Neuro - auditory system 1 + 2 Flashcards
What ear structures in the outer ear are involved in frequency amplification?
Which frequencies are affected?
What is important about these frequencies?
Structures: Pinna and Channel
Frequencies: 3000 Hz
Importance: this frequency range is important for speech
What is the role of Pinna? (2)
1) Passive process - Filters and channels sound to external canal
2) Amplifies frequency around 3000 Hz
What structures make up the middle ear? (4 - in order from outer to inner)
1) tympanic membrane
2) Malleus
3) Incus
4) Stapes
What is the structure of the tympanic membrane? (3)
3 layer translucent membrane
1 cm^2 vibratory surface
cone shaped
What causes the impedance mismatch?
Transmitting sound pressure wave from air (outer ear) to fluid (inner ear) is difficult and requires higher force
What are the mechanisms for Impedance matching? (3)
1) area ratio
2) lever action
3) buckling of tympanic membrane
What is the total gain of middle ear impedance matching?
31 dB
How does area ratio help impedance mismatch? What is the gain from this process?
The tympanic membrane is 14 times bigger than the oval window. Force is transduced through the Malleus –> Incus –> Stapes –> Oval window
Force is concentrated onto small area causing higher pressure
Gain: 23 dB
How does lever action help impedance mismatch? What is the gain from this process?
The length of the malleus is longer than the process of the incus creating a lever action. Ratio is 1.3 to 1
Gain: 2 dB
How does buckling of tympanic membrane help impedance mismatch? What is the gain from this process?
Buckling of the TM when it vibrates applies almost twice the force to the malleus
Gain: 6 dB
What are the three chambers of the cochlea?
Scala vestibuli
Scala media
Scala tympani
Where is Reissner’s membrane located?
Bottom part of scala vestibuli
Where is the Basilar membrane?
In between the Scala media and Scala tympani.
Where is the Organ of corti?
Sits on the basilar membrane
Where are the cells for hearing (inner and outer hair cells, etc)?
Makes up the Organ of corti
Where is the stria vascularis?
What is its role? (2)
The lateral edge of the Scala Media
Role: blood supply to cochlea and responsible for ionic concentration and cochlea potential
Known as the “Cochlear battery”
Where is the spiral ligament?
Lateral most of scala media
What are Deiters cells?
These cells support each outer hair cell
What is the reticular lamina?
Keeps the outer hair cells and inner hair cells seperate from the fluid in the scala media
What are the Pillars of corti?
separate the 1 layer of inner hair cells with the 3 layers of outer hair cells
What is the tunnel of corti?
Space between 1 layer of inner hair cells and 3 layers of outer hair cells
Where is the Tectorial membrane?
On top of the stereocilia of the hair cells
What is the sprial lamina and what does it contain?
Bony shelf coming out from the modiolus and attaches to basilar membrane. Auditory nerve fibers go through and innervate nerve cells.
What is the structure of the Basilar membrane?
when uncurled: the base is narrow and stiff, the apex is wider and less stiff
How does the Basilar membrane work?
A sound wave will travel up the basilar membrane from the oval window - base to apex. The point of maximal deflection will be the pitch you want to encode
What is tonotopic organization?
Different frequencies maximally displace the basilar membrane at different points
High frequencies at base to low frequencies at apex
Which direction does an upward phase of a sound wave deflect the hair cells and stereocilia?
Hair cells: pushes up
Stereocilia: Lateral force of tectorial membrane pushing stereocilia away from modiolus
Which direction does a downward phase of a sound wave deflect the hair cells and stereocilia?
Hair cells: downward
Stereocilia: Lateral shear force of the tectorial membrane pushing stereocilia towards modiolus
What is the active mechanism of the cochlea?
Outer hair cells have motile properties - dynamic microtubules and microfilaments along membrane that allow contractile behaviors
How is the mechanical energy of the hair cells translate into electrochemical energy?
ANSSEWER
Which cells do afferent fibers of the spiral ganglion cells synapse?
How do these afferent fibers synapse?
95% Inner hair cells (multiple fibers per inner hair cell)
5% outer hair cells (one fiber synapses on multiple outer hair cells)
What is the fluid in the scala tympani called? What is it made of?
Name: perilymph
Made of: low K+, 0 mV
What is the fluid in the Scala media called? What is it made of?
Name: endolymph
made of: High K+, 80 mV
What is the fluid in the inner hair cells made of?
Low K+ and -45 mV
How is fluid seperated between the inner hair cells and scala media?
reticular lamina
How does a downward wave defection effect the stereocilia? (3)
1) Deflects stereocilia towards the modiolus (away from taller stereocilia)
2) Tip links closes pores
3) K+ can’t get in and hair cells are hyperpolorized
How does an upward wave effect the stereocilia?
1) stereocilia deflected away from mediolos (towards taller stereocilia)
2) Tiplinks open pores and positive K+ ions from endolymph flow into hair cells
3) hair cells depolarize and release neurotransmitter
What is an audiogram?
Hearing level in patients is plotted to determine what they can hear.
Softest sound that can be detected at each frequency is plotted. Anything below line (and thus louder at each frequency) can be heard
What are the types of hearing loss?
Conductive loss
Sensorineural
What is conducive hearing loss?
Treatments?
Something blocking the external or middle ear
Treatments: surgery, medication, sometiems can’t be treated
What is sensorineural hearing loss?
Causes? (4)
Treatment?
Involves structures of inner ear and cochlea.
Causes: Congenital, Noise exposure/trauma, Medication, age
Treatment: permanent, not much can be done to cure, can’t restore structures
Where do auditory nerves synapse?
Cochlear nucleus
What are the functions of the two pathways that originate in the cochlear nucleus?
1) Recognition of sound patterns
2) Localizing sounds
What is the pathway for recognition of sound patterns starting from the cochlear nucleus?
Dorsal + Ventral coclear nucleus (DCN and VCN) –> Contralateral Inferior colliculus –> medial geniculate nucleus (MGN) –> primary auditory cortex
What is the pathway for localizing sounds starting from the cochlear nucleus?
Ventral cochlear nucleus (VCN) –> Superior olivary complex (SOC on both sides) –> Inferior colliculus (IC) –> Medial geniculate nucleus (MGN) –> primary auditory cortex
What is the lateral lemniscus?
carries sound pattern information from Dorsal and Ventral cochlear nuclei to contra-lateral inferior colliculus
Where is the first place for binaural convergence?
Superior olivary complex
Where is the superior olivary complex (SOC)?
In the pons
Where is the medial geniculate nucleus?
In the thalamus
Where is the auditory cortex?
Broadman’s area?
Area of brain?
Broadmans area: 41
Area of brain: superior temporal lobe - Heschel’s gyrus
Why is topography and laterality of limited use for diagnosis of pathology? (2)
1) Because the only place in the brain NOT binaural is cochlear nucleus
2) duplication of pathways makes it difficult to selectively cut afferents from one ear alone (unless at periphery or from cochlear nucleus)
Where is Wernicke’s area and what does it do?
Where: left hemisphere in superior gyrus of temporal lobe
Purpose: speech interpretation
What is the characteristic frequency of a cell?
It is the frequency in which a cell maximally responds
What is a frequency tuning curve?
A graph that describes how well a cell responds to higher and lower frequencies
What is the result of an INCREASE in sound intensity in regards to a tuning curve?
the curve broadens
What is the result of an DECREASE in sound intensity in regards to a tuning curve?
the curve narrows
What is the purpose of broca’s region?
Important for speech production (the motor aspect of speech)
Where is area 41/A1 (auditory cortex)?
buried in the lateral sulcus
What is unique about cells in the auditory cortex relative to cells in other parts of the auditory pathways?
Cells in auditory cortex can be selectively responsive to complex features of sounds
Example: some cells are selective for speech components (dynamic beginning followed by sustained tone)
What are the two factors that allow the brain to localize sound?
1) Extra distance between the sounds going to each ear
2) “shadowing” produced by the head makes sound going into one ear more intense than the other
Which brain region contains the cellular structures to localize a sound?
Superior olivary complex (SOC)
What is temporal coincidence circuitry?
Mechanism for which the superior olive can create a neural code for a location in space.
A cell in the superior olivary complex will fire maximally if a nerve impulse fire on it at the same time from both ears. Cells are arranged to receive inputs from each ear at different times.
