Exam 3 Flashcards
What are the baseline electrical potential levels in the inner ear?
In the absence of an auditory stimulus…
- Endolymphatic potential = +80 mv
- Hair cell potential = -70 mv
Which electrical potentials are created when the inner ear is stimulated?
In the presence of an auditory stimulus…
- At hair cells:
- Cochlear microphonic (CM)
- Summating potential (SP)
- At auditory nerve:
- Action potential or all-or-none potential
Charge levels in the endolymphatic fluid & organ of corti
- Scala media
- +80 mv
- Has endolymphatic fluid
- Organ of corti
- -70 mv
- Hair cell potentials
Afferent vs. Efferent Neurons
- Afferent neurons (AKA sensory neurons): bring the stimuli from the sensors (e.g., skin, eyes, ears) to the CNS
- Efferent neurons (AKA motor neurons): bring the responses from the brain to the muscles and the glands
Cochlear Microphonic vs. Summating Potential
- Electrical potentials produced when the hair cells move.
- SP Is unwanted electrical potential and is not very significant in normal hearing subjects.
(Stereo)cilia anatomy
- Stereocilia make contact with the tectorial membrane
- Movement of cilia is transmitted through the side links
- Transduction channel = ionic channel
- Connected through tip link, which opens channel
Anatomy of a neuron
Terminal end bulb contains synaptic vesicles
What is the role of the myelin sheath?
Increases the speed of conduction of an action potential by insulating the axon.
Threshold of a neuron
Lowest level at wich a neuron triggers an action potential.
- Avg resting potential is -80 mv
- Avg threshold is -40 mv
Like putting pressure on a trigger – needs a specific pressure before you can release it
Absolute Refractory Period
Lasts 1 ms (true of all neurons in the body)
Amplitude is always about +50-53 mv, regardless of sound intensity/frequency
Neurons cannot fire again during ARP
- Luckily, we have billions of neurons
Relative Refractory Period
Time interval between threshold and resting potential
Neuron can fire again, but the intensity of stimulation has to be greater than in ARP
The auditory action potential is the __________ correlate of the input sound (i.e. the acoustical energy).
electrical
How does an action potential travel through a single neuron?
- The dendrite is hooked up below a hair cell
- Axon produces action potentials at every node of ranvier
- Ionic exchange keeps happening until AP reaches terminal end bulb & synaptic vesicles w/ neurotransmitters
- stimulates them to release chemicals
Ionic changes during AP firing
Ions move from high to low concentration
- At rest…
- Interior has neg. charge (compared to exterior)
- Na+ (K+)
- At moment of firing…
- Na+ rushes in, K+ rushes out
- Interior has pos. charge compared to exterior (depolarization)
- K+ (Na+)
- After firing…
- Interior restored to neg. charge =
- Re-establishes equlibrium (repolarization)
- Na+ (K+)
Sounds from the middle ear enter the _____ _________ through the oval window.
Scala Vestibuli
Frequency-place principle
- AKA tonotopical organization
- Based on the frequency, there is a specific place in the cochlea where the traveling wave will have the maximum amplitude
- High at base, low at apex
What different types of energy is a sound converted into as it travels through the ear?
- Middle ear: mechanical energy
- Inner ear:
- Hydrodynamic: fluid moving back & forth in inner ear
- Mechanical: fluid moves hair cells
- Electrochemical: potassium/sodium ionic echange @ level of the hair cells
- Central auditory nervous system: electrochemical (APs)
How does sound go from the oval window to the hair cells?
- Enter ME through oval window to Scala Vestibuli
- Wave starts at base & moves to apex
- At place of maximum amplitude…
- Disturbs perilymphatic fluid (bony labyrinth)
- Disturbs Reissner’s and tectorial membrane
- Disturbs tectorial and basilar membrane
- Disturbs hair cells
How does the movement of cilia transmit signals to the brain?
- Cilia movement produces Cochlear Microphonic (CM) potential
- Flow of K+ dictates amt. of CM produced
- Direction cilia move controls K+ flow
- CM potential stimulates vesicles to release NTs into Synaptic Cleft (no physical contact of neurons)
- If CM quantity is sufficient to reach auditory nerve threshold, an AP is triggered
- The AP travels through the auditory nerve to the auditory brainstem and finally to the brain (temporal lobe)
What is the role of the round window of the cochlea?
Fluid can’t be compressed w/o a fluid release outlet. Round window bulges out to accommodate the compression of the fluid.
What happens to the cilia when the footplate of the stapes on the oval window moves inward vs. outward?
What is happening in diagrams A, B, and C?
- A: Footplate of stapes moves otward. Basilar membrane moves up, gates open, and more ionic exchange/CM potential happens. Will produce AP at the level of the auditory nerve
- B: At rest (no sound coming in).
- C: Stapes moves inward. Basilar membrane moves down, gates close, and less ionic exchange happens. Auditory nerve doesn’t get an AP.
What happens at the level of the cilia when the basilar membrane moves down vs. up?
- Cilia make physical contact through side links, so that when the stereocilia move, all others move with it
- Transduction channel = ionic channel
- When OW moves in, BM moves down and channels close (inhibition)
- When OW moves out, BM moves up and channels open letting K+ ions in (excitation)
How do the cochlear mechanics account for intensity changes?
- At higher intensities, the height of the traveling wave is larger
- Basilar membrane deflection increases
- Increase in number of hair cells stimulated increases
- More neurons are recruited
Why do we lose frequency specificity at higher intensities?
- For a high intensity sound, there is a lack of sharpness of the peak of the wave
- Sound is louder because more hair cells are moving at more locations
- More nerve fibers are recruited, but we lose frequency specificity
How do the cochlear mechanics account for frequency changes?
≤ 1000 Hz:
- Use time interval b/w spikes (lower the freq., longer time b/w spikes)
> 1000 Hz:
- Use rate of firing (higher freq. = higher rate of firing
