1.1 Sound Conduction & Transduction

Question 1

The hair cell is the receptor cell for hearing:
- Apical: _______________ (finger-like projections of cytoplasm)
- Basal: Innervation by axon terminals of the axons from the ___________________
Sound waves deflect the stereocilia in a certain direction, triggering molecular pathways in the body of the hair cell:

• Culminates in release of _____________ (excitatory) from the base of the cell → depolarisation of axon terminal → action potentials carried to the brain
• Cells are bathed in fluid in a small fluid-filled chamber (cochlea) deep in the base of the skull → sound waves must be conducted to the _________ before they can be detected and transduced

Answer 1

Stereocilia;

cochlear nerve (CN VIII);

glutamate;

inner ear

Question 2

DIVISIONS OF THE
EAR The ear can be divided into the outer, middle, and inner parts (refer to “Anatomy of the Ear”):
- Outer: Visible part (auricle) to tympanic membrane (eardrum)
- Middle: Air-filled chamber between the tympanic membrane and oval/round windows:
• ______________: connects middle ear to the back of the nasopharynx
Inner: ________________ (balance), __________ (contains hair cells; hearing)
*Impulses generated by hair cells travel in the cochlear nerve; impulses from vestibular apparatus travel in vestibular nerve → combine to form ________________

Answer 2

Eustachian tube;

Vestibular apparatus; cochlea;

vestibulocochlear nerve (CN VIII)

Question 3

Sound waves are produced by the vibration of a source (e.g. vocal cords, prongs of tuning fork):
• Generates alternating areas of compressed air and less compressed (rarefied) air
• Visualised as a sine wave of _________________

Frequency (no. of cycles per second): _________ of sensation

Amplitude of stimulus: __________ of sensation

Answer 3

air pressure against distance/time ;

Pitch;

Loudness

Question 4

what is the threshold of hearing

Answer 4

0db

Question 5

what is the db rating for whisper

Answer 5

30db

Question 6

what is the db rating for normal conversation

Answer 6

50-60

Question 7

what is the db rating for shouting

Answer 7

90

Question 8

what is the db rating for gunshot

Answer 8

120

Question 9

what is the db rating for pneumatic drill

Answer 9

140

Question 10

The external ear carries sound waves to the _________________ (along the passageway of the external ear) and does not serve many other functions.

Answer 10

tympanic membrane

Question 11

The wall of the middle ear mostly consists of bone, with the tympanic membrane on the outer wall and the ________________ on the inner walls:
• Three ossicles (malleus, incus, stapes) bridge across the air-filled chamber → important for sound amplification
• Functions: __________ of low intensity sounds, ______________ of high intensity sounds, ________________

Answer 11

round and oval windows;

amplification; dampening down; equilibration of air pressure

Question 12

The malleus is firmly attached to the tympanic membrane (at the umbo), while the ___________________ is attached to the oval window membrane:
• Sound waves travel along the outer ear → vibrations of tympanic membrane → carried through ______________ to oval window → in and out vibrations of oval window → pressure wave inside fluid-filled cochlea (inner ear)

Answer 12

footplate of stapes;

ossicular ridge

Question 13

There are 2 main mechanisms of sound amplification leading to an amplification of ~30dB:

Mechanism
- “Lever” mechanism of ossicles: Three ossicles are articulated by _________ → allow lever system where vibrations are amplified as they pass through
• Necessary for most types of sound to reach the threshold for detection by the hair cells

Passive mechanism of surface area of membrane ratios: Tympanic membrane is many times larger in area than the oval window → all the force of the sound wave impinging on the tympanic membrane becomes focused on a much smaller area

Answer 13

synovial joints

Question 14

The middle ear has 2 safety mechanisms against very loud noises (hair cells are vulnerable and easily damaged) and unequal pressures (on either side of tympanic membrane):

Very loud noises (acoustic reflex): __________ anchors the tympanic membrane (via the malleus) to the wall of the chamber; _____________ anchors the stapes:
• Very loud noise → very large amplitude vibration of tympanic membrane → reflex contraction of two muscles (pulls two ends of ossicular bridge ________________)
• Dampens the normal amplification mechanism (reduced movement at joints) → guards against large vibrations
- Useful for natural sounds (reflex action which requires _______________ to occur → most natural sounds take that long to reach peak amplitude):
• Not as effective for manmade sounds (which reach peak amplitude much more quickly e.g. explosions)
• Prolonged exposure to very loud sounds (e.g. pneumatic drill): strict regulations to protect against these loud noises (mechanism not particularly effective)

Unequal pressures: Equal pressure in the _________________ is required for the tympanic membrane to vibrate naturally:
• May not always be the case (e.g. take-off in aeroplane → atmospheric pressure is gradually reduced causing lower external ear pressure than in the middle ear)
• Mechanism: _____________ (normally closed) momentarily opens (during yawning/swallowing/manipulating lower part of the face) → air rushes out/into middle ear to equilibrate pressure on both sides of the tympanic membrane

Answer 14

Tensor tympani; stapedius;

downwards;

> 50ms for actual contraction;

outer and middle ear chambers;

auditory tube

Question 15

If the aforementioned safety mechanisms fail, conductive deafness results:

• Wax (most common): Build-up of wax in the outer ear presses on the tympanic membrane → cannot vibrate properly
• Otitis media: Middle ear infections cause secretion of fluid → interferes with amplification mechanism
  • URTIs (e.g. cold, sore throat) can migrate _______________ to the middle ear

Otosclerosis of ossicles: ____________ grow on the ossicles → interferes with articulation at ______________ → hinders leverage (amplification mechanism)

• Perforated eardrum: May occur due to mechanical damage (e.g. something inserted into the ear) or unusually intense pressure wave (e.g. from explosion)
• Congenital malformation: Shape of middle ear/ossicles affected → affects amplification mechanism

Answer 15

back along the auditory tube;

Little spurs of bone;

ossicular joints

Question 16

SOUND TRANSDUCTION

The cochlea is a bony tube tightly coiled into a spiral, which is divided into three compartments (scala vestibuli, media, tympani) by the vestibular and basilar membranes:

Scala vestibuli& Scala tympani

• Attached at the __________ by a narrow channel connecting the two
• Perilymph (___________)

Scala media
- Completely separated from the other two - Endolymph (high _______)

Answer 16

apex;

tissue fluid;

[K+]

Question 17

The stapes (abutting the base of cochlea) vibrates in a certain direction due to the sound wave carried through the ossicles, setting up a pressure wave at the ____________:

Perilymph: ___________ → around the apex → ____________
*Safety mechanism: compensatory __________________ membrane → equilibrate pressure throughout the system
• Prevents potentially harmful large pressure from building up elsewhere in the system

Endolymph: Vibration of _______________ → scala media → ______________ (moves up and down) → movement of hair cells sitting along the basilar membrane (stereocilia bathed in endolymph)
• Pressure eventually passes along the scala tympani and dissipates

Answer 17

oval window;

Scala vestibuli; scala tympani;

outward movement of round window;

vestibular membrane (very flexible);

basilar membrane

Question 18

The cochlear organ of Corti (spiral organ) includes all the structures on the basilar membrane:
• Supporting cells do not participate in auditory mechanisms
• ________________ is only attached at one side → cantilevered over hair cells (along the whole length of the scala media)
• Innervation of hair cells: peripheral axons of CN VIII (cell bodies in the spiral ganglion → centre of cochlea within the ___________

Answer 18

Tectorial membrane;

modiolus)

Question 19

Inner hair cells

• location
• number
• innervation

Answer 19

• closest to modiolus
• fewer
• very rich (≥ 10 ganglion cells to each individual inner hair cell)

Question 20

outer hair cells

• location
• number
• innervation

Answer 20

• further away
• more numerous
• Sparse (several hair cells share branches of the same axon)

Question 21

Both the inner and outer hair cells can functionally respond to sound by generating impulses in their adjacent axons if a pressure wave deflects their stereocilia:
• Only information from the inner hair cells are used by the brain to __________________
• Information from the outer hair cells are used to ___________________

Answer 21

generate the sensation of hearing;

refine the resolution of the information passing up to the brain (not directly contributing)

Question 22

The vibration of the organ of Corti is divided into the upwards and downwards phases:
• Movement of basilar membrane is much larger than the movement of tectorial membrane (more _________ → does not respond to pressure waves as much)

Upwards phase: Difference in movement of membranes causes shear force in the direction _____________ modiolus → ______________ in stereocilia membrane
• K+ moves into stereocilia (K+-rich endolymph) → _________________ the hair cell → Ca2+ influx through Ca2+ channels → release of _____________ from cell base → depolarises axon terminals → train of impulses carried along the axon through CN VIII to brainstem
*Deflection of stereocilia away from the modiolus causes depolarisation

Downward: Difference in downward movement of membranes causes shear force in the direction ____________________ → no depolarisation of hair cells
• No Ca2+ influx → no glutamate release → no impulses carried along the axons to the brainstem
*Deflection of stereocilia towards the modiolus prevents depolarisation

Answer 22

gelatinous;

away from the;

opening of K+ channels;

depolarises;

glutamate;

towards modiolus

Question 23

The stereocilia on a single hair cell are not uniform in length and are carefully organised on the _________ surface (tallest one tends to be furthest from the modiolus):
• Not free to move in any direction → tip links (physical links between the tips)
• Very sensitive mechanism → minimum deflection is < 1/1000 of diameter to produce sensation of hearing

Upward phase: Tip links comes under more tension (movement towards ___________________) → opens K+ channels → depolarises axons

Downward: Tension becomes relaxed → closes K+ channels → prevents depolarisation

Answer 23

apical ;

taller stereocilia

Question 24

The brain can distinguish sounds of different frequencies due to the structure of the basilar membrane in the cochlea:
• Basilar membrane composed of transverse fibres (________________ at the apex, _________________ at the base) → gradual gradation
• Sound waves passing down the scala vestibuli causes vibration of the basilar membrane (only at a particular position where there is resonance depending on the frequency of the pressure wave)

Answer 24

wider and more floppy;

narrower and stiffer

Question 25

Hair cells lie along the length of the basilar membrane, and in sounds of certain frequencies, only the hair cells of the ________________ have deflection of stereocilia:
• Axons from the basilar membrane are arranged in a very orderly spatial arrangement in the ____________ → maintained throughout the nerve and as information is relayed through the various relay structures in the central pathway (to auditory cortex)
• Enables auditory cortex to determine the_________________ (based on which part of basilar membrane it originates from) → acts as a frequency analyser
• Most important part is the middle part of membrane (covers the frequencies most often used in speech)

Answer 25

areas with resonance;

CN VIII

frequency of sound

Question 26

AUDITORY PATHWAY The central auditory pathway begins at the cochlea:
• Cochlear nerves (CN VIII) come from each cochlea (spiral ganglion) → _________________ (on surface of brainstem) → ________________ (deeper in brainstem)
• Relays through __________________ i (in midbrain) → _________________ (thalamic nuclei specialised for auditory information) → auditory cortex (temporal lobe)

Answer 26

cochlear nuclei;

superior olivary nuclei ;

inferior collicul;

medial geniculate body

Question 27

Characteristics of central auditory pathway

• Bilateral pathway: Information coming into the cochlear nucleus goes to both the contralateral and ipsilateral ______________→ both auditory cortices receive information from both ears
  • Extensive injury is required to both pathways/auditory cortices to have complete deafness due to central injury → injury to one of the cortices will affect hearing (but still have information from both ears)
• Tonotopy: Allows cortex to determine the _______________ (detecting which part of the basilar membrane it originated from)
• Inferior colliculi: Involved in_________________ → connections with visual and motor systems
• Descending feedback: Descending feedback loops at every level (even back to cochlea) → continually improve _______________ of auditory system

Answer 27

superior olivary nuclei;

frequency of the sound;

auditory reflexes (e.g. head turn or startle reflex);

resolution

Question 28

The primary auditory cortex is divided into different areas receiving information from a particular part of the basilar membrane (representing a particular range of ______________):
• Cells within these areas can respond to different features of the incoming information (e.g. responding only when the sound starts, only when it stops, for the whole duration)
• Information analysed by the primary auditory cortex is projected to the _____________________→ higher level of analysis
o Includes analysis of quality of sound, beginning of analysis of music
o Strong connections to _______

Answer 28

frequencies;

secondary auditory cortex (surrounding) ;

Wernicke’s area

Question 29

Senso-neural deafness (sensory: affects hair cells in inner area)

• _______________: Effect of aging → hair cells become defective and eventually die after the third decade of life (esp. at higher frequencies)
• Loud noises: Causes hair cell death (very vulnerable)
• Meniere’s disease: Defective maintenance of ______________ → affects both balance and hearing (functions of inner eneuralar)
• ______________: Toxicity to hair cells (e.g. antibiotics)

Answer 29

Presbycusis;

endolymph;

Ototoxicity

Question 30

Senso-neural deafness (sensory: affects hair cells in inner area)

Acoustic neuroma: Benign tumour developing on CN VIII within a very narrow canal through the base of the skull carrying the nerve from the inner ear to the brainstem:
• Usually begins on the ____________
• Compresses auditory axons (cochlear nerve) as it grows in size → hearing loss
• Usually diagnosed in time to be removed (but normally results in some reduction in hearing)

Viral infection: CN VIII passes through partially a canal shared with CN VII:
• CN VII opens into the external environment behind the ear (potential route for infection)

Answer 30

vestibular part of CN VIII (balance);