Auditory System

Question 1

What is sound?

Answer 1

Sounds are audible variations in air pressure
- Many sources of sound produce variations in air pressure that are rhythmic

Question 2

What is sound frequency?

Answer 2

Sound frequency is the number of sound cycles per second (Hertz)
- We perceive high- frequency waves as having a higher pitch
- We perceive high-intensity waves as louder

Question 3

What frequencies do people hear at?

Answer 3

Our auditory system can respond to pressure waves over the remarkable range of 20 Hz to 20,000 Hz

Question 4

What is pitch?

Answer 4

high or low tone
- determined by the frequency

Question 5

What is intensity?

Answer 5

amplitude
- Sound intensity determines the loudness we perceive
- loud sounds having higher intensity

Question 6

What sound intensity damages the ears?

Answer 6

1. Noise above 70 decibels (dB) over a prolonged period of time may start to damage your hearing.
2. Loud noise above 120 dB can cause immediate harm to your ears

Question 7

Describe the outer ear?
Function?

Answer 7

• Pinna to tympanic membrane/ear drum
• The “pre amp” – makes sound waves stronger
• Pinna for humans has a poor design (too flat on head) so try “cupping” ears and see how sound quality improves

Question 8

Describe the tympanic membrane?

Answer 8

• A thin semi-transparent partition between the outer ear and middle ear.
• Covered by epidermis and lined by simple cuboidal epithelium.
• Tears usually heal in a month

Question 9

Histology of the external auditory canal?

Answer 9

The external auditory canal is lined with hairs and specialized sweat glands called ceruminous glands
- Ceruminous glands secrete cerumen /earwax

Question 10

2 important functions of earwax?

Answer 10

1. Keeps skin in ear canal soft (lubrication)
2. Keeps bugs out (they don’t like the taste)

Question 11

Parts of the middle ear?

Answer 11

1. Back of tympanic membrane
2. 3 small bones (malleus, incus and stapes)
3. Eustachian /auditory/ pharyngotympanic tube
   - Sound waves change to mechanical energy in the middle ear
   - Separated from inner ear by the round and oval windows

Question 12

Describe the set-up of the ossicles in the middle ear?

Answer 12

• The handle of the malleus is attached to the inner wall of the tympanic membrane
• The head of the malleus is attached by a tiny synovial joint to the incus, which is in turn attached by a tiny synovial joint to the stapes.
• The foot plate of the stapes is held in place in the oval window by a flexible annular ligament

Question 13

Function of the ossicles?

Answer 13

Transmit vibrations from the tympanic membrane to the oval window

Question 14

Why can`t sound waves simply directly move the membrane at the oval window?

Answer 14

• the cochlea is filled with fluid, not air
• The fluid in the inner ear resists being moved much more than air does (i.e., fluid has greater inertia), so more pressure is needed to vibrate the fluid than the air can provide.
• The ossicles provide this necessary amplification in pressure

Question 15

When will the pressure at the oval window become greater than the pressure at the tympanic membrane?

Answer 15

1. the force on the oval window membrane is greater than that on the tympanic membrane
2. the surface area of the oval window is smaller than the area of the tympanic membrane.

Question 16

How does the middle ear increase pressure at the oval window?

Answer 16

force applied onto a larger area (tympanic membrane) is concentrated onto a smaller area (oval window)
- The force at the oval window is greater because the ossicles act like levers
- increases pressure by 20 times

Question 17

What are the muscles of the middle ear?

Answer 17

1. The tensor tympani muscle
   - innervated by the trigeminal nerve
   - is anchored to bone in the cavity of the middle ear at one end and attaches to the malleus at the other end
2. The stapedius muscle
   - innervated by the facial nerve
   - also extends from a fixed anchor of bone and attaches to the stapes

Question 18

Function of the muscles of the inner ear?

Answer 18

When these muscles contract, the chain of ossicles becomes much more rigid, and sound conduction to the inner ear is greatly diminished

Question 19

What is the attenuation/acoustic reflex?

Answer 19

The onset of a loud sound triggers a neural response that causes the tensor tympani and stapedius muscle to contract the chain of ossicles becomes much more rigid, and sound conduction to the inner ear is greatly diminished
Note: Sound attenuation is much greater at low frequencies than at high frequencies
- Since the reflex suppresses low frequencies more than high frequencies, it is thought to be activated when we speak, so we don’t hear our own voices as loudly as we otherwise would

Question 20

What is a limitation of the attenuation reflex?

Answer 20

the attenuation reflex has a delay of 50–100 msec from the time that sound reaches the ear
- Does not offer much protection from very sudden loud sounds
- damage might already be done by the time the muscles contract.

Question 21

Functions of the Eustachian/auditory tubes?

Answer 21

1. Drain cells/tissue
2. Equalize air pressure between outside air and middle ear cavity
   - Normally closed, opened when yawning, chewing, swallowing

Question 22

What are the clinical correlates for the Eustachian tube?

Answer 22

The most common type of ear infections (otitis media) is caused by pathogens that travel from the throat and nose via the auditory tube
- Children more susceptible because their auditory tubes are shorter, narrow, almost horizontal (45° tilt in adults) which decreases drainage efficiency

Question 23

Describe the structure of the auditory tube?

Answer 23

The first third of the tube beginning at the middle ear is surrounded by bone and the remainder is surrounded by an incomplete ring of elastic cartilage
- The meeting point of the bony and cartilaginous portions is called the isthmus

Question 24

Describe the importance of the auditory tube?

Answer 24

Cells lining middle ear absorb air (negative pressure) thus constant supply from Eustachian tube is necessary

Question 25

What happens when the auditory tube is blocked?

Answer 25

When the tube is blocked, the eardrum is sucked into the middle ear (pain, pressure, hearing loss)

Question 26

What causes blockage of the auditory tube?

Answer 26

1. inflammation of nasal lining narrows opening or passageway 2. nasal allergy 3. obesity (excess fat deposits along passageway)

Question 27

Fullness in the ears can be alleviated by? How?

Answer 27

Fullness in the ears (negative pressure in middle ear) can be alleviated by ‘popping’ the ears - Yawning or swallowing opens the eustachian tube by contracting the tensor palatini muscles Note: The hook-shaped arrangement of the Eustachian tube cartilage keeps the tube in the normally closed state. - The tube is opened when the cartilage hook is uncurled by action of the tensor palatini muscle

Question 28

What is the cochlea?

Answer 28

Snail-shaped, pea-sized, filled with fluid

Question 29

What does the cochlea consist of?

Answer 29

1. the scala vestibuli - perilymph similar to CSF - low K+ (7 mM) and high Na+ (140 mM) concentrations] 2. the scala media - endolymph with ionic concentrations similar to intracellular fluid - high K+ (150 mM) and low Na+ (1 mM). 3. the scala tympani - perilymph similar to CSF - low K+ (7 mM) and high Na+ (140 mM) concentrations

Question 30

Describe the cochlea’s role in hearing?

Answer 30

- At the base of the cochlea, the scala vestibuli meets the oval window and the scala tympani meets the round window. - Inward motion at the oval window pushes perilymph into the scala vestibuli. - Because the fluid pressure has nowhere else to escape, the membrane at the round window bulges out in response to the inward movement of the membrane at the oval window

Question 31

What is the cochlear aqueduct?

Answer 31

- A small canal in the bony labyrinth that contains the perilymphatic duct, which drains perilymph into the cerebrospinal fluid of the subarachnoid space - The lateral end arises from the scala tympani

Question 32

What is the endolymphatic sac and duct?

Answer 32

- In the membranous labyrinth - drain endolymph from the auditory and vestibular apparatus - the sac acts as both a reservoir for endolymph and the site for reabsorption into the epidural space

Question 33

Describe the organ of Corti?

Answer 33

1. on the surface of the basilar membrane 2. Contains a series of electromechanically sensitive cells, the hair cells (20,000). - Hair cells are specialized epithelial cells - Each hair cell has about 100 stereocilia on its apical border - They extend above the reticular lamina into the endolymph - They are sandwiched between the basilar membrane and a thin sheet of tissue called the reticular lamina 3. The rods of Corti (pillar cells) span these two membranes and provide structural support

Question 34

Name and describe the cells of the organ of corti?

Answer 34

1. inner hair cells - Hair cells between the modiolus and the rods of Corti (about 4500 form a single row) - tips of sterocilia end just below the tectorial membrane 2. outer hair cells - Cells farther out than the rods of Corti are called outer hair cells (12,000–20,000 arranged in three rows) - tips of sterocilia end in the gelatinous substance of the tectorial memmbrane

Question 35

Function of hair cells?

Answer 35

Generate nerve impulses in response to sound vibrations

Question 36

What is an important structural component of hair cells that aids in auditory signal transduction?

Answer 36

1. Stereocilia become progressively longer on the side of the hair cell away from the modiolus 2. The tops of the shorter stereocilia are attached by thin filaments to the back sides of their adjacent longer stereocilia

Question 37

Describe auditory signal transduction?

Answer 37

1. Whenever the cilia are bent in the direction of the longer ones, the tips of the smaller stereocilia are tugged outward from the surface of the hair cell. - This causes a mechanical transduction that opens 200 to 300 cation-conducting channels 2. K+ influx from the surrounding scala media fluid into the stereocilia, depolarization of the hair cell membrane 3. The entry of K+ depolarizes the hair cell, which opens voltage-gated calcium channels. 4. Ca2+ influx triggers the release of the neurotransmitter glutamate, which activates the spiral ganglion fibres lying postsynaptic to the hair cell

Question 38

Describe the centralauditory pathways?

Answer 38

Afferents from the spiral ganglion enter the brain stem in the auditory–vestibular nerve. At the level of the medulla, the axons innervate the dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea where the axons originated From the cochlear nucleus, auditory info is transmitted to superior olivary nuclei (pons, decussation) lateral lemniscus (midbrain) inferior colliculus (midbrain) medial geniculate nuclei (thalamus) All levels of the central auditory system receive and process information from both the ipsilateral and contralateral sides

Question 39

What is otitis externa?

Answer 39

inflammation of outer ear

Question 40

What is otitis media?

Answer 40

inflammation of middle ear

Question 41

What is cholesteatoma?

Answer 41

benign tumour) a skin-lined cyst that begins at the margin of the eardrum and invades the middle ear and mastoid

Question 42

What is hearing loss?

Answer 42

1. Conductive hearing - damage to or obstruction of the outer or middle ear 2. Sensorineural hearing loss - gradual loss of hearing resulting from an abnormality in either the inner ear, the auditory nerve, or both

Question 43

What is tinnitus?

Answer 43

ringing in the ear 1. exposure to loud noise 2. normal aging 3. infections