Neuro 10: Sound conduction and transduction Flashcards
Define sound,
Sound = pressure wave in air
Define frequency
cycles/ sec (Hz), perceived as pitch
Amplitude
intensity, perceived as loudness
Decibel range
log scale of loudness (i.e. amplitude)
What structures include the outer ear
Pinna (auricle) + external auditory meatus
Function of the outer ear
Collects and conducts sound waves towards tympanic membrane
What are the boundaries of the middle ear
Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially
What structures are in the inner ear
Cochlea + organs of balance
Function of the inner ear
Hair cells transduce mechanical energy of sound into electrical signal in cochlear nerve
What is contained within the air space in the middle ear
The 3 bones of the ear: the maleus, incus and stapes
Which structure contiues on from the tympanic cavity (i.e middle ear)
The eustachian tube which links to the nasal cavity (=AUDITORY TUBE)
What is the function of the middle ear (and ear in general)
AMPLIFY the sound (conduction through middle ear amplifies sound by 30dB)
How is amplification achieved
lever system of articulated ossicles and ratio of area of tympanic membrane to oval window
Protective mechanism
- Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles.. these muscles stabilise the tympanic membrane and the stapes respectively
- Auditory tube allows equilibration of air pressure on either side of tympanic membrane
Define conductive hearing loss
Sound prevented from reaching cochlea
5 types of conductive hearing loss
- wax
- otitis media
- otosclerosis of ossicles -perforated tympanic membrane
- congenital malformations
When rolled out, what is the top compartment of the chochlea called
scala vestibuli
When rolled out, what is the middle compartment of the chochlea called
Scala media (bound by membrane)
When rolled out, what is the lower compartment of the chochlea called
scala tympani
State which fluid is present in the scala vestibuli, scala media and scala tympani
vestibuli and tympani is perilymph, media is endolymph
Whcih membrane separates the scala vestibuli and scala media
vestibular (=Reissner’s) membrane
Whcih membrane separates the scala tympani and scala media
basilar membrane
Where does the stapes bone articulate to the cochlea
at the oval window (it pushes it in and out, which sends vibration through the scala vestibuli then the scala tympani, so the round window pops out)
What is the round window
The area of cochlea which can pop out when stapes pushes in.. is is the fluid in scala tympani pushing out into the round window (see video)
Where are cochlear hair cells attached to
BASILAR MEMBRANE
What comprises the organ of corti
Hair cells
Tectorial membrane (which overlies the hair cells… hair cells lie between tectorial membrane and basillar membrane)
Spiral ganglion
Stria vascularis
Where is the spiral ganglion located
ebedded int he modiolus….. innervates hair cells
Function of stria vascularis
secretes endolymph
Chemical properties of endolymph
secretes endolymph (high in K+, low in Na+)
What is the ultrastructural property of the hair cell
Have many stereocilia per cell
State 2 types of hair cell
inner hair cell – about 3,500 cells arranged in single row, densely innervated by about 10 sensory axons/cell
outer hair cell – about 20,000 cells arranged in 3 rows sparsely innervated: one axon inervates several cells
Which hair cell provides information to the brain
Inner hair cells
Role of outer cells
Efferent nerve causes cell to change shape, which in turn amplifies the response to sound of adjacent inner hair cells at the centre of the vibration… ELECTROMOTILITY (mechanical response which drives oscillations in the cell’s length, which occur at the frequency of the incoming sound and provide mechanical feedback amplification)
TRANSDUCTION:
what does the basilar membrane do in response to sound
Vibrates
TRANSDUCTION:
what happens in upward movement of the basilar membrane
Upward movement displaces stereocilia away from modiolus → K+ channels open → K+ enters from endolymph → hair cell depolarises
TRANSDUCTION:
what happens in downward movement of the basilar membrane
Downward movement displaces stereocilia towards modiolus → K+ channels close → hair cell hyperpolarises
Why is stria vascularis important in sound transduction
Because movement of K+ into cell depends on maintenance of endolymph at +80 mV by stria vascularis
TRANSDUCTION: Effect of depolarisation in the body of the hair cell
Opens Ca++ channels in body of hair cell
TRANSDUCTION How is a nerve activated attached to the inner hair cell
Depolarisation leads to Ca2+ release inside cell, this causes glutamate to be released from base –> depolarises axon of spiral ganglion cell → action potential
Where in the chochlear are different frequencies picked up
Basilar membrane acts as frequency analyser
High frequencies vibrate basilar membrane nearer to base
Low frequencies vibrate membrane nearer to apex
TONOTOPIC ORGANISATION
At what range can humans hear
20 Hz – 20 kHz
most sensitive 1-3 kHz
Define tonotopy
spatial organisation of response to frequency is preserved throughout pathway
Outline the auditory pathway from cochlea to cortex
Spiral ganglion –>
ipsilateral ventral and dorsal cochlear nucleus (still tonotopic organisation in this nucleus) –>
From cochlear nucleus, fibres project to ipsilateral AND contralteral superior olivary nuclei –>
So then from superior olive to inferior colliculus (now containing mix from both ears… still inbrainstem –>
To medial geniculate body in the thalamus –>
Auditory cortex in the cerebral hemisphere
http://pages.ucsd.edu/~johnson/COGS17/17Mnemonics.html
Where else do fibres project apart from this central pathway
Collateral pathways to reticular formation and cerebellum
Function of lateral inhibition in the ascending pathway
ascending pathway enhances resolution of similar frequencies
Function of inferior colliculi
reflexes eg. startle, head turn
T/F.. there is no descending pathway in the auditory pathway
FDescending pathways provide feedback at all levels
How is primary auditory cortex organised
subdivided according to frequency response
What do cells in the PAC respond to
pecific features of sound e.g. on/off, duration, repetition, intensity + some more complex sound patterns e.g. rising/falling frequencies, animal vocalisations
Function of secondary cortex
Secondary cortex – neurones respond to more complex sound patterns
Causes of sensorineural deafness (sensory)
presbyacusis Exposure to loud noise Ménière’s disease Toxicity e.g. some antibiotics Hereditary disorders
Causes of sensorineural deafness (neural)
Acoustic neuroma
Viral infection
Causes of sensorineural deafness (central)
Demyelination in MS
Injury to central auditory pathway (unlikely to cause serious deafness unless both auditory cortices affected)
What is the auditory reflex
involuntary muscle contraction that occurs in the middle ear in response to high-intensity sound stimuli or when the person starts to vocalize.
The stapedius and tensor tympani muscles of the ossicles contract.[1] The stapedius stiffens the ossicular chain by pulling the stapes (stirrup) of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the tympanic membrane when it pulls the malleus (hammer) in toward the middle ear.
Stapedius=facial
tensor tympani= V3
What is sound intensity and how is that measured
the power carried by sound waves per unit area
(joules/m2)
Decibel scale is used, which is a log scale of loudness
What is the impedance
The reluctance of a system to receive energy from a source. Ossicles have to match the impedance of the sound wave and reduce energy loss when transferring the sound from the ear bones to the cochlea.
Outline the impedance of the cochlea
The impedance of the basilar membrane varies along its length, meaning that so does the local resonant frequency
What is the resonant frequency
The frequency at which the
impedance of the system is
minimal is called the
resonant frequency.
What is mechanotransduction
This is referring to the point at whch vibrations cause deflection of the hair cells against the tectorial membrane, leading to depolarisation (if the cells are deflected AWAY from the modiolus, leading to K+ influx)
