1. Hearing The Ability to Sense Vibrations in the Air
Process known as
Mechanosensory Transduction

2. Mechanosensory Transduction
The process by which mechanical energy in the vibration of sound waves traveling through air are converted into electrical signals that the brain can process and understand

3. Sound Audible variations in air pressure
Objects that move make sound; as the object moves toward a patch of air it compresses (increases density of air molecules)
Object moving away from a patch of air it rarefies (makes molecules less dense)
Speed of sound travels at 343 m/sec 767 mph

4. Sensitivity to Sound
At threshold of hearing, the air molecules are moving 10 picometers.
Hearing more sensitive than vision

5. Sound waves Sound waves are periodic changes in air pressure
Sound is a sine wave moving up and down

6. Sonic Boom
aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship's bow.
When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or "release" point.
The sound heard on the ground as a "sonic boom" is the sudden onset and release of pressure after the buildup by the shock wave or "peak overpressure."

7. 4 Features of Sound Waves
Waveform=amplitude vs time
Phase=completion of 1 cycle
Amplitude=intensity=loudness decibels dB
Frequency=cycles/second=pitch

8. Frequency of soundwave
is the number of compressed/rarefied air patches that move past ear/second.
Audible range 20 Hz-20,000 Hz
Frequency of sound wave determines the Pitch:
Low organ note=20Hz; high piccolo note is 10K Hz
Double the frequency raise the pitch one octave

9. Frequency of Sound Wave
Humans hear 20 cycles/sec= Hz to 20,000 Hz
Greatest sensitivity is Hz
Each spiral ganglion sensory neuron having a synapse with a hair cell is “tuned in” or most sensitive to a particular frequency

11. WaveFrequency= Pitch Ultrasound = above 20KHz Infrasound = below 20 Hz
Unheard sounds can have subconscious effects causing dizziness, headache, nausea (carsick) due to low frequency sound of car at high speed
High intensity low frequency sound can damage internal organs by resonating the body cavity

12. WaveHeight= Intensity/Loudness=
Difference in pressure between the peak compressed and peak rarefied patch of air
Determines loudness of sound expressed in decibels
Loud sounds have higher intensity
To double loudness, intensity increases 10fold

13. Loudness-decibels Logarithmic scale 20dB=whisper
65 dB=normal speaking voice
100dB=near jet engine
120dB=pain
Is represented by the height amplitude sound wave
Encoded by number of neurons activated not height of spike or number of spikes/time

14. Phase
Used to locate sound in space by comparing the in and out of phase waveforms

15. Anatomy of the EAR Mechanosensory cells=hair cells
Located within the cochlea=a spiral shaped bony enclosure filled with fluid.
Air vibrations impact the tympanic membrane stretched across the ear canal.
Transmitted to cochlea through 3 bones in middle ear

18. Ossicle Amplification
Ossicles amplify the sound wave in air to produce a force on the oval window 5 times greater than on the tympanic membrane so that the fluid in the cochlea is moved
stapes transduces air waves into water waves since the cochlea is a fluid filled chamber
1000 ft/sec sound through air
5000 ft/sec sound through water

19. Oval Window
Oval window= connection of middle ear stapes bone with opening of cochlea
Is a flexible membrane

20. Cochlea Separated into 2 regions by the basilar membrane.
A pressure wave reaches the oval window and pushes it inward and increases pressure above the basilar membrane
Basilar membrane moves downward as pressure is released by bulging out the round window at base of cochlea

21. Cochlear Compartments
Scala vestibuli is connected to oval window where sound waves enter cochlea
Scale tympani is compartment connected to round window
Intervening compartment is scala media that is bounded by basilar and vestibular membranes

22. Basilar Membrane Architecture
Narrow at base near oval window
Wide at apex
Hair cells sit along the basilar membrane, have cilia will depolarize to different extents in response to frequency of sound wave
High frequency hair cells respond maximally to high frequency sound with high frequency oscillation of membrane potential

24. Basilar Membrane
Moves up and down in response to waves of pressure impinging on oval window and transmitted through to round window.
Hair cells sit atop the basilar membrane
Hair cells connect to sensory neurons that live in spiral ganglion inside cochlea
Axon travels to CNS through auditory nerve ie CN8

25. Organ of Corti
Contains hair cells and rest on the basilar membrane and move up and down with sound waves
Composed of outer and inner hair cells
Three rows of outer hair cells, 1 row of inner hair cells. Exist at ratio of 5:1 or 20K to 4K

27. Hair Cells Mechanoreceptors for vibration
Have cilia which are deflected by vibrations
Deflection change membrane potential of hair cells

29. George Van Bekesy
Nobel Prize in 1961

34. Ion Channels
Changes in membrane potential of hair cells is caused by movement of cilia that changes ion permeability
Cilia on hair cells are tethered to each other at the tips by connecting filaments that act like springs that transmit tension to cation channels in membrane of cilia

37. NT release Potassium channels open
Potassium comes in and depolarizes membrane
Voltage sensitive calcium channels open
Increased calcium causes NT release onto spiral ganglion neurite

39. Cochlear Fluid Perilymph: Same ionic In Scala vestibuli and tympani
Composition as CSF
7mM K
140mM Na
Bathes the hair cells
Endolymph:
In Scala Media
Hi K concentration
150mM K
1mM Na
Bathes stereocilia of hair cells
Inward K+ flux leads to depolarization

40. Outer Hair Cells Outer are larger with more cilia
Embedded in overlying membrane ka tectorial membrane
Cilia are deflected by shearing forces generated by movements of basilar and tectorial membranes

41. Function of Outer Hair Cell
To amplify movement of tectorial membrane so that inner hair cell will respond more strongly
Outer hair cells do this by increasing and decreasing their length thus amplifying movement of basilar membrane at area that matches the frequency of sound

43. Inner Hair Cells Are not directly connected to tectorial membrane
Cilia move in response to motion of fluid within cochlea transmit caused by outer hair cells

44. Functions of Inner Hair Cell
Afferent cells that transmit information to the sensory neuron
90% of all synapses with sensory neurons occur with inner hair cells
1 inner hair cell can connect to 20 spiral ganglion neurons

45. Sensory Neuron Connections
Almost all spiral sensory ganglion neurons contact inner hair cells
15K HC & 30K SGN
20:1 ratio of inner cells to outer cells contacted by neurons
20 outer hair cells synapse with 1 neuron whereas 1 inner hair cell contact 5-10 neurons
More information reaches CNS from inner hair cell

46. Differences Between Outer and Inner Hair Cells
Outer HC are larger than Inner HC & have more cilia that attach to tectorial membrane above
Inner hair cells do not directly touch the tectorial membrane and fluid alone causes cilia movement

47. Active Movements
Hair cells elongate and shorten in height to amplify basilar membrane movements
Depolarization shortens the hair cell
Hyperpolarization lengthens hair cell
Involves changes in actin filament lengths and is not well understood

48. Contractions of Hair Cells
Amplifies movement of basilar membrane

50. Damage to Ear Mechanical or Neural
Mechanical=damage to tympanic membrane or ossification of middle ear bones
Neural=shearing off or sticking of hair cell cilia and damage to auditory nerve CN8
Birds regenerate hair cells humans do not

51. END

55. Hearing Sound Sound is a sine wave moving up and down
Frequency of the sine wave determines the pitch of sound
Each spiral ganglion nerve axon is tuned to respond to a particular frequency maximally and less well to higher and lower frequencies

56. Afferent Connections
Refer to hair cells sending info to spiral ganglion neurons that bring info to the CNS