Auditory System Flashcards
Explain how a fluid wave generated by the stapes acting on the membrane of the oval window results in hearing.
Wave flows through scala vestibuli and scala tympani, which are above and below the organ of corti. As the wave travels through these structures with a specific amplitude and frequency, it disturbs the basilar membrane on which the organ of corti sits. This disturbance has the same amplitude and freqency as the wave in the perilymph.
Hair cells, which are mechanoreceptors, are sitting on top of the basilar membrane and they are attached to the tectoral membrane also. They are more securley attached to the tectoral membrane than they are the basilar membrane, so when the basilar membrane is deformed the hair cells shear and move the tectoral membrane with them in a specific manner that causes the depolarization or hyperpolarization of the hair cells and results in an IPSP or EPSP.
Explain what happens inside a hair cell to result in transmission of a neural signal to the brain.
Hair cells are bent –> K+ enters through mechanically gated K+ channels –> generates larger depolarization that travels down to base of hair cell –> Voltage gated Ca2+ channels open –> Ca2+ enters –> Ca2+ helps traffic vesicles containing glutamate to the synapse of the hair cell with an afferent nerve of CN 8 that will carry the information to the brain.
What is the structure and function of the middle ear?
What is their relationship to the tympanic membrane?
Structure: ossicles = small bones (malleus, incus, stapes)
Function: amplify sound that hits tympanic membrane
Malleus is attached to tympanic membrane.
Why is sound measured in decibels?
Decibels is a unit that is a conversion of pressure, or pascals. Sound amplitude is measured in pascals, but the order of magnitude of pressure for the range of human hearing can be very small so it’s hard to express in Pa. As such, it is converted to decibels as: dB = 20 log (actual pressure/reference pressure).
In general, what is the function of the superior olivary complex?
To be able to know where sound is coming from in space.
Describe the tonotopic organization of the basilar membrane and the cortex.
The basilar membrane is contained within the cochlea. If the chochlea were to be unrolled, there would be an end closest to the oval window and an end furthest away. The part of the basilar membrane that is closest to the oval window is more sensitive to high frequency soundwaves and the part farthest away is more sensitive to low frequency soundwaves. This organization is maintained in the cortex with part of the auditory cortex being more sensitive to these types of soundwaves that come from hair cells along the basilar membrane.
Depolarization of hair cells is caused by […]
K+
Describe the structure of the outer ear and its function in hearing.
The pinna are the folds of the outer ear. They direct waves into the auditory canal to vibrate the tympanic membrane.
What is the role of the primary auditory cortex compared to the secondary auditory cortex (i.e. auditory association cortex)?
Explain the pathway that sound takes from the afferent nerve endings in the left ear to the right auditory cortex.
Left Afferent nerve endings –> left spiral ganglion –> projects to left ventral and dorsal cochlear nuclei in rostral medulla
- Axons from left dorsal cochlear nuclei ascend to mid pons, cross to right, and ascend to right inferior colliculus via the lateral lemniscus. From there, they synapse on a neuron that projects to the right MGN of thalamus, which synapses on a neuron that projects to right auditory cortex
- Axons from left ventral cochlear nucleus bifurfate and send projections to both the left and right superior olivary complexes. From there, they ascend in the left and right lateral lemnisci to the left and right inferior colliculi where they synapse on neurons that project to ipsilateral MGN of thalamus and then synapse on neurons that project to ipsilateral auditory complexes.
Describe the pathway of information from hair cells to spiral ganglion to cochlear nucleus.
- One inner hair cell projects to multiple spiral ganglion cells.
- Multiple outer hair cells project to one spiral ganglion cell
- All sprial ganglion cells project to cochler nucleus
- Efferent projections from superior olivary complex to spiral ganglion cells to outer hair cells to fine tune frequency coding by altering stiffness of tectorial membrane
Describe the tonotopic organization of the primary auditory cortex as you move more anteriorly.
More posterior = higher frequency
More anterior = lower frequency
Sound amplitudes above […] dB cause pain in the ear.
140
What are 2 ways that we are able to localize sound?
- Intraural time difference - sound coming from my left will take slightly longer to reach my right ear than my left and the brain is able to detect that very small difference in time
- Intensity difference - sound coming from my left will be processed as louder by my left ear than my right ear
Describe the function of the nasopharynx and the eustachian tube.
The middle ear functions best when the pressure inside is the same as the pressure outside. The nasopharynx is usually closed but it opens during swallowing, yawning and when flying in an airplane to relieve pressure in the middle ear.
Explain how sound is amplified from the tympanic membrane to the oval window.
The sound wave has a specific amplitude, or pressure associated with it. It strikes the tympanic membrane, which has a finite area. As such, each sound wave transmits a specific force to the malleus, which transmits it to the incus and to the stapes, which then transmits the same amount of force (if a perfect system with no loss) to the oval window which has a smaller surface area. Same force divided by smaller surface area = bigger pressure aka bigger amplitude –> amplification.
Where is Wernicke’s area?
What does it do?
In secondary auditory cortex, just posterior to primary auditory cortex
Comprehending speech
In the organ of corti, there are inner and outer hair cells. What is the difference between these cells?
The inner hair cells are the actual sensory receptors, the outer hair cells sharpen the frequency tuning by changing the stiffness of the tectoral membrane.
Damage to bones of the inner ear results in what condition?
Conduction deafness
The cochlear duct is filled with […] and contains the […], which is the sensory organ of the ear.
Endolymph
Organ of corti
What is the function of the inner ear?
It contains the neural machinery of the cochlea
What is the function of the round window?
The vibration from the membrane of the oval window has to be able to generate a wave in the fluid of the cochlea or else there will be no transduction of the sound wave to the part of the ear that is responsible for hearing. Fluid cannot be moved if the system were closed, so the round window is another membrane enclosed opening that allows a place for the vibration from the fluid to leave the cochlea.
Describe the pathway of sound from external to internal ear structures.
Sound wave –> directed to external auditory canal by pinna –> hits tympanic membrane with specific amplitude and frequency –> moves malleus –> moves incus –> moves stapes –> knocks on membrane of oval window with specific amplitude and frequency –> generates waves in fluid of inner ear with specific amplitude and frequency –> fluid moves hair cells of cochlea –> hair cells have mechanoreceptors that generate neural impulses in response to movement and send to cochlear nuclei in brainstem and up to MGN of thalamus and up to primary auditory cortex of brain
- What is a soundwave?
- What is the amplitude of a soundwave?
- A succession of compressions (higher density of air molecules) and rarefactions (lower density of air molecules)
- Amount of air pressure
What are 4 sources of damage to hair cells and thus hearing?
Noise damage
Age related hair cell loss (high frequency)
Antibiotics
Chemo and other drugs
If sound is directed immediately in front of us or directly behind us, can we tell where the sound is coming from?
No
The bony labyrinth of the cochlea is filled with […]
Perilymph
- What 2 components of sound is the ear responsible for capturing?
- What component of sound is the brain responsible for capturing?
- Frequency (pitch) and intensity (loudness)
- Localization of the sound in space
Conductive hearing loss
- How can a person develop this problem?
- What test can be used to check for conductive hearing loss?
- How do you perform the test and interpret the results?
- Wax build up or problem with ossicles
- Rinne test
- Ring a tuning fork and hold it next to their ear. Then ring it again and hold it against their mastoid process. Ask them which sounded louder. If the person is healthy - the tuning fork should sound loudest when held next to the ear. If the person has conduction hearing loss, the tuning fork will sound loudest when held to the mastoid process.
Sensorineural hearing loss
- What can cause this condition?
- How can you test for this condition?
- How do you conduct the test and interpret the results?
- Disorder of the cochlea or CN 8
- Weber test
- Ring the tuning fork and hold the base of it against their forehead at the midline of the skull. Ask the patient in which ear it sounds loudest. A normal person will say it sounds equal in both ears. A person with this condition will say that it sounds louder on one side (the unaffected side)
