auditory system

Question 1

range of human hearing

Answer 1

20Hz - 20Khz

Question 2

what range is the ear most sensitive at

Answer 2

1000-4000 Hz

Question 3

outer ear: pinna and ear canal

Answer 3

• pinna – cartillagenous structure
• formed from pharyngeal arches 1 & 2 (6x Hillocks of His)
• forms between 10th and 18th weeks in utero
• directs soundwaves towards ear canal
• high pitch > low pitch
• ear canal – 1/3 cartilage & 2/3 bone
• sound enters through pinna then enters ear via external auditory canal

Question 4

outer ear: tympanic membrane

Answer 4

• 8 x 10mm diameter
• 14mg
• 84mm2 – 55mm2
• Broadly split into 2 parts – pars tensa(3 layers inc. fibrous layer) , pars flaccida (only inner and outer layer – more prone to structural damage)
• As air molecules push against tympanic membrane it vibrates at the same frequency as the sound wave

Question 5

the middle ear

Answer 5

• Bones: Malleus, incus and stapes
• Muscles: tensor tympani & stapedius
• Tubes: eustachian tube
• Air-filled cavity in temporal bone of skull
• Sensation: glossopharyngeal nerve
Role: amplification of the airborne sound vibration

Question 6

transmission of sound in middle ear

Answer 6

• Pressures in the external auditory canal and middle ear cavity are normally equal to atmospheric pressure
• Middle ear is exposed to atmospheric pressure via the eustachian tube which opens into the pharynx through a slit like opening which is normally closed except when swallowing, yawning or sneezing
• Difference in pressures occurs due to changes in altitude- when this happens middle ear pressure initially remains constant because eustachian tube is closed – constant pressure can stretch tympanic membrane – pain – relieved by yawning/swallowing – pressure equilibrate
• Vibrations of the tympanic membrane are transmitted to the inner ear through the ossicles: malleus, incus, stapes
• These bones have synovial joints between them
• They act as a piston and couple the vibrations of the tympanic membrane to oval window
• Total force of a sound wave applied to the tympanic membrane is completely transferred to the oval window – oval window is much smaller than the tympanic membrane – force per area is larger so it can transmit the sound energy through the fluid filled cochlea
• Energy transmitted can be reduced (loud noise) by contraction of tensor tympani (malleus (mad div. CN V)) and stapedius (stapes (CN VII))

Question 7

inner ear: vestibulocochlear apparatus

Answer 7

• A set of fluid filled sacs, encased in bone
• Cochlear- responsible for hearing
• Labyrinth- responsible for balance
• Innervation: Vestibulocochlear nerve

Question 8

inner ear: cochlea

Answer 8

• 2.5 turns filled bony tube
• 2 openings- round window & oval window
• 3 compartments ( Scala Tympani, Scala Media & Scala Vestibuli)
• 2 Ionic fluids
• Completely divided lengthways by cochlea duct – contains sensory receptors of the auditory system

Question 9

cochlea fluids

Answer 9

• Endolymph - High K+
• Perilymph- Like ECF and CSF, Na+ rich
• Gradients maintained by: Na, K-ATPase & NKCC1 CIC-K chlorine channels
• Ion channel abnormalities- deafness.

Question 10

Helicotrema

Answer 10

small hole allows communication from scala vestibuli to scala tympani so sound wave can travel through

Question 11

transmission of sound through inner ear

Answer 11

• Oval window moves in and out of scala vestibuli – creates wave of pressure
• Majority of waves are transmitted across the cochlear duct with some transmitted towards helicotrema into scala tympani where the pressure is relieved by the movements of the membrane of the round window
• The side of the cochlea duct closest to the scala tympani is formed by the basilar membrane, upon which sits the organ of corti – contains the ears sensitive receptor cells

Question 12

basilar membrane

Answer 12

in scala tympani
high feq at base
low freq at apex

Question 13

hair cells of the organ of corti

Answer 13

• Inner Hair Cells- Mechanical transduction – single row
• Outer Hair Cells- fine tuning – 4/5 rows
Base attached to basilar membrane
Stereocillia anchored to tectorial membrane.
Shearing forces at the stereocilia
Some antibiotics can damage the stereocilia of the hair cells

Question 14

inner hair cells

Answer 14

Body of cell within perilymph
Hair cells (stereocilium) in endolymph
Convert pressure waves into receptor potentials
From waves to sparks:
• Movement of the sterocillia
• Rapid response required
• Mechanically gated K+ channels opened causing depolarization ( K+ rich endolymph)
• Depolarization results in opening of voltage gated Calcium channels
• Release of neurotransmitter- Glutamate (plus others)
• Repolarization through K+ efflux ( into K+ poor perilymph)

Question 15

outer hair cells

Answer 15

• Stereocilia are embedded in the overlying tectorial membrane – mechanically alter its movement to sharpen frequency tuning at each point along the basilar membrane
• Each nerve responds maximally at a specific frequency.
• But our ability to discriminate different frequencies is not fully explained by this theory.
• Outer Hair Cells can alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and dampened response at another.
• Increased resolution

Question 16

central auditory pathway

Answer 16

E.C.O.L.I = 
•	Eighth nerve 
•	Cochlear nucleus 
•	Olive 
•	Lateral leminiscus 
•	Inferior colliculus

Question 17

how is sound info encoded

Answer 17

• FREQUENCY (PITCH) Encoded in nerves by location along the basilar membrane
• INTENSITY (LOUDNESS) Encoded in nerves by numbers responding and by firing rate
• SOUND TRANSDUCTION Inner Hair Cells (and OHCs)
• AMPLIFICATION Outer Hair Cells

Question 18

types of hearing loss

Answer 18

• Conductive hearing loss = defective outer/ middle ear – improve amplification e.g. hearing aid
• Sensorineural hearing loss = defective inner ear – stimulate, cochlea implant