COSI 325 Chapter 10: Physiology of Hearing

What are the general jobs of each section of the ear?

• Outer ear: collects sound and shapes frequencies

• Middle Ear: match the acoustic signal with fluid in the cochlea 

• Inner Ear: temporal and spectral analysis happens

Describe the first step in the hearing process:

1. Acoustic energy is funneled in by the pinna where the sound is localized and amplified

Describe the second step in the hearing process:

2. Sound is sent to tympanic membrane where the acoustic energy will be converted into mechanical energy → vibration from TM will move across the ossicles → ossicles also amplify the sound

Describe the third step in the hearing process:

3. Footplate of stapes that goes into oval window send vibration through fluid filled cochlea

Describe the fourth step in the hearing process:

4. Vibration of the cochlea vibrates the basilar membrane which will excite the hair cells. Inner hair cells → transmit acoustic/sound; Outer cells → amplifies sound and alerts basilar membrane

Describe the fifth step in the hearing process:

5. Tectorial membrane and inner hair cells will convert mechanical energy into electrical energy

Describe the 6th step in the hearing process:

6. Electrical signal is moved to the cochlear nucleus of the medulla in the brain stem

Describe the 7th step in the hearing process:

7. From the brainstem it will move to the olivary nucleus → assist with localizing sound (processing and analyzing for localization of sound) + direction of sound 

Describe the 8th step in the hearing process:

8. From olivary nucleus we move to the midbrain (moving superiorly up the brainstem) → move to inferior colliculus of the midbrain → continues to process the sound and helps with our auditory reflexes

Describe the 9th step in the hearing process:

Move to the medial geniculate body of the thalamus → sends information to the cortex for higher level processing → then goes to our auditory cortex

Describe the 10th step in the hearing process:

10. Sound makes it to the superior temporal gyrus and this is going to process things like frequency and pitch and intensity

What does the pinna do?

• Pinna collects the sound and localized the sound → because of funnel shape that allows for pulling in sound from the environment 

  • Helps with letting us know where in the environment is the sound coming from (brain does most of this but pinna helps)

  • Pinna collects sound and sendss it through EAM

What does the external auditory meatus do?

• Pinna collects sound and sendss it through EAM and here the sound will be enhanced and shaped → tube will be a resonator → it selectively enhances specific frequencies of sound and keeps others the way they ate 

  • The frequencies it enhances are the frequencies of speech → allows for our ears to make distinctions between different pitches and frequencies 

What happens after sound goes through the EAM?

• Then sent to tympanic membrane → wave sent to the TM→ the vibration through here goes across the ossicles and 

Why is it important that the intensity of the sound is increased to match the density of the fluid filled cochlea?

• We need to make sure the intensity is increased so it can match the density of the fluid filled cochlea

  • If it does not have an amplified intensity it will bounce off and not make it through to the fluid filled space → this is why it is important for our ossicles to amplify the sound 

What happens if there is something impeding the TM from vibrating?

• If there is something in the EAM (ear wax) that impedes the ™ from really vibrating it → sounds will be muffled because the vibration is not enough for the ossicles to amplify and therefore does not create the impedance matching 

What is impedance and why is important?

• Impedance: the resistance to movement 

  • Importance of matching impedance through middle ear space 

  • With the amplification of sound, pressure is also increased → pressure will match the pressure of the inner ear, and this is what allows for the impedance matching 

    • Makes sure sound is efficiently getting from the environment to the ear so that it can go to the brain and it can be analyzed 

• The middle ear mechanism is designed to increase the pressure approaching the cochlea, thereby overcoming the resistance to the flow of energy, termed impedance.

  • That is, to increase pressure, you must either increase the force or decrease the area over which the force is being exerted. The middle ear mechanism uses the latter as the primary means of matching the impedance of the outer and inner ear.

• PRIMARY FUNCTION OF MISSLE EAR IS THE MATCH IMPEDANCE OF TWO SYSTEMS

• WE NEED TO INCREASE PRESSURE

What are three mechanisms of impedance matching?

1) Ratio of TM area to oval window area

2) lever effect of ossicle lengths

3) buckling of curved TM at point of malleus attachment

What is the ratio of the TM area to oval window area? Why is this important?

25dB ratio of TM area to oval window Area

• pressure can be increased by decreasing the area over which force

  is distributed

• Sound energy reaching the TM is “funneled” to the much smaller area of the oval window, so there is a gain of 17:1, which translates to a signal amplification of about 25 dB

How does the lever action increase the signal?

When the malleus moves is displaces the incus and then the stapes → lever action → increases the force that is applied to the oval window → +2dB occurs because of this

How does the buckling of the TM increase the signal amplification?

The shape of the tympanic membrane is curved and is attached to the malleus at a very specific point of that curve shape → when eardrum vibrates and moves with malleus → it buckles in a very specific way that moves inward and pulls the malleus with it → This results in a reduction in velocity of displacement of the malleus, with a resulting increase of force that provides a 4 to 6 dB increase in effective signal

What is the total signal gain from the three mechanisms of impedance matching?

31dB

What would happen if the middle ear were removed?

Because the middle ear is responsible for impedance matching if it were to be removed then a signal entering the EAM would have to be 31dB more intense to be heard. Any process that reduced the effectiveness of this function can have a serious impact on the conduction of sound to the inner ear.

What are some conditions that impact middle ear impedance-matching impact hearing?

• Otosclerosis: when the ossciles have increased calcification which makes them stiff → prevent vibration → stiff at the joint 

• Tumor: takes up more space in cavity 

• Otitis Media → ear infection → if there is fluid in the middle cavity → eustachian tube infection → cant get rid of the fluid and is backed up so it builds up in the middle ear cavities

Explain the frequency and spectral analysis that occurs in the inner ear

Frequency and spectral analysis: Inside cochlea we have a bunch of tiny hair cells and they are sensitive to certain frequencies → as sound goes through cochlea it displaces the hair cells at very specific locations along that membrane → depending on the frequency of that sound the hair cells will be displaced at certain areas → our inner ear and hair cells are not at that point processing what that certain frequency is → instead when the sound travels along the membrane based on where it reaches that determines the frequency it is

Which frequencies reach more towards the base of the basilar membrane?

higher frequencies

Which frequencies reach more lower towards the apex of the basilar membrane?

Lower frequencies

Explain timing temporal analysis

Timing Temporal analysis → analyzed the timing of the sound → can detect changes in sound pressure and then convert the changes into an electrical signal

Why do we need both frequency/spectral analysis and timing temporal analysis?

We need both of these in order for us to analyze things like music and certain aspects of speech → cant have one without the other

What two types of analysis make up the acoustic analysis function of the inner ear?

• frequency and spectral analysis

• Timing Temporal Analysis

What are the two things the inner ear is responsible for?

1. Acoustic Analysis

2. Balance

Explain the balance component of the inner ear

The other half of the system is responsible for sense of balance → canals and otolith organs, utricle and saccule → when we move our head the fluid is displaced and the hair cells pick up on that movement → info is sent to the brain and this is important so we dont fall and we have a sense of space within our own body and gravity

Does the inner ear work in isolation?

The inner ear does not work in isolation → integrated within other sense in our body → vision and proprioception (bodys sense of position → making sure where our body is in space and making sure that our sight and our vision are aware of what is around us

What is another name for the scala media?

Cochlear Duct

What type of fluid is found in the scala media? Why is it particularly important for it to be in the scale media compares to the scale vestibuli and tymapani?

Endolymph in scala media is the opposite composition of perilymph (provides cushion and buoyancy in the system) → want to protect the organ of corti because it is a very sensitive and tiny system (hearing organ) → need to make sure scala media is protected so it is sandwiched by two perilymph filled sections

How does the basilar membrane function?

• Basilar membrane is also our vibrating floor 

  • As the wave moves along that membrane it will determine what information about frequency it will send to the brain 

    • Where does the traveling wave peak the highest and the location along the membrane when that happens 

    • When the vibration enters into the cochlea it will vibrate BM but will not vibrate the whole membrane –< the peak along that membrane and where is what will determine high or low frequency sounds 

Describe the organization of the basilar membrane

• BM organized in a specific way → we have amore narrow and stiffer base of the BM and the apex is wider and looser 

  • Base has higher frequency → stiff narrow less mass (whistle)

  • Apex has lower frequency → loose and wide more mass (hitting a low drum)

What happens to the vibration after it hits its peak in the basilar membrane?

Beyond the point of the highest amplitude (peak) vibration will diminish

What is tonotopic representation?

• Tonotopic representation → specific areas along the membrane are sensitive to specific frequencies

  • Depending on where it peaks that is the information that is going to be sent to the brain

What is the frequency range of the auditory system?

~10 octaves

What is the relationship between cavities and hearing?

• Cavities have characteristics that make them particularly responsive to specific frequencies

  • Selective enhancement of certain frequencies

    • When a cavity is excited by a input stimulus it will tend to select energy at its resonant frequency while tending to reject energy at frequencies other than resonant frequency

What kind of effects does the outer ear have on sound?

• Outer ear only has passive effects on sound because there are no moveable elements 

  • The pinna acts as a sound funnel, focusing acoustic energy into the external auditory meatus (EAM), and the EAM funnels sound to the tympanic membrane (TM)

    • Both of these structures, however, have shapes that boost the relative strength of the signal through resonance,

Why is the middle ear important?

• The cochlea is a fluid-filled cavity, and were it not for the presence of the middle ear mechanism, talking to each other would be like trying to talk to someone under water: The sound energy would reflect off the oval window because of the vast differences in the liquid and gaseous media of perilymph and air.

What do the semicircular canals do?

• Semicircular canals are uniquely designed to respond to rotary movements of the body 

  • Orientations of the canals are so that all movement of the head can be mapped by combinations of outputs of the sensory components (crista ampullares)

    • Activation of this sensory element comes from inertia 

      • As your head rotates the fluid in the semicircular canal tends to lag behind 

        • This results in the cilia being stimulated by relative movement of the fluid during rotation

What is the difference between the semicircular canals and the utricle + saccule?

Utricle and Saccule sense acceleration of the head rather than rotation when the body or head is tilting

What are the vestibular maculae?

• The vestibular maculae are two sensory epithelia housed in the utricle and saccule of the inner ear that provide sensory input to the reflex circuits underlying balance and posture

  • Macula of the Utricle is horizontal to plane of movement (forward or backward movement will be sensed here)

  • Saccule is Vertically oriented (upwards and downwards)

Explain the major components of the semicircular canals

• Each inner ear has 3 semicircular canals (horizontal (lateral), anterior and superior)

  • Each canal has a crista ampullaris and a cupola atop each crista

    • Each crista has 50 stereocilia protruding from it which are embedded in the cupola 

      • When cupola moved it deflects (causes to change in direction) the stereocilia → this causes the sensation of movement 

What are the movements detected by each semicircular canal?

• Horizontal Canal: senses movement related to head rotation → turning your head to signal no

• Anterior Canal: sense movement in the vertical place → nodding head yes

• Posterior Canal: senses movement o the head towards the shoulders 

Explain the sensory element that is associated with the canals?

• Sensory element associated with movement in canals is dependent on inertia 

  • Fluid in the canal lags because of inertia 

  • The lag provides the sensation that you perceive as the movement 

How is information about acceleration sensed by the semicircular canals?

• Information about acceleration sensed by the semicircular canals is transmitted by the VIII vestibulocochlear nerve to the vestibular nuclei of the brain stem

What are the three scalae? What fluid is found within them?

1. Scala vestibuli —> perilymph

2. Scala Media —> endolymph

3. Scala Tympani —> perilymph

what kind of charge do hair cells have when at rest?

negative charge

what do we see an influx of when ion channels open?

when ion channels open we see an influx of K and Ca which causes depolarization making the hair cells positively charged

What happens when inner and outer hair cells are depolarized?

Depolarizing the inner hair cells causes excitation of the VIII nerve as

a result of glutamate release, while depolarizing the outer hair cells causes a

motor response that actually moves the basilar membrane

What triggers the release of glutamate?

triggered by K and Ca²⁺

When are hair cells activated?

when the basilar membrane is displaced towards the scala vestibuli. when they are displaces towards the scala tympani the electrical activity of the hair cell is inhibited

• when the basilar membrane moves the hair cells upwards they shear against the tectorial membrane which causes the hair cells to get excited and open the ion channels

When do the ion channels open?

When the cilia are mechanically deflected toward the highest stereocilia, the channels are drawn open.

• When the cilia are pivoted toward the tallest of the stereocilia (the kinocilium), the hair cell depolarizes; when it is tilted away from the kinocilium, the hair cell is hyperpolarized (cannot fire)

What is the importance of glutamate being released?

the release of glutamate puts the whole auditory tract into motion to send messages to the brain

• used as a signal to the VIII nerve fiber

• the tract moves as a result of this release

What happens after the glutamate is released?

Tracts leave from the hair cells and travel through the internal auditory meatus foramen.

• Tracts from the hair cells form a bundle that creates the CN VIII vestibular cochlear nerve (specifically the cochlear branch)

• tracts then go to the cochlear nucleus in the medulla —> first way station in the auditory pathway

What are the 3 nuclei that make up the cochlear nucleus?

1) Dorsal Nuclei

2) Posterior Ventral Cochlear Nuclei

3) Anterior ventral cochlear Nuclei

Where will the tract from the dorsal nuclei in the cochlear nucleus go?

Most tracts will go to the nucleus of the lateral lemniscus (pons) and a small amount will go to the inferior colliculus(midbrain)

• (moves. contralaterally)

What are the two cells in the dorsal cochlear nuclei?

1. Principle Cells (pyramidal cells) —> help process higher level cells —> frequency, timing, music

2. Stellate Cells (inhibitory cells) —> prevent certain thens from happening, dampen neural signals that don’t matter or dampen signals that may interfere w principle cells being able to do their jobs

Where will the tracts from the posterior ventral nuclei in the cochlear nucleus go?

moves contralaterally. A few of them will synapse on the lateral lemniscus (pons) but most will keep moving upward into the inferior colliculus (midbrain)

What kind of cells are found in the posterior ventral cochlear nuclei?

Octopus cells: have bifurcations and branches that come off of them

Where does the tract from the anterior ventral cochlear nuclei go?

moves bilaterally

1) internal acoustic stria —> moves ipsalaterally to the superior olivary complex

2) Trapezoid body —> moves contralaterally to the superior olivary complex

What are the cells found in the anterior ventral cochlear nuclei?

1. stellate cells

2. Bushy cells

What are the two parts of the superior olivary complex?

1. Medial Olivary Complex —> assists with timing of sound

2. Lateral Olivary Complex —> responsible for intensity of sound

Where do the tracts from the superior olivary complex go?

1) there is a contralateral loop that goes between the two superior olivary complexes

2) Tract from the olivary goes to the hair cells in the middle ear

• elongates hair cells towards basilar membrane

• tells hair cells to chill out and not get excited as much —> causes the hair cells not to shear as much with the tectorial membrane so not all signals get through —> this protects against loud sounds

Where do tracts from the inferior colliculus go?

They travel to the medial genicula body of the thalamus

What is the name of the descending tract coming from the inferior colliculus?

Tectospinal tract

• motor tract (efferent)

What is the function of the tectospinal tract?

recieves sound and send info to our motor nerves —> moves head and neck to look at where the sound is coming from —> AUDITORY REFLEX

Are most tracts ascending or descending?

most tracts are sensory which means they are ascending!

What is the reticular formation and what protective function does it enact?

The reticular formation is an important mass of nuclei that spans the medulla, pons, and midbrain.

• grey matter

When there is a loud sound it puts things in place to protect up

• send out CN V and CN VII to the middle ear

• CN VII innervates the strapedius

• CN V innervates the tensor tympani

• stiffens the muscles so the ossicles do not perform the lever action

Where do tracts from the reticular formation go?

sends fibers to the middle ear responsible for protecting our middle ear space

What is wernickes area?

area in the temporal lobe responsible for comprehension of sound and language

What is brocas area?

area in frontal lobe that is responsible for the production of speech

Do wernickes area and brocas area need to interact?

YES!!! Their interactions is how we communicate and respond!

What is the superior temporal gyrus?

The superior temporal gyrus (STG) is involved in auditory processing, including language, but also has been implicated as a critical structure in social cognition.