Lecture 7 + Assignment 6 Flashcards
Sound waves
Longi vs transverse
Wave of compression (particles close) and rarefaction (particles far)
Longitudinal: Pulse and vibration the same direction
Transverse: Pulse left/right, vibration up/down
Sound wave measurements
Speed = distance/time = wavelength/period
Frequency (Hz or s^-1) = 1/period
Speed = wavelength x frequency
Human voice frequency and wavelength
Men
- 100 Hz
- 3.44 m
Women
- 200 Hz
- 1.72 m
2 things that allow for sound localization
- Interaural intensity difference
- Interaural time difference
Interaural intensity difference
- sounds louder in the ear that it’s near
bc head muffles sound
Interaural time difference
difference in speed of sound entering one ear vs. the other
brain calculates
Ascending auditory pathways
Destination: primary auditory complex in temporal lobe
Nucleus 1: in medulla cochlear nuclei
Nucleus 2: pons nuclei (ITD and IID) sup. olive
axons from the ear to the 8th cranial nerve (auditory nerve)
- spiral ganglion
Cranial nerves
General
Most rostral = 1
Most caudal = 12
don’t enter the spinal cord
enter through fossa holes in the cranium bone surrounding brain stem
Auditory cortex
tonotopic representation based on frequencies
closer to front corresponds with apex of cochlea
closer to back corresponds to base of cochlea (20 000 Hz)
Humans can hear what frequencies
20 Hz - 20 000 Hz
The superior olivary complex
Where is it
contains 2 nuclei for sound localization:
- LSO (lateral superior olive)
- interaural loudness/level difference - MSO (medial superior olive)
- interaural time difference
in mid-pons
Lateral superior olive
interaural intensity difference
for high frequency sounds > 2000 Hz
smaller wavelengths than the diameter of the head (20cm)
LSO sound localization
each cochlear nucleus
excites:
- the ipsilateral LSO (same side)
- the contralateral MNTB
MNTB inhibits the ipsilateral (to itself) LSO
Medial superior olive
monitors interaural time difference for low-frequency sounds
under 2000 Hz
where there is NO head shadow
so do time instead of intensity
MSO sound localization
Jeffress Model
- MSO gets input from both sides
- different axon lengths to same nucleus in MSO
- one will get EPSPs from both sides at the same time
coincidence detection = tells you location from each ear
Parts of the human ear
pinna
concha (shell/bowl)
external auditory meatus
tympanic membrane
Middle ear:
malleus
incus
stapes
Eustachian tube
Inner ear:
oval window (membrane)
vestibule
cochlea
round window
semicircular canals
hair cells
What does the eustachian tube do
- equalizes pressure in the middle and outer ear
- opens into throat
- for tympanic membrane
- allows it to not bulge and burst
- so it can vibrate
Attenuation reflex muscles (2)
Tensor tympani muscle
- between cochlea bone and malleus
Stapedius muscle
- between bone and stapes
reduce ossicle vibration as to not damage hair cells in loud environment (constrict)
Pressure amplification in the middle ear (2 mechanisms)
- The oval window is much smaller than the tympanic membrane
- force funneled into a smaller area = increased pressure - Stapes displaces the oval window much less than the tympanic membrane, but with more force
- decrease distance, increase force (lever system)
Archimedes
- could move earth with a big enough lever
287-212 BCE
- Greek guy
mechanical advantage principles
The cochlea - basilar membrane
bm
- like a series of tuning forks
smallest near oval window
= base
= stiffer
= responds to the highest frequency
although the cochlea is bigger there, bm smaller
tonotopic map
150 microns at base
500 microns at apex
Resonant frequency
some things only resonate at one frequency
ex. tuning forks or locations on the basilar membrane
Tonotopy of the basilar membrane
- different axons respond to different frequencies along the bm best
The cochlea
3 chambers
- scala vestibuli
- touches the oval window
- connected to the scala tympani
- makes the round window bulge out - scala media
- does acoustic transmission
- not connected to others
- different type of fluid = endolymph
- contains hair cells with their tips touching the tectorial membrane - scala tympani
- connected to the vestibuli and round window
Endolymph vs perilymph
Endolymph
- in scala media
- high K concentration
Perilymph
- in other scalas
- low K concentration
Arrangement of inner and outer hair cells
- stereocilia on top of hair cells touch tectorial membrane
- basilar membrane houses supporting cells
- when bm bounces, so do they
- causes hairs to bend
- opens mech gated ion channels in stereocilia
= acoustic transduction
Organ of corti
15 000 hair cells
3500 are inner
1:5 ratio
95% of afferent axons innervate inner hair cells
outer do motor function
Basilar membrane movement and hair cells
bm moving up
- shearing force of tectorial membrane up moves stereocilia towards long one
bm moving down
- shearing force moves stereocilia towards short one
Threshold hair bundle deflection
displacement of 0.3 nm
Mechano-acoustical transduction
- have tip links
= gating springs - connected to Ca/K ion channels
- pull them open and depolarize
- causes voltage gated Ca channels to open and transmit to afferent axon that goes to the auditory nerve
- glutamate transmits
OPEN WHEN DEFLECTED TO LONG END
= increases tension
causes outer hair cell to contract
Outer hair cell contraction motors
Prestin: the motor protein in the outer hair cell membrane
depolarization causes OHC to contract
hyperpolarization causes OHC to lengthen
Conductive hearing loss
- vibration impeded from reaching inner ear
pre-oval window
ex.
- wax
- otitis media (middle ear infection in kids)
- otosclerosis (stapes fused to oval window bone)
Sensorineural hearing loss
- neural processing compromised
post-oval window
ex.
- occupational deafness (loud sounds)
- presbycusis (with age, high frequency BM not bendy)
- antibiotic neuroma (damage hair cells)
- acoustic neuroma tumor (aka vestibular schwannoma)
Thomas Edison
- bilateral conductive hearing loss
- inventor of records
- read a lot
Hearing loss today
- hair cells don’t regenerate
- cumulative damage
more decibels = more danger
protect your hearing!
Immunofluorescence - direct method
- Using antibodies visualise certain proteins and see where they are in neurons
- Antigens (foreign) are recognized by antibodies in the immune system
- Requires that the primary antibody be fluorescently tagged
- More difficult bc you need to get the primary antibodies from the animal and tag them
Immunofluorescence - INdirect method
- More efficient
- Different primary antibodies have the same tail region
- Tagged secondary antibody can serve as an all-purpose labeller
(Other animal makes antibodies) - Several tagged antibodies can bond to the same primary antibody tail = more light
Immunofluorescence - detecting noise damage
- Label synaptic ribbon that occurs between the inner hair cells and the afferent auditory nerves AND in the IHC nuclei → red
- Label neurofilament protein (axons) → green
After sound exposure: synapse loss + disordered cell body appearance + axon problems
Max firing rate of a typical neuron
500 AP / s
1 AP / 2ms
due to refractory period
Why do rats hear much higher frequencies than humans
- their BM is much narrower at both ends
= higher resonance frequencies
Cochlear implants vs. normal hearing aids
Cochlear implants
- activate the auditory nerve axons by electrically stimulating them with electrodes threaded through the cochlea
- profound sensorineural hearing loss
mic on head -> speech processor -> transmitter -> receiver-stimulator -> electrode stimulation
Conventional hearing aids
- amplify sound
Tinnitus + most common cause
ringing or buzzing in the ears though no sound present
- phantom sound/hallucination
most common cause
- hearing loss caused by exposure to loud noise
- ringing heard at the frequency where hearing was lost
- also age, congenital hearing loss, ear infections, etc.
- maybe like phantom limb = spontaneous activity
Barn owl audition
- asymmetrical ears
right = points up
left = points down
allows for vertical localization (elevation)
as opposed to just horizontal (azimuthal)
