The Peripheral Auditory System is involved in what function?

Conduction and Transduction

= "sound reception"

What is conduction?

Conduction is when the outer and middle ears conduct sound vibration to the inner and central system

What is transduction?

Transduction takes place in the inner ear, where sound is transduced from vibration to neural impulses

Which parts of the ear make up the outer ear?

Describe these 3 parts

 Pinna - which is a visible flap

Concha - bowl-like

External Auditory Canal - an S-shaped curve, 2.5 cm long (on average)

What is the function of the External Auditory Canal?

Funnels sounds to the tympanic membrane (ear drum)

P and C collect sound from the environment

then,

the EAC funnels that sound to the tympanic membrane

Protection

Localization

Resonance

What role does 'Protection' play in conduction?

The s-shape of the external auditory canal helps to keep out both water and foreign objects

-Wax impactation also keeps out the aforementioned objects

What is 'localization'?

In sound localization, binaural localization is the best (simply means that it is best with both ears)

-In localizing sound, we use the differences in arrival times of sound waves and their differences in intensity

-Compare time of arrival (phase) & dB information between the ears to determine location

What are the 2 primary cues of localization?

1. Interaural level difference (ILD)  inter-aural timing differences mostly due to headshadow

-Headshadow is a region of reduced amplitude of a sound because it is obstructed by the head. Sound may have to travel though and around the head in order to reach an ear.

The head shadow causes particular difficulty in sound localization in people suffering from unilateral hearing loss.

2.  Inter-aural time differences- mostly due to the distance between ears

The external auditory canal as a resonator,

Ear canal as open-closed tube

Which resonance formula do we use for the external auditory canal?

-Open closed tube

-Resonance of 2.5 cm

-Speed of sound = 344 m/sec

(344m/sec)/(4*0.25) = 3440 HZ

The ear canal can amplify Hz between X and X?

How many Hz is Speech energy?

Ear canals amplify between 2000-5000 Hz

-Speech energy is mostly 500-5000Hz

Name the key structures to the Middle Ear

Tympanic Membrane (Eardrum - border b/t OE and ME)

Ossicles (Malleus, Incus, and Stapes)

Eustachian Tube 

Oval and Round Windows (border b/t ME and IE)

What is the role of the Eustachian Tube?

-Connects middle ear to nasopharynx

- Equalizesaire pressure inside and outside the ear

-Ear 'popping' is the Eustachian tube opening

-Links the air in the the middle ear with outside air

The Eustachian tube is normally closed.

Why does it open?

Opens to equalize pressure in the middle ear

The ossicles provide a mechanical linkage between....

the Tympanic Membrane and the Oval Window

Transitions: Oval and Round Windows,

the Malleus, Incus, and Stapes are connected to each connected to a separate part of the ear,

What are they?

Malleus - connects to the TM

Incus - middle bone

Stapes - footplate connects to oval window

What is Impedance?

Is the opposition to energy flow

- Diferent mediums have different impedances (i.e. air lower impedance than fluid)

Air and Fluid do not have similar impedances

Efficient flow of energy requires SIMILAR impedances

How much energy would be lost if impedance went straight from the air-filled outer ear to the inner ear?

99.9% of sound energy (if no middle ear existed)

How does the middle ear act as an impedance matcher?

-Helps overcome the impedance MISMATCH between air and fluid

-Adequate transmission of energy

-How? Middle Ear increases pressure by ~30dB

Surface area differences between TM & oval window
   REMEMBER:  P = F / A    
  Pressure & area INVERSELY PROPORTIONAL
  Force at TM = Force at OW 
  How does pressure change due to difference in areas.  
  Surface of TM is approx. 17x’s greater than OW
 

What is the role of the Middle Ear as a protector against loud sounds?

ME works as an acoustic reflex: protects the inner ear from excessive and intense sound BY acoustic reflex (which is a contraction of muscles in the ME)

Basic functions of the Outer Ear

(Review)

– Collects/Funnels Sound from
Environment
– Protection
– Localization
• ITD
• ILD
– Resonance
 

Basic functions of the Middle Ear

(review)

What is the primary function of the Inner Ear?

to transduce vibrations (for hearing) or movement (for balance) into neural impulses

What are the two main parts of the Inner Ear?

1. Vestibular system (semicircular canals)

2. Cochlea

Anatomical Orientation of the IE,

where is the Basal end, the Apical end?

-Basal (base of cochlea) - closest to middle ear

-Apical end: scala vestibuli, scala media, organ of corti - sit on top of BM, includes hair cells, basilar membrane - border b/t scala media and tympani

Primary function of the Cochlea is...

For hearing

What are the key structures in Transduction?

-Oran of Corti (spiral organ) : organ of hearing

-4 rows of hair cells - stereocilia

-1 inner row (~3500 cells)

-3 outer rows (~12000 cells)

Tectorial Membrane

Physiology of Transduction

-Stapes pumps in & out of cochlea, basilar membrane vibrates (tonotopically)

-Structures sitting on top of BM vibrate (in particular, organ of corti and the hair cells within it)

-Vibration of hair cells cause stereocilia to ben ("sheared" by tectorial membrane)

-Bending causes change in the cells' polarity

-Polarity changes result in release of neurotransmitter at bottom of hair cell bauses nerve fibers in Auditory never to fire

How does the function of outer hair cells and inner hair cells differ?

(Both hair cells contribute to hearing)

-Inner: transduce mechanical vibration into an electrical signal
-Outer: shrink and expand to fine tune (amplify) the frequency response of the cochlea
 

What is a traveling wave?

How is frequency and amplitude depicted in a traveling wave?

a traveling wave provides information about sounds

-amplitude of sound = height of a traveling wave

-freq. greater db SPL = greater height

What is tonotopic organization?

Frequency organizaed by place along basilar membrane

Each point on the Basilar membrane are tuned to different frequencies, the traveling waves of high and low frequencies will peak where?

Traveling waves of high freq. will PEAK near BASE

Traveling waves of low freq. will PEAK near APEX

What contributes to tonotpoic organization?

What are the two mechanical properties of this organization?

Mechanical properties of BM:

1st  property: Stifness gradient (base is stiff, Apex is leff stiff)

2nd Property: Width gradient (base is narrow, apex is wide)

The amount of damade to the auditory system depends on what two things?

-TIME in noisy environments (durations)

-LEVEL of noise exposed to 

What are the signs of noise exposure?

-must raise voice to be heard

-can't hear someone at a distance of 2-3 feet away from you

-speech sounds muffled or dull after leaving a noisy area

-experience pain or ringing after the noise exposure

True or False:

No one fiber extends all the way from the cochlea to the auditory cortex, Instead the fibers make connections with other nerve fibers along the way (called synaptic junction)

True.

Obviously.

What is the difference between Afferent and Efferent fibers?

Afferent = something that conveys an impulse to the brain

Efferent = something that conveys a feeling from the brain to the rest of the body

Generally, what hair cells do afferent fibers synapse with? Efferent fibers?

Afferent = inner hair cells

Efferent = outer hair cells