Auditory System I & II Flashcards
waveform
amplitude of pressure vs. time
Intensity
amplitude, measured in decibels where dB = 20log (p/p0) (where p=amplitude)
p(0)
0.0002 dynes/cm^2
frequency
cycles per second (Hz)
physiology vs. physics
loudness:pitch::intensity:frequency – a big increase in pressure does not result in a comparable increase in loudness
equal loudness curves
on a plot of intensity vs. frequency, dip where humans have the most acute hearing (1000-3000Hz)
pitch and frequency
correlated but not related, 2pitch=3frequency
pitch and amplitude
pitch is partially based on amplitude
pitch in general
is more dependent on frequency than amplitude
sound localization
uses temporal (phase) and intensity (amplitude) cues
external outer ear composition and function
pinna (shadow at high frequencies - low frequencies slide through) and concha: together work as a resonator
tympani membrane
boundary between outer and the middle ear
middle ear
malleus, incus, and stapes (oval window). transfer of energy from a low to high impedance medium. HUGE amplification (mainly due to the size of the tympanic membrane vs. the oval window)
2 muscles in the ear that decrease the effect of loud stimuli
tensor tympani (malleus, tension), stapedius (stapes, retraction)
eustachian tube
connects the middle ear and the pharynx – pressure differences can cause pain/difficulty hearing
cochlea
three chambers: scala vestibuli, scala tympani, scala media (organ of corti)
organ of corti
is at the base of the scala media – includes structures from the tectorial to the basilar membrane. has inner and outer hair cells (efferent 75%)
hair cells
have stereocillia at the apex connected by tip links. stimulated by the shearin gforces of the tectorial and basilar membranes. mechanical displacement yields action potentials
endolymph vs. perilymph
+80mV, high K+ vs.
potassium flow through the hair cell
apex (mechanosensitive, in, depolarization), base (Ca2+ activated, out, hyper-polarization) coupled with the opening of voltage gated Ca2+ channels. this de/hyperpolarization allows for a full sine curve at low frequencies
phase locking
1:1 relationship between the sound wave and neuronal firing (1000 impulses/second)
volley theory
a way of encoding pitch at low frequency (phase locking)
sound input process
tympanic membrane compression –> bone lever –> push at oval window –> travel to helicotrema then to round window –> basilar membrane supports the wave from the oval window –> rarefraction pulls the stapes so fluid moves back towards the oval window (overall the compression followed by rarefraction causes the basilar membrane vibration along the organ of corti)
basilar membrane
thickest and widest at the apex, apex stimulated by lower frequencies, hard to get resolution at low frequencies (handled by volley theory anyways)
characteristic frequency
the frequency at which a given nerve is most sensitive (seen via a tuning curve)
cochlear implants
require hair cells, utilize the tonotopic map of frequencies
tonotopic map of frequencies is…
maintained all the way to the primary auditory cortex
low frequency
spatial localization via phase shift, frequency encoding via volley theory
high frequency
spatial localization via shadowing (intensity), frequency encoding via the tonotopic map
3 theories for the encoding of sound intensity
- neuron recruitment 2. increased probability of neural firing 3. loss of spatial resolution from loud stimuli 4.
Sensorineuronal deafness
problem in bone conduction pathway, CN VIII
conduction deafness
problem in the middle ear conduction
rinne’s test
tuning fork, mastoid process to ear (normal: ear heard after bone, conduction deafness: bone > ear, sensorimotor deafness: both forms are diminished
weber’s test
vertex of skull, normal: heard equally, conduction deafness: heard in abnormal ear first (wow!), sensorimotor deafness: sound louder in normal ear
efferent auditory pathway
through the outer hair cells (75%), not related to tinnitus, correlated with a otoacoustical emission, thought to sharpen acuity and increase frequency resolution
auditory nerve to cortex pathway
1.cochlear nuclei 2. superior olive 3. lateral lemniscus 3. inferior colliculus 4. medial geniculate 6. auditory cortex (superior olive/lateral lemniscus are not innervated by all fibers), most fibers ascend bilaterally, therefore a post cochlear lesion tends to result in a bilateral deficit
MSO
medial superior olive, time based sound localization at low frequencies
LSO & MNTB
lateral superior olive & the medial nucleus of the trapezoid body – sound localization, shadowing, high frequencies, excited by ipsi input, inhibited by contra input
lateral lemniscus
only receives contralateral input, “onset” and “duration” of sounds
inferior colliculus
auditory space map creation for sound localization, higher sound processing (i.e. speech)
medial geniculate complex
temporal comparison
primary auditory cortex
has tonotopic map, columnar organization(EE, EI - two vs. one ear excitation)
secondary auditory cortex
inferior temporal lobe, complex sound processing (speech recognition centers)
non dominant hemisphere (R)
processes emotional tone and inflections (L dominant processes speech)
wernicke’s area lesion
word salad, inability to comprehend own deficit
