Lecture 6: Auditory System Flashcards
Describe the physics of sound
Sound is the result of pressure waves produced by vibrating air molecules.
As an object vibrates it produces first condensation of air molecules and than rarefication as the object moves in the opposite direction. This wave of increasing and than decreasing pressure spreads from the source in three dimensions.
Properties of sound
- *Frequency** – pitch; cycles of waves per second (Hz)
- *Amplitude** – loudness; expressed on the log scale of decibels (dB)
- *Waveform** – amplitude over time; can be plotted as a sine wave
Human sound perception is between 20 Hz and 20 kHz, although the max for most adults is around 15 kHz.
What range is the human ear designed to be particularly sensitive to?
3-5 Hz, a range that is particularly important to human hearing
What is the typical hearing range of animals that echolocate
Animals that echolocate, use very high frequencies (bat range is 20 kHz to 200 kHz).
Trend in animal size to frequency range
Different animals hear in different frequency ranges. Smaller animals with smaller hearing apparatus operate in higher frequency ranges, with the reverse true for larger animals. Like humans, frequency ranges corresponding to vocalizations are emphasized (mice vs. elephants).
Describe the structure of the external ear.
Focuses sound energy onto the eardrum, particularly boosts sounds 3 kHz range (as much as a 100-fold).
Selectively filters sound frequencies in order to detect elevation of sound source. Selectively permits more high frequency sounds from an elevated source, which the brain can translate into positional information.
Describe the middle ear and function.
Converts sound wave features (pitch, volume) carried in the air to the aqueous environment of the inner ear.
Air is low impedance (is not resistance to sound wave propagation) whereas water is high impedance (more resistive). Normally, sound energy would be almost entirely reflected in a transition from air to aqueous medium.
This is avoided in the ear by
(1) concentrating all the sound energy to a specific spot, the oval window, where sound is transmitted to the inner ear
(2) The lever action of the small ear bones (ossicles) that conduct sound from the tympanic membrane to the oval window.
Conductive hearing loss
Caused by damage to the middle or external ear that compromises its ability to transfer and amplify sound to the inner ear.
Sensorineural hearing loss
due to damage/dysfunction of sound receptors in the inner ear (hair cells) or to the auditory nerve
Describe the cochlea
A fluid-filled ,coiled tube that narrows from the basal to the apical end. Contains the actual sensory neurons (hair cells).
Inside the cochlea, the tube is split down the middle by the cochlear partition, which is flexible and supports the basilar and tectorial membrane (more on these last two later).
Sound causes the ear bones (ossicles) to push on the oval window. This pushing on the oval window causes the fluid of the inner ear to move/displace. This causes the round window to move outward, which deforms the cochlear partition (vibrates).
This movement/vibration of the *cochlear *partition is translated by the basilar membrane, which leads to the stimulation of the hair cells.
Movement of the basilar membrane bends the _____ of the hair cells. Describe the process
“stereocilia”
As the basilar membrane vibrates (↑↓) this causes a relative lateral movement (shearing) between the hair
cells and the tectorial membrane above.
Hair cells are the actual auditory receptive cell.
This movement causes the stereocilia of the hair cells to bend.
Upward motion depolarizes the hair cells
Downward motion hyperpolarizes the hair cells
Describe the hair bundle in cochlear and vestibular hair cells.
Consist of many (30-100s) stereocilia and a single kinocilium
Kinocilium is only true cilia (9 + 2 microtubule structure); precise function isn’t known
Stereocilia are the actual transducers of auditory stimulus
Not true cilia; have a simple actin fiber based structure
Taper as they enter the hair cell; allows them to pivot
Are graded in height and arranged in rows with shorter ones in front of taller ones
Tip links – a fiber that runs from the tip of one stereocilium to the tip of an adjacent taller one
How do hair cells tranduce sound into electrical signals (mechanoelectrical transduction)?
When the stereocilia move in the “taller” direction, the tip links are stretched and open cation-selective ion channels which depolarize the hair cell (stretch-activate ion channels). Note that depolarization is the result of K+ influx.
When the stereocilia move in the opposite direction, the tip links relax and the ion channels partially close, repolarizing the cell.
Hair cells produce grade potentials (no action potentials), so synaptic transmission is proportional to level of depolarization (via activation of voltage-gated Ca2+ channels).
At rest, a portion of the tip channels are open, so that there is some degree of membrane depolarization/synaptic transmission.
When the stereocilia bend in the “shorter” direction, more channels are closed and the hair cell hyperpolarizes.
This means that the hair cells have biphasic responses to sound waves.
depolarization of hair cells in the cochlea is the result of what ion movement?
result of K+ influx in the stereocilia region
Note: the endolymph is high in K
Explain how we are able to have hair cells depolarize with K+ influx and why this doesn’t change the action potential propagation.
The apical and basal surfaces of the hair cells are separated by tight junctions.
The apical surface in bathed in a high K+, low Na+ solution (endolymph) and the external K+ concentration is quite higher then the K+ concentration inside the hair cell.
This favors a passive flow of K+ into the cell through the stereocilia channels, depolarizing the hair cell.
The basal surface of the hair cell is bathed in a more typical low K+, high Na+ solution (perilymph) and the K+ concentration inside the hair cell is higher compared to the external K+ concentration of the basal solution.
So K+ flows out of the hair cell on the basal side through somatic voltage-gate K+ channels.
This along with K+ efflux through Ca2+-activate K+ channels mediates repolarization of the hair cell.
perilymph
low K
surrounds basal membrane
Describe what happens in the cochlea when different intensities of sound hit the hair cells
First (B)
Displacement of the hair bundles produces a depolarization of the hair cell. The greater the displacement/movement, the greater the depolarization.
Now (A)
As the hair bundles vibrate with the frequency of the sound wave, the membrane potential of the hair cell oscillates (depolarizes and repolarizes) in synch with the vibration frequency.
Pitch is communicated via this hyperpolarization or depolarization. Depends on if hair cell is moving toward kinocilia or away from kino.
What property of the wave changes when it reaches an area of the cochlea tuned for that frequency?
amplitude increase!
Describe how cochlear implants can help reduce sensorineural hearing loss.
Can be used in cases where people have lost cochlear hair cells, but the auditory nerve is still intact.
Consists of a microphone, a digital signal processor that converts sound into an electrical signal, a multi-electrode stimulation array.
The array threads through the cochlea where it stimulates the auditory nerve endings in a tonotopic manner.
The digital processor directs which parts of the stimulating electrode are activated; remember the position in the cochlea that is maximally sensitive to a given frequency changes along its length.
The array uses only the tonotopic organization of the cochlea and cannot reproduce the frequency transduction properties of the hair cells.
What are the major auditory pathways?
Bipolar cells in the spiral ganglion project a peripheral process to a hair cell & a central process to the cochlear nuclei in the medulla (via the auditory nerve/vestibulocochlear nerve/ CN VIII).
There are three distinct cochlear nuclei; each has distinct sensory processing properties, but the tonotopic organization of the cochlear input is maintained to each of these nuclei.
Ascending projections from the cochlear pathways actually bifurcate in the brainstem (pons) to innervate targets on both the ipsilateral and contralateral side.
Because of this bilateral innervation, central lesions of the central auditory pathways always produce loss of hearing in both ear.
CN VIII via the cochlea (auditory nerve, vestibulocochlear nerve) enters at medulla
signal bifurcates at superior olive in the midpons
- some interneurons invoved*
- At medial geniculate nucleus, projects to primary auditory cortex*
Show how MSO computes the location of a sound by interaural time differences.
