Week 44- Balance and Hearing Flashcards
What is sound?
Air molecules compressed with forward movement and becoming less dense when the object moves back
What is Hertz?
Cycles per second:
Distance between successive regions of compressed air molecules (peaks in the wave)
What frequencies of sound can be heard in humans?
20-20000Hz
What determines the loudness/softness of a sound?
Pressure wave –> more pressure means loud (higher amplitude)
What is the entire role of the hearing apparatus?
Converts sound –> vibrations –> neural signals
What structure of the ear collects sound pressure waves?
The pinna of the ear –> directed along auditory canal to the tympanic membrane
What is the role of the tympanic membrane?
Vibrates in response to the sound pressure wave
Separates the outer ear from the middle ear
What is both the outer and middle ear filled with?
Air filled
What is the role of the ossicles?
They are small bones that transfer the vibrations of the tympanic membrane to the oval window (and amplify them)
Where is the oval window located?
Base of the cochlea of the inner ear –> vibrations are converted to neural signals here
What is the general path for sound detection?
Waves of air pressure –> moves tympanic membrane back and forth
Tympanic membrane –> vibrates ossicles
Ossicles amplify the force greatly –> transfer the oval window
Movement in the oval window moves fluid in the cochlea
Fluid movement in cochlea converted to neural signals
Neural signals go to the brainstem then onwards to the auditory cortex
What is the ossicle which contacts the oval window?
Stapes
What does the cochlea wrap around?
Central pyramid –> containing nerve cell bodies of the spiral ganglion
How many spiral ganglion cells is there in humans?
35,000-50,000
What does a cross section of a cochlea look like?
3 chambers separated by tissue membranes
Where do the sensory detector cells sit within the cochlear?
Basilar membrane
What area of the cochlea is on the other side of the oval window?
Scala vestibuli
What area is the basilar membrane located?
Scala media –> adjacent to the scala vestibuli chamber
What is the structure of the membrane between the scala vestibuli and the scala media like?
Very flexible –> so when pressure waves propagate through the scala vestibuli both of the membranes vibrate
What is the structure of the cochlea at its base (near the oval window)?
Narrow and stiff
What is the structure of the cochlea at its apex (away from the oval window)?
Wide and floppy
What is resonance?
A property of a system to oscillate at a greater amplitude at some frequencies than at others –> known as resonant frequencies
What happens at a systems resonant frequency?
The system absorbs vibrational energy –> even small forces can produce large amplitude oscillations
How are different frequencies of sound detected by the basilar membrane?
Resonant frequencies on the basilar membrane vary along the length of the cochlea from base to apex
Where are high frequency sounds detected?
The base of the cochlea –> the start
Where are low frequency sounds detected?
The apex of the cochlea –> the end
What basilar membrane structure is needed for high frequency sound?
Narrow and stiff
What basilar membrane structure is needed for low frequency sound?
Wide and floppy
What is on the basilar membrane that actually detects the fluid wave?
The organ of Corti
What does the organ of Corti detect?
The location of maximal deformation for a frequency –>converting it to a neural signal at that point
What are the two hair cells within the organ of Corti?
Inner hair cells
Outer hair cells
What is the role of the outer hair cells in the organ of Corti?
They act as amplifiers
What is the role of the inner hair cells in the organ of Corti?
The terminals of the spiral ganglion cells are mostly located on the base of the inner hair cells
How does the hair cells of the organ of Corti detect pressure waves?
Cilia on the apical membrane of the hair cells is deflected in response to sound/pressure waves
What membranes deflect the cilia within the organ of Corti?
Relative movement between the tectorial membrane and the reticular lamina
How is the cilia movement converted into a neural signal?
Cilia movement drives the hair cell to depolarise or hyperpolarise –> depending on direction of cilia movement
Depolarisation –> neurotransmitters released –> terminals on the spiral ganglion depolarise –> axons on the spiral ganglion enter the brain –> give info on pitch and loudness
How are loud sounds converted and conveyed as loud in the brain?
Loud sounds –> pressure waves of greater amplitude
Basilar membrane vibrates with greater amplitude
Larger area of maximal deformity –> more hair cells depolarise
Terminals on the spiral ganglion depolarise and fire action potentials at greater rate
How does a cochlear implant function?
- External sound processor captures sound and converts to digital format
- Signals sent through the coil to the internal implant
- Internal implant converts signals to electrical impulses –> bypassing damaged hair cells within the cochlea
- Hearing nerve delivers the signal to the brain –> allowing sound to be heard
What is the path from the cochlear to the auditory cortex?
Spiral ganglion CN8 --> vestibulocochlear Brainstem --> Ventral cochlear nucleus then superior olive Midbrain --> inferior colliculus Thalamus Auditory cortex
What side of the cochlear nucleus cells do the spiral ganglion cells synapse at?
Ipsilateral
What is tonotopy in the brain?
There are auditory pathways that are activated by the same frequencies –> they end at the auditory cortex in certain positions –> giving a “frequency map” on the brain or “tonotopy”
What sides does the cochlea nucleus receive information?
From both ipsilateral and contralateral superior olive region