lecture 23 - nervous system 3: hearing & the ear

Question 1

what is sound?

Answer 1

pressure waves travelling through air or another medium

defined by:
• frequency
• amplitude

Question 2

what is frequency?

Answer 2

pitch of sound

measured in Hertz cycles per second

Question 3

what is amplitude?

Answer 3

intensity - loudness

measured in dB

log scale

Question 4

what happens when a sound creates a pressure wave?

Answer 4

it travels through a medium

air molecules are alternatively compressed and rarefied

the wave travels from source to ear through the air but the gas molecules themselves remain in their locations

Question 5

what is the ear composed of?

Answer 5

outer ear

middle ear

inner ear

Question 6

structures of the outer ear

Answer 6

pinna

auditory canal / auditory meatus

tympanic membrane

Question 7

structures of the middle ear

Answer 7

air filled space

comprised of the malleus (malleus, incus and staples)

connected to the nasopharynx by the Eustachian tube

Question 8

structures of the inner ear

Answer 8

cochlea

organs of balance

nerve fibres leaving the cochlea go though a hole in the skull into the brain

Question 9

role if the pinna

Answer 9

directs sound to ear canal

externalisation of sounds
• listen without pinna (headphones)
• sound seems to be inside head

localisation of sounds in vertical plane
• above/below or in front/behind head
• interference of sound

Question 10

role of the auditory canal/meatus

Answer 10

leads to ear drum

protects sensitive machinery of ear

resonance - amplifies sounds in frequency of range of speech

Question 11

what are the 3 bones in the middle ear

Answer 11

malleus

incus

stapes

Question 12

what is the tympanic membrane?

Answer 12

the ear drum

Question 13

structure of the middle ear

Answer 13

malleus attached to tympanic membrane and allows sound to be conducted through small bones

footplate of stapes attached to window in cochlea called oval window which allows sound to be taken to cochlea

it is an air filled cavity - pressure needs to be the same as the ear canal

Question 14

how do you adjust the pressure in the middle ear?

Answer 14

by swallowing

opens the Eustachian tube connected to the nasopharynx

allows pressure to equalise with the ear canal

Question 15

why do we need a middle ear?

Answer 15

problem of transferring energy from 1 medium to another

inner ear is fluid filled

without middle ear there would be poor transfer of energy from air to cochlea fluids

Question 16

function of the middle ear

Answer 16

overcomes air-fluid mismatch

helps transfer sound more efficiency

Question 17

structure of the cochlea

Answer 17

consists of a spiral of 2 and a half turns

3 membranous tubes that separate large tube into 3 divisions

Question 18

what are the 3 divisions of the cochlea?

Answer 18

top one - Scala vestibuli

lower one - Scala tympani

between them is the Scala media

at each level, cochlea nerve fibres come down

cell bodies of the fibres are in the spiral ganglion

Question 19

what is the Scala media and the Scala tympani separated by?

Answer 19

basilar membrane

on the basilar membrane in the organ of corti - contains cellular machinery of the ear

Question 20

what is the Scala media and the scala vestibuli separated by?

Answer 20

Reissner’s membrane

Question 21

how is the fluid in the different cochlea compartments different?

Answer 21

in the vestibule and tympani there is a fluid called perilymph that has a high Na and low K concentration

in the media there is the endolymph which is produced by the stria vascularis which secrete K+ into the space

Question 22

organ of corti

Answer 22

sits on the basilar membrane

complex structure

has hair cells - stereocillia

Question 23

what are the 2 types of hair cells in the organ of corti?

Answer 23

inner and outer hair cells

only 1 row of inner hair cells

3 rows of outer hair cells

they have nerve fibres associated with them

Question 24

what lies over the hair cells in the organ of corti?

Answer 24

the tectorial membrane - stereocillia of the outer hair cells are embedded in it

Question 25

what is the fluid in the organ of corti?

Answer 25

fluid above the hair cells and below the tectorial membrane in endolymph

perilymph baths the area between the basilar membrane and the basolateral membranes

Question 26

what does the stapes do in the separation of sound frequencies in the basilar membrane?

Answer 26

introduces sound vibrations into the cochlea

it moves forwards and backwards to introduce pressure into the scar vestibuli (outer chamber)

when you increase pressure, it has to be accommodated - round window bulges outwards

when you apply pressure it begins at the base and travels as a wave that gradually increases and then decreases to the apex

this is called the travelling wave

Question 27

frequency analysis by the cochlea

Answer 27

low frequencies produce maximal displacement at the apex of the cochlea

high frequencies produce maximal displacement at the base

displacement of the basilar membrane activates the hair cells

Question 28

what happens when hair cells are activated by displacement of the basilar membrane?

Answer 28

cell depolarise and K+ enters when bundle bends towards tallest stereocillia, Ca++ entry and neurotransmiite release

cell hyperpolarises when bundle moves towards shortest steroecillia

they only generate graded potentials not APs

Question 29

what do inner hair cells do when activated?

Answer 29

release neurotransmitter glutamate

activate cochlea nerve fibres associated with inner hair ells in region where basilar membrane displacement is maximal

Question 30

what do outer hair cells do?

Answer 30

shorten and lengthen is response to depolarisation and hyperpolarisation

brought about by motor proteins called prestin

increase the sensitivity of our hearing