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Year 2 - neuroscience > Sound conduction > Flashcards

Sound conduction Flashcards

1
Q

Describe the pathway of sound from the outside, inside the ear

A
  • The outer ear localises sound, and channels this sound into the ear via the ear canal (external auditory meatus
  • Sound reaches the tympanic membrane
  • The middle ear is the eardrum and the 3 little bones
  • After the eardrum are 3 little bones: the stapes, the incus and the malleus
  • After passing the middle ear, sound is transferred to the cochlea – the organ of hearing
  • From here, signals are transferred via the vestibulocochlear nerve (CN VIII) into the central pathways
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2
Q

What are the 3 main parts of the ear?

A
  • outer ear (auricle, auditory canal and outer eardrum)
  • middle ear (eardrum, cavity and ossicles)
  • inner ear (oval window, semicircular ducts, cochlea and auditory tube)
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3
Q

What are the three ossicles and how are they attached?

A

malleus (or hammer) - long handle attached to the eardrum

incus (or anvil) - the bridge bone between the malleus and the stapes

stapes (or stirrup) - attached to cochlea

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4
Q

What is the passage of sound from the ear to the auditory cortex?

A

Sound goes into the ear -> the cochlea -> along the auditory nerve fibre -> ipsilateral cochlea nucleus -> superior olivary nucleus -> inferior colliculus -> auditory cortex

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5
Q

What is sound?

A

A change in air pressure – voice, explosion, guitar

Vibration propagating as audible waves of pressure

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6
Q

How is frequency changed?

A

By the changing the amount of compression/rarefaction between air particles.
So the frequency of sound is the number compressed or rarefied patches of air that pass our ears /s

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7
Q

What is sound intensity?

A

Difference in pressure between the compressed and rarefied air regions - amplitude determines it

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8
Q

What frequencies can the human ear hear?

A

Between 20- 20000 Hz

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9
Q

What does the cochlea contain?

A

Hair cells that can depolarise and hyperpolarise to transfer frequency as a neural signal

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10
Q

What does the movement of fluid in the cochlea cause?

A

Generates a response in sensory neurones

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11
Q

What are the 3 chambers within the cochlea?

A

scale vestibule
scala media
scala tympani

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12
Q

What is the eustachian tube?

A

A tube that links the nasopharynx to the middle ear. It is a part of the middle ear.

Pressure in the middle ear is the same as pressure in the nasal cavity. This is equalised by the eustachian tube.

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13
Q

What is the oval window?

A

A membrane-covered opening that leads from the middle ear to the vestibule of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear.

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14
Q

What is the role of the ossicles and how do they do their job?

A
  • Ossicles amplify the sound pressure
  • Fluid in the inner ear resists movement – so the ossicles need to give it a kick -> amplify sound pressure
  • Pressure at the oval window is bigger than at the tympanic membrane as it has a smaller surface area
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15
Q

What is the basilar membrane?

A

It separates the scala media and the scala tympani

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16
Q

What is the reissner’s membrane?

A

It separates the scala vestibule and the scala media

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17
Q

What are the fluids filling the chambers called - composition?

A

perilymph (CSF like – low K, high Na) and endolymph (high K, low Na)

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18
Q

Describe the structure of the basilar membrane

A

Wider at the apex than at the base

Stiffest at the base and most flexible at the apex

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19
Q

What does the movement of the stapes cause in the cochlear?

A

Causes the endolymph to flow, causing a travelling wave in the membrane

The higher the frequency, the further along the wave will travel

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20
Q

How do different parts of the basilar membrane differ at different frequencies?

A

Different locations of the basilar membrane are maximally deformed at different frequencies

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21
Q

What does pressure at the OW lead to?

A

It pushes the perilymph into the scala vertibule. The cochlear membrane is rigid so increase in pressure goes from the scala vestibule through the helicotrema and down the scala tympani to the round window. The pressure has nowhere to go so the RW bulges and depending on how far the wave travelled, the bulge varies (larger if further).

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22
Q

Where are the hair cells found?

A

On top of the basilar membrane

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23
Q

What are the types of hair cells?

A

Inner and outer hair cells - sensory and contain stereocilia

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24
Q

What happens to the hair cells following waves?

A
  • There are waves that cause movement of hairs in either one direction, or the other direction
  • So you get either depolarisation of hyperpolarisation
  • When you get depolarisation, action potentials are transmitted along the auditory nerve
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25
Q

What is the function of the hair cell?

A

Amplify and improve the clarity of the sound. The extent of movement of hair cells depends on frequency.

26
Q

What are the inner hair cells?
Number
What they do
What stimulates them?

A
  • Primary sensory cells
  • 3500
  • Generate APs in the auditory nerves
  • They are stimulated by the fluid movements
27
Q

What are the outer hair cells?
What they do
What happens when they depolarise and hyperpolarise?

A
  • 20,000
  • On depolarisation, they become short
  • On hyperpolarisation, they become long
  • They increase the amplitude and clarity of sounds
28
Q

Describe the auditory pathway from the hair cells to the auditory cortex

A
  • Hair cells form synapses with neurones in the spiral ganglion -> axons form the vestibulocochlear nerve
  • From there, signals transcend up the auditory pathway
  • Afferents enter brainstem at the level of the 8th CN, you innervate both the dorsal and ventral cochlear nuclei ipsilateral
  • On the same side afferents go from the cochlear nucleus -> superior olivary nucleus -> lateral lemniscus -> inferior colliculus - > medial geniculate nucleus in thalamus-> AUDITORY CORTEX
29
Q

What is tonotopy and its importance?

A

The spatial arrangement of where sounds of different frequency are processed in the brain. It is maintained throughout the auditory nuclei

30
Q

The basilar membrane isn’t sensitive to frequencies below 200Hz, so how do we hear them?

What happens at different frequencies - which mechanisms are used?

A
  • Phase locking mechanism: matching amplitudes to a certain phase of a wave
  • Neurone fires at a particular phase

Very low frequencies: phase locking

Mid frequencies: mix of phase locking and tonotopy

Very high frequencies: tonotopy alone

31
Q

What is the interaural time difference?

Why is ti important?

A

The difference in arrival time of a sound between two ears.

It is important in the localisation of sound as it provides information relating to the direction or angle from the head.

32
Q

Describe the interaural time difference and how this is used by the brainstem

A
  • 20 cm distant between ears
  • 0.6msec between sound recognition
  • This difference is detected by neurones in the brainstem
  • There are IA intensity differences
33
Q

How else can sound be transmitted?

A

Directly through the cochlear via bone conduction (instead of air conduction)

34
Q

Why is the fact that sound can be conducted in 2 ways important?

A
  • This is clinically important in people with hearing loss because it allows us to detect where the problem is
  • If bone conduction hearing is better than the air conduction hearing, the problem is anywhere between the pinna, through the external acoustic meatus, all the way to the inner ear (cochlea)
  • There is a blockage somewhere (wax, perforated tympanic membrane, ossicles)
35
Q

How can we test hearing?

A
  • By playing sounds of different frequencies into the ear
  • Frequencies range from 200 to 8,000 Hz
  • Every time the subject hears it, they press a button as a response
  • Hearing thresholds are used (lowest sound heard 50% of the time in decibels)
  • Anything below 20 decibels is normal hearing
  • Anything above progressively deafer
36
Q

What are the 2 ways in which hearing can be tested?

A
  • Via the air conduction route (using headphones)

- Vibrate the skull (using a mastoid vibrator) to stimulate the cochlea directly

37
Q

What does it mean if there is an air bone gap?

A

This means that there is a problem typically before the cochlear.

38
Q

What does it means if both the air and bone conduction are poor?

A

auditory nerve after the cochlear is damage

39
Q

How is a pure tone audiogram reported?

A
  • is hearing loss unilateral or bilateral?
  • degree of hearing loss
  • shape of audiogram
  • type of hearing loss (which part affected)
40
Q

What are the different types of hearing loss and explain them? (5)

A

Conductive (CHL): prevents sound transmission through the external or middle ear

Sensorineural (SNHL): permanent hearing loss as a result of cochlea +/- auditory nerve dysfunction affecting sensitivity to sound as well as quality of sound detection – affects the nerve or the cochlea.

Mixed: SNHL and CHL

Auditory neuropathy spectrum disorder (ANSD): dysfunction in neural transmission of sound from the IHCs to lower brainstem resulting in poor speech discrimination beyond expected

Auditory processing disorder (APD): dysfunction of the central auditory pathways resulting in difficulty in processing (i.e. interpretation) of sound information despite good peripheral hearing e.g. understanding speech in background noise, localising sound. Requires more extensive testing

41
Q

What are behavioural hearing tests (children) and describe the ages they are used for and what is done?

A

Behavioural observation Young babies <7 months or children with complex needs. Very observer dependent and must be supported by objective tests. Sounds made and response recorded.

Distraction testing Babies 4-18 months or children with complex needs. Sounds made and child tries to locate it.

Visual reinforcement audiometry 6-24 months.
Child seated between two visual reward boxes with a speaker on each. A sound is played on each side and if they look they are rewarded. Frewuencies can be tested.

42
Q

What are objective hearing tests - examples and what they test?

A

Specialised

Tympanometry – test of middle ear function

Flat tymp: glue ear or eardrum perforation

Peaked tymp: and negative middle ear pressure = Eustachian tube dysfunction

Otoacoustic emissions – test of outer hair cell function (reflects cochlea function)

Electrocochleography – test of cochlear function and CN VIII

Auditory brainstem response – test of retrocochlear function for synchronous firing of auditory nerve fibre/asymmetrical latencies/frequency-specific threshold estimation

Stapedial reflex – test of auditory nerves and part of brainstem function (reflex present/elevated/absent)

43
Q

What are the causes of pre-lingual hear loss (born with it)?

A
  • Acquired (20-25%)
  • Bacterial meningitis
  • Head injury
  • Ototoxicity
  • Congenital infection (CMV, rubella, syphilis)
  • Perinatal cause (hypoxic event, kernicterus, ototoxicity)
  • Unknown (25-50%
44
Q

What are the causes of hearing loss in adults?

A
  • Presbycusis (most common type of sensorineural hearing loss). Natural aging of auditory system
  • Vascular, Meniere’s, Autoimmune inner ear disease
  • Genetic
  • Noise-induced
  • Ototoxicity (e.g. aminoglycosides, chemotherapy)
  • Infection (e.g. bacterial meningitis, measles)
  • Tumours (brain, acoustic neuroma)
  • Post-ear surgery
  • Head injury
  • Otosclerosis
  • Cholesteatoma
45
Q

What is noise induced hearing loss?

A

Hearing impairment resulting from exposure to loud sound. People may have a loss of perception of a narrow range of frequencies, impaired cognitive perception of sound, or other impairment, including sensitivity to sound or ringing in the ears – happens at 4,000 Hz.

46
Q

What is the management for hearing loss?

A
  • hearing aids, hearing tactics, assistive listening devices
  • supporting development of speech and language in children
  • manage co-morbidities e.g. psychological
47
Q

What are the protective mechanisms of the ear?

A
  • Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles
  • Protects against natural sounds but maybe not against man-made sounds
  • Auditory tube allows equilibration of air pressure on either side of tympanic membrane
48
Q

What is conductive hearing loss and its causes?

A

Sound prevented from reaching cochlea.

Causes:
Wax, Otitis media, otosclerosis of ossicles perforated tympanic membrane, congenital malformations.

49
Q

What is the structure of the cochlea?

A
  • Hollow tube in bone, curled into spiral
  • Divided longitudinally into 3 compartments, separated by 2 membranes (vestibular at top and basilar at bottom)
  • Sound wave causes vestibular (Reissner’s) and basilar membranes to vibrate
  • Cochlear hair cells are attached to basilar membrane
50
Q

Where is the perilymph and where is the endolymph?

A

perilymph - above vestibular membrane and below the basilar

endolymph - in between the two membranes

51
Q

What is the organ of corti and describe its structure?

A
  • Spiral organ, thought of as the microphone
  • In the cochlea
  • On the basilar membrane and has hair cells
  • Above hair cells is tectorial membrane (gelatinous and does not vibrate with sound)
  • Spiral ganglion is embedded in modiolus and innervates hair cells
  • Stria vascularis (vessels) secretes endolymph (high in K+, low in Na+)
52
Q

What are the 2 types of hair cells?
How are they arrange and innervated?
Which send information to the brain?

A

Inner hair cell – about 3,500 cells arranged in single row, densely innervated by about 10 sensory axons/cell

Outer hair cell – about 20,000 cells arranged in 3 rows sparsely innervated: one axon innervates several cells.

Both types of hair cell respond to sound but inner hair cells are the ones that provide information for brain.

53
Q

Describe the transduction mechanism from basilar membrane

A
  • Basilar membrane vibrates in response to sound
  • Upward movement displaces stereocilia away from modiolus → K+ channels open → K+ enters from endolymph → hair cell depolarises
  • Downward movement displaces stereocilia towards modiolus → K+ channels close → hair cell hyperpolarises
  • Depolarisation opens Ca++ channels in body of hair cell
  • Glutamate released from base depolarises axon of spiral ganglion cell → action potential

Highly sensitive mechanosm, which depends on the maintenance of endolymph at +80 mV by stria vascularis

54
Q

How does the basilar membrane distinguish frequency?

A

High frequencies vibrate basilar membrane nearer to base

Low frequencies vibrate membrane nearer to apex (lower frequencies have more force and more force is needed to move the thicker apex)

55
Q

How is the primary auditory cortex arranged and what does it do?

A
  • Subdivided according to frequency response
  • Cells respond to specific features of sound e.g. on/off, duration, repetition, intensity + some more complex sound patterns e.g. rising/falling frequencies, animal vocalisations
56
Q

What does the secondary auditory cortex do?

A

neurones respond to more complex sound patterns

57
Q

What is sensory deafness?

A

Presbyacusis, exposure to loud noise, Ménière’s disease, toxicity e.g. some antibiotics, hereditary disorders

58
Q

What is neural deafness?

A

Acoustic neuroma, viral infection

59
Q

What is central deafness?

A

Demyelination in MS, injury to central auditory pathway

60
Q

What is the decibel scale?

A

Logarithmic scale for loudness

Year 2 - neuroscience (13 decks)

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