The auditory system

Question 1

What is the anatomy of the outer, middle and inner ear?

Answer 1

• Outer = pinna, ear canal + tympanic membrane
• middle = ossicles + oval window
• inner = Vestibular system + cochlea

Question 2

Perilymph vs endolymph

Answer 2

P: lots of sodium, low potassium (like ECF) - in bony labyrinth
E: Lots of K, low Na (like ICF) - in membranous labyrinth

Question 3

What are the 3 chambers called inside the cochlear?

Answer 3

• Scala tympani (bottom)
• cochlear duct
• Scala vestibuli (top)

Question 4

What are the membranes called that separate the 3 chambers?

Answer 4

• Vestibular membrane between vestibuli and duct

- Basilar membrane between duct and tympani

Question 5

What sits on the basilar membrane?

Answer 5

Spiral organ, with the tectorial membrane lying over the top of that.

Question 6

What is the spiral organ?

Answer 6

hair cells and pillar cells

- hair cells are the auditory receptors - have stereocilia which are responsible for sound transduction

Question 7

What is the sound pathway in the ear?

Answer 7

Sound > Canal > tympanic membrane > ossicles > oval window > fluid vibrates in the cochlear > vibration of vest. membrane > vibration of cochlear duct > vibration of basilar membrane

Question 8

What complications can we get in the sound pathway?

Answer 8

• Blocked canals
• Tympanic membrane trauma
• Stiff ossicles

Question 9

How is sound transduced?

Answer 9

• when stereocilia move, they will tug on the tiplinks - transduced into an AP
• at the base of the tiplinks are mechanically gated channels - pulled open by tip links and allow K ion influx - moves down electrochemical gradient
• when sterocilia move towards longest cell, tip link opens the channel - causes depolarisation -> APs
• when stereocilia move towards the smallest one, they close and hyperpolarise -> no APs

Question 10

Detecting sounds

Answer 10

• inner hair cells are responsible for sound discrimination

- outer hair cells physically contract, pulling the basilar membrane in such a way to amplify the sounds

Question 11

What complications can we have with sound transduction?

Answer 11

• Endolymphatic hydrops = increased pressure in endolymph
• Menier’s disease = endolymphatic hydrops causes mixing of the 2 lymphs which damages inner hair cells
• Ototoxic drugs can damage stereocilia of hair cells (irreversible) - less APs - e.g. NSAIDs, chemo, gentamycin

Question 12

How are different qualities of sound encoded by the system?

Answer 12

• loudness = louder sounds produce larger vibrations, bigger receptor potentials and hence more APs
• pitch = based on the structure of the basilar membrane. At different points of the cochlea, hair cells oscillate at different frequencies
• right in the middle of the spiral = 20Hz
• at the end of the cochlea = 20kHz as it is more floppy

Question 13

Complications with sound discrimination

Answer 13

Presbycusis - loss of high-frequency hearing as you get older

Question 14

What is the primary auditory pathway?

Answer 14

• cochlear nerve travels through dorsal and ventral cochlear nuclei
• crosses midline to inferior colliculus
• up to medial geniculate nucleus
• up to primary auditory cortex on top of the temporal lobe

Question 15

How do we localise sound?

Answer 15

• determined by comparing the sound detected by the two ears - separate for high and low-pitched sounds
• The lateral superior olivary nuclei compare loudness of high frequency sounds
• the medial superior olivary nucleus compare the timing of low frequency sounds

Question 16

Complications with sound localisation

Answer 16

• acoustic neuroma - auditory nerve runs in the internal auditory meatus - can be damaged by SoL
• Vestibular schwannoma - uncontrolled schwann cell growth in internal auditory meatus - nerve compression
• 1st symptoms - unilateral tinnitus, later get dizziness and fainting.