auditory system

Question 1

what is sound?

Answer 1

A wave of air particles knocking into one another, resulting in areas of rarefied air (few particles) and compressed air (lots of particles)

Frequency in Hz

Intensity - the difference between rarefied and compressed (same as amplitude really) measured in dB (logarithmic scale)

Question 2

what is the range of human hearing?

Answer 2

Human hearing range 20 Hz - 20,000 Hz
Sound waves cause a one dimensional movement of the ear drums

Question 3

describe the gross anatomy of the ear

Answer 3

Very outer ear = pinna, very convoluted in order to locate whether sound is coming from above or below

External auditory canal leads to tympanic membrane/ eardrum, acts like a drum skin, this is the end of outer ear, beginning of middle ear

Middle ear contains your ossicles (three ear bones) with a membrane at the end - oval window

Next is inner ear, with the cochlear

Question 4

what are the three bones of the middle ear

Answer 4

The three ossicles, moving in from the tympanic membrane, are the malleus/hammer, the incus/anvil and the stapes/stirrup

Question 5

explain how the middle ear transduces sound

Answer 5

compression phase/peak of a sound wave pushes the tympanic membrane, which moves the malleus down and out and therefore pushes the incus out too, which causes the stapes to push the oval window inwards

This concentrates the force of the sound wave in order to actually move the fluid in the cochlea (as fluid has a greater impedance than air) otherwise it would take much greater intensities of sound to actually hear something, the air alone would barely move the fluid

The tympanic membrane is pulled back in the rarefaction phase of the sound wave, moving the malleus etc.. pulling the oval window out

Question 6

what is the function of the round window while the oval window moves in and out with the sound waves?

Answer 6

round window moves in opposite way to the oval window, to keep pressure balanced so as not to harm cells

Question 7

what are the three sections of the cochlea?

Answer 7

scala vestibuli - filled with perilymph (similar to ECF), connects to oval window at large end

scala media - filled with endolymph (very high in K+)

scala tympani - filled with perilymph and connects to round window at large end

Question 8

what are the stria vascularis? how are they important in terms of hair cells?

Answer 8

cells in cochlea that actively pump K+ into the scala media endolymph, causing an accumulation of positive charge that creates the endocochlear potential of around +80 mV

Hair cells in organ of corti = resting membrane potential of -60mV, so this means there is a huge electrical gradient between the hair cells and the endolymph

Question 9

what is the organ of corti? include its features

Answer 9

lying between the scala media and tympani, the organ of corti has inner and outer hair cells with projections called stereocilia - hairs in hair bundles of short, medium and tall hairs

Question 10

how do inner hair cells work at the peak of a sound wave?

Answer 10

Peak of sound wave - excitation:

The peak of sound wave pushes oval window, fluid moves around the cochlea, moving the stereocilia one way (TOWARDS the tall hairs), which essentially moves the tip links out of the way of a mechanically-gated non selective cation channel (MET channels for mechanoelectrical transducer channels)

These are on the shorter row of stereocilia, the cell depolarises, voltage gated calcium channel opens, vesicles exocytosis, glutamate released and afferent neuron is activated

note - its a GRADED POTENTIAL in hair cell, then Ca influx at synapse releases glutamate cause EPSP in afferent neuron, if large enough = AP

Question 11

how do inner hair cells work at the trough of a sound wave?

Answer 11

n the trough/rarefaction of a sound wave when the oval window is pulled and fluid goes the other way, the hairs also move the other way closing the channel, hyperpolarizing it, so it’s like on/off etc…

VG-K+ channels are present in order to let K+ back out of the hair cell when sound isn’t present - repolarise the cell

Question 12

what happens in sustained sound?

Answer 12

cycle of depolarisation and hyperpolarisation at the sound frequency, generating pulses of NT release and therefore afferent activity

Question 13

what is the basilar membrane and how does it work?

Answer 13

organ of corti rests on the basilar membrane (with the tectorial membrane above)

It’s these that move up and down with the fluid, resulting in the side to side motion of the stereocilia

A particular sound frequency causes a ‘max movement’ of the basilar membrane at one location, called the ‘characteristic frequency location’ or CF (always same place for same frequency)

Question 14

the basilar membrane is narrow at one end and wide at thew other. Why?

Answer 14

The basilar membrane is narrow and stiffer at one end (base end), where higher frequency sounds cause movement (- they have a short wavelength, low energy)

the Apical end is wider and thinner, where lower frequencies are detected (longer wavelength higher energy, travels further)

Question 15

what is mean by the phrase ‘tonotopic organisation’ in terms of the basilar membrane?

Answer 15

Tonotopic organisation is like a piano - from low at one end to high at the other
Each ‘key’ represents an individual inner hair cell coding a narrow frequency band, if the piano had 3-4000 keys
It is the position of the active inner hair cell that encodes the sound frequency ( it’s position along the basilar membrane) - known as place-frequency code
Sound frequency is therefore not related to firing rate of the nerve fibres - this is used to encode sound intensity instead

Question 16

tonotopic organisation is P_____?

Answer 16

Tonotopic organisation preserved throughout the auditory pathway

Cochlea - auditory areas of brain stem, midbrain, all the way to the auditory cortex

Question 17

what is the place-frequency code?

Answer 17

Each inner hair cell is like a key on a piano, in that it is responsible for a narrow frequency band

the position of the active inner hair cell therefore encodes the sound frequency (it’s position along the basilar membrane) - known as place-frequency code

Sound frequency is therefore NOT related to firing rate of the nerve fibres - this is used to encode sound intensity instead

Question 18

what do outer hair cells do?

Answer 18

Don’t have main sensory role, they amplify the cochlea

They shorten and lengthen in time with the sound frequency - known as electromotility

Question 19

why do outer hair cells have only one or two afferent fibres?

Answer 19

Only one or two afferents as less information is being passed onto the brain

Question 20

what do outer hair cells have, in terms of features, in common vs different to inner hair cells?

Answer 20

in common - Still have transducer channels at tips, with tip links between,
Still have VG-K+ channels in the membrane for repolarization

different - their hair bundle is V shaped
PRESTIN - a protein in their cell membrane that allows them to shorten when depolarised and lengthen when hyperpolarised

Question 21

why is the cochlea spiralled?

Answer 21

the extend hearing range - allows for more sensory cells in a small space

Question 22

what is the role of the outer hair cells?

Answer 22

their electromotility amplifies the movement of the basilar membrane at the certain ‘characteristic frequency’ region, increasing stimulation of the inner hair cells (like positive feedback?)

at rest, there is a transducer current where the cell is at -40 mV, then the OHCs depolarise in excitatory bundle deflection and shorten…