Hearing Science: Anatomy & Physiology

Sagittal/medial plane

Divides the body into two parts, left and right

Coronal plane

divides body into front and back

Transverse plane

horizontal division of the body into upper and lower portions

midline

divides body into left and right; up and down middle

medial

toward the midline

lateral

away from the midline

distal

away from the point of attachment

proximal

Closer to the point of attachment

superior

above

inferior

below

posterior

toward the back/behind

anterior

toward the front/in front

The eyes are ____ to the ears

medial

The heel is ___ to the toes

posterior

the arms are ___ to the midline

lateral

The hand is ____ to the shoulder

distal

The knee is ____ to the ankle

proximal

peripheral auditory system divisions

1. outer ear

2. middle ear

3. inner ear

What are the major divisions of the outer ear?

- pinna (auricle)

- external auditory/acoustic meatus (ear canal; EAC)

How do we assess the outer ear?

otoscopy

otoscopy steps:

1. inspect the pinna

2. inspect the EAC

3. inspect the tympanic membrane

What is the composition and orientation of the pinna?

- flexible cartilage with various folds (known as landmarks)

- skin (epithelial cells) and fat

- 30 degree orientation

structure of the EAC

outer 1/3: cartilaginous

inner 2/3: osseous (bony)

How long is the EAC?

~2.5 cm

What is the ridge between the cartilaginous/bony portions of the EAC called?

isthums; 1st bend in the EAC

What shape is the EAC?

S-shaped

What makes up the EAC?

- hair follicles (keratin)

- glands (apocrine and sebaceous)

- temporomandibular joint (TMJ)

What do the hair follicles in the EAC do?

move and gently push ear wax out of the ear over time

Where is ear wax/cerumen produced?

the cartilaginous portion of the ear (where there are hair follicles); produced by the apocrine and sebaceous glands

Why is it important to not use Q-tips?

Cerumen is produced in the cartilaginous portion of the outer ear. Using Q-tips pushes ear wax back into the osseous portion, where it can get impacted and won't get pushed back out by the hair follicles. Using a Q-tip can also perforate your tympanic membrane.

What is the lateral wall of the middle ear?

tympanic membrane

What is the part of the malleus that connects to the tympanic membrane called?

umbo

What holds the tympanic membrane in place?

annulus/annular ligament

What are the two parts of the tympanic membrane?

pars tensa and pars flaccida

Where does the cone of light come from?

otoscope reflection in a healthy ear

How can we tell which ear we are looking at by looking at the tympanic membrane?

Look at the cone of light (if on left: left ear; right: right ear)

What are the acoustic functions of the outer ear?

sound transmission

aids localization

What is the non-acoustic function of the outer ear?

protection

sound transmission

- pinna

- gathering, funneling, and amplifying sounds

- shape of pinna catches more sound waves than the EAC alone can

transfer functions

- compare inputs to outputs

- can look at to see how the structures of the outer ear influence sounds

gain

how much "bigger" the output is from the input

output - input

How much gain does the EAC provide?

+ ~ 10 dB at 2.5 - 3.5 kHz

How much gain does the pinna provide?

+ ~ 10 dB at 5 kHZ

What contributes to the gain that the EAC provides?

resonance (EAC is a 1/4 wavelength resonator)

How does the pinna provide gain (amplify signals)?

- relationship between amplitude and phase properties when they encounter structures of the pinna (resonance)

- constructive and destructive interference coming in at different spots and interacting

- how much energy is "gathered": concha ~ 5 kHZ & helix/anti-helix +3 dB ~4 kHz

inverse square law (pinna)

- pinna collects and funnels sounds

- provides gain to high-frequency inputs

- general idea: the larger the surface area of the pinna, the more sound will be collected (more gain)

What do you do if you want to "hear" someone/thing better?

cup your ears

- creates a bigger collective "funnel"

- extension of the pinna

- can assist with directionality as well

- adds ~8 dB to mid-high frequency sounds

What is sound localization?

where in a space is a sound coming from

azimuth

angle on horizontal plane (looking straight forward; L/R from nose)

0 degrees: in front of you

90 degrees: to the side of you

elevation

angle on a vertical plane

distance (inverse square law)

as distance doubles, intensity is reduced by ~6 dB

monaural localization

Localization of sound with one ear

- vertical localization (elevation)

- HRTFs

binaural localization

comparison between both ears (horizontal localization)

- ITD/IPD and IID/ILD

Does the OE decode/interpret localization cues?

No, the OE is not responsible for decoding/interpreting these cues. The OE simply helps collect the signals and aids the ability for a listener to gather these cues

interaural timing (phase) difference (ITD/IPD)

- horizontal localization (L/R)

- more efficient for low frequencies (< 850 Hz)

- comparing the timing/phase of a signal from one ear to another

- when wavelength is bigger than the distance between ears (such as for low frequencies), it is easier to localize using this cue; if wavelength is smaller than the distance between ears, it does not work as well

interaural intensity (level) difference (IID/ILD)

- horizontal localization (L/R)

- head shadow effect: reduction in loudness for farther away ear

- more efficient for high frequency sounds

- when wavelength is smaller than the width of your head, you get an acoustic shadow, which reduces the volume in the farther away ear (this is why ILD is more efficient for high frequency sounds)

head related transfer function (HRTF)

- vertical localization

- combined influence from structures of the head, neck, and torso on incoming sounds --> especially in the vertical plane

- direction and frequency dependent ratio of sound pressure (elevation changes the way sound is collected by the pinna)

- each of us have unique HRTFs --> map of localization cues

- modifying the pinna/concha/EAC (i.e. plastic surgery) changes one's HRTF: localization becomes poorer, but you can "learn" to readjust because of neural plasticity, but the HRTF will be different

- spectral cues/shifts in spectral valleys

How does the outer ear provide protection?

- protects from foreign objects/outside atmosphere (especially the tympanic membrane)

- Shape of the EAC (S-shaped) prevents foreign objects from getting all the way to the TM

- production of cerumen and hairs: outward migration (hairs push ear wax out); collects particles (dust/debris)

What energy is in the outer ear?

acoustic energy

What energy is in the middle ear?

mechanical energy

What energy is in the inner ear?

hydrodynamic/electrical

What does the peripheral auditory system do?

transmit and transduce signals

conductive portion of the peripheral auditory system

outer and middle ear

sensory portion of the peripheral auditory system

inner ear

central auditory system

brainstem all the way up to auditory cortex

What are the functions of the middle ear?

- mechanical transducer (transduce acoustic to mechanical energy)

- impedance matcher (overcome impedance mismatch)

What does the middle ear do?

The middle ear conducts sounds from the environment to the inner ear (cochlea)

Why does the ME act as an impedance matcher?

- The inner ear (cochlea) is filled with fluid

- Fluid has a higher impedance than air (when sounds go from areas of low impedance - air - to areas of high impedance - water/fluid - energy is lost and doesn't travel as efficiently)

- As sounds move from air to fluid, energy is lost

- The ME must compensate for what is lost by making signals bigger (adding gain - dB SPL)

- the ME must add gain to the signal to overcome what is attenuated

How does the middle ear act as a mechanical transducer?

Transmits acoustic into mechanical energy through movements of the ossicular chain

What frequencies does mass work against?

High frequency transmission

What frequencies does stiffness work against?

Low frequency transmission

impedance (Za)

- opposition to flow of energy

- measured in Ohms

admittance (Ya)

- measured in clinic

- ease of transmission of acoustic energy

- measured in Mhos (Y)

- opposite of impedance; what can go through

Does the tympanic membrane contribute to mass or stiffness?

mostly stiffness, but can be mass

Do the ossicles contribute to mass or stiffness?

mass

Do the ossicular ligaments/joints contribute to mass or stiffness?

stiffness

Does the middle ear space contribute to mass or stiffness?

stiffness (air filled)

What is the front boundary of the pars flaccida?

anterior fold

What is the posterior boundary of the pars flaccida?

posterior fold

composition of the tympanic membrane

- outer layer: skin - cuticular layer

- middle layer: fibrous layer

- inner layer: mucosal layer/lining (covers boundaries of the middle ear space)

- middle/fibrous layer has looser weaving in the pars flaccida (loose, mass) and tighter weaving in the pars tensa (stiffness)

otitis media stages

- with effusion = with fluid

- middle ear infection

1. retracted: negative pressure pulls fluid out, TM is sucked backward

2. effusion beginning: TM being pushed out with fluid filling middle ear space; redness

3. effusion and bulging: full effusion = full fluid; fully bulging; if fluid can't go anywhere, can rupture TM

beginning of middle ear

tympanic membrane

end of middle ear

oval window

ossicular chain movement

- acoustic energy will vibrate the tympanic membrane

- vibrations travel along ossicular chain (ossicles move with TM)

- vibrations of the stapes footplate at the oval window introduce vibrations to cochlea

- ossicles are crucial for transmission of sound in ME

What are the three ossicles?

malleus, incus, stapes

incudomalleolar joint

connects the incus and malleus

incudostapedial joint

joint between incus and stapes

What two ligaments are on the malleus?

anterior malleus ligament

superior malleus ligament

What two ligaments are on the incus?

superior incus ligament

posterior incus ligament

What muscle is on the malleus?

tensor tympanic muscle

What muscle is on the stapes?

stapedius muscle

What cranial nerve innervates the tensor tympani?

CN V (trigeminal)

What cranial nerve innervates the stapedius muscle?

CN VII (facial)

chorda tympani

- CN VII

- branch of the facial nerve running through the middle ear space

- gives sensation of taste

aditus ad antrum

opening to mastoid air cells (porous portion of temporal bone)

tegmen tympani

bony roof of the middle ear

epitympanum

upper middle ear space

promontory

first turn of the cochlea that can be seen in the middle ear

pyramidal eminence

point of attachment for stapedius muscle

eustachian tube

- runs out of the anterior wall of the middle ear, connects with the nasopharynx

- allows air in and out (equalizes pressure)

- drainage of fluid build-up