L12 - Physiology of the Ear

Question 1

What are pinnae? (outside of ear)

Answer 1

• Help to pick up sound over large area and concentrate over the eardrum, can be moved around
• Can also be stationary, those are not horn shaped
• Increase sound level and sensitivity in hearing in humans
• Mammals have pinnae = good at high frequency hearing and can focus sound, others do not and so have a smaller range and can’t cope with longer wavelengths

Question 2

Why are owls different from other birds?

Answer 2

Have a structure analogous to pinnae as eyes have sound reflection, makes them a bigger head

Question 3

What is the middle ear? (ossicles)

Answer 3

• Cochlea (inner ear) is full of fluid and can detect vibration
• Sound must get from outside and into the fluid and sound is reflected from water extremely
• Made of three bones: mechanical system and take weak waves in the air and turn them into forceful waves to enter the fluid
• Small area making small movements and large area making large movements to increase force conducts airbourne sound into cochlear fluid making hearing 30db more sensitive
• Bones are Malleus, Incus and Stapes
• Differs in mammals and birds: birds have one bone
• Evolution of 3-bone middle ear allowed for more hearing range

Question 3

What is the reason for the ear canal?

Answer 3

• Protect the eardrum: long tube surrounded by bone
• Has a resonance frequency = makes things less intense at the eardrum
• Responsible for the peak sensitivity in absolute threshold

Question 4

Why are mammals hearing similar but shifted along?

Answer 4

• Lowest frequency determines the volume limit
• Highest frequency determined by mass limit (bigger bones if bigger animal)
• Smaller the bones, the higher the frequency you can hear

Question 5

Describe the cochlea:

Answer 5

• Cochlea has a lot of turns = in a spiral structure
• Bottom part = scala tympani, middle = scala media, top = scala vestibuli
• Top and bottom are joint together
• Sound will travel in circular pattern all the way up from the bottom to the top, exit through a hole in the top called the helicotrema and back down
• Fluid is not compressible, pressure wave has to go somewhere = membrane called round window opens into middle ear so it can move back and forth to release pressure waves
• As sound moves, it makes the basilar membranes move up and down
• Cell fibres from auditory nerve follow the pattern of the sound in the cochlea = called the spiral ganglia
• High frequencies excite the base and low frequencies excite the apex, the membranes sort the frequencies out, they excite at different places in the cochlea

Question 6

How is the basilar membrane like a harp?

Answer 6

• Harp has short light strings at one end and long, heavy ones at the other and results in different notes
• Similarly, the basilar membrane is narrow, light and stiff at the bottom and is wide, heavy and floppy at the top
• If you drop something in the room, harp strings will vibrate in response to the frequency of the drop = same as cochlea = called passive resonance
• Mammals have this change systematically in their basilar membrane

Question 7

What is inside a turn inside the cochlea?

Answer 7

• Scala tympani and scala vestibuli are joined and have the perilymph as the liquid (in the lymphatic system)
• Scala media has endolymph as the stria vascularis pumps potassium ions into the ears causing a charge difference (100mv) = feeds nerve fibres to basilar membrane to the organ of Corti

Question 8

What is the organ of Corti?

Answer 8

• Have 2 kinds of hair cells: different shapes and are positioned differently, one inner row and then 3 rows of outer hair cells - have hairs (stereocilia) and detect vibrations/movements
• Tectorial Membrane sitting on top: a flap
• Stiff fibres in the middle and tunnel of Corti
• Nerves going to the inner and outer hair cells
• Afferent: extends info from outside to brain
• Efferent: from the brain to periphery

Question 9

How does the Organ of Corti work?

Answer 9

• Sound waves travels through the cochlea
• Passes above/below basilar membrane causing it to vibrate
• Oval window = pressure pushed in above basilar membrane, and round window releasing pressure move complementarily to each other (moves up and down)
• Diff places along length of it go up/down depending on frequency

Question 10

What is the effect of basilar membrane motion on the bundles of stereocilia?

Answer 10

• Pressure waves come in, they also leave from the other window
• Tectorial membrane has a shearing action compared to the basilar membrane
• This excites a hair cell and they respond electrical and react to dendrites attaches = allows transduction
• Hairs themselves: arranged in short to tall links and tip links each stereocilium in the bundle to the next, they pull open ion channels, allowing K+ into the cell to change voltage and respond = allows auditory to react quickly = dependent on pressure

Question 11

What do OHC do?

Answer 11

• Only receive efferent inputs = do not info to brain
• They also move in response to change in voltage = change length depending on voltage
• Second mechanism of tuning in the ear: Outer hair cells that like particular frequencies and change length - push the basilar membrane and make it move more - amplification system

Question 12

How is it an amplification effect?

Answer 12

• Sound comes in = basilar membrane moves a bit = inner hair cells change their voltage = excite auditory nerve
• Outer hair cells = change voltage and change length = push basilar membrane and move it more than it would do = more inner hair cell activation
• Outer hair cells allow 60db sensitivity (positive feedback amplification)

Question 13

What does the auditory nerve do?

Answer 13

• Has a lot of axons along its length = takes info to the brain
• Basilar membrane analyse sounds into diff frequencies = auditory nerve fibre picks up info about particular frequencies
• In a guinea pig, you can see the range of frequencies and intensities auditory nerve fibres responded to = some nerves are more sensitive to certain frequencies/intensities
• Increase sound level = respond to a wider range of frequencies

Question 14

How is this data collected? (for auditory nerve)

Answer 14

• Sweep sound across frequency at diff sound levels and record all spikes that go up auditory nerve fibre
• Low level sound = as it goes up in frequency = narrow range of frequency where nerve fires
• Increase sound level = wider range of frequency where nerve fires

Question 15

What are the two mechanisms for tuning?

Answer 15

• Passive: broad tuning = less sensitive and less precise
• Active: more finely tuned = spikey bit with the outer hair cells with amplification

Question 16

What is phase locking?

Answer 16

• Nerves and fibres connecting to diff parts of cochlea and responding to diff frequencies = allowing a frequency intensity map
• Auditory nerve fibres are unique as they respond to the timing of the waveform they are exposed to
• Wave form is being transmitted to the auditory nerve with its information = important for sound localisation
• Phase locking is critical for encoding of integral time delays

Question 17

How do nerve fibres represent entire waveform (because of refractory period)

Answer 17

• Have more than one nerve fibre to fire independently = can represent more cycles of the waveform = called the volley principle
• Lots of nerve fibres combine to send a volley of fire up auditory nerve

Question 18

What is auditory transduction (summary)

Answer 18

1) Sound waves set the ear drum in motion (tympanic membrane)
2) Vibrations passed through middle ear bones (ossicles) to inner ear
3) Pressure waves in inner-ear fluid set basilar membrane into motion - diff frequencies in diff places
4) Stereocilia of hair cells on basilar membrane flex back/forth
5) Tip links of stereocilia pull open and close ion channels
6) Current flows into hair cells in time to vibration
7) Outer Hair Cells change length amplifying basilar membrane vibration
8) Inner Hair Cells stimulate phase-locked action potentials on auditory nerve