8.5. Physiology of hearing. Flashcards
I. Auditory system
1. What is the role of auditory system?
- Responsible for sensing of hearing
- This system detects the rapid vibration of surrounding air (sound waves) in the range of 20 – 20 000 Hz
I. Auditory system
2. What is the range of rapid vibration of surrounding
air (sound waves) that auditory system can detect?
The range of 20 – 20 000 Hz
I. Auditory system
3. What are the receptors of auditory system? What is their role?
Hair cells are the receptors in this system.
- They are used to convert mechanical stimulus of sound into APs to be processed by the CNS
II. Basic anatomy review
1A. How does sound travel in auditory system?
- Sound travels through the external ear to meet the tympanic membrane, causing it to vibrate.
- This membrane conducts the vibration to the middle ear by vibrating the chain of ossicles (malleus, incus, and stapes).
- The movement of the ossicles can be reduced by the contraction of stapedius and tensor tympani muscles.
- The stapes connects to the oval window covering the inner ear.
- The cavity of the inner ear contains the cochlea and the vestibular apparatus
II. Basic anatomy review
1B. What is the role of tympanic membrane?
This membrane conducts the vibration to the middle ear by vibrating the chain of ossicles (malleus, incus, and stapes
II. Basic anatomy review
1C. What does the cavity of inner ear contain?
The cavity of the inner ear contains the cochlea and the vestibular apparatus
II. Basic anatomy review
2A. What is cochlea?
- Cochlea is the organ of hearing and is formed by the bony + membranous labyrinths.
- It coils around 2,5 times and is divided into 3 compartments: Scala vestibuli, Scala tympani, Scala media
II. Basic anatomy review
2B. What are the features of Scala vestibuli of cochlea?
- First part of the cochlea to receive the vibration form the oval window.
- It extends to the end of the 2,5 cochlear turns, until it reaches the end space called the helicotrema.
- Scala vestibuli is separated from scala media by the Reissner’s membrane
II. Basic anatomy review
2C. What are the features of Scala tympani of cochlea?
- At this point, the canal becomes the scala tympani and it coils back around to reach a structure called the round window.
- Scala tympani is separated from scala media by the basilar membrane (contains hair cells), which moves in response to vibration in scala tympani
II. Basic anatomy review
2D. What are the features of Scala media of cochlea?
This is where the organ of Corti is located, which receives the stimulus for hearing.
- It contains rows of 1 inner and 3 outer hair cells, that are used to convert mechanical energy into APs
III. Sound
1. Give the definition of sound
Sound is a periodic, longitudinal wave of low and high pressure that propagates in the air.
III. Sound
2. What is the speed of sound?
330 m/s
III. Sound
3. What are the characteristics of sound waves?
- Frequency (1/f): most important characteristic. Recognizable as unique type of sound
- Amplitude: corresponds to intensity
- Phase difference: comparison between 2 tones
=> Can detect the sound + origin of the sound by these properties
III. Sound
4A. What are the 3 types of sound?
- Pure tone
- Real sound
- Noie
III. Sound
4A. What are the 3 types of sound?
- Pure tone
- Real sound
- Noie
III. Sound
4B. What are the characteristics of pure tone?
- Pure tone: single frequency -> sinusoidal wave.
- Artificial = not normally produced in nature.
- Frequency = 20 – 20 000Hz
III. Sound
4C. What are the characteristics of Real sound?
- Fundamental frequency (pitch) along with all of its overtones.
- Sound of humans, animals and instruments
III. Sound
4D. What are the characteristics of Noise?
Noise: has no recognizable periodic elements = just random change
III. Sound - Intensity of sound
5. How is intensity of sound determined?
Determined by the amount of pressure – measured in dB
III. Sound - Intensity of sound
6. Calculate dB
dB = 20 * log (actual pressure / reference pressure)
III. Sound - Intensity of sound
7A. What is reference pressure?
Reference pressure = 20microPascals = average threshold of human hearing in the case of 2000 Hz pure tone (one single frequency).
IV. Amplification by the middle ear
1. What happen in middle ear?
In middle ear, there is an increase in the intensity of sound
=> this amplification is due to Decrease in area and Lever system of the ossicles
IV. Amplification by the middle ear
2. How can decrease in area be involved in Amplification by the middle ear?
Oval window is 10-20x smaller than Tympanic membrane
IV. Amplification by the middle ear
3. How can Lever system of the ossicles be involved in Amplification by the middle ear?
- Ossicles are used to exert a large force over a small distance at one end of the lever (oval window) by applying a smaller force over a longer distance at the opposite end (tympanic membrane)
- Contraction of Stapedius and Tensor tympani muscles can reduce the effect of this amplification -> reducing the intensity of louder soundsof the ossicles
V. Hair cells
1. What types of cells are hair cells?
Modified epithelial cells
V. Hair cells
7. Make a schematic diagram to demonstrate structure of hair cells shown in both endolymph and perilymph
V. Hair cells - Production of endolymph
1. What are the features of Production of endolymph?
- Generated by perilymph
- Trans-epithelial transport is responsible for the production of endolymph in the stria vascularis
- The transporters will be expressed in the perilymphatic surface
- Have a K+-channel which can be found on the endolymphatic surface of the cell
- The final result: K+-ions will be transported from perilymph to endolymph (reason for ↑[K+]) -> the K+-current will be responsible for generating the positive electric potential
+) the endolymph will fill the vestibular system, but it is far away from the place of the production -> the [K+] remains the same, but the electric potential will disappear (0mV), because the distance is too large and it will be attenuated
V. Hair cells - Production of endolymph
2. Make a schematic diagram for the production of endolymph?
VI. Mechano-electrical transduction in the hair cells
1. What happen in mechano-electrical transduction in the hair cells?
- The cells will release transmitters, and for that we need Ca2+- channels
- The stereovilli are connected at the tips via tip links (thin filaments), which will be responsible for the synchronized movement of the stereovilli
- The tip links are connected to channels found on top of the tip links -> TRP (transient receptor potential) channels -> electrotonic potential channels
- Based on the increasing length of the stereovilli, it will determine what stimuli can activate the hair cells
-> it is a mechanical stimulus which will cause deflection of the stereovilli and should be directed towards the increasing length of the stereovilli to activate them
VI. Mechano-electrical transduction in the hair cells
2. Make a schematic diagram to demonstrate mechano-electrical transduction in the hair cells
VI. Mechano-electrical transduction in the hair cells
3. How is electrochemical gradient be determined in hair cells?
- Will determine the movement of ions through the TRP-channels when they are open
VI. Mechano-electrical transduction in the hair cells - Electrochemical gradient
4. What are the features of -TRP-A1 channel (non-specific cation channel)?
- Permeable for Na+, K+, Ca2+-ions (only K+-ion is present)
- The K+-ions will move if the channel becomes permeable for the ions
- Have to calculate the electrochemical gradient for K+-ions
- We actually have no chemical gradient, since [K+] inside and outside is the same
-> we only have electrical gradient - 80mV - (-40mV) = 120mV -> huge electrochemical gradient in the cochlear cells -> whenever the TRP channels are activated, they open -> K+-ions move from outside to inside
- Since electrochemical gradient is very high, the cochlear hair cells will be very sensitive for the activation of the TRP channels
VI. Mechano-electrical transduction in the hair cells - TRP channels = mechanosensitive channels
5. How do TRP channels = mechanosensitive channels work?
The entire process starts with mechanical stimulus:
- The stimulus will cause the deflection of stereovilli (in direction of the longest one)
- The deflection will increase the distance between the tips of stereovilli
- The ↑ distance will ↑ the tension the tip links -> ↑tension will be detected by channels, which will activate/open them
- Open TRP channels = permeable for ions (K+-ions) -> K+-influx -> depolarization of cells -> activate VG Ca2+-channels -> Ca2+-signal
->↑[Ca2+]IC -> exocytosis -> glutamate release
=> If the mechanical stimulus appears in the other direction, then everything will be OPPOSITE (tension of tip links↓ -> TRP channels close -> K+-ions↓ -> hyperpolarization -> no NT release)
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7A. What is the 8-step Mechanism of activation (of auditory nerve cell)?
- Pressure in the middle ear↓ -> stapes moves outward -> ↓pressure in scala vestibuli (perilymph) = ↑pressure in scala tympani
- Basilar membrane moves upward – causing the organ of Corti (located on bas.membr) to move towards the tectorial membrane
- Since the stereocilia of the outer hair cells are connected to the tectorial membrane, this movement will tilt the stereocilia in the direction of the longer stereocilia. This leads to opening of the TRPA1 channels -> K+-influx and depolarization of outer hair cells - Contraction of outer hair cells will require a motor protein = Prestin
Prestin is part of the anion transporter family, but not an anion transporter
- Does not require ATP or Ca2+
- Conformational change-> contraction of outer hair cells (= electrochemical transducer, since they convert electrical signal of depolarization into mechanical signal)
- Hyperpolarization causes opposite effect -> dilation/elongation of outer hair cells - Contraction of outer hair cells enhances upward movement of basilar membrane
-> this is why outer hair cells acts as a cochlear amplifier (if Prestin is knocked out, hearing is not possible = deaf) - Since the basilar membrane is close to the tectorial membrane, the compartment between is reduced -> the endolymph produced will flow out from the inner sulcus
- Flow will have an effect on the inner hair cells -> the hair cell bundle will bend toward the longest stereocilium -> inner hair cells get activated
- Depolarization – activation of VD-Ca2+-ch. -> exocytosis -> release of glutamate
- Afferent neurons are activated
and can generate APs, which are transmitted to the CNS
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7B. What happen if basilar membrane moves upward?
Basilar membrane moves upward – causing the organ of Corti (located on bas.membr) to move towards the tectorial membrane
- Since the stereocilia of the outer hair cells are connected to the tectorial membrane, this movement will tilt the stereocilia in the direction of the longer stereocilia. This leads to opening of the TRPA1 channels -> K+-influx and depolarization of outer hair cells
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7B. What happen if basilar membrane moves upward?
Basilar membrane moves upward – causing the organ of Corti (located on bas.membr) to move towards the tectorial membrane
- Since the stereocilia of the outer hair cells are connected to the tectorial membrane, this movement will tilt the stereocilia in the direction of the longer stereocilia. This leads to opening of the TRPA1 channels -> K+-influx and depolarization of outer hair cells
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7C. What happen if Pressure in the middle ear↓?
Pressure in the middle ear↓
-> stapes moves outward
-> ↓pressure in scala vestibuli (perilymph) = ↑pressure in scala tympani
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7D. What does Contraction of outer hair cells require?
Contraction of outer hair cells will require a motor protein = Prestin
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7E. What are the features of Prestin?
- Prestin is part of the anion transporter family, but not an anion transporter
- Does not require ATP or Ca2+
VI. Mechano-electrical transduction in the hair cells - Mechanism of activation (of auditory nerve cell)
7F. What are the consequences of Contraction of outer hair cells?
Contraction of outer hair cells enhances upward movement of basilar membrane
-> this is why outer hair cells acts as a cochlear amplifier (if Prestin is knocked out, hearing is not possible = deaf)
VII. Sound identification
1. What is RATE CODING?
The rate of AP frequency will be increased with more intense sound. This is called “rate coding” because the frequency of action potentials correlates with intensity (as opposed to amplitude)
VII. Sound identification
2. What is PLACE CODING?
Movement of (specific areas of) the basilar membrane depends on the frequency of the sound. This is called “place coding”
1. The structure of basilar membrane is not totally the same throughout its length.
- Width is increased closed to apex. Also, it becomes stiffer at origin compared to the end.
- Lower frequency sound will cause movement of the basilar membrane close to the apex.
- Higher frequency sound moves basilar membrane at the basal end
2. It is important to distinguish between the fact that the frequency of APs corresponds to the intensity of sound, while the frequency of sound waves corresponds to the place of displacement in cochlear membrane.
VII. Sound identification
3. What are features of spatial localization?
- There is a difference in the distance of when the sound reaches the ears, with lower frequency sounds reaching longer.
- This results in a phase difference. Because of this phase difference, the CNS interprets the difference in origin of sound, and is then able to localize it
VIII. Innervayion
1. Describe afferentation of auditory system?
- 95 % afferent fibers are from the inner hair cells – so these are the real receptors
- Body of neurons forms the spiral ganglion, while the axons of nerve cells form the cochlear nerve.
- First nucleus is cochlear nucleus – for simple processing of auditory information.
- Then it goes to both superior olivary nuclei – for complex processing.
- At last, it goes to the auditory cortex – Brodmann 40/41.
VIII. Innervayion
2. Describe efferentation of auditory system?
Efferentation: Has a protective role, with two targets:
1. Muscles – Tensor tympani, Stapedius – contraction reduces movement of ossicles.
2. Outer hair cells – Receive efferent fibers (inner hair cells do not). They receive ACh as NT, and outer hair cells have nicotinic receptors. ACh leads to depolarization, causing Ca2+ influx and activation of Ca2+-dependent K+ channels -> hyperpolarization. This works against any future depolarization of the outer hair cells, which means the electromechanical transduction is decreased.
