Exam 4: Auditory and Vestibular systems Flashcards
T/F sounds start as air vibrations?
true
how fast do hearing mechanisms work?
pretty fast -within 5-10 ms
- The range we hear is in KHz
what are the properties of sound aka air pressure waves? (3)
- frequency
- intensity
- diffraction
frequency
measured in cycles per second ⇒ Hertz/Hz
- one hertz is the same as one cycle per second
wavelength
speed of sound/frequency => distance between high points on wave signals
- Speed of sound = 343 m/s
- Speed of light 300,000,000 m/s
diffraction
sound bends around objects
- sound consists of alternating changes in local air pressure is simply evident when a loud noise rattles a window
T/F most sounds are mixtures of multiple frequencies?
True
- pure tones put together in a single sine wave
- when you get to the auditory cortex, individual neurons can respond to complex sounds
what kind of frequency does wind have? Music?
a low frequency; range of frequencies
what are human ears sensitive to regarding hearing frequency?
1-10 KHz
- Ear receptors are not sensitive to very high frequencies
- ultrasound in medical imaging = MHz ⇒ too high frequency to be audible
are sound intensity and sound pressure level equal or correlated?
correlated
- intensity measured relative to threshold, using a log scale
- Units of intensity: Bels
- 0 dB = silence, whisper = 15 dB
- 120 dB is the threshold for pain
what are the anatomic components of the ear? (3)
- external ear
- middle ear
- inner ear
external ear
Air filled; open chamber
- Low pressure changes sufficient for movement of tympanic membrane
what does sound do to the tympanic membrane?
increases and decreases in air pressure push and pull effectively on the membrane, moving it inward and outward
middle ear
includes the ossicles: malleus => incus => stapes
- the stapes pushes on the oval window and the round window is directly beneath it
inner ear
aka cochlea
- Liquid filled; closed chamber; little fluid movement
Auditory tube (eustachian tube)
pressure equalization mechanism
- There is air in the middle ear which needs to be pressurely equalized
how does sound get transmitted to the cochlea?
the oval window via the footplate of the stapes
- Fluid flows through the tube to the apex and then out at the round window
what is the order of anatomy after the tympanic membrane ignoring the ossicles? (2)
oval window (round underneath) => basilar membrane
- across 33 mm in total
- the basilar membrane divides the area in half
what happens to the basilar membrane with compression and rarefaction?
with compression the basilar membrane pushes downward closer to the round window plane while during rarefaction it pushes upward closer to the oval window plane
what sits on top of the basilar membrane?
the hair cells which signal to the spiral ganglion cells lateral to the basilar membrane
stereocilia
tip of the hair cell which are responsible for opening ion channels
- 5nm in diameter so they are fragile
like a lever for opening a mechanoelectrical ion channel in the hair cells
tectorial membrane
deflects the stereocilia on hair cells ⇒ the piece of jelly on top of the hair cells
base of the basilar membrane; apex?
the base responds to high frequency soundwaves; the apex responds to low frequency soundwaves
which side of the basilar membrane is thicker/tense; thinner/flaccid?
the base; the apex
- high resonant frequency at the base and low at the apex
what is a low frequency wave? Where will it show up? high frequency? where does that show up?
low frequency would be 100 Hz at the apex; high frequency would be 10,000 Hz at the base
T?f complex sounds can cause multiple parts of the basilar membrane to vibrate at once?
True
- The receptors are aligned on the membrane
Signal transduction (auditory)
occurs in the inner ear by hair cells to the spiral ganglion cells
what are the 2 types of hair cells?
- inner
- outer
inner hair cells
carry the signals going into the brain
- Encode sounds and lead to perception in the cortex
- Movement of stereocilia on hair cells open ion channels that depolarize hair cells
TMC1
protein that forms the pore of the transduction channel on the inner hair cells
outer hair cells
for top down modulation
- Modulate (amplify or attenuate) via the brainstem feedback inputs
which type of hair cells improve our ability to discriminate specific frequencies?
inner hair cells
- up/down basilar membrane movements translate to lateral/shearing movements of the tectorial membrane
when the basilar membrane goes upward, which direction do the stereocilia go? downward?
to the right and depolarizing the hair cells; to the left which does not depolarize the hair cells
T/F the tectorial membrane provides bidirectional activation of hair cells?
False, unidirectional
What channel gets opened by stereocilia?
MET channel
what is the most common cause of deafness?
mutation of TMC1
- cause human deafness
- knockout in mice blocks transduction current
how is TMC1 formed?
Homodimers form the ion channel
what does cross linking do to TMC1?
blocks transduction current
what is the dark current in hair cells?
persistent baseline K+ channel tone enables a biphasic receptor potential
- Allows for a relative depolarization to signal increases and decreases in membrane potential
- You activate calcium voltage gated channels responsible for vesicle fusion and transmitter release ⇒ the transmitter is glutamate
what direction is the K+ ion flowing regarding the hair cell?
down its current but going into the cell
what happens when the hair cells get depolarized?
K+ comes in and so does Ca2+ on the lateral sides of the hair cell which allow for glutamate vesicle fusion and release to afferent neurons
- membrane potential is relatively depolarized which allows for depolarization and hyperpolarization more easily
T/F the hair cell fires an action potential?
FALSE
- The hair cell DOES NOT fire action potentials and doesn’t have specialized channels
- The nerve axons will then fire action potentials to the brain
where is there high and low potassium in the cochlea?
low potassium in the perilymph which is in the Scala vestibule and Scala tympani
- these sections are the ones above and below the regions containing the endolymph in the middle which has the basilar membrane and connects to the spiral ganglion
- the scala media has the endolymph and tectorial membrane + hair cells which has high K+
what is the type of response by the hair cells called?
graded response
- Time in microseconds is very fast in responses
how do hair cells adapt to prolonged stimulation?
the auditory system will try to minimize damage ⇒ if there is a persistent sound the excitation current will be adapted and reduced via myosin
- Tip links have myosins and with persistent stimuli myosin moves down the stereocilia so the deflections cannot as easily open ion channels (channels get more floppy)
T/F hair cells are tuned to specific frequencies?
True
- Basilar membrane is to the only mechanical source of frequency selection
do low or high frequency hair cells have longer bundles of sterocilia?
low frequency cells
what device is needed if hair cells are absent? how does it work?
cochlear prosthesis; stimulation electrodes along the length of the scala tympani can activate axons of the spiral ganglion ⇒ lots of supporting tissue around the cochlea
- Each electrode stimulates local axons with a specific frequency
- Across the range of electrodes, all sound frequencies can be covered
- the probes along the length of the auditory membrane can be driven by electrical activation which allows for different sets of hair cells to activate what we want based on frequencies
- there is a microphone that grabs sound but math will calculate what needs to drive different parts of the cochlea
T/F the auditory system loudness does not change the auditory frequency tuning like the visual system stimulus brightening does not affect orientation of tuning of neurons?
False; auditory system loudness does change auditory frequency tuning
what is the tuning curve?
inverted curve with a minimum spot corresponding to the best stimulus
- The softest stimulus at a given frequency drives the neuron or the receptor ⇒ the most quiet sound
- there is a voltage threshold change in membrane potential since the hair cells don’t fire action potentials
how do we create the tuning curve?
We pick one stimulus and vary intensity to find the change in mV by 1
- Then we can see what is the best stimulation
Central frequency
the spot on the tuning curve with one specific frequency for which the intensity required to elicit a given response is minimal (threshold)
- Threshold could be 1 mV for hair cells and a few action potentials for an auditory neuron
- this occurs at a certain frequency and the stimulus magnitude in DB will determine the direction that the hair cell responds to the for frequencies just outside of the CF on either side
what is the trend with increasing stimulus magnitude from the smallest DB at the central frequency?
it goes up in both directions relating to frequencies higher and lower than the CF
- U shaped
how can the brain send feedback to reduce sensitivity?
efferent feedback to the hair cells can minimize sheering and reduce sensitivity of the tuning curve
how are auditory nerve fibers tuned?
the amount of action potentials fired at different stimuli frequencies => low vs high
- the nerves will respond at particular frequencies and fire more action potentials
- The weakest stimulus that can do this will help to determine different axons on the basilar membrane ⇒ maps the tonotopography on the membrane
what is the auditory pathway?
cochlea (inner hair cells) => brainstem (cochlear nuclei, medial nucleus of trapezoid body, superior olivary complex, nucleus of lateral lemniscus, inferior colliculus) => MGN thalamus => auditory cortex (A1)
Primary auditory cortex
area 41 (A1) ⇒ main termination at MGN
is low frequency anterior/posterior in A1?
anterior and high frequency is posterior
what do most A1 neurons respond best to?
bilateral input
Core areas
in the auditory cortex (A1) receive MGN input and respond to pure tones
Belt areas
in the auditory cortex (A1) and sometimes receive MGN input but respond best to complex sounds
T/F one neuron across different days respond to a particular complex sound over others?
True
- layers 2/3 auditory nerves respond to specific complex sounds consistently
what is special about the auditory cortex and mothers?
mothers have cells tuned to ultrasonic vocalizations (USV) from their babies
- there are USV preference cells which spike at particular times for USV
dorsal auditory stream
location of the sound
ventral auditory stream
identity of the sound
where does the core project? what does this do?
A1 projects to the anterior temporal lobe which projects to the prefrontal cortex
- sound identification (ventral)
where does the belt project to?
A1 projects to the posterior parietal (PP) lobe which projects to the pre-frontal cortex
- speech perception (Wernicke’s) and sound localization (Dorsal)
which is anatomically anterior in the temporal lobe: the core or the belt?
the core
- the belt is posterior but they are right next to one another
the parabelt (PB) cortex is right under the core cortex
projects to T2/T3 which projects to the pre frontal cortex ventrally
what does the where pathway associate with? what?
dorsal; ventral
T/F hearing and balance are both part of the labyrinth?
True
T/F both hearing and balance systems have endolymph?
True
- in the ear the identical type of hair cells responsible for the vestibular system to hearing are in different locations
- Mechanical forces on one set of hair cells are different
vestibular
balance, acceleration, sense of motion/position
what is the anatomy of the labyrinth? (2)
- otolith organs
- utricle
- saccule - semicircular canals
what do the hair cells detect? (2)
- Orientation
- bending/displacement
what are the functions of the labyrinth? (3)
- Linear vs angular movement
- Sustained vs transient
- Constant velocity vs acceleration
what 3 systems do the semicircular canals have?
- horizontal/lateral canal
- posterior canal
- superior/anterior canal
Horizontal/lateral canal
spinning around
- like looking from left to right
- rotating around a vertical axis
Posterior canal
tilting heat L/R
superior/anterior canal
tilt head front and back
what is another term for semicircular canals? what do they contain?
ampullae; hair cells at the base
- also in the utricle and saccule
T/F the ampullae, utricle, and saccule mix axons?
False
how do hair cells get depolarized in the vestibular system?
from shorter to taller movements based on endolymph, same MET channel, K+ mediated depolarization
cupula
The jelly is encapsulating the hair cells in the ampullae
utricle
responsible for horizontal movement
- linear acceleration forward and backward
saccule
responsible for vertical movement
- acceleration up and down
T/F in the ampullae and otolith organs the hair cells are oriented in one direction?
False; they are in the ampullae, but in the utricle and saccule the have some facing each way
- for the ampullae, each ear has different canals that are oriented in the opposite direction with their hair cells in order to detect head movement one way or the other
Otoconia (+4 properties)
calcium carbonate and otoconia protein that act like rocks in the otolith organs to help determine direction and movement
- 60% calcium carbonate and 40% otoconia protein
- Fully forms shortly after birth
- Probably cannot be replaced
- Enable detection of changes in head position with respect to gravity ⇒ falling or ascending
- Detect constant tilt to heat
when does the utricle vs saccule fire action potentials standing upright? Laying down?
no signals from utricle standing up, but saccule fires; no signals from saccule laying down, but utricle fires
- same thing as propelling forward while standing up, utricle will activate due to the movement
- saccule will activate going up and down like in the elevator
T/F the otolith hair cells can detect slight differences the way the visual system can?
False, its not as good
- If you are moving at an angle and your eyes are closed then you cannot sense that as well
- Its not as accurate as some of the other sensory systems
T/F otolith hair cells have a high baseline firing rate for vestibular nerve afferents?
True
how do the otolith hair cells also sense head tilt?
when the head tilts back there is a deflection which triggers action potential firing
- there are other hair cells that have the opposite direction for when you tilt your head forward to signal this as well
- signal the constant head tilt that we have unit the head is back in its upright position ⇒ different from rotational acceleration signaled by the semicircular canals
what is the baseline firing of hair cells?
40 spikes/s
- You don’t see that in the cortex
T/F if you are tilting your head back and then forth, you can get almost no firing for the backward sensing hair cells at the forward tilt?
True
T/F the otolith cells adapt?
False, they constantly will signal a head tilt
what do the semicircular canals sense aside from rotation?
the direction and speed of angular acceleration
- this means they activate at the acceleration and then deactivate even more at deceleration
- they will fire at the same rate during a constant velocity
how does pairwise activation/inhibition work in the semicircular canals for turning left and right? Which canal?
- Turn left ⇒ horizontal canal left activated and right inhibited
- Turn right ⇒ horizontal canal right activated and left inhibited
which horizontal canals are in the same planes?
left anterior canal (L side) and left posterior canal (R side); right posterior canal (L side) and right anterior canal (R side)
if the head moves to the left, what is the activation mechanism?
when looking top down on the horizontal canal, if the head moves to the left the fluid in the semicircular canal goes in the opposite direction to the right
The hair cells have the right orientation that they will be depolarized on the left side of the body
- There is an increase in firing due to the stereocilia
- The opposite is happening in the right side ⇒ endolymph still moves to the right but hair cells are in opposite orientation so they have decreased/inhibited firing (opposite happens if you look to the right)
which way do your eyes move if you turn your head to the left?
they move right
- you can rotate your head and fixate your eyes on something and not lose perception of the focused spot
what is the vestibular sensory nerve pathway?
vestibular nerve cells (Scarpa’s ganglion) ⇒ vestibular nuclei in brainstem ⇒ ventral posterior nuclear complex of the thalamus ⇒ cortical 3a, 2v, PIVC for self motion perception
what is the vestibular motor nerve pathway?
vestibular nerve cells (Scarpa’s ganglion) ⇒ vestibular nuclei in brainstem ⇒ abducens (brainstem) ⇒ eye muscles abducens in brainstem ⇒ oculomotor nucleus in brainstem ⇒ eye muscles
what is different about the thalamocortical pathways of the vestibular system compared to the visual system?
there are multiple pathways instead of just 1 ending at V1
Vestibular ocular reflex (VOR)
produces eye movements that counter head movements
- VOR drives the eyeballs to compensate for any jerking movements we do
which two eye muscles move in pairs for left to right head movements?
Lateral and medial rectus muscles on opposite eyes
what is the general pathway of neurons in the movement of eyes? (4)
- left semicircular horizontal canal fires
- Vestibular nucleus brainstem (pons)
- Abducens in the brainstem (pons) and this will crossover to lateral muscles
- Crossover to lateral muscles at the medial longitudinal fasciculus and keep going without synapse
- oculomotor nucleus in the midbrain before going to the medial muscles
what is the mechanism for the vestibular ocular reflex?
- endolymph must move clockwise due to laws of inertia when moving the head to the left
- Vestibular nerve afferents leaving the base of the left horizontal semicircular canal fire action potentials
- Activates the left vestibular nucleus in the pons brainstem
- this activates the left abducens nucleus in the pons
- this crosses over at the medial longitudinal fascicles and activates the right oculomotor nucleus
- these relay neurons go to the medial rectus of the left eye
- the neurons from the left abducens go to the lateral rectus of the right eye to contract
- The vestibular nerve afferent leaving the base of the right horizontal semicircular are inhibited ⇒ no spiking to contract the left lateral muscle and right medial muscle
- The eyes will move to the right
does the lateral or medial pathway have 1 extra synapse?
medial
End card
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