Physiology of Auditory and Vestibular Systems Flashcards
What is the audible range of frequencies?
20-20,000 Hz
Sound waves over ___ dB can cause pain and damage to the cochlea
120
What kind of fluid is contained within the membraneous labyrnith?
endolymph
What kind of fluid surrounds the membranous labyrinth between the bony labyrinth?
perilympth
What is characteristic of endolymph?
it is low in [Na+] and high in [K+]
what is characteristic of perilymph?
it is high in [Na+] and low in [K+]
Why do sound waves need to be amplified?
Differences in density between air and liquid or else there would only be 1% transmission of sound energy
describe the process of hearing in the middle ear
- sound waves cause the tympanic membrane to vibrate
- in turn, the ossicles (malleus, incus and stapes) vibrate, pushing the stapes into the oval window of the cochlea and displacing cochlear fluid
What is impedance matching and what does it do?
amplifying the sound wave to improve transmission
Where is the cochlea and what does it contain?
It is located in the inner ear within the bony labyrinth and contains 3 fluid filled canals called scala vestibuli, scala tympani, and scala media
What is significant about scala vestibuli and scala tympani?
both contain perilymph (high in [Na+] and low in [K+])
what is significant about about the scala media?
It contains endolymph (high [K+] and low [Na+]) and borders the basilar membrane which is the site of the organ of Corti
what is the organ of Corti?
Contains inner and outer hair cells embedded in the tectorial membrane that are receptors for auditory stimulus
Describe the differences in inner and outer hair cells within the organ of corti
Outer hair cells are more numerous but inner hair cells have more innervation and are the receptors connected to the CNS. Inner hair cells are though to be the hair cells that actively “hear” sound
What is the mechanism of sound transduction?
- Sound waves cause a vibration of the organ of Corti
- This causes vibration of the basilar membrane and the hair cells bend by a shearing force as they push against the tectorial membrane
- Bending of the cilia causes a change in K+ conductance of the hair cell membrane
- Due to a combined +140 mV total potential difference (+60 mV potential difference of perilymph and inner hair cell added to +80 mV potential difference of endolymph to peilymph) this favors movement of a cation into the cell if there is an open channel for it
What causes the opening or closing of ion channels in cochlear transduction?
the links between the cilia, called tip-link adhesion molecules, depending on the direction fo the cilia
Describe the electrical process and result of cilia bending towards the tallest member
- opening of stretch-activated K+ channels
- K+ enters the hair cell and depolarizes it
- Voltage-gated Ca+ channels open resulting in increased neurotransmitter release and overall increased rate of AP towards the axis of polarity
Describe the electrical process and result of cilia bending toward the smallest member
- Closing of stretch-activated K+ channels
- No K+ entry into the hair cell so it becomes hyperpolarized
- Ca+ channels close and there is no neurotransmitter release and thus no APs away from the axis of polarity
What is the Place theory Coding System?
Based on the basilar membrane, the base is stiffer and more responsive to high-frequency sounds whereas the apex is more compliant and more responsive to low-frequency sounds. Used for above 2000 Hz.
What is the Volley Theory Coding System?
Only used for low tones, frequencies under 400 Hz. Each neuron responds to a sound by firing APs slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain
What is temporal summation?
occurs when one presynaptic neuron releases neurotransmitters over a period of time to fire an action potential
What is spatial summation?
occurs when multiple presynaptic neurons release neurotransmitters together to fire an action potential in the postsynaptic neuron
Where are receptor hair cells found in the semicircular canals?
in the crista ampullairis, located within the ampulla and the cilia are embedded in a gelatinous mass called the cupula
what is the kinocilium?
the longest cilia of a hair cell
What is the axis of polarity for the horizontal semicircular canal?
towards the utricle
what is the axis of polarity for the anterior and posterior semicircular canals?
away from the utricle
What do semicircular canals detect?
angular or rotational acceleration
Describe the mechanical transduction mechanism that occurs within semicircular canals
- rotation of the head causes movement of the endolymph in the opposite direction
- the endolymph pushes against the cupula and bends the hair cell fibers
- when constant velocity is reached, both the head and endolymph are traveling in the same direction at the same velocity
- stopping rotation does not stop the endolymph as quickly, resulting in the post-rotational response
What does the utricle detect? and provide examples
linear acceleration in the horizontal plane. examples include tilting of the head, and forwards/backwards movement
what does the saccule detect? and provide examples
linear acceleration in the verticle plane. examples include gravity and up/down movement
What are the otolith organs and where are they contained?
Otolith organs are the utricle and saccule, contained within a thickened area called the macula. Cilia are embedded in the otolith membrane covered by otoconia
Describe the mechanical vestibular transduction mechanism that occurs in the otolith organs
- Tilting of the head causes movement of the endolymph to displace the otolith membrane and otoconia
- hair cell fibers bend
- because the striata depression changing the axis of polarity, there are opposite discharge effects on the two sides of the striata
