Auditory Flashcards
What is the Auditory System?
Determines the frequency, intensity and other properties of sounds
Interprets meaning of sound
Identifies source of sounds - sound localization
What is sound?
Audible variations in air pressure
- mechanical energy measured as the calculated pressure that a wave makes against an object (ear drum).
Sound Waves
Produced by vibrations of air molecules
Tonotopy
Mapping of sound frequencies on basilar membrane - Principle of adequate stimulus
Frequency Coding at Basilar membrane
Hair cells along basilar membrane have receptive fields differing in frequency preference
Frequency Coding or Tonotopy
Neural encoding of pitch (place codes)
Base of Basilar Membrane
Short and Stiff Fibers
Apex of Basilar Membrane
Long Floppy Fibers
Resonance
The tendency of a system to oscillate at a greater amplitude at some frequencies than at others.
Resonant Frequency
Small periodic driving forces can produce large amplitude oscillations because the system stores vibrational energy
High Frequency
Waves vibrate stiff fibers best (at base): travel short distance
Low Frequency
Waves vibrate floppy fibers best (at apex): travel entire distance
TRPA Channel
A family of transient receptor potential ion channels
- TRP channel blocks K+ ions and others @ tip links
- Stretch-activated channel opens when mechanically stimulated
Perilymph
In the Scala Vestibuli and Scala Tympani
Low K+, High Na+
Endolymph
In Scala Media
High K+, Low Na+
Endocochlear Potential
Endolymph is 80 mV more positive than perilymph
Rapid Responsiveness ( Temporal Fidelity)
TRPA1 channels respond to stretch with 10 us speed, faithfully traduced high frequency signals
TRPA1 Channel Sensitivity and Range
High Sensitivity
Range: soft to loud sounds (displaces stereocilia from 0.3 nm to 20nm)
Biphasic Receptor Potential
Mimics sound waves - contains bi-polarity that resembles the peak and trough of a sound wave
Inner Hair Cells
Type of Spiral Ganglion cell
95% of SGC receive auditory information from inner HCs
High Fidelity
Myelinated Fibers
Outer Hair Cells
Type of Spiral Ganglion Cell
5% of SGC receive auditory info from outer hair cells
Many HCs feed to one SGC - low fidelity
Unmyelinated Fibers
Primary Auditory Area
Brodmann’s Area 41
Secondary Auditory Area
Brodmann’s Area 42
Isofrequency Bands
Cortical columns with different tonal receptive fields
Frequency Coding in CNS
Basilar Membrane (PNS)-> Spiral Ganglion -> Auditory Nerve -> Cochlear Nucleus (CNS)
Binaural Auditory Neurons
Found at Superior Olive and above.
Each cochlea is bilaterally represented in the brain.
Receptive Fields of Central Auditory pathway
Complexity of stimuli detected increases as you ascend the pathway : Cochlear nucleus and MGN
Further processing
Inferior Colliculus sends signals to superior Colliculus (integration of auditory and visual info) and cerebellum
Extensive Feedback from Central Auditory pathway
- Descending Efferents from Brainstem (to superior olive) contact and modulate OHCs in cochlea
- Efferent storm auditory cortex send axons to MGN and inferior Colliculus
Ventral Cochlear Nucleus
Receives auditory signals ipsilaterally
- if severed, you will e deaf on the same side
What are the 3 mechanisms of Intensity Coding?
- Stronger Displacement - increase firing rate to affected sensory n. (Temporal summation)
- Wider area displaced - more auditory sensory n. activated (spatial summation)
- Special Hair Cells that have a High Threshold - only intense sound will make the AP Fire.
Place Code
Displacement of Basilar Membrane = a function of pitch (frequency) and intensity
- can interfere with Tonotopy because louder sounds can have a different frequency
Phase Locking
Consistent firing of a cell at the same phase of a sound wave
- alt method for frequency coding
- works well at low frequencies (20 - 200Hz)
Phase Locking with Volley
@500Hz - 4kHz frequencies are represented by pooled activity of several auditory n, each member firing in a phase locked manner
- shared wave among population of neurons
Very Low Frequency sound coding mechanism
20 - 200 Hz
Phase Locking
Intermediate Frequency Sounds coding mechanism
Phase Locking (200Hz - 500Hz [limit for phase locking])
Phase Locking with Volley (500Hz - 4kHz)
Tonotopy
High Frequency sound coding Mechanism
Tonotopy (>4kHz - 20 kHz)
Duplex Theory of Localization
Two Different Processes: Interaural Time Delay and Interaural Intensity Difference
Interaural Time Delay (ITD)
Sudden sounds of all frequencies and continuous sounds of 20 - 2000Hz
Interaural Intensity Difference
Continuous Sounds with high frequencies (2 - 20kHz)
Sound Shadow
Head casts a sounds shadow to the ear further from the sound, Lower intensity sound at one ear is a cue that the origin of sound is in the other direction
