Sound Conduction and Transduction Flashcards
What fraction of people in the UK are affected by healing loss?
10%
Hearing range in humans
20Hz - 20 kHz
Frequenzy
1 hertz = 1 cycle per second
What does the impedance of the system depend on?
- mechanical properties -> changes impedance
Conductive hearing loss
The ear is not capable of transmitting the vibration of sound to the cochle.
- e.g. fluid accumulation in children
- 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)
basilar membrane
- vibrates at different positions of the lengths
Where fo all asscending auditory pathways in vertebrate converge?
The inferior colliculus
Precendence effect
Your brain filters out sounds that are not strictly necessary to localise the sound.
Filter out sounds with lower sensitivity.
What makes up the outer ear?
- auricle
- external acoustic meatus (lateral third surrounded by cartilage, medial 2/3s surrounded by bone)
What makes up the middle ear?
- boundary: eardrum / tympanon
- auditory ossicles (malleus, incus and stapes) -> smallest bones in your body -> Stapes interacts with the inner ear through the oval window
- muscles of the middle ear: stapedius and tensor tympanic
Broadly, how does the inner ear work?
- receives mechanical signals (vibration) from the stapes through the oval window
- hair cells -> fluid vibration in cochlea moves hair cells -> signals sent to brain.
- signals that come in have to come out in order to adjust the pressure -> through the round window
What makes up the inner ear?
- 3 semicircular canals
- vestibule
- cochlea -> contains hair cells
Vestibulocochlear nerve
- made up of vestibular and cochlear nerve joining
- passes through the internal acoustic meatus
- CN8
- facial nerve (CN7) also travels along and passes through the internal acoustic meatus, also originates in pons next to CN8.
What are the main causes of hearing loss?
- Loud traumatic sounds: military, industrial, clubs
- 200 genetic conditions that cause hearing problems
- Infections like meningitis or congenital ones such as rubella or syphilis
- Drugs: used for severe heart infections and chemotherapy
- Ageing
Hellen Keller on hearing loss
blindness deprives us of the contact with things, hearing deprives us of the contact with people
Pitch
the perception of frequency
timbre
what distinguishes two sounds at the same frequency and intensity.
how small moevements can the ear detect?
The internal ear can detect movements large as a fraction of a nanometer, roughly the size of a water molecule.
Why do we use the DeciBel scale?
We want to compact a large range into a more manageable scale. Instead of measuring the intensity I with respect to the faintest perceivable intensity of sound 𝐼0, we compare their logarithms:
- bel scale defines the sound level
- x10 = decibel sclale
Speech - frequencies and intensities
- Speech is a complex cocktails of frequencies and intensities
- higher frequencies: consonants
- lower frequencies: vowels
What is the sensory receptor of the inner ear?
Hair cells
Hair bundle
- The hair bundle is a cluster of modified microvilli called stereocilia.
- hair cells take their name from the hair bundle
Impedance matching in the ear
- The three ossicles transmit the vibration of the tympanic membrane onto the cochlea, which is a snail-shaped organ filled with liquid.
- Their role is to match the impedance and reduce the loss in energy as the vibration goes from the air to the cochlea.
Impedance
The impedance measures the reluctance of a system in receiving energy from a source.
- the impedance of a system depends on its mechanical properties.
Resonant frequency
The frequency at which the impedance of the system is minimal
-> maximal transmission energy
Cochlea
- The cochlea is a liquid-filled snail-shaped organ
- The motion of the stapes generates a difference in pressure between the two liquid-filled chambers of the cochlea, which in turns causes the vibration of the basilar membrane.
Organ of Corti
The Organ of Corti includes the basilar and tectorial membranes, the hair cells
and supporting cells
The basilar membrane
- works as a frequency analyser
- vibrates at different positions along the length in response to different frequencies
- mechanical properties differ along the length
- an 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 the energy of each component frequency along its length
- therefore sensory receptors along the whole length of the basilar membrane are needed in order to detect all frequencies: these receptors are the hair cells.
Impedance of the basilar membrane
- The impedance of the basilar membrane varies along its length, meaning that so does the local resonant frequency.
- narrow+tough (near ossicles) —> broad and floppy (along the length)
Where in the cochlea do hair cells sit?
- in the organ of Corti
What lies above and below the hair cells?
- above: tectorial membrane
- below: basilar membrane
The relative motion of the 2 membranes in response to sound vibration is what triggers the hair cells.
How do vibrations act in hair cells?
- The motion of the basilar membrane deflects the hair bundles of the hair cells, that act as sensors.
- 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. This is called Mechano-transduction (MT).
Mechano Transduction (MT)
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
- The tip links project the force of the stimulus onto ion channels
- Stereocilia are connected by filamentous linkages called tip links. They work as small springs stretched by the stereocilia’s sliding.
- Historically, scientists observed:
• Tip links share their location with ion channels
• Their disruption abolishes mechanotransduction - They concluded that the response currents are the result of the opening of ion channels activated by the stretching of the tip links.
Hair bundle - active or passive process?
- active!
- The opening of MT ion channels in response to an external stimulus, relaxes the tip link and, in turn, the whole hair bundle.
- A healthy hair bundle actively complies with the direction of the stimulus: the measured stiffness becomes negative when channels open!
- The hair bundle has the capacity to do work. This points to the existence of an active process in hair cells.
negative stiffness
- bundle is actively injecting work into the system
- it moves itself into the direction dictated by the stimulus actively
- amplification mechanism
What are the 4 main aspects of the active process of the hair bundle?
The sensitivity and the sharp frequency selectivity of the cochlea cannot be explained solely by passive mechanical properties: basilar membrane (BM) impedance. 4 aspects of the active process:
- amplification
- frequency tuning
- compressive non-lineraity
- spontaneous otoacoustic emissions
Amplification (active process)
A particular segment of a living basilar membrane (red curve) vibrates far more in response to its resonant frequency, than a dead BM. This instead behaves as a passive system (blue curve).
Frequency tuning as an active process
A dead basilar membrane produces a broad response and it is not tuned for a specific frequency (blue curve). A living BM instead selectively amplifies single frequencies.
Compressive non-linearity as an active process
The motion of the BM is augmented 100-fold during low-intensity stimulation, but amplification diminishes progressively with the increasing intensity of the stimulus.
Spontaneous otoacoustic emissions as an active process
- 70% of normal humans ears emit one or more pure tones when in a quiet environment
- This is only possible in healthy cochleas
- This is due to the work performed by the ear in normal conditions to counteract the viscous drag in the cochlea.
What are the 2 types of hair cells?
- inner hair cells
- outer hair cells
How many hair cells are there in the human cochlea?
- Inner hair cells (IHCs): ~3500 per human cochlea
- Outer hair cells (OHCs): ~110000 per human cochlea
How are the hair cells linked to the brain?
- 95% of afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from IHCs. IHCs provide sensory transduction.
Eselsbrucke: AI (artificial intelligence = afferent inner) - Most of the efferent projections (from the brain to the cochlea) connect to OHCs. What is the role of OHCs? What do they do?
Eselsbrucke: OE (otoemissions -> outer efferent)
How many layers of inner and outer hair cells are there in the Organ of Corti?
- inner: 1s
- outer: 3
Electromotility of hair cells
- The origin of the cochlear amplification and otoacoustic emissions might be the OHCs.
- Their cell body shortens and elongates when their internal voltage is changed.
- This is called electromotility.
- It is due to the reorientation of the protein prestin.
-> video of cell dancing to music
Cochlear ganglion
- Nerve fibres transmit information to the 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 begins.
- Ganglion cells in a particular area of the spiral ganglion respond best to the resonant frequency of the basilar membrane in that same area.
Tonotopic
sound - map
Sensoneurinal hearing loss
- when the problem is rooted in the sensory apparatus of the Inner ear or in the vestibulocochlear nerve (retrocochlear hearing loss).
- This is the most widespread type of hearing loss by a large margin.
What are the causes of sensoneurinal 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).
Cochlear implants
- Hearing loss is primarily due to the loss of hair cells. These do not regenerate in mammals
- One solution is to bypass the dead cells and stimulate the nerve fibres directly: detect sounds, break them down into their constituent frequencies and send the signal directly to the auditory nerve via antennas.
- An elongated coil is inserted into the cochlea with pairs of electrodes corresponding to single frequencies.
- Early models: 4 channels.You need 20 channels to understand speech well.
Do hair cells regenerate in mammals?
NO
The ventral cochlear nucleus
Nerve fibres convey information to the cochlear nucleus where different kinds of neurons are arranged tonotopically
- low frequencies ventrally
- high frequencies dorsally
(HD - high frequencies dorsally)
Detecting sounds from different locations
- The dorsal cochlear nucleus locates sounds in the vertical plane
- Sounds of high frequencies produce intensity differences between the two ears. High-frequency sounds (wavelengths ~ size of the head and ears) produce interference with the body.
- The ears detect and affect differently sounds coming from different directions due to their asymmetrical shape.We call these spectral cues.
- our body affects the sounds we receive
Dorsal cochlear nucleus
- there are different cells that are good at different things
- e.g. initiation of sound
SOC
superior olivary complex
What is the function of the superior olivary complex?
- compares the bilateral activity of cochlear nuclei.
Medial superior olive
- Here the interaural time difference is computed: sounds are first detected at the nearest ear before they reach the other one.
- A map of interaural delay can be formed due to delay lines.
Lateral superior olive
- The LSO detects differences in intensity between the two ears (>2 kHz in humans due to head size).
- Interaural level difference is computed to localise sounds in the horizontal plane.
- Excitation that arrives ipsilaterally must arrive at the same time as inhibition from the contralateral side.
- The contralateral inhibitory signal is carried out via large axons with large synapses (the large calyces of Held). The axons that carry ipsilateral excitations are smaller and conduct more slowly.
What is the function of the large calyces of Held?
- The large calyces of Held provide fast contralateral inhibition
- they have large axons with large synapses
How does the superior olivary complex send feedback to the cochlea?
- SOC neurons send feedback to the hair cells.
- Activity in these efferent fibres increases the representation of signals in noise and protects it from damage by loud sounds.
- The feedback is used to balance the responses from the two ears, but also to reduce the sensitivity of the cochlea.
Sensorineurinal hearing loss in the brain
Hearing loss can be due to the malfunctioning of the auditory pathway in the brain.
- demyelination (can be due to inflammation or viral, most common cause is MS)
- blast injuries can cause disruption in the balance between inhibition and excitation.
What fibers to the hair cells are myelinated / unmyelinated?
- Myelinated (from the SOC bilaterally)
- Unmyelinated (from the SOC ipsilaterally)
Inferior Colliculus
- All ascending auditory pathways converge here
- In mammals: central nucleus, dorsal cortex and external cortex. Only central nucleus is tonotopically organised.
- The more we ascend towards the cortex the more neurons become responsive to complex sounds. In the IC many carry information about sound location. Precedence effect.
Superior colliculus
- All ascending auditory pathways in vertebrate converge here
- Here auditory and visual maps merge. Neurons are tuned to respond to stimuli with specific sound directions.
- The auditory map here created is fundamental for reflexes in orienting the head and eyes to acoustic stimuli.
Auditory complex
- In the auditory cortex neurons respond to complex sounds.
- The primary auditory cortex A1 is located in the superior bank of the temporal lobe.
- This is the central area of the AC and it is tonotopically mapped. Loudness, rate and frequency modulation also seem to be mapped in A1.
Superior auditory cortex
In the visual system the outputs of the primary visual cortex are segregated. We can identify a “What” and “Where” stream in the auditory system (primates). In the visual pathway this is clearly defined.
