The Ear and Auditory System

Question 1

Pitch vs. Timbre?

Answer 1

Pitch - perception of frequency

Timbre - what distinguishes between two sounds at the same frequency & intensity

Question 2

Anatomy of Outer-ear?

Answer 2

o Focuses on the TYMPANIC MEMBRANE

• it boosts sounds
• and creates pressure waves

Question 3

What is the sensory receptor of the internal ear?

Answer 3

Found of the COCHLEA
Hair bundle is a cluster of modified microvilli

STEREOCILIA

Question 4

Ossicles?

Answer 4

3 - stapes, incus & malleus

Transmits vibration from the tympanic membrane to the cochlea (snail-shaped filled w. liquid)

Question 5

What is the function of the ossicle?

Answer 5

Match the IMPEDANCE and reduce loss of energy as the vibration goes from the air to the cochlea

Question 6

Anatomy of Middle-ear?

Answer 6

INCREASES PRESSURE of VIBRATION by:
o focusing vibrations from the larger tympanic membrane to the smaller oval windows
o Incus has a flexible joint w. the stapes, so the ossicles can use leverage to increase force on the small oval windows

Oval window = (or fenestra vestibuli) is a membrane-covered opening that leads from the middle ear to the vestibule of the inner ear.

Question 7

Protective mechanism of the ear to loud noises?

Answer 7

The
o stapedius muscles
AND
o tensor tympani muscles

can CONTRACT when noise is loud to RESTRICT the movements of the ossicles (malleus & incus)
- protects inner ear from excessive volumes

Question 8

Impedance and resonant frequency?

Answer 8

Impedance - measures the reluctance of a system in recieving energy from a source

Resonant frequency - frequency at which the impedance of the system is minimal

Question 9

Anatomy of inner-ear?

Answer 9

Cochlea - transduce vibration into nervous impulses

3 compartments of the inner ear:
o Scala vestibuli - contains perilymph fluid
o Scala tympani - contains perilymph fluid
o Scala media - contains ENDOLYMPH fluid
- (stria vascularis secrets this - high K+, low Na+)

Basilar membrane - sensitive to different frequencies at different points along its length (high proximal, low distal)

Question 10

Cochlea?

Answer 10

Transduce vibrations into nervous impulse

By doing this, cochlea produces a frequency/pitch and intensity analysis of the sound

Question 11

What causes the vibration of the basilar membrane?

Answer 11

Montion of the stapes generates a difference in pressure between the two liquid-filled chambers of the cochlea causing the vibration of the basilar membrane

Question 12

Organ of Corti?

Answer 12

Includes the
 o basilar membrane
 o tectorial membrane - gelatinous and does NOT vibrate w. sound
 o hair cells
 o supporting cells

Question 13

What is needed for basilar membrane to function?

Answer 13

Breaks down complex sounds by distributing the energy of each frequency along its length

It therefore needs sensory receptors along the whole length
i.e. HAIR CELLS

Question 14

2 types of hair cells?

Answer 14

Hair cell - have many stercocilia per cell
o Inner hair cell
- 3500 cells arranged in a single row
- densely innervated by ~10 sensory axons
o Outer hair cells
- 20,000 cells arranged in 3 rows
- sparsely innervated by 1 axon

Both types response to sound BUT its the INNER CELLS that transmit signals to BRAIN

Question 15

Mechanism of transduction in the ear?

Answer 15

INNER EAR!

1. Basilar membrane vibrated to sound
2. UPWARD movement displaces stercocilia away from modiolus
   o K+ channels open
   o K+ enters from endolymph
   o hair cell DEPOLARISES
3. Depolarisation opens Ca2+ channels in body of hair cell
4. Glutamate released from base depolarises axon of SPIRAL GANGLION CELL
   o causes an AP
5. DOWNWARD movement displaces stercocilia towards modiolus
   o K+ channel close
   o hair cell HYPERPOLARISES

Question 16

Conditions required for the mechanism of transduction in the inner ear?

Answer 16

Mechanism is HIGHLY SENSITIVE
- threshold sound requires 0.3nm deflection

System depends upon the maintenance of the EDOLYMPH
- being at +80mv by the stria vascularis

Question 17

Protective mechanism for the inner-ear?

Answer 17

Auditory tube allows equilibrium of air pressure on either side of the tympanic membrane

Question 18

Central auditory pathway?

Answer 18

SPIRAL GANGLION CELLS
from each cochlea project via.
CN VIII (vestibulocochlear nerve)
to the IPSILATERAL COCHLEAR NUCLEI (monoaural neurons)

After that, all connections are BILATERAL!

Question 19

Deafness in ONE ear?

Answer 19

MUST be caused by problems in the
o Cochlear nucleus
OR
o CN VIII (rare)

AS all connection are BILATERAL after the ipsilateral cochlear nuclei

Question 20

Ventral and dorsal cochlear nucleus?

Answer 20

Nerve fibres convey info. to the COCHLEAR NUCLEUS!

Different kinds of neurons arranged TONOTOPICALLY
low frequencies = ventral ; high frequencies = dorsal

Question 21

Central auditory pathway?

Answer 21

1. Cochlear nerve travels to the ipsilateral cochlear nucleus
2. Cochlear nucleus goes to the SUPERIOR OLIVARY NUCLEI BILATERALLY
3. SON to the INFERIOR COLLICULUS UNTILATERALLY
4. IC to the MEDIAL GENICULATE NUCLEUS
5. MGN to the AUDITORY CORTEX

Question 22

Types of hearing loss?

Answer 22

1. Conductive
   o an outer OR middle ear problem
   o i.e. cannot transmit vibrations of sound waves onto the cochlea
2. Sensorineural
   o and inner ear problem
3. Mixed hearing problem

Question 23

Causes of OUTER-EAR conductive hearing loss?

Answer 23

1. Congenital malformation
   o e.g. congenital atresia - collapse/closure of the ear canal
2. Impacted wax
3. Foreign bodies
4. External otitis - inflammation of outer-ear passage
   o Otorrhea - abnormal fluid
   o Pain on mobilisation of the ear and tragua can lead to systemic symptoms
5. Exostosis - benign bony growths in the ear canal due to repeated exposure to cold water
   o usually multiple & bilateral - leads to accumulation of ear wax and external repeat ear infections

Question 24

Causes of MIDDLE-EAR conductive hearing loss?

Answer 24

1. Acute otitis media - inflammation of the middle-ear
2. Otitis media w. effusion - inflammation of the middle-ear with fluid accumulation
3. Chronic otitis media - can be 2 forms:
   o NO cholesteatomatous
   o Cholesteatoma
   (ONENOTE!!)
4. Otosclerosis - soft, spongy growth of new bone mostly near the oval window
   o normally no symptoms
   o can reduce mobility of stapes = hearing loss so can preform a stapedectomy

Question 25

Sensorinueral hearing loss?

Answer 25

Interests the COCHLEAR & NERVE

i.e. the sensory apparatus of the inner ear
OR
the vestibulocochlear nerve (CN VIII)

Question 26

Causes of Sensorineural hearing loss?

Answer 26

1. Prebyacusis - age-related hearing loss
2. Sudden hearing loss
   - unexplained rapid loss of hearing usually in one ear
3. Ototoxic drugs e.g. diuretics, B-Bs, TCAs, Antibiotics
4. Infections e.g. MM, chicken pox, influenza
5. Noise-induced hearing loss - 2 forms
   o Acoustic trauma
   o Long-term noise exposure

Question 27

Cochlear Implants?

Answer 27

Primary cause of hearing loss is due to the LOSS OF HAIR CELLS
(do NOT regenerate in mammals)

Can put an elongated coil into cochlea which bypasses the dead cells and stimulates the nerve fibres