Sound Conduction and Transduction Flashcards
What scale is used to measure how loud a sound is?
Decibels (logarithmic scale) From Bel scale
What is the audible range for humans in terms of frequency?
20-20,000 Hz
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 auditory meatus and narrows to the tympanic membrane
This focuses the noise and increases the pressure on the tympanic membrane
Is the tympanic cavity fluid-filled or air-filled?
Air-filled
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
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 pressurewave
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 (V3): The tensor tympani acts to dampen the noise produced by chewing. When tensed, the muscle pulls the MALLEUS medially, tensing the tympanic membrane and damping vibration in the ear ossicles and thereby reducing the perceived amplitude of sounds.
Stapedius (VII)- prevents excess movement of the STAPES, helping to control the amplitude of sound waves from the general external environment to the inner ear
What is the name given to this reflex?
Auditory reflex
What is the latency period (time between the stimulus and a reflex response) of this reflex?
50-100 ms
What is hyperacusis?
Heightened sensitivity to sound, with aversive or pained reactions to normal environmental sounds.– can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy - stapedius is innervated by facial nerve)
Which test is used to determine the site of damage to the auditory system, that is causing hearing loss?
Weber Test
What are the 2 specialised membranes of the cochlea?
Oval Window
Round Window
What are the three compartments of the cochlea in 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 structure connects the two perilymph compartments?
Helicotrema
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 endolymph in the scala media leading to vibration of the basilar membrane.
The round window vibrates as well to equalise the pressure in the cochlea. (since perilymph fluid is mostly water which is incompressible, and so the pressure ‘inserted’ at the oval window needs to be relieved at the round window via vibration as well)
Describe the difference in sensitivity of different parts of the basilar membrane.
Higher frequency sounds = base
Lower frequency sounds = apex
What is the Organ of Corti?
The sense organ of the cochlea, which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Where is the Organ of Corti found?
It lies on top of the basilar membrane and beneath the tectorial membrane, this is all within the Scala Media
What are the two types of cell in the organ of corti?
Inner and outer hair cells
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 fluid of the cochlea does the stereocilia of the hair cells project into?
Endolymph, hence movement of endolymph in the scala media will trigger movement of stereocilia on the hair cells
What internally generated sounds are the outer hair cells responsible for?
Otoacoustic emissions
These are sounds made by the cochlea and can be measured using microphones. They are measured in newborn babies to see if they have hearing problems. This is partly because otoacoustic emissions is closely associated to the cochlear amplifer (the cochlea can amplify the intensity of the sound that you hear and hence make it more audible)
What are stereocilia connected by?
Tip links
What bony conical structure is found at the middle of the cochlea?
Modiolus
Describe what happens when the basilar membrane is displaced upwards.
Depolarisation Upward displacement moves the stereocilia away from the modiolus K+ channels open K+ enters from the endolymph Hair cells depolarises
Describe what happens when the basilar membrane is displaced downwards.
Hyperpolarisation
Stereocilia move towards the modiolus
K+ channels close and hair cells hyperpolarises
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
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
Up to what point is the auditory pathway from one ear ipsilateral?
Cochlear nuclei
Beyond this point there is bilateral representation
The inferior colliculus receives input from both cochlea. What is the inferior colliculus responsible for?
What is the role of the superior colliculius?
IC: All ascending auditory pathways converge. Eg different sound frequencies converge
Reflex associations – turning your head towards loud noise
SC: Where auditory and visual pathways merge- also important in reflexes involving sound and vision eg turning your head and looking towards the direction of a loud noise
Describe a phenomenon that is involved in sharpening the signal coming from the cochlea.
Lateral inhibition
To which parts of the CNS do collaterals from the auditory pathway go?
Reticular formation
Cerebellum
In which lobe is the primary auditory cortex?
Temporal
What is the secondary auditory cortex responsible for?
Responding to sounds coming off/on
Responding to the duration of sound
What is the name given to the axons that project from the medial geniculate nucleus to the primary auditory cortex?
Acoustic radiations (they travel via the internal capsule)
NB optic radiations from the visual pathway travel from lateral geniculate nucleus to the primary visual cortex
How do you localise short sound burst?
Interaural time delay
How do you localise continuous sound?
Interaural intensity/ level difference
difference in loudness and frequency distribution between the two ears. (intensity of sound is dependent on frequency and amplitude of the sound wave)
What is conductive hearing loss?
Any problem at places before the INNER EAR. Involved in conducting the sound waves anywhere along the route through the outer ear, tympanic membrane (eardrum), or middle ear (ossicles).
eg When diseases of the middle ear damage the ossicles or stiffen theirjoints so that the amplification system is eliminated – results in conductive hearing loss
What is sensorineural hearing loss and what can it be caused by?
When the cochlea (ie the inner ear) or cochlear nerve get damaged, the signal transmitted to the primary auditory cortex is reduced or lost
It can be caused by acoustic schwannoma (tumour of the cochlear nerve) or cerebellar tumours expanding and putting pressure on thecochlear nerve
What is the term used to describe loss of hearing due to the death of hair cells in normal ageing?
Presbyacusis
presby= old/aging
acusis= hearing
Name 2 types of second order neurons found in the cochlear nucleus
T- Stellate cells
Bushy cells
These are both neurons in the cochlear nucleus. First order bipolar neurons of the cochlear nerve (part of CNVIII) will synapse with these neurons. They will then project to eg the superior olivary complex (this is all part of the auditory sensory pathway)
explain frequency, pitch, intensity, volume
Frequency: no of waves per second, measured in Hz
Pitch: Higher frequency = higher pitch.
Sound intensity: units is W/m2. Watt= joules/second so intensity = J/s/m2 so think of it as the amount of energy per second that passes through 1 square metre of space
Volume: Sound intensity is perceived as sound volume. Higher intensity = higher volume
Timbre
Distinguishes two sounds at the same frequency and intensity
What happens after the ear detects sound waves in the air
Via series of mechanical couplings, stimuli is projected onto hair cells- sensory receptors of the internal ear
Hairbundle is
Cluster of modified microvilli – stereocilia
How many ossicles in the middle ear? function?
3
Transmit the vibration of the tympanic membrane onto the cochlea– snailshaped
Cochlea function
Match the impedance and reduce the loss in energy as the vibration goes from the air to the cochlea.
impedance
measures the reluctance of a system in receiving energy from a source
resonant frequency
frequency at which the impedance of the system is minimal
Conductive hearing loss- ear is not capable of transmitting vibration onto the cochlea.
Causes
In children, fluid accumulation in the inner ear is a commo
Cerumen, infections such as otitis, tumors can all affect transmission.
A perforated tympanic membrane is a form of conductive hearing loss.
• An abnormal growth of bone (otosclerosis) can obstruct the ear canal.
• Barotrauma is a temporary form of conductive hearing loss. (Valsalva maneuver to reopen the Eustachian tubes)
Cochlea is
Cochlea is is liquid filled snail shaped
Organ of Corti includes?
the basilar and tectorial membranes, the hair cells
and supporting cells
Role of basilar membrane
Frequency analyser
elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies.The basilar membrane breaks complex sounds down by distributing the energy of each component frequency along its length. We need therefore sensory receptors along the whole length of the basilar membrane in order to detect all frequencies: these receptors are the hair cells.
Motion of basillar membrane
deflects the hair bundles of the hair cells,
Mechano-transudction
The bending of stereocilia towards the tallest stereocilium changes the internal voltage of the cell, ultimately producing an electric signal that travels towards the brain.
Tip links are?
Role?
filamentous links that link stereocilia.
Project the force of the stimulus onto ion channels by working as small springs stretched by the stereocilia’s sliding.
Response currents are the result of the openingof ion channels activated by he stretching of the tip links
Tip links share their location with ion channels
disruption abolishes mechanotransduction
External stimulus
Opening of MT ion channels, relaxes tip link and eventually the whole hair bundle
Active process in hair cells leads to
A healthy hair bundle actively complies with the direction of the stimulus: the measured stiffness becomes negative when channels open
4 aspects of the active process
Amplification
Frequency tuning
Compressive nonlinearity
Spontaneous otoacoustic emission
Two types of hair cells
Inner and outer(more than inner per human cochlea)
95% of afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from
IHC
They provide sensory transduction
Most of the efferent projections (from the brain to the cochea) connect to
OHC’s
OHC role
Origin of the cochlear amplification and otoacoustic emissions
What happens to OHC when their internal voltage is changed?
what is this process called
What is this due to
Cell body shortens and elongates
Electromotility
Reorientation of Protein prestin
Where do nerve fibres transmit information to
To the cochlear nucleus
Where do hair cells form synapses with sensory neurons in
Cochlear ganglion (spiral ganglion)
Ganglion cells in a partiuclar area of teh spiral ganglion
best respond to the resonant frequency of the basilar membrane in that same area
Sensorineural hearing loss– problem occurs where
How common?
Rooted in the sensory apparatus of the inner ear or in the vestibulocochlear nerve (retrocochlear hearing loss.)
Most widespread type of hearing loss
Causes of sensorineural hearing loss
Loud noises, headphones at high volume can cause temporary or permanent hearing loss (Club: ~100 dB, Rock concert: ~120 dB)
• Many genetics mutations affect the Organ of Corti
• Aminoglycoside antibiotics are toxic for hair cells
• Congenital diseases (rubella, toxoplasmosis)
• Acoustic neuroma (tumor on the cochlear nerve)
• Ageing (presbycusis).
Causes of sensorineural hearing loss
Loud noises, headphones at high volume can cause temporary or permanent hearing loss (Club: ~100 dB, Rock concert: ~120 dB)
• Many genetics mutations affect the Organ of Corti
• Aminoglycoside antibiotics are toxic for hair cells
• Congenital diseases (rubella, toxoplasmosis)
• Acoustic neuroma (tumor on the cochlear nerve)
• Ageing (presbycusis).
Hearing loss is primarily due to
Loss of hair cells which do not regenerate
Cochlear implants function
Bypass dead cells, stimulate nerve fibres directly: detect sounds, break them down by their constituent frequency constituents and send signal directly to the auditory nerve via antennas
Elongated coil inserted into the cochlea wit pairs of electrodes corresponding ot single frequencies.
need 20 channels to understand speech well
Ventral cochlear nucleus- How are neurons arranged
Tonotopically-low frequencies ventrally, high frequnencies dorsally
Ventral cochlear nucleus- How are neurons arranged
Tonotopically-low frequencies ventrally, high frequnencies dorsally
Dorsal cochlear nucleus locates sounds from where
vertical plane
How is a high sound classified and how does it affect body?
wavelengths size of head and ears
produce interference with the body
Spectral cues
Spectral cues –ears detect and affect differently sounds coming from different directions due to their asymmetrical shape.
Function of the superior olivary complex
Compares the bilateral activity of the cochlear nuclei
Medial superior olive function
Place in which the interaural time difference is computed: sounds first detect at the nearest ear before reaching the other. A map of interaural delay can be formed due to delay lines
Lateral superior olive function
detects differences in intensity between the 2 ears. Interaural level difference is computed to localise sounds in the horizontal plane.
The large calyces of Held provide
provide fast contralateral inhibition
Lateral superior olive
Excitation that arrives ipsilaterally must arrive at the same time as inhibition from the contralateral side.
