Cochlea and Auditory System

Question 1

Structures involved in hearing

Answer 1

• Outer ear
• Middle ear
• Inner ear

Question 2

Outer ear

Answer 2

• Pinna, directs sound waves into the ear cannal

- Ear lobe

Question 3

Middle ear

Answer 3

• Tympanic membrane (ear drum)
• The ossicles (malleus, incus, stapes)
• Two tiny muscle that attach to the ossicles

Question 4

Inner ear

Answer 4

• Cochlea

- Labyrinth (part of the vestibular system)

Question 5

Cochlea

Answer 5

• Two membrane covered holes, round window and oval window

- Three fluid filled chambers; scala vestibuli, scala media, scala tympani

Question 6

Reissner’s membrane

Answer 6

Separates the scala vestibuli from the scala media

Question 7

Basilar membrane

Answer 7

Separates scala tympani from the scala media

Question 8

Organ of Corti

Answer 8

• Sits upon the basilar membrane

- Contains auditory receptor neurons

Question 9

Tectorial membrane

Answer 9

Hanging over the organ of Corti

Question 10

How is frequency encoded?

Answer 10

In nerves by location along the basilar membrane

Question 11

How is intensity encoded?

Answer 11

In nerves by numbers responding and by firing rate

Question 12

Sound transduction

Answer 12

• When the basilar membrane moves in response to a motion at the stapes, the entire foundation supporting the hair cells moves
• Recordings from hair cells indicate that when the stereocilia bend in one direction the cells depolarise and in the other direction it hyperpolarises
• When a sound wave causes the stereocilia to bend back and forth, the hair cell generates a receptor potential that alternates from hyperpolarised to depolarised from resting potential of -70mV
• Causes a response in the auditory nerve which sends information to the brain

Question 13

What is special about the K+ concentration in the endolymph?

Answer 13

It is unusually high

Question 14

Amplification of sound

Answer 14

• By outer hair cells
• Act like tiny motors that amplify the movement of the basilar movement during low-intensity sound stimuli
• Key to this is motor proteins found in membranes of OHCs
• Motor proteins can change the length of the hair cells
• When OHCs amplify the response of the basilar membrane, the stereocilia on IHCs bend more and increase the transduction process and produces a greater response in the auditory nerve

Question 15

Which regions can have a tonotopic map?

Answer 15

Anterior ventral (AVCN), posterior ventral (PVCN) and dorsal (DCN) cochlear nuclei

Question 16

Superior olives

Answer 16

Sound localisation

Question 17

Medial superior olive

Answer 17

Low frequency analysis, interaural time differences, receives inputs from calyx of Held on both sides

Question 18

Lateral superior olive

Answer 18

High frequency analysis, interaural intensity differences

Question 19

Inferior colliculus

Answer 19

• Site of convergence of projections with complex frequency responses
• Monoaural input from DCN
• Binaural input from SO
• Tonotopic maps
• Responsible for attention reflexes, startle response, learned reflexes

Question 20

Cortex

Answer 20

• Tonotopic map retained
• Neurons with sensitivity to features in complex sounds
• Auditory space maps, selective attention, inhibition of inappropriate motor responses, recognition of stimuli, discrimination of temporal patterns and short-term auditory memory