answer 52

An adult human's cochlea is about 5 mm in height and 9 mm in length. If the cochlea were "rolled out," it would be about 35 mm long.