L5-L7 Flashcards
How is sound produced?
physical vibration of objects (a purely mechanical phenomena)
Compression vs rarefaction
air molecules bunch together (high air pressure); air molecules spread apart (low air pressure)
Frequency
rate of fluctuation of sound pressure measured in cycles/second or Hertz (Hz)
Phase
part of the cycle the sound pressure wave has reached at a given point in time; measured in degrees until 360°; often used to compare the timing of 2 sound waves
Amplitude
maximum pressure change of wave above normal atmospheric pressure that determines sound intensity
Pure tones
represented by single sine wave and produced by a tuning fork, wherein sound pressure level corresponds to loudness and frequency corresponds to pitch
Fourier’s theorem
most sounds are complex and can be described as a set of sine waves
Fourier analysis
mathematical procedure for separating a complex pattern into component sine waves that vary over time (hearing) and space (vision)
Fundamental frequency
lowest sine-wave frequency in a complex sound that usually determines perceived pitch
Harmonics
higher frequency sine-wave components; integer multiples of fundamental frequency
What do differences in frequency and amplitude in harmonics determine?
the psychological attribute of quality or timbre (i.e. explains why instruments sound different when playing the same note)
Function of the shape of the pinna
helps with sound localization
Function of the length and shape of the ear canal
enhances sounds 2000-6000 Hz (range of frequency where humans are most sensitive)
Function of the middle ear
impedance matching and protecting the inner ear from potentially harmful loud sounds through acoustic reflex
Impedance matching
middle ear amplifies sound energy to reduce loss due reflection at the oval window (air/fluid boundary)
Acoustic reflex
tensor tympani and stapedius muscles contract to reduce the magnitude of the auditory signal transmitted to the inner ear, in response to prolonged sounds
Function of the eustachian tube
equalizes air pressure between the middle and outer ear
3 canals in the inner ear
vestibular canal (scala vestibuli), middle canal (cochlear duct or scala media), tympanic canal (scala tympani)
What do the vestibular and tympanic canals contain?
fluid called perilymph
What does the middle canal contain?
fluid called endolymph and organ of corti that contains auditory receptors (where mechanical energy is transduced into neural signals)
Function of the tunnel of corti
locating 2 different hair cells: 1 row of 3500 inner hair cells on one side and 3 rows of ~10500 outer hair cells on the other
Location of inner hair cells vs outer hair cells in the cochlea
single row of stereocilia that rests against the tectorial membrane (gelatinous roof of the organ of corti); V-shape arrangement of stereocilia with the tallest embedded in the tectorial membrane
Function of inner hair cells
act as sensory receptors that convey information about sound to the brain
have no axons but release neurotransmitters at the synapse with afferent fibers
Function of outer hair cells
modulate sensitivity and frequency-tuning of cochlear partition
General function of hair cells (stereocilia)
transduction
How does movement of the basilar membrane cause transduction?
when it moves up and down, hair cells bend back and forth against the tectorial membrane, causing neurotransmitter release into the synapse with the dendrites of the auditory nerve fibers
What comprises the auditory nerve (part of vestibulocochlear nerve)?
also called cochlear nerve
axons of the auditory nerve fibers
2 kinds of auditory nerve fibers
afferent fibers and efferent fibers
Afferent fibers
carry sensory information to the CNS and fires action potentials; 90% is connected to the inner hair cells and 10% is connected to the outer hair cells
Efferent fibers
carry information from the CNS to the inner ear and comprises of most neurons synapsing with the outer hair cells
Graded potential vs action potential
slow change in membrane voltage that varies in size (not all-or-nothing) and occurs in hair cells; rapid depolarization that occurs in auditory nerve fibers
Place code for sound frequency
different places on the cochlea are tuned to different frequencies; sound frequency is converted into position on the basilar membrane and sound wave peaks at different places for each frequency of sound
Where do high frequency vs low frequency sounds peak on the basilar membrane?
high frequency peaks closer to the oval window (stiff and narrow membrane near cochlear base); low frequency peaks closer to the helicotrema (wide and loose membrane near cochlear apex) and more of the basilar membrane is activated
Black curves on the sound wave of a pure tone
represents the basilar membrane response to a pure tone at 2 instants in time
Red curve on the sound wave of a pure tone
envelope of all possible travelling waves for the pure tone that shows the maximum amplitude for each position
Cochlear amplifier
an active process is sharpening and amplifying the frequency response of the basilar membrane (peak is higher and narrower in the living ear) compared to the passive process based on its physical properties
What causes the active process in the basilar membrane response?
electromotility of the outer hair cells, which produces otoacoustic emissions as a byproduct
Electromotility
ability of outer hair cells to lengthen and contract in response to changes in electric potential
3 changes that occur in outer hair cells during electromotility
(1) depolarization causes contraction; (2) hyperpolarization causes elongation; (3) voltage-sensitive protein (prestin) changes shape
Otoacoustic emissions
sounds emitted by healthy ears
2 kinds of otoacoustic emissions
evoked emissions and spontaneous emissions
Evoked emissions
occurs in response to auditory stimulation and depends on the frequency of the stimulating sound; used clinically as a quick indicator of inner ear damage
Spontaneous emissions
occurs without stimulation and very weak (less than 20 dB, 100-2000 Hz); taken as evidence for outer hair cell involvement in the cochlear amplifier
How do you reduce spontaneous emissions?
aspirin (reduces activity of the outer hair cells but not the inner)
Characteristic frequency
frequency that increases the firing rate of the afferent fiber at the lowest intensity
lowest absolute threshold at lowest point of threshold tuning = high sensitivity
Contribution of outer hair cells to afferent fibers (which synapse with inner hair cells)
improves their sensitivity (threshold intensity for firing above spontaneous rate) and frequency selectivity (sharpness of tuning curve)
Two-tone suppression
firing rate of AN fiber to its characteristic frequency for a test tone decreases when a suppressor tone of similar frequency is presented simultaneously
When are suppression effects on an AN fiber pronounced?
when the second (suppressor) tone has a lower frequency than the first (test) tone
Isointensity curves
show the average firing rate of the neuron in response to different intensities over the same frequency range
Rate saturation
point at which an afferent fiber is firing as fast as possible and further stimulation can’t increase firing rate (i.e. loss of frequency tuning at higher intensities)
What is the frequency at which the basilar membrane has poor frequency discrimination?
< 500 Hz where the entire basilar membrane is involved in its response and there is no peak
Phase-locking
AN fibers tend to fire at a particular point (or phase) in the cycle of a sound wave, which provides a temporal code for frequencies below 1000 Hz
Volley principle
combined firing of a group of afferent fibers matches the frequency of an incoming sound (1000-4000 Hz) to provide a temporal code for frequency
afferent fiber can’t fire on every cycle at frequencies above 1000 Hz
3 aspects of afferent fiber firing that code sound frequency
(1) timing of firing for frequencies below 500 Hz; (2) place of maximum firing for frequencies above 4000 Hz; (3) temporal and place information at frequencies between 500 and 4000 Hz
How does pattern of firing code for sound intensity?
more auditory nerve fibers of each type (low, mid, and high spontaneous) from a particular region of the basilar membrane fire as sound intensity increases
Sound intensity threshold of low spontaneous fibers
high threshold (above 60 dB) to reach maximum firing rate (above spontaneous level)
Sound intensity threshold of high spontaneous fibers
low threshold (below 60 dB) to reach the maximum firing rate (above the spontaneous level)
How does pattern of firing code for a frequency?
auditory nerve fibers from different regions of the basilar membrane fire when intensity is constant but frequency is changed
Monaural neurons vs binaural neurons
receives input from only one ear; receives input from both ears
Threshold tuning curves
defines the absolute threshold intensity of individual auditory nerve fibers as a function of frequency
i.e. the lowest intensity necessary for neuron to fire above its spontaneous rate at each frequency
3 subdivisions of the cochlear nucleus
dorsal, posteroventral, anteroventral
Dorsal cochlear nucleus
axons of neurons that project to the superior olive (but don’t synapse) cross over to the opposite side of the brain
Posteroventral cochlear nucleus
axons synapse in the contralateral superior olive
Anteroventral cochlear nucleus
axons synapse in the contralateral or ipsilateral superior olive
3 functions of olivocochlear bundle
efferent fibers that protect the inner ear from loud sounds; suppress continuous background noise for easier sound detection; help with sound localization
Where do fibers from the medial superior olive in the olivocochlear bundle project to?
outer hair cells (mainly contralateral), which reduces its electromotility and otoacoustic emissions
Where do fibers from the lateral superior olive in the olivocochlear bundle project to?
dendrites of type I afferents (mainly ipsilateral)
3 subparts of the auditory core region
primary auditory cortex (A1), rostral core, and rostrotemporal core
2 belt regions surrounding A1
belt and parabelt
Tonotopic map
neurons that respond to different frequencies are organized anatomically in order of frequency to which they are most sensitive
Where are tonotopic maps found in the auditory system?
all structures (cochlear nucleus, superior olives, olivocochlear bundles, inferior colliculus, medial geniculate)
Tonotopic map in the belt
cruder and incomplete; neurons respond to complex sounds
belt has 6 subregions!
Tonotopic map in the parabelt
some tonotopy; neurons respond to speech sounds; involved in multi-sensory integration
“Where” pathway in the auditory cortex
posterior parabelt to posterior parietal cortex to dorsolateral prefrontal cortex; more activated by detecting a location
“What” pathway in the auditory cortex
anterior parabelt to orbitofrontal cortex; more activated by recognizing pitch
