Hearing

The Ability to Sense Vibrations in the Air

Process known as

Mechanosensory Transduction

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

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

Sensitivity to Sound

• At threshold of hearing, the air molecules are moving 10 picometers.

• Hearing more sensitive than vision

Sound waves• Sound waves are periodic changes in air

pressure

• Sound is a sine wave moving up and down

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."

4 Features of Sound Waves

• Waveform=amplitude vs time

• Phase=completion of 1 cycle

• Amplitude=intensity=loudness decibels dB

• Frequency=cycles/second=pitch

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

Frequency of Sound Wave

• Humans hear 20 cycles/sec= Hz to 20,000 Hz

• Greatest sensitivity is 1000-4000 Hz

• Each spiral ganglion sensory neuron having a synapse with a hair cell is “tuned in” or most sensitive to a particular frequency

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

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

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

Phase

• Used to locate sound in space by comparing the in and out of phase waveforms

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

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

Oval Window

• Oval window= connection of middle ear stapes bone with opening of cochlea

• Is a flexible membrane

• 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

Cochlea

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

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

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

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

Hair Cells

• Mechanoreceptors for vibration

• Have cilia which are deflected by vibrations

• Deflection change membrane potential of hair cells

George Van Bekesy

Nobel Prize in 1961

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

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

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

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

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

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

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

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

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

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

Contractions of Hair Cells

• Amplifies movement of basilar membrane

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

END

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

Afferent Connections

• Refer to hair cells sending info to spiral ganglion neurons that bring info to the CNS