Week 7 Flashcards
Middle ear
Two cubic centimeter cavity separating inner from outer ear
Middle ear contains three ossicles:
Malleus
Incus
Stapes
Malleus
Moves due to the vibration of the tympanic membrane (eardrum)
Incus
Transmits vibrations of malleus
Stapes
Transmit vibrations of incus to the inner ear via the oval window of the cochlea
The outer and middle ear are filled with _____.
Air
The inner ear is filled with ____ that is much ____ than air.
Fluid; Denser.
Pressure changes in the air transmit ____ Inyo the denser medium
Poorly
- Less than 1% of the vibration can transmit directly.
The ossicles act to ____ the vibration for better transmission to the fluid.
Amplify
- Can do this up to a factor 50.
Pinna
(Outer ear)
Helps with sound location and amplification and is responsible for steering signal into auditory canal.
Auditory Canal
Tube-like structure just long enough to protect the tympanic membrane (eardrum) at the closed end.
- sound waves reflect off the end of the canal and create resonance.
Acoustic reflex of the ossicles
Can reduce transmission of loud sounds by about 20 dB.
Middle ear muscles can dampen the ossicles’ vibration to protect the inner ear from potentially damaging the stimuli.
The main structuere of the inner ear is the ___.
cochlea
Cochlea
Where sound waves are traduced into neural signals.
- fluid-filled, snail-shaped.
- sound waves are transduced into neural signals within the cochlea.
- vibrates with motion of the stapes and eardrum.
-signals will be transmitted out of the cochlea via the auditory nerve.
The ___- pushes against the _____, transmitting atmospheric vibrations directly to the inner ear.
Stapes; oval window.
Structure of the cochlea
- Hard, bony
- Divided into the scala vestibule (above) and scala tympani (below) by the cochlear partition.
- Cochlear partition extends from the base (stapes end) to the apex (top end). Base > Apex
What is contained within the cochlear partition (in the scala media)?
Organ of Corti
Where does transduction occur?
In the organ of Corti
- sits on the basilar membrane
- basilar membrane vibrates in response to sound and supports the organ of Corti
- inner ((~3500) and outer hair cells (~12000) are the receptors used for hearing.
Transduction takes place by:
Stereocilia
Stereocilia
Hair cells
Heir cells bending in response to pressure changes from vibration of the Organ of Corti against the tectorial membrane or fluid surrounding it.
Movement of inner hair cell Stereocilia in one direction
Opens ion channels via tip links.
Movement in other direction closes the channels
Outer hair cells amplify this.
Two ways nerve fibers can signal frequency:
- Which/Where nerve fibers are responding.
- specific groups of hair cells on bail ar membrane activate a specific set of nerve fibers.
- place theory - How/When fibers are firing
- rate or pattern of filing of nerve impulses.
- temporal coding
Place theory
Georg Von Bekesy’s place theory of hearing says that the frequency of sound is indicated by the location/place (of hair cells) along the cochlea that has the highest firing rate.
Two ways Bekesy determined place theory:
- Through the direct observation of the vibrational properties of the basilar membrane in cadavers.
- By building a model of the cochlea using the physical properties of the basilar membrane.
Basilar Membrane
- The base of the membrane is 3 or 4 times narrower than at the apex and 100 times stiffer than at the apex.
- Both the model and observation (Bekesy) showed that the vibrating motion of the membrane is a traveling wave (like snapping a rope)
Envelope of the traveling wave
- Most of the basilar membrane vibrates in response to a traveling wave, but the displacement is largest in one place.
- Hair cells at this maximum point are stimulated the strongest, leading to the nerve fibers firing the most at this location.
- The envelope shows the entire displacement caused by a traveling wave and its peak.
Envelopes at different frequencies
- Show that different maximum vibration points, which indicate the place where most hair cell activity is predicted.
-**Low* frequencies activate hair cells toward the apex, while high frequencies activate hair cells near the base.
If I play a pure tone at 100 Hz, it will be transduced by ____ hair cells nearest the _____ of the cochlea.
Inner; Apex
Electrode recordings have supported Place Theory
- Electrodes were placed outside the cochlea of guinea pig while different pure frequency tones were presented.
- Cochlea shows an orderly map of frequencies along its length.
-Apex responds best to low frequencies.
-Base responds best to high frequencies.
-This is called a tonotopic map*
Neural frequency curves provide physiological support for Place Theory.
- Pure tones are used to determine the threshold for specific frequencies measured at single neurons.
- Record action potentials from one auditory nerve fiber along the cochlea and play tones at different frequencies.
- Plotting threshold for frequencies results in tuning curves.
Frequency to which the neuron is most sensitive (lowest threshold) is the:
Characteristic frequency
Auditory masking experiments
- Thresholds for a number of frequencies are determined.
- Then an intense masking frequency is presented at the same time that the thresholds for the original frequencies are re-determined.
- The masking effect (raised threshold) is seen at the masking tone’s frequency and spreads to higher frequencies more than lower ones.
Understanding the masking effect:
- This effect aligns with the place theory’s prediction about how he basilar membrane responds to sound..
Evaluating the Basilar Membrane
- Physical properties of the basilar membrane are key to understanding place theory.
- Base of the membrane is narrower and stiffer, leading to a traveling wave for sound transduction.
Outer hair cells, cochlear amplifier, responds to sound by …
Slight tilting and a change in length.
These cells enhance the sensitivity and frequency selectivity of the cochlea.
Better Bekesy
- Original place theory predicted less sensitivity to frequency difference.
- Live membrane research shows outer hair cells respond by tilting and changing length, acting as a cochlear amplifier.
Cochlear Amplifier
Outer hair cells enhance the ear’s sensitivity and selectivity for sound frequencies
Encoding sound intensity
Sound intensity is encoded by movement of the basilar membrane and the activity of the cochlear amplifier.
Encoding Sound Intensity:
Inner hair cells are activated _____ with_______ sound intensity, and neighboring cells also respond ___.
More; increased; more
Encoding Sound Intensity
Impact of outer Hair Cells:
Selective destruction of outer hair cells shows a decrease in the firing rate of inner hair cells, indicating their role in amplifying characteristic frequency.
Complex Tone Analysis
- The basilar membrane acts like an acoustic prism, separating complex tones into their harmonic components.
- Each peak in membrane vibration corresponds to a harmonic frequency.
Fourier Analysis in Hearing
- Fourier analysis can break down complex waves into individual components, mirroring how the ear processes complex sounds.
Phase Locking
- Nerve fibers fire in bursts at or near the peak of the traveling wave, locking in phase with the wave.
- The refractory period post-firing establishes a limit for individual signaling capacity.
Auditory Nerve Fiber Firing
Illustration of inner hair cells with auditory nerve fiber firing patterns synchronized with the sound stimulus.
Fibers may fire at different ____, allowing a group fo neurons to encode a broader range of frequencies.
Peaks
Combining neuron activity can create frequencies beyond a _____ neuron’s capacity
Single
Place coding provides information for
the broad frequency range of hearing.
Temporal coding with phase locking us effective for frequencies up to ___ kHz.
4-5
Coding Frequency and Pitch
Both coding strategies work together, with temporal coding playing a significant role in pitch perception.
Types of Hearing Loss
Conductive hearing loss
Sensineural hearing loss
Conductive hearing loss
Involves blockages or garage to the outer or middle ear structures
Sensineural hearing loss
Results from damage to the hair cells or auditory nerve
Chemical Ablation
Selective destroying of outer hair cells
If I play a complex sound into teh ear, teh traveling wave vibrating the basilar membrane will have:
A. A single peak at the highest amplitude frequency
B. A peak for every frequency
C. No peak
D. A peak at the fundamental frequency
B
Amplitude modulated noise
The primary change in intensity results in the perception of pitch
Presbycusis (old hearing)
- Age related hear loss (ARHL)
- Greatest loss is at higher frequencies (>12 kHz)
- Affects men more
- May change with behavior.
What can presbycusis be cause by?
Accumulated exposure to damaging noises o drugs over a lifetime.
Mosquito Mk4
Old person use mosquito noise to keep away teenagers lol.
Mosquito ringtone
Teachers (ppl older than 30) can’t hear text/call ringtone.
Noise-Induced Hearing Loss
- Loud Noise can severely damage the Organ of Corti (loss of inner and outer cells)
- Damage associated with certain types of work (factory, heavy machinery)
- Leisure noise can also cause hearing loss (earbuds!!!)
Pitch neurons
Cell just outside A1 and responds to complex tones with the same fundamental frequency
Bendor & Wang
What happened when Norman-Haignere used fMRI to measure responses to complex tones perceived as a pitch (110Hz) and frequency-matched noise?
- The noise stimuli contained all the frequencies, but wasn’t perceived as having a specific pitch.
- Areas in anterior auditory cortex were more responsive to pitch.
