Auditory system 10. Flashcards
What is sound?
A transverse wave consisting of compressed or rarefied air characterised based on frequency and pitch
Loudness based on amplitude
Measured in decibels which is a logarithmic scale.
What range of sound can humans hear?
20-20,000 Hz
Log scale allows huge range of sound to be plotted more easily
What happens as you get older?
Hearing decrements
What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?
Pinna
Describe the shape of the outer ear and its importance.
It is conical, starts off wide at the external acoustic meatus and narrows to the tympanic membrane.
This focuses the noise and increases the pressure on the tympanic membrane
What are the main causes of hearing loss?
10% of UK population affected
Main causes are:
Loud traumatic sounds(military, industrial, clubs)
200 genetic conditions
Infections - meningitis or congenital ones such as rubella or syphillis
Drugs use for severe heart infections and chemo
Ageing
What is pitch?
The perception of frequency
Distinguishing two sounds at the same frequency and intensity
Is the tympanic cavity air filed or fluid filled?
Air filled
What does the shape of the pinna indicate?
Gives you an idea about the elevation of the sound
Floor level or ceiling level, depends on how frequencies bounce through pinna
How does ear detect sound?
In air
series of mechanical couplings projects stimuli to hair cell which is sensory receptor in the internal ear
Define the boundaries of the middle ear
Between tympanic membrane and the cochlea
What are ossicles and how are they involved in transmission
Smallest bones in the body
malleus, incus, stapes
Improve signal from vibration of the tympanic membrane and transmit onto the cochlea
State 2 ways in which the ossicles increase the pressure of vibration of the tympanic membrane.
Focussing the vibrations from the large surface area of the tympanic membrane to the small surface area of the oval window – this decrease in surface area means that the pressure is increased
The incus has a flexible joint with the stapes, such that the ossicles use leverage to increase the force on the oval window
This amplifies the sound by 30 dB
Define impedance
Measures the reluctance of the system in receiving energy from the source
What is resonant frequency?
The frequency at which the impedance of the system is minimal
When transfer of energy is optimal
What is role of the ossicles?
They match the impedance and reduce loss of energy from air tympanic to cochlea (fluid filled)
- What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?
The cochlea contains fluid, in which you are trying to induce a pressure wave
If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance
Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction
What 2 muscles are involved in making sure that the ossicles aren’t damaged by excessive vibration due to loud noise?
Tensor Tympani
Stapedius
Contraction of these muscles reduces movement of the ossicles
AUDITORY REFLEX
What is the latency period of the auditory reflex?
50-100ms
What is conductive hearing loss and what can cause it?
When ear is not capable of transmitting the vibration of sound waves onto cochlea
Cerumen, infections(otitis), tumour can all affect transmission
Fluid accumulation in kids
Tympanic membrane puncture
otosclerosis ( growth of bone) obstructing canal
Barotrauma which is temporary
What is hyperacusis?
Painful sensitivity to low intensity sounds – can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy)
What are the 2 specialised membranes of the cochlea?
Oval Window
Round Window
What are the three compartments of the inner ear?
Scala Vestibuli
Scala Media
Scala Tympani
Which types of fluid do each compartment contain?
Scala Vestibuli + Scala Tympani = perilymph
Scala Media = endolymph
What is the significance of motion of stapes?
Generates a difference in pressure between two liquid filled chambers of the cochlea which causes vibration of basillar membrane
What does the organ of corti contain and what is its function?
Basilar and tectorial membranes
Inner and outer Hair cells
Supporting cells
converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Describe how the cochlea functions.
The vibration of the tympanic membrane is conducted and amplified to a vibration of the oval window by the footplate of the stapes.
This vibration induces a pressure wave in the perilymph in the scala vestibuli.
This vibrates the scala media leading to vibration of the basilar membrane.
The round window vibrates as well to equalise the pressure in the cochlea.
The cochlea is made of bone so fluid is in compressible thus, when oval window vibrates inward the round window (below it) vibrates out to equalise the pressure.
What structure connects the two perilymph compartments?
Helicotrema
Is impedance same along basilar membrane?
No
Local resonant frequency will also change as a result
Describe the difference in sensitivity of different parts of the basilar membrane.
Higher frequency sounds = base
Lower frequency sounds = apex
Where is the Organ of Corti found?
It lies on top of the basilar membrane and beneath the tectorial membrane
Why is the basilar membrane described as a frequency analyser?
Elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies.
Breaks complex sounds down by distributing energy of each component frequency along length (Tonotopic map)
What are the receptors on basilar membrane and why are they all across the membrane?
They are hair cells
Needed all across due Tonotopic mapping allowing us to detect all frequencies
Organ of corti detects deflection in basilar membrane
What is mechano-transduction?
The bending of stereocilia towards the tallest stereocilium changes the internal voltage of the cell, produces an electric signal that travels to brain
What is the role of tip links?
Stereocilia connected by filamentous linkages called tip links, work as small springs stretched by stereocilia sliding
Opening of MT ion channels relaxes tip link and whole hair bundle
Healthy hair bundle complies with direction of stimulus, measured stiffness becomes negative when channels open
ACTIVE PROCESS
Describe the features and function of inner hair cells.
Found on their own
Not in contact with the tectorial membrane
Send impulses to the brain
They have stereocilia that move in response to the movement of endolymph in the scala media
Roughly 3500 in the body
Describe the features and function of outer hair cells.
Found in groups of three
They are in contact with the tectorial membrane
They receive input from the brain
Electromotile so can expand and contract to amplify the amount of vibration (this is the basis of the cochlear amplifier)
Damage can result in sensorineural hearing loss
Roughly 20,000 in the body
Which compartment of the cochlea does the stereocilia of the hair cells project into?
Endolymph
base is in the perilymph
What happens when an inner hair cell moves due to movement of endolymph?
Depolarises and K+ channels open
UPWARD movement of basilar:
displaces stereocilia away from modiolus, K+ open, K+ into endolymph Hair cell depolarises
DOWNWARD BM = K+ close, hyperpolarise
Describe the difference in K+ and Na+ concentration in the different compartments of the cochlea.
Scala Media = High K+ and Low Na+
Scala Tympani = High Na+ and Low K+
NOTE: stria vascularis maintains this concentration
What is significance of inner hair cells and outer hair cells with regards to projections?
95% of afferent projections are from IHCs
Most efferent projections (from brain to cochlea) connect to OHCs, OHCs are responsible for otoacoustic emissions noises that ear makes itself
What is one of the sources of active process of OHCs?
Their cell bodies shorten and elongate when their internal voltage is changed which is called electromotility to reorientation of protein prestin
What is sensorineural hearing loss and what are some of its causes?
Problem in cochlea (sensory apparatus) or vestibulocochlear nerve, most widespread type of hearing loss
Loud noises
Genetic mutations affecting organ of corti
aminoglycoside antibiotics toxic for hair cells
congenital disease - rubella
acoustic neuroma - tumour of cochlear nerve
Ageing- presbycusis
acoustic schwannoma (tumour of the cochlear nerve)
HAIR CELLS DONT REGENERATE IN MAMMALS
Discuss nerve fibre transmission to cochlear nucleus
Hair cells form synapses with sensory neurons in the cochlear ganglion (spiral ganglion). Each ganglion cell
responds best to stimulations at a particular frequency. The tonotopic (sound-location) map continues.
Describe the auditory pathway from the cochlea to the primary auditory cortex.
Spiral ganglion cochlear nuclei superior olive inferior colliculus medial geniculate nucleus primary auditory cortex
What is the tonotopical organisation of in the ventral cochlear nucleus?
Low frequency ventral
High frequency dorsally
What are the projection of fibres like?
At level of medulla/cochlear nuclei there is ipsilateral however, everything becomes bi-lateral mid-pons and onward
THUS
Loss of hearing in one ear = problem with cochlear nucleus or auditory nerve
Beyond this point hearing loss will be in both
Where do all pathways converge?
Inferior colliculus (midbrain)
Receives input from both cochlei
Reflex associations - turning head toward loud noise
Carries information about sound localisation
Lateral inhibition make middle neuron signal more sharp/stand out
What are some of the collateral pathways auditory pathway?
Reticular formation
Cerebellum
What is the function of the superior olivary complex?
Compares the bilateral activity of cochlear nuclei
At the level of the pons
Function of medial superior olive?
Interneural time difference is computed
Sounds are detected at nearest ear before they reach the other one.
Function of lateral superior olive?
Detects difference in intensities between the two ears, interneural level difference is computed to localise sounds in the horizontal plane.
Discuss the excitation and inhibition of lateral superior olive
Excitation must arrive ipsilaterally at the same time as inhibition from contralateral side
Contralateral signal carried out by large axons with large synapses - large caylces of Held, those that carry excitations are smaller and slower
How can lateral superior olive contact the cochlea?
SOC neurons send feedback to hair cells, activity in efferent fibres increases the representation of signals in noise and protects from damage from loud sounds
BALANCES RESPONSE OF 2 EARS
REDUCES SENSITIVITY OF COCHLEA
What are the two ways in which auditory pathway can mulfunction?
SENSORINEURAL HEARING LOSS
Demyelination = loss of myelin, inflamm. or viral, most common in multiple sclerosis MS
Blast injuries: disruption to balance between stimulation and inhibition signals
What happens at superior colliculus
Auditory and visual maps merge
respond to stimuli with specific sounds
important in reflex head orientation and eyes to acoustic stimuli
Where is primary auditory cortex located?
A1 located in superior bank of the temporal lobe, central area of AC tonotopically mapped
LOUDNESS, RATE AND FREQ modulation
Gaze control in response to complex tasks
