Physiology of the Auditory System- Graf

Question 1

What consists of the external, middle, and inner ear?

Answer 1

External: pinna, ear canal, tympanic membrane

Middle: malleus, incus, stapes, pharyngotympanic tube (eustachian tube) to the pharynx

Inner: semicircular canal, vestibular apparatus, cochlea, nerves, labyrinth

Question 2

The oval window and round window separate what?

Answer 2

fluid-filled inner ear from the air filled middle ear

Question 3

What is sound?

Answer 3

• a pressure disturbance of air molecules (alternating areas of high and low pressure) originating from a vibrating object
• composed of areas of rarefactions and compression
• represented by sine wave in wavelength, frequency, and amplitude

concert hall have very good acoustic and so everything vibrates

Question 4

Properties of sound:

What is the frequency and pitch of sound?

Answer 4

frequency: the number of waves that pass a given point in a given time
pitch: perception of different frequencies (we hear from 16-20,000 Hz)

Hz = one cycle per second

Question 5

What types of waves are sound waves?

Answer 5

they are pressure waves with alternating peaks of compressed air and valleys where molecules are farther apart

tuning fork makes compression and vibration of air molecule (tool used to test hearing)

Question 6

How are sounds waves distinguished?

Answer 6

by their amplitude which is measured in decibels (dB) and their frequency which is measured in hertz (Hz)

Question 7

Pure tones vs complex tones:

All natural sounds are of what tone?

Answer 7

all natural sounds are complex tones (combination of different sound frequencies)

tuning fork sounds are pure tone

Question 8

What is the absolute hearing threshold in humans?

Answer 8

2 x 10^-5 Pa

Question 9

Measurement of Sound

Answer 9

decibels is on a logarithmic scale:
increase of 6 dB = doubling (2x) of sound pressure

increase of 20 dB = 10x sp

increase of 40 dB = 100x sp

increase of 60 dB = 1,000x sp

Human hearing range: 0-130 dB

Human language: 40-70 dB

Disco music, jack hammer: 100 dB

Jet engine: 140 dB

Danger level: >90 dB

Pain level: >130 dB

Speed of sound (c): 340 m/sec in air (= 1 Mach); 1,400 m/sec in water

Pitch (frequency of sound waves, n): Hz. Human hearing = 16 Hz – 20 kHz

Wave length: lambda measured in (m)

Relationship: c = n x l

Bats produce 100 kHz @ 100 dB!!

Question 10

What is pitch?

Answer 10

the frequency of sound waves

• higher pitch: more wave disturbances= more frequency
• lower pitch: less wave disturbances= less frequency

Question 11

Describe the transmission of sound to the inner ear.

Answer 11

The route of sound to the inner ear follows this pathway:

• outer ear: pinna, auditory canal, eardrum
• middle ear: malleus, incus, stapes to the oval window in the cochlea
• inner ear: scalae vestibuli through the helicotrema to the scalae tympani to the scalae media (cochlear duct)
• cochlear duct has the organ of corti; stimulation of organ of corti which is overlaid by the tectorial membrane will move the outer and inner hair cells; AP go out ot the spiral ganglion; generation of impulses in the cochlear nerve (mechanoelectrical conductions to generate sound waves into APs)

Question 12

The transduction of sound from outer to inner ear

Answer 12

First transduction: Sound waves strike the tympanic membrane and become vibrations

The sound wave energy is transferred to the three bones of the middle ear, which vibrate.

Second transduction: The stapes is attached to the membrane of the oval window. Vibration of the oval window create fluid waves within the cochlea.

Third transduction: The fluid waves push on the flexible membranes of the cochlear duct. Hair cells bend and release neurotransmitter.

Fourth transduction: Neurotransmitter release onto sensory neurons creates action potentials that travel through the cochlear nerve to the brain.

Energy from the waves transfers across the cochlear duct into the tympanic duct and is dissipated back into the middle ear at the round window.

Question 13

What are the 3 mechanical characteristics that contribute to the impedance adjustment of the air to fluid penetration of sound waves?

Answer 13

the sounds which we hear are actually much lower in intensity that what we produce; there is attenuation of the sound in the ear which increase the pressure of the sound waves up to 10x

• the surface of the tympanic membrane is about 30 times larger than the surface of the oval window. Thus pressure is amplified by about his factor
• lever action of the ossicular chain: about 1.3 times
• increase of sound pressure through the conical form of the tympanic membrane

Question 14

What is the result of the combined effect of middle ear impedance adjustment?

Answer 14

an increased hearing efficiency of 20 dB

the sounds which we hear are actually much lower in intensity that what we produce; there is attenuation of the sound in the ear which increase the pressure of the sound waves up to 10x

Question 15

Describe the flow of sound from the oval window to the round window.

Answer 15

The sound travels from the oval window to the scala vestibuli towards the helicotrema, and from the helicotrema towards the round window in the scala tympani.

Question 16

Describe the structure of the Organ of Corti in the scala media.

Answer 16

-tectorial membrane
-support cells
-lamina spiralis ossea
-N. cochlearis
-ganglion spirale
-1 row inner hair cells: hear cells
-3 rows outer hair cells (for amplification of the sound from the low sound pressure level environment
(hair cells sit on the basilar membrane)
-basilar membrane : gets vibrating up and down bending the hair in which the stereocilia gets pushed up against the tectorial membrane and ion channels (mechanically gated) are open and thus AP in the cochlear nerve

Question 17

Membrane potential of hair cell corresponds to the action potentials the primary sensory neuron

Answer 17

At rest: About 10% of the ion channels are open and a tonic signal is sent by the sensory neuron

Excitation: when the hair cells bend in one direction, calcium channels open allowing NT release, the cells depolarizes, which increases action potential frequency in the associated sensory neuron

Inhibition: If the hair cells bend in the opposite, ion channels close, the cell hyperpolarizes, and sensory neuron signaling decreases

Question 18

What are other names for scala vestibuli, scala media, and scala tympani?

Answer 18

• scala vestibuli: vestibular duct
• scala media: cochlear duct
• scala tympani: tympanic duct

Question 19

The basilar membrane stretches from what two structures?

Answer 19

basilar membrane stretches between osseous spiral lamina and cochlear spiral ligament

Question 20

What are the two membrane that arise from the spiral limbus?

Answer 20

• tectorial membrane

- Reissner’s membrane

Question 21

What moves the cilia on the hair cells?

Answer 21

movement of the tectorial membrane

Question 22

Describe hair cell transduction.

Answer 22

• fluid wave moves
• basilar membrane moves
• tectorial membrane moves
• steriocilia move
• ion channels open
• depolarization
• neurotransmitter release
• sensory nerve action potentials

Question 23

Describe the excitation of hair cells in the organ of corti.

Answer 23

Bending cilia:

• opens mechanically gated ion channels
• causes a graded potential and the release of a neurotransmitter (probably glutamate)

the neurotransmitter causes cochlear fibers to transmit impulses to the brain, where sound is perceived

Question 24

Where do auditory nerve fibers emerge from?

Answer 24

from the cochlear

the cochlear duct is coiled; the auditory nerve fibers emerge from the cochlea and innervate nuclei in the brain stem

Question 25

Explain the traveling sound pressure wave and the representation of different sound frequencies along the cochlea.

Answer 25

Sound activates the cochlea in complex, rapidly changing patterns.

• High pitch sounds are detected towards the base of the cochlea
• low pitch sounds towards the apex

tonotopic pattern!!!

Question 26

The basilar membrane has variable sensitivity to sound wave frequency along its length.

Answer 26

High frequency (high pitch) at the stiff region near round window

Low frequency (low pitch) at the flexible region near helicotrema (distal end)

tonotopic pattern in the basilar membrane!!!

so it vibrates differentially based on the pitch sound it receives

Question 27

Cochlear nuclei have four major groups of principal cells.

Answer 27

neurons live in the spiral ganglion and then project to the cochlea nuclei which have different types of cells:

• fusiform
• octopus
• stellate
• bushy

Question 28

Auditory nerve fibers pass information to multiple parallel, ascending pathways

Answer 28

-superior olivary complex
-ventral nucleus of the lateral lemniscus (VNLL)
-Inferior colliculus
-thalamus
auditory cortex

in the auditory system we have the inferior colliculus project to the medial geniculate nucleus in the thalamus to the auditory cortex

the visual system is a parallel system that goes from the inferior colliculus to the LGN and then to the visual cortex (superior colliculus does NOT project to LGN)

Question 29

What type of cells of detect spectral cues for localizing sounds in elevation?

Answer 29

fusiform cells

-fusiform cells also integrate proprioceptive information acoustic information

Question 30

Which cells detect the coincident firing of auditory nerve fibers?

Answer 30

Octopus cells

-octopus cells are coincidence detectors for inputs with cochlear delays and participate in “binding”

input from many sharply tuned fibers results in broad tuning

• when there is a deficit trumpet will sound like a donkey
• this can be related to visual agnosia which refers to an inability to identify or recognize even common objects presented in the visual modality, with intact semantics and recognition of objects through other sensory modalities.

Question 31

Which cells report the presence of energy in narrow frequency bands?

Answer 31

stellate cells

• stellate cells as a population encode the spectra of sounds
• dendrites of stellate cells lie parallel to the auditory nerve fibers which come from different parts of the basilar membrane

Question 32

Which cells participate in localizing sounds in azimuth?

Answer 32

Bushy cells

• Bushy cells mediate the comparison of timing at the two ears by the superior olivary complex
• assessment of interaural phase is the basis for localizing sound in azimuth: phase locking
• azimuth is horizontal plane
• sounds arrive almost at the same time making a depolarization which then signal sounds that comes from a certain direction

Question 33

Spectral components of natural sounds are bound into a single percept by?

Answer 33

by converging information and processing channels

• fusiform deals with localization in elevation
• octopus cells deal with binding
• stellate cells deal with spectrum
• bushy cells deal with localization in azimuth

Question 34

What is the role of outer hair cells?

Answer 34

• they act as amplifiers of sound
• outer hair cells have very little afferent fibers that go the brainstem compared to inner hair cells

-efferent fibers that come from the cochlea has lots of output to the outer hair cells (very little to the inner hair cells)

Question 35

Outer hair cells shorten when __________and lengthen when ____________causing oscillations.

Answer 35

depolarized
hyperpolarized

we able to hear sound louder through the outer hair cells when they depolarize; they let the tectorial membrane come closer to the inner hair cells to bend the kinocilia and let more ions in getting more APs so sound is louder.

Question 36

What protein in the outer hair cell membrane reconfigures in response to changing electric fields?

Answer 36

prestin

Question 37

How do you determine the loudness of sound?

Answer 37

• by the amplitude of vibration of basilar membrane
• by the number of hair cells being stimulated (more hair cells stimulated, more APs in the cochlear nerve, so you hear sounds louder)

Question 38

Directional localization of Sound

The direction from which sound emanates is determined by?

Answer 38

• time lag between entry of sound into one ear and its entry into opposite ear (timing, coincidence detectors)
• differences in the intensities of sound at the two ear (intensity, interaural intensity difference)

important to know where sounds are coming from to prevent yourself from being hunted against, etc.

Question 39

What is the result of a lesion to the facial nerve that is innervating the stapedius muscle?

Answer 39

hyperacusis, causes normal sounds to be perceived as very loud. Paralysis of the stapedius muscle may result when the nerve to the stapedius, a branch of the facial nerve, is damaged, or when the facial nerve itself is damaged before the nerve to stapedius branches.

tensor veli tympani tenses the tympanic membrane for it to vibrate properly

Question 40

How can you have impaired hearing loss through the ossicles in the middle ear?

Answer 40

they can undergo osteoporosis which will affect the joints and thus their function impairing hearing

Question 41

Describe hair cells damage.

Answer 41

it is permanent and cumulative so when you do not have a lot of hair cells

-damaged by sound, chemicals, and medication like streptomycin

Question 42

How many photoreceptors do we have? How many hair cells do we have?

Answer 42

• 100 million photoreceptors

- 4,000 inner hair cells in the cochlea (we only have so much so lets not damage it)

Question 43

Where does the stapedius muscle insert?

Answer 43

foot of the stapes

Question 44

What is the difference between perilymph in the vestibular and tympanic duct vs. endolymph in the cochlear duct?

Answer 44

Perilymph:

• has an ionic composition similar to blood plasma (high in Na and low in K)
• in vestibular and tympanic duct

Endolymph:

• high in K and low in Na
• unlike any fluid in our body
• in cochlear duct
• hair cells bathe in this fluid
• has ion channels where K flows into cell along with calcium

Question 45

Can fluids be compressed?

Answer 45

fluids cannot be compressed, thus when they start to vibrate everything around them will vibrate including the endolymph and the basilar membrane to which the organ of corti is sitting on

Question 46

What is the endocochlear potential?

Answer 46

• driven by the potential difference between the stereocilia (-130 mV) and the basolateral pole of the hair cells (-45 mV)
• 45 -(-130) = 85 mV which is the charge in the endolymph (high conc of K so positive charge in the endolymph and low conc of K in the hair cells)
• it is about 85 mV

-potassium is generated by stria vascularis and essential for the normal function
of hair cells

Question 47

If the stria vascularis does not work , why do you get hearing loss?

Answer 47

• stria vascularis produces potassium
• if you do not produce potassium to be in the endolymph
• no endolymph and thus hair cells do not work well leading to hearing loss

some diuretics can cause hearing loss; diuretics may alter the potassium gradient between the chambers of the cochlear, affecting its function

Question 48

What are the two sound localization cues for azimuth and elevation respectively?

Answer 48

azimuth: interaural time differences and interaural sound pressure level differences
elevation: monaural pinna cues

Question 49

Adult and baby barn owl. Malleability of the representation of the auditory world in adult brain is greater than expected

Answer 49

• the owl can detect left, right, up, and down sound with extreme precision

Question 50

Remember this

Answer 50

the inferior colliculus projects to the medial geniculate nucleus

so there is a serial pathway that goes to the primary auditory cortex through Hescl’s gyrus

-there is a massive BILATERAL auditory projection; so if you have a unilateral damage the effect is not so severe except when you damage one nerve of cochlear nuclei, then you cannot have the bilateral projection anymore

tonotopic organization is preserved in the primary auditory cortex

Question 51

Bats have remarkable hearing.

Answer 51

• ears reflect not only left, right but up and down very effectively
• in their auditory cortex they have what is termed an auditory fovea where the sound which they emit is processed with high precision
• they put out 100,000 Hz with about 100 dB

Question 52

Auditory cortex: auditory belt in different animals

Answer 52

• A1 and A2 primary auditory cortex around Heschl’s gyrus

- auditory belt not only what help to determine what sounds mean but how to react to sound

Question 53

The ventral and dorsal streams of the auditory cortical system

Answer 53

VISUAL SYSTEM

dorsal stream:
-The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. Also known as the parietal stream, the “where” stream, or the “how” stream, this pathway stretches from the primary visual cortex (V1) in the occipital lobe forward into the parietal lobe. It is interconnected with the parallel ventral stream (the “what” stream) which runs downward from V1 into the temporal lobe.

ventral stream:
The ventral stream is associated with object recognition and form representation. Also described as the “what” stream, it has strong connections to the medial temporal lobe (which stores long-term memories), the limbic system (which controls emotions), and the dorsal stream (which deals with object locations and motion).

AUDITORY SYSTEM

Ventral stream
Along with the visual ventral pathway being important for visual processing, there is also a ventral auditory pathway emerging from the primary auditory cortex. In this pathway, phonemes are processed posteriorly to syllables and environmental sounds. The information then joins the visual ventral stream at the middle temporal gyrus and temporal pole. Here the auditory objects are converted into audio-visual concepts.

Dorsal stream
The function of the auditory dorsal pathway is to map auditory sensory representations onto articulatory motor representations. Hickok & Poeppel claim that the auditory dorsal pathway is necessary because, “learning to speak is essentially a motor learning task. The primary input to this is sensory, speech in particular. So, there must be a neural mechanism that both codes and maintains instances of speech sounds, and can use these sensory traces to guide the tuning of speech gestures so that the sounds are accurately reproduced.

Question 54

What is an important aspect of sound processing?

Answer 54

• language
• there is asymmetry of language areas
• Wernicke’s: lesions here means pts cannot talk properly anymore; semantic context of sound and speech; on the left side; sensory aphasia where pts don’t understand anything;

Question 55

What is motor aphasia?

Answer 55

• broca’s area on the LEFT

- can still write and understand everything but cannot respond appropriately (bad, bad, tom, tom, tom)

Question 56

Speech production is complex

Answer 56

just need to know that

Question 57

What is Broca and Wernicke’s areas and what type of aphasias are seen in both conditions?

Answer 57

Broca area (motor processing of language): motor aphasia (mechanical part of articulation is impaired)

Wernicke area (Sensory processing of language): sensory aphasia (cognitive deficit in language production i.e. misuse of words and out of context: paraphasia, and incapability to use exact grammar (paragrammatism)

In the majority of the population i.e right-handed persons these areas are found in the left hemisphere. A lesion of both areas result in a global aphasia.

Damage to the right temporal “Wernicke” area leads to disturbances in comprehension of the emotional content of language.

Question 58

What is conduction deafness?

Answer 58

something hampers sound conduction to the fluids of the inner ear (e.g. impacted earwax, perforated eardrum, osteosclerosis of the ossicles)

impairment of the transmission of sound coming through the ear going to the sensory cells

Question 59

What is sensorineural deafness?

Answer 59

results from damage to the neural structures at any point from the cochlear hair cells to the auditory cortical cells (presbycusis; acoustic neuromas)

damage to the hair cells, nerves, or something centrally

Question 60

What is Menière’s syndrome?

Answer 60

labyrinth disorder that affects the cochlea and the semicircular canals causing vertigo, nausea, and vomiting, tinnitus and deafness

-thought to be leaking of endolymph on the vestibulocochlear nerve, endolymph has lots of potassium; if you put potassium outside of the cell you get nerve block because the cell cannot repolarize

Question 61

What is tinnitus?

Answer 61

ringing or clicking sound in the ears in the absence of auditory stimuli

Question 62

What is central deafness?

Answer 62

due to lesions of the cochlear nuclei and their connections with the primary auditory processing areas in the temporal lobes

Question 63

What is sudden onset of bilateral sensorineural hearing loss?

Answer 63

sudden hearing loss following cardiopulmonary bypass surgery, hysterical events or incidents just after general anesthesia

Question 64

What is pure word deafness?

Answer 64

impairment of auditory comprehension, but comprehension of written language is preserved (by contrast to Wernicke’s aphasia). Lesions are found bilaterally in the middle third of the superior temporal gyri, thus severing the bilateral connectivitie

Question 65

Are the ossicles involved in air conduction or bone conduction?

Answer 65

air conduction because its the natural pathway which goes to the cochlea

Question 66

What is loudness recruitment?

Answer 66

Heightened perception of loudness once the (increased) hearing threshold has been exceeded: thus the patient usually responds: “You don’t have to shout at me” when the examiner raises the voice. Selective destruction of low-intensity elements subserved by outer hair cells are thought to underlie this pathology, while high intensity elements are preserved, so that loudness is only appreciated at high intensities

-impairment of the outer hair cells which amplify sounds

Question 67

What is the difference between air and bone conduction?

Answer 67

Air conduction hearing occurs through air near the ear, and it involves the ear canal and eardrum. Bone conduction hearing occurs through vibrations picked up by the ear’s specialized nervous system. A Weber test is another way to evaluate conductive and sensorineural hearing losses.

Question 68

What is the difference between Weber and Rinne tests?

Answer 68

they both use a vibrating tuning forks

Weber: compares both ears

Rinne: detects only one ear

Rinne and Weber tests are exams that test for hearing loss. They help determine whether you may have conductive or sensorineural hearing loss. This determination allows a doctor to come up with a treatment plan for your hearing changes.

A Rinne test evaluates hearing loss by comparing air conduction to bone conduction. Air conduction hearing occurs through air near the ear, and it involves the ear canal and eardrum. Bone conduction hearing occurs through vibrations picked up by the ear’s specialized nervous system.

A Weber test is another way to evaluate conductive and sensorineural hearing losses.

Question 69

What are other hearing tests?

Answer 69

audiometry: makes also use of differentiation between air conduction and bone conduction

brainstem auditory evoked potentials/response (BAEP/BAER): clicks are delivered to the ears and response waves are then recorded via scalp electrodes

Question 70

As one become older what are differences in hearing one could experience?

Answer 70

• loss of high pitch sounds
• have lower hearing threshold
• because you lose hair cells; osteoporosis of the ossicles

Question 71

Describe hearing prothesis

Answer 71

• amplify the sounds
• a microphone that is attached to a processor with little wires implanted in the cochlear nerve or even in the brainstem; the processor detects and makes an analysis of the sound and tries to mimic what your cochlea would have processed

Question 72

Explain the Lancet Commision on Dementia, 2017

Answer 72

Hearing loss triggers cross-modal plasticity as a compensation for degraded auditory signal (frontal/prefrontal cortices- visual areas) –> activate auditory cortex–> decreased cognitive reserves