L16 Hearing and Vestibular Function Flashcards Preview

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Flashcards in L16 Hearing and Vestibular Function Deck (43):
1

Learning outcomes (for general perusal)

•To elucidate the importance of hearing to the human

•To describe how mechanical soundwaves are converted into neural signals by the ears

•To explain how sound amplitude (loudness) and frequency (pitch) are coded and perceived

•To show how deafness occurs and describe some hearing tests

2

What are the two main qualities of sound waves?

Amplitude and Frequency, interpreted as loudness and pitch

3

What is conductive hearing loss?

Hearing loss because of problems with the carriage of sound waves to the auditory hair cells 

4

What is sensorineual hearing loss?

Hearing loss as a result of problems with the neural structures underlying hearing itself 

5

What structures are in the 

  1. Outer ear
  2. Middle Ear 
  3. Inner ear

  1. pinna (auricle), ear canal, tympanic membrane

  2. Ossicular lever system -malleus (hammer), incus (anvil), stapes (stirrup)

  3. cochlea (hearing) and vestibule + semicircular

       canals (balance)

6

What is the function of the

  1. Outer ear
  2. Middle ear
  3. Inner ear

  1.  Directs sound to the tympanic membrane (eardrum) and is specifically shaped to catch sound waves
  2. which acts to amplify the sound and relay the vibrations to the inner ear 
  3. transduce the mechanical stimulus of sound vibrations to electrical impulses in the cochlear nerve.

7

Middle Ear

What are the 3 functions of the middle ear?

  1. Critical Dampening
  2. Amplification
  3. Impedance Matching

8

Middle Ear

Function 1 : Critical Dampening 

  1. Why is the eardrum a 'resonator'?
  2. How is it critically damped?
  3. What is the latency for the reflex? What can a consequence mean?

  1. The tympanic membrane vibrates at the same frequency as the sound waves on its surface.
  2. By being attached to the three bones of the middle ear. The tensor tympani and stapedius muscles undergo a pre-programmed or reflex contraction either before you speak (pre-programmed) or on hearing loud sounds (reflex)
  3. 30-160ms. Gunshots/explosions will cause damage to the ear, no protective mechanism.

9

Middle Ear

Function 2: Amplification

  1. ​Outline the mechanism behind amplification in the middle ear
  2. By what factor is the force multiplied?
  3. Why is amplification required?

  1. The sound pressure arriving at the oval window is increased because of the lever action of the muscles and the bones. There is a large area (tympanic membrane) vibrating a small area (oval window).
  2. x1.3
  3. to vibrate the inner ear membranes because they operate in a fluid medium unlike those of the outer and middle ear 

10

Middle Ear

Function 3: Impedance Matching

  1. Why is amplifiation required?
  2. Why is this process called impedance matching?

  1. to vibrate the inner ear membranes because they operate in a fluid medium unlike those of the outer and middle ear. This fluid has a much greater impedance than air and so is harder to vibrate. So the weak vibrations of the large area of the eardrum in air need to be magnified by the ossicles and concentrated into strong vibrations over small area of the oval window to provide the necessary force to displace the fluid and membranes of the inner ear. 

  2. because it matches impedance in an air environment (outer ear) with that of a fluid environment (inner ear). 

11

Give an overview of how the vibrations pass after reaching the oval window

vibrations of the oval window compress the fluid of the inner ear and set up vibrations in the basilar membrane before being absorbed by deformations of the flexible round window 

12

Where are movements of the oval window transmitted to?

Scala Vestibuli and Scala Media

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13

What is the cochlea divided up into?

What do they contain?

 

How are vibrations in the basilar membrane converted to electrical signals?

Three chambers

  • Scala Vestibuli and Scala Tympani (continuous) 
    • a fluid rich in Na+ ions (perilymph

  • Scala media 
    • fluid rich in K+ ions (endolymph

 

 by hair cells embedded within it 

14

Explain this image

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This is a transverse section of the sensory area of the cochlea

  • there are 3-4 rows of outer haircells about 12-20,000 in number - modulators, alter how much the inner hair cells vibrate to given frequencies)
  • 1 row of inner hair cells = transducers (about 3500 in number)

15

Where do most of the afferent fibres in the cochlear nerve come from?

 

Where do most of the efferent fibres in the cochlear nerve go to?

 

Inner hair cells (90%) - main transducers

 

The outer haircells - function as modulators of sensitivity of inner hair cells. 

16

How are vertical vibrations of the basilar membrane  converted to lateral motion of the hair cells ?

By the arrangement of the tectorial membrane and basal lamina (above) in which the hair cells are embedded 

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17

  1. Describe the hair cells in the organ of corti
  2. What happens as the hairs vibrate laterally?
  3. What happens to the potassium that enters the hair cell?

  1. Rows of sterocilia that get progressively smaller as one goes across the rows 
  2. filaments linking one hair cell with its
    neighbour (right) pull open a trapdoor, allowing
    potassium to enter the hair cell, depolarize it, and so set up action potentials in the VIII nerve.

  3. recycled back into the endolymph via the stria
    vascularis.

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18

  1. What are sound waves?
  2. What speed do they travel at?
  3. What are their two main properties?

  1. longitudinal waves formed by vibrations in the fluid medium of the external environment. 
  2. 344m.sec-1 

  3. Amplitude and frequency

19

  1. What is frequency (pitch) coded by?
  2. Explain the mechanism behind this with reference to the basilar membrane
  3. What is the basilar membrane near the oval window maximally vibrated by?
  4. What is the basilar membrane near the helicotrema maximally vibrated by?

  1. which portion of the basilar membrane is maximally stimulated by the travelling sound wave set up by the vibrations of the oval window (which hair cell along the membrane is maximally stimulated)

  2. The basilar membrane is narrow and stiff near the oval window of the cochlea and wide and floppy near the helicotrema

  3. maximally vibrated by high frequency (pitch) sounds (high resonant frequency) - narrow and stiff

  4. therefore maximally vibrated by low frequency (pitch) sounds (low resonant frequency)  - wide a floppy

20

  1. What is loudness coded by?
  2. How can it be expressed?
  3. What is a decibel?
  4. What does threshold (0dB) to max (140dB) actually represent?
  5. What does a change of 10dB represent?
  6. What is hearing damage proportional to?
  7. What is the level at which hearing loss can occur?
  8. At what level will pain occur?
  9. What is the loudest recommended exposure WITH hearing protection?

  1. the frequency of action potentials of the hair cell

  2. in terms of the pressure change on the eardrum, but for convienience, it is given on a relative scale - the decibel.

  3. the logarithm of the ratio of the intensity of a sound to a standard sound. 0 dB is that sound that is barely perceptible by an average young adult.

    1. ​1dB = 10 x log10 actual sound intensity/reference sound intensity

  4. a 10 million fold change in sound intensity

  5. sound is interpreted as twice as loud

  6. the intensity of sound causing the damage by the time occuring before the activation of the tensor tympani and stapedius muscles

  7. 90-95dB

  8. 125dB

  9. 140dB

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21

How is directionality of sound detected?

By 2 means

Frequencies about 3000Hz - the loudness difference detected by the ears give as clue as to the direction of the sound (sound is loudest in the ear closest to the source)

Frequencies less than 3000Hz - the timing difference in picking up the sound between the 2 ears is the clue used. Although this may be as low as 0.5 msec, its still detectable. 

 

22

  1. Where is the auditory cortex located? Generally, what does it contain?
  2. What is the primary auditory cortex?
  3. What are the secondary auditory areas?
  4. Where does language processing occur? Outline the two areas

  1. the superior portion of the temporal lobe, embedded in the sylvian fissure. 
  2. a topographical columnar map of the basilar
    membrane. Each column is repeated several times different locations of sound picked up in
    different columns. High frequencies = posterior, Low frequencies = anterior

  3. Association areas are found close to the
    primary auditory cortex. These associate sounds with other somatosensory information.

  4.  

    Wernickes area, COMPREHENSION of speech.

    1. damage =  receptive, or fluent aphasia (randoms ad)

      •Broca’s area, PRODUCTION of speech, the motor expression of the sense. 

      1. Damage =  expressive or non – fluent aphasia.

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23

  1. What is conductive deafness?
  2. What is sensorineural deafness?

Give examples of both

  1. impaired sound conduction from the pinna to the inner ear (wax in the ear canal, to a perforated eardrum (common in sport diving and with very loud sounds), fluid in the middle ear from infection, or damaged ossicles, either from disease or loud sounds )

  2. damage to the hair cells or neural pathways (with loud sounds as hair cells are sheared and not regenerataed (iPods, rock concerts). The basilar membrane can actually rip with very loud sounds. Some antibiotics (streptomycin and gentamycin) can enter the channels of the hair cells and disrupt their function and cause them to degenerate resulting in deafness. Tumours in the auditory nerves and vascular damage to the medulla also result in deafness. )

24

  1. What is audiometry based upon?
  2. What does nerve deafness generally result in?

  1. detecting threholds for hearing at different frequencies 

  2. hearing loss at higher frequencies

25

(Was Beethovens deafness conductive or sensorineural?)

Don't revise

Conductive - wooden block between his head and the piano indicating some

sound was being perceived, post-mortem findings, inflammation possible otosclerosis, inflammation of the ossicles responsible. 

Sensorineural- possible syphilis, the famous lock of hair showing lead poisoning high frequency problems, pain with hearing 

26

Summing Up 16a Hearing (for general perusal)

•Sound, a mechanical disturbance in air is converted to nerve signals by the complex lever and fluid system that is the auditory apparatus (outer middle and inner ear)

•Frequency (pitch) and amplitude (loudness) of sound waves are well characterised, pitch, by what portion of basilar membrane and auditory cortex is stimulated, amplitude by frequency of AP’s in the cochlear nerve

•Brain areas processing sound may be associated with other areas via the secondary cortex

•Deafness may be conductive or sensorineural

27

  1. What is rotational acceleration detected by?
  2. What is horizontal and vertical acceleration detected by?

  1. semicircular canals
  2. utricle and saccule

28

Learning Outcomes 16b (for general perusal)

•To elucidate the importance of vestibular function to the human

•To describe how the semicircular canals and the vestibule inform us of the position of our head in space, and of rotational and linear acceleration

•To show how, like with hearing, these are transducers, converting mechanical energy into nerve signals

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29

What serves the balance modality?

the semi circular canals and the vestibule (divided into the utricle and saccule)

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30

How is acceleration detected?

  • three semi-circular canals detect rotational acceleration in 3 planes (X, Y and Z) 
  • Utricle detects horizontal acceleration
  • Saccule detects vertical acceleration
  • the utricle and saccule switch functions when a person is SUPINE

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31

Semi Circular Canals

  1. What is located at the base of each SC canal?
    1. What does it do?
  2. What happens when the head rotates?
  3. How do these hair cells differ to those in the cochlea?
    1. What is this feature's function?
  4. What happens at the start of rotation with the above mechanism in mind?

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  1. A swelling called the ampulla with a gelatinous ridge (crista ampullaris) within it
    1. Mounts hair cells (process within the gelatinous mass - the cupula) 
  2. The fluid within the SC canals moves relative to the labyrinth and displaces the cupula
  3. they have a non motile hair (the kinocilium) as the largest cilium. 

    1. As the other hairs bend toward the kinocilium they cause increased discharge in the nerve, as they bend away, they reduce firing rate in the nerve 

  4. firing rate in the nerve increases as fluid bends the hairs toward the kinocilium 

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32

SC Canals

  1. Explain how changes in rotational acceleration are signalled to the brain
  2. How is the information sent to the brain an accurate second to second representation of rotational displacement in all planes?

  3. What is the tonic level of discharge?

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  1. At the start of rotation, firing rate in the nerve increases as fluid bends the hairs toward the kinocilium. Then as we abruptly stop rotating the hairs are bent back the other way and so discharge rate decreases. 

  2. there is a pair (right and left) of SC canals serving each plane of rotation (3x2 canals)

  3. 100hz (impulses per second)

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33

Utricle and Saccule

  1. What do they detect?
  2. What is on the floor of the utricle and saccule?
  3. How is the macula of the utricle orientated?
    1. And in the saccule?
  4. What overlies the hair cells?
  5. What is the tonic firing rate of these hair cells?
  6. How is linear acceleration (either by movement or gravity) signalled to the brain?

  1. Linear Acceleration
  2. otolithic organ (macula)
  3.  horizontal postion (in upright human)
    1. oriented vertically.
  4. Gelatinous layer in which is embedded crystals of calcium carbonate (statoconia, otoliths or ear dust)
  5. 100 impulses.sec-1

  6. as hairs move towards the kinocilium they increase nerve dischare rate, moving them away from the kinocilium decreases their impulse generation. 

34

CNS Connections

Outline the projections from the vestibular nucleus of the medulla to the CNS?

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  • Cerebellum - concerned with balance and setting a background against which predicted rapid movements can take place 
  • The Spinal cord (via the vestibulospinal tract) - to muscles involved in maintenance of equilibrium 
  • Reticular Formation - causing general arousal and increasing conduction speed and sensitivity in all motor nerve pools 
  • Equilibrium Cortex (in the sylvian fissure opposite the hearing area) - This is important for conscious sensations of equilibrium and for the vestibule ocular reflex (VOR) and nystagmus. 

35

What is Nystagmus?

a pursuit movement of the eye during head rotation where the eye fixates on a single object (pursuit) and then flicks back to its original position when the eye has reached the limit of its rotation (saccade).

36

What is the limit of rotation of the eye called?

Saccade

37

What is nystagmus controlled by?

the semicircular canals 

38

 

Nystagmus

What happens when we rotate a person?

When can this effect be seen in other conditions?

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  • the nystagmus first occurs in one direction when movement starts
  • then it stops during rotation
  • then it occurs in the opposite direction after rotation (postrotatory nystagmus). 

This effect can also be seen when warm saline is injected down the external auditory meatus. 

39

What can pathologic nystagmus be due to?

lesions in the CNS, vestibular nerves or apparatus. 

40

  1. When does motion sickness occur?
  2. How can we alleviate dizziness?

  1. when there is a conflict between 2 signals between the vestibular system and the visual system, eg when reading a book within a moving car, ones vestibular system signals horizontal acceleration while ones visual system is signalling that one is stationary 

  2. have the 2 sensory modalities agree with one another as much as is possible ie. Don’t read in a car, look at the horizon on a heaving deck so that you don’t heave 

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41

Where have hearing and vestibular systems evolved from?

the lateral line organ in the fish which is open to water for detection of disturbances in  their fluid environment

42

Summing Up 16b (for general perusal)

•The vestibular system has evolved to give the human information about orientation of the head and space as well as details of linear and rotational acceleration

•It does this using a mechanism similar to that used to convert sound waves into electrical signals in the organ of corti.

•This information is used in the maintenance of posture and balance, in the control of muscle and in important reflexes (eg, the VOR)

43

What is the function of the VOR?

Vestibular Ocular Reflex

to stabilise the retinal image during rotations of the head

When the head rotates, the eyes should rotate at the same speed but in the opposite direction (gain -1)

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