Auditory 1

What is sound?

pressure wave - elastic vibrations which can compress and refract

What are pure tone sound wave relationship with pitch and loudness?

• frequency is directly related to pitch

• amplitude related to percived loudness

Because there is a logarithmic relationship between frequency and pitch what does this mean?

can distinguish between lower frequencies more easily

Fourier spectrum

What is a narrow band sound?

• sounds in which a relatively small number of components contain most of the energy

• periodic and may evoke identifiable pitch

What is a broad band sound?

• sounds containing very many components of similar amplitude and often do not evoke a strong pitch

• noises and clicks

What is a spectrogram?

• complete description of sound if the frequency composistion of the sound is constant over time

• frequency x time

Neurogram

What is the human ear composed of?

• external ear

• middle ear

• inner ear

What does the external ear do?

• act as a funnel and filter

• certain features of the sound are attentuated and other amplified before enter

  • helps with vertical localisation of sound

  • direction of arrival affects attenuation and amplification

Where is the middle ear located?

middle the tympanic membrane and the inner ear

What does the eustachian tube connect and what is its role?

• middle ear and back of throat (nasopharynx)

• equalising air pressure, yawning and chewing gum

  • responds to changes in air pressure

What is the function of the middle ear?

to minimise loss of energy because acoustic impedances of air and fluid (in middle ear) are very different - fluid higher than air

How does impedance matching take place?

• collects sound over large area of eardrum

• concentrates on much smaller area of stapes footplate

• lever forced by malleus longer than that of incus

  • resulting in further increase in pressure by 1.3

• increases sensitivity of ear by about 30dB

What is the stapedius relfex?

• activated by intese, low frequnecy sounds

  • contracted through relfex

• also activated when we vocalise

• allows us to perceive higher frequency sounds such as speech and ignoring background sounds

Does the stapedius reflex help protect us from sudden loud noises?

no - gunshots eg very dangerous

What is the inner ear comprised of?

fluid filled chambers

3 features of the cochlea?

• coiled tube enclosed in hard bone shell

• filled with 2 fluids

• contains hair cells

What are the 3 chambers in the cochlea?

• scala vestibuli

• scala media

• scala tympani

What is the fluid in the scala vestibuli and scala tympani?

perilymph - Na+

What is the fluid in the scala media?

endolymph - K+ (+80mV which sets up electric voltage grab between chambers)

What does Reissner’s membrane separate?

scale vestibuli and scala media which prevents flow of ions between chambers and maintains potential

Where is the Basilar membrane?

between scala media and scala tympani

What are the two ‘openings’ (membrane instead of bone) of the cochlea?

oval window and round window

How does propagation of mechanical energy in the cochlea take place?

• eardrum moved by sound waves

• transmitted and amplified via 3 bones of middle ear

• stapes pushes on oval window

• causes round window to bulge out

• change in pressure causes movement of basilar membrane

  • from waves in fluid of cochlea

• travelling waves reach peak at different positions on basilar membrane depending on frequency

How does the basilar membrane establish tonotopy?

• high frequency sounds at the base as more rigid

• low frequency sounds at apex

• vibration in different places establishes ‘place code’

Define tonotopy?

• concept of sounds with similar frequency contents processed by geographically close regions

• established at basilar membrane and maintained throughout most structures

What do complex sounds cause on the basilar membrane?

deflections at several positions along the membrane

What sits along the basilar membrane?

organ of Corti with roof called tectorial membrane

How are hair cells arranged on the organ of corti?

• 3 rows of outer hair cells

• 1 row of inner (closer to cochlea spiral centre) hair cells

What are bundles on hair cells called?

stereocilia

What does up-down movement of the basilar membrane do to the organ of corti?

• causes the tectorial membrane to slide sideways over the membrane, causing a sideways displacement of the hair cell bundles in the cochlear hair cells

• direction of deflection depends on up or down movement

Steps for transduction in hair cells?

• stereocilia of the hair cell bundle are connected by ‘tip links’

• movement of bundle thought to change the tension on the tip links, thereby opening or closing stretch sensitive K+ channels

• K+ channel opening causes an influx of K+ from the endolymph, depolarisation of the hair cell membrane, opening of V-gated Ca++ channels and an increased probability of transmitter release

Do hair cells fire APs?

no

How is the membrane potential different for low frequencies and high frequencies?

• at low freq, membrane potential of hair cells follows every cycle of stimulus (AC response)

• at high freq, membrane potential instead remains depolarised throughout duration of stimulus (DC response)

Why does the DC mode come about?

• asymmetry in effects of displacing stereocilia

  • opening channels can depolarise membrane more than closing them hyperpolarises it

Fun fact for outer hair cell?

changes shape in response to depolarisation and hyperpolarisation

• shorten when depolarise and lengthen when hyperpolarise

What are 2 active mechanisms contributing to cochlear mechanics?

• somatic motility

• hair bundle motility

  • deflection of hair bundles causes them to ‘twitch’ thus amplifying the stimulus

What is somatic motility?

• OHCs change length when stimulated

• depolarisation triggers massive change in length

• length change is mediated by a motor protein (prestin) in OHC membrane

• OHC motility produces a localised amplification of the basilar membrane motion

  • leading to higher sensitivity and sharper frequency tuning

• source of non-linearity: weak stimuli are amplified more than strong ones

1991 Ruggero and rich

outer hair cells as being responsible for the high sensitivity and frequency selectivity of basilar membrane responses?