Anatomy of hearing

Question 1

3 main divisions of the ear:

Answer 1

Outer Ear

Middle Ear

Inner Ear

Question 2

Outer ear parts and their functions

Answer 2

Outer Ear

Pinna - catches sound, deflects them into canal

Auditory canal - gathers and amplifies sound to focus it onto the eardrum

Tympanic Membrane (eardrum) - Vibrates to transmit sound energy to ossicles

Question 3

Middle ear parts and their functions

Answer 3

3 Ossicles (bones)

Malleus (hammer) - Receives vibrations from the tympanic membrane

Incus (anvil) - Acts as a lever that passes vibrations between the malleus and the stapes

Stapes (stirrup) - conducts vibrations from the incus to the cochlea via the oval window

Question 4

Inner ear parts and functions

Answer 4

• Cochlea
• energy from pressure waves is transduced into neural signals
• Hollow, filled with lymphatic fluid
• Vibration on the oval window vibrates fluid in the cochlea

Question 5

Cochlea’s chambers

Answer 5

Scala vestibuli: One end is the oval window

Scala media: Contains the Organ of Corti

Scala tympani: One end is the round window

Question 6

Organ of Corti membranes

Answer 6

These membranes are floating in the scala media

Tectorial membrane: Stereocilia of hair cells are embedded in this membrane

Basilar membrane: Bases of the hair cells are embedded here

Vibration of fluid in the cochlea creates a Shearing force between the membranes causing the stereocilia of the hair cells to bend and move against the membranes

Question 7

Hair cells, stereocilia, and Auditory afferents

Answer 7

1. Sound waves cause stereocilia to bend against tectorial membrane
2. Stretches the tip links
3. Pulls open mechanically gated K+ channels
4. Scala media fluid has high concentration of K+, K+ moves into the stereocilia
5. Causes EPSP
6. Opens voltage gated Ca2+ channels
7. Release of neurotransmitter
8. DOES NOT send APs

Auditory Afferents

9) Send the signal to the brain with APs to represent sound

Question 8

Inner Hair Cells

Outer Hair Cells

Answer 8

Inner hair cells:
• One row
• Activate afferent neurons whose axons
run in the auditory nerve
• Responsible for 90-95% of auditory
signal
Outer hair cells:
• Three rows
• Receive efferent input from the brain
• Fine tune sound input by contracting or
relaxing the length of the cells to
stiffen/loosen the flexibility of the
tectorial membrane
• This then influences the shearing effect
on the inner hair cells!

Question 9

How do hair cells code for amplitude?

How do hair cells code for frequency?

Answer 9

Amplitude/Loudness

The bigger the sound wave = the more displacement of cilia = more neurotransmitter release = more firing of APs

Coded by rate of AP firing

Frequency/Pitch

Variations in air pressue travels down the cochlea and hits the basilar membrane (base for high frequencies and apex for low frequencies) which affects the hair cells’ responses.

Coded by which afferent is firing

Question 10

Hearing in a nutshell

Answer 10