The Ear and Auditory Pathways

Question 1

what is the action of the outer ear?

Answer 1

focuses sound onto tympanic membrane to create pressure waves

Question 2

what is the action of the middle ear?

Answer 2

increases pressure of vibration:

1) focuses vibrations from larger tympanic membrane to the smaller oval window
2) incus with flexible joint with the stapes so the ossicles can use leverage to increase force onto the oval window

Question 3

how are the ossicles protected when there are loud noises?

Answer 3

the stapedius and tensor tympani muscles contract when noise is loud

this restricts the movement of the ossicles to protect the inner ear

Question 4

what is the function of the cochlea (inner ear) ?

Answer 4

transduce vibration into nervous impulses

produces a frequency/pitch and intensity analysis of the sound

Question 5

what are the 3 compartments of the inner ear?

Answer 5

scala vestibul (contains perilymph fluid)

scala tympani (contains perilymph fluid)

scala media (contains endolymph fluid)

Question 6

how does basilar membrane sensitivity change?

Answer 6

is sensitive to different frequencies at different points along its length

proximal, narrow and tough (close to oval window)
distal, broad and floppy (at the apex in the cochlea)

Question 7

what is the function of the auditory tube in relation to the tympanic membrane?

Answer 7

allows equilibrium of air pressure on either side of the membrane

Question 8

what is the organ of Corti?

Answer 8

collectively the hair cells surrounded by the supporting cells

Question 9

what feature of the inner ear secretes endolymph?

Answer 9

stria vascularis

high K+, low Na+

Question 10

how are inner hair cells arranged?

how many? rows? innervation?

Answer 10

3500 cells arranged in a single row

density innervated by ~10 sensory axons

Question 11

how are outer hair cells arranged?

how many? rows? innervation?

Answer 11

20000 cells arranged in 3 rows

sparsely innervated by one axon for several cells

Question 12

which of the two hair cell types transmit signals to the brain?

Answer 12

the inner hair cells

Question 13

what is the effect of higher amplitudes on the stereo cilia?

Answer 13

greater deflection

K+ channel opening

Question 14

mechanism of transduction

Answer 14

1. Basilar membrane vibrates to sound.
2. Upward movement displaces stercocilia away from modiolus:
   K+ channels open –> K+ enters from endolymph –> hair cell depolarises.
3. Depolarisation opens Ca2+ channels in body of hair cell.
4. Glutamate released from base depolarises axon of spiral ganglion cell –> action potential.
5. Downward movement displaces stercocilia towards modiolus:
   K+ channels close –> hair cell hyperpolarises.

Question 15

sensitivity of the transduction mechanism? how much deflection is required?

Answer 15

high sensitive

sound requires 0.3 mm deflection

Question 16

what does the system depend on to remain sensitive?

Answer 16

the endolymph needs to maintained at +80mV by the stria vascularis

Question 17

how do the spiral ganglion cells of the cochlea reach their ipsilateral cochlear nuclei?

Answer 17

via the vestibulocochlear nerve

Question 18

what is the significance of the ipsilateral connection made by CN VIII?

Answer 18

all connections from the cochlear nuclei onwards are bilateral
deafness in one ear must be caused by problems in the cochlear nucleus or CN VIII

Question 19

hearing organisation in the brain

Answer 19

tonotopy is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain

Question 20

range of sound in humans

Answer 20

20-20000 Hz

Question 21

what range is human hearing most sensitive

Answer 21

1000-3000 Hz

Question 22

where do higher frequencies vibrate the basilar membrane?

Answer 22

closer to the base

Question 23

where do lower frequencies vibrate the basilar membrane?

Answer 23

closer to the apex

like a swimming jumping board, the end (apex) is least stiff and widest, the base is tough and narrow

Question 24

decibel scale

Answer 24

logarithmic
0–>120
60= conventional speech
120= thunder

ears response to sound is not linear

Question 25

hearing loss

Answer 25

with age

difficulty hearing the louder frequencies associated with speech (2000-5000 Hz)

Question 26

how is the tympanic membrane clinically evaluated?

Answer 26

otoscopy

Question 27

what does the tuning fork test enable you to do?

Answer 27

differentiates between conductive (outer/middle ear) and sensorineural (inner ear) hearing loss

Question 28

what is audiometry?

Answer 28

measuring the acuity for variations in sound intensity and frequency.

An audiometer produces sound of varying intensity and frequency and a pure tone audiogram can be generated to discover where the hearing loss is.

Question 29

what is tympanometry?

Answer 29

examination used to test conduction of middle ear, mobility of tympanic membrane and the conduction bones by creating variations of air pressure in the ear canal.

Question 30

what are the possible results on a tympanogram?

Answer 30

Tympanograms can commonly show;

• A (normal compliance)
• C (negative middle ear pressure)
• B (non-vibration of tympanic membrane) – non-vibration of TM can be due to:
• middle ear effusion
• perforation of TM
• Eustachian tube dysfunction - occluded ear canal.

Question 31

what is spontaneous otoacoustic emission?

Answer 31

OAEs are low-intensity sounds generated by the cochlea outer hair cells.

Question 32

3 types of hearing loss

Answer 32

• conductive hearing loss (outer/middle ear problem)
• sensorineural hearing loss (inner ear)
• mixed hearing loss

Question 33

5 main causes of outer-ear conductive hearing loss

Answer 33

1) congenital malformation e.g. congenital atresia (collapse/closure of ear canal)
2) impacted wax
3) foreign bodies
4) external otitis
5) exostosis

Question 34

what is external otitis?

examples?

Answer 34

inflammation of the passage of the outer ear

e. g. otorrhea- abnormal fluid
e. g. pain in mobilisation of the ear and tragus–> systemic symptoms

Question 35

4 main causes of middle ear conductive hearing loss?

Answer 35

1) acute otitis media - inflammation of middle ear
2) otitis media with effusion- same but with fluid accumulation
3) chronic otitis media
4) otosclerosis

Question 36

the two forms of chronic otitis media (of middle ear)

Answer 36

1) no cholesteatomatous:
- without perforation or with perforations

2) cholesteatoma- destructive and expanding growth consisting of keratinising squamous epithelium in the middle ear and/or mastoid process

Question 37

what is otosclerosis?

Answer 37

soft, spongy growth of new bone mostly near the oval window

90% of cases, no symptoms
can reduce mobility of stapes –> hearing loss therefore perform stapedectomy

mostly affects women

Question 38

5 main causes of inner ear sensorineural hearing loss

Answer 38

1) prebyacusis- age related hearing loss
2) sudden hearing loss
3) ototoxic drugs
4) infections
5) noise-induced

Question 39

prebyacusis

Answer 39

Is gradual and symmetric and is due to the ageing European population.

It affects frequencies of speech for the 5th decade of life.

Men are 2x more affected and tinnitus is often associated.

Question 40

what is sudden hearing loss?

Answer 40

greater than 30dB hearing reduction over >3 contiguous frequencies, occurring over a period of 72 hours or less.

there is rapid unexplained loss of hearing usually in one ear

Question 41

examples of ototoxic drugs

Answer 41

diuretics
beta blockers
TCAs
antibiotics

Question 42

infections that can affect inner ear conductivity

Answer 42

mumps
measles
chickenpox 
influenza
syphilis

Question 43

what are the two forms of noise-induced hearing loss?

Answer 43

1) acoustic trauma- brief exposure to very intensive sounds. Can cause severe hearing loss, but recovery can be great
2) long term noise exposure - common in occupational settings