Auditory System Flashcards
How does the sound look like?
concentric waves
sinusoidal wave
Amplitude
Volume
Frequency
Pitch
External ear
Collects sounds and boosts frequencies around 3 kHz (human speech 2-5 kHz)
Middle ear
- 200-fold sound energy amplification
- Pressure focus from large tympanic membrane to small oval window
- ossicles
Inner ear
Cochlea: sound transduction to neural signals
How does Cochlea detect sound?
The cochlea contains sound detectors: hair cells
What is the importance of the basilar membrane?
Movement of the basilar membrane bends the stereocilia of the hair cells.
• Basilar membrane vibrates with the frequency of sound
•Tectorial membrane sits just above stereocilia of the hair cells, forming a cover
•Inner and outer hair cells: transduce/convert mechanical motion of basilar membrane into neural signals
The cochlea: two compartments with different cation concentrations
Scala media
Scale Tympani
Scala media
Scala media contains endolymph that has high K+ and low Na+concentration, this drives K+ into the cells
Scala tympani
Scala tympani contains perilymph (high Na+, low K+)
Organ of Corti
potential difference:125mV
Large driving force for K+ to enter the cell
Hair cells have high K+concentration
K+ can both hyperpolarize and depolarize
What happens during stereocilia movement?
Tip-links open and close K+ ion channels
Ribbon synapses
enable neurons to transmit signals over a dynamic range of several orders of magnitude in intensity.
The hair bundle on a cochlear hair cell senses variations in sound-induced pressure in the cochlea. The resulting, voltage-dependent signal is transduced and passed via the afferent nerve fibre and the auditory nerve to the brain, where it is perceived as sound.
Changes in air pressure are transmitted to
changes in fluid pressure in inner ear
Sound energy is filtered and amplified in all 3 parts:
in external ear (shape), middle (ossicles) and inner ear
Sound transduction
These changes induce movement of basilar membrane and stereocilia of hair cells
Hair cells contain mechanoreceptors that open and permit K+ flow into the cell.
This causes depolarization because of differences in K+ concentration in endolymph and perilymph.
The changes in membrane potential lead to neurotransmitter release and APs in afferent fibers
Outer haircells
Signal amplification
Inner hair cells
detect movements of atomic dimensions (0.3 nm) and millisecond precision and comprise 95% of fibers projecting to the brain
Signal amplification
Basilar membrane vibrates 100-fold more than predicted Outer cells (3 rows) receive projections from superior olive, adjust the basilar membrane motion and act as an amplifier
Sound amplification
Hair cells change shape in response to voltage change
What senses voltage changes and contracts in response?
Prestin is the motor protein that senses voltage changes and contracts in response
In which 3 parts is sound energy filtered and amplified?
In external ear by the shape of pinna and meatus.
In middle ear by the ossicles.
In inner ear the vibration of the basilar membrane is amplified by outer hair cells and motor protein prestin
Tonotopy
Tuning of basilar membrane or tonotopy: cochlea as frequency analyzer.
Hair cells at the base of cochlea are sensitive to high-frequency sounds,hair cells at the apical end to low-frequency sounds.
What does tonotopy allow?
Tonotopy allows for conducting higher frequency sounds
Auditory pathway: Why do fibers cross?
Significant number of nerve fibers cross the brain and make connections with neurons on the side opposite from the side of the ear in which they begin. This happens very early on in the auditory system. Inter-aural comparisons are an important source of information for the auditory system about where a sound came from
How do we locate sound? (1)
Medial Superior Olive computes the location of a sound by interaural time differences.
Inputs from the two ears into MSO are delayed by various amounts relative to one another by the length of the axons
How do we locate sound? (2)
Lateral superior olive encodes sound location through interaural intensity differences
interaural intensity differences
The projections from one ear will excite the ipsilateral (same side) LSO and at the same time inhibit contralateral LSO (opposite side)This arrangement results in a good detection of sound location when it is directly lateral to the listenerIt takes two LSOs to represent horizontal positions
What about high frequencies?
Time differences are very difficult to detect
•There is a different mechanism – detection of interaural intensitydifferences
•Head acts as an acoustical obstacle, “shadow” of lower intensity
Sound location detection by 2 mechanisms:
- Medial superior olive detects interaural time differences: it contains neuronal map where each neuron responds most strongly when APs from two ears arrive simultaneously
- Lateral superior olive detects interaural intensity differences of sounds: ipsilateral excitation and at the same time contralateral inhibition
How is tonotopical representation achieved by the auditory system?
- Different parts of cochlea respond to different frequencies of sound: high frequencies at the base and low frequencies at the apex
- Both medial and lateral superior olives share this feature, afferent fibers from high and low frequency areas in cochlea project selectively to high and low frequency parts in superior olives
Inferior Colliculus
Inferior Colliculus integrates the cues for localizing sounds in space and contains topographical representation of the audible space
Auditory thalamus
Auditory thalamus detects specific combinations of spectral and temporal combinations of sounds (important for speech recognition)
How is the primary auditory cortex organized and what is its function?
Auditory cortex is tonotopically organized and is essential for frequency discrimination and sound localization, also plays an important role in processing of communication sounds
What are the belt areas?
The belt areas of the auditory cortex have a less strict tonotopic organization and also process complex sounds, such as those that mediate communication
What does the primary auditory cortex contain?
- Contains patches of EE cells (excited by inputs from 2 ears) and EI cells (excited by input from one ear and inhibited by input from the other ear)
- Remind the ocular dominance columns in visual cortex
- Combination-sensitive neurons – neurons that respond to specific sound combinations. (Remind simple cells in visual cortex)
- Cells that respond to specific temporal sequences of sounds (as in communication or music)
Just like in visual system, auditory system contains 2 streams from primary auditory cortex:
- ventral (object processing) and
* dorsal (space and motion)
How are Increasingly complex combinations of sound processed?
increasingly complex combinations of sounds are processed from cochlea to auditory cortex
