Neurology 10 - Sound Conduction and Transduction Flashcards
List the main causes of hearing loss
- Traumatic loud sounds
- Genetic conditions
- Infections like meningitis, rubella or syphillis
- Drugs (used for heart infections and chemotherapy)
- Aging
Compare the hearing and vision range in humans
- Hearing range from 20Hz to 20kHz
- Vision - static images changing at a rate of 20 times a second are percieved as continuous. The ear works at 20000 times a second
What is pitch?
The perception of frequency
What is timbre?
What distinguishes two sounds at the same frequency and intensity
What movements can the internal ear detect?
Movements as small as a fraction of a nanometer (size of a water molecule)
Describe the volume range of the ear
- Volume is the same as intensity
- Faintest intensity is 10^-12 w/m^2
- Loudest is 12 orders of magnitude larger
How does the ear receive sound?
- The ear detects sound waves in the air and via mechanical couplings, projects the stimuli onto the hair cells
- Hair cells are the sensory receptor of the internal ear
What are hair bundles?
A cluster of modified microvilli called stereocilia, present on the hair cells
What is the function of the ossicles of the ear?
- Three oscicles (malleus, incus and stapes)
- Transmit vibration of the tympanic membrane (caused by air) to the cochlea (filled with liquid)
- Role is to match the impedance and reduce the loss in energy as the vibration goes from the air to the cochlear
What is impedance?
- A measure of the reluctance of a system in receiving energy from a source
- When a sound is recieved, one sound is accepted and one is reflected
What is the resonant frequency?
- The frequency at which the impedance of a system is minimal
- Transmission of energy is maximal
How is the tension of the tympanic membrane controlled?
Tympanic muscle and stapedius muscles adjust the malleus and incus
What is a conductive hearing loss?
- When the ear is not capable of transmitting the vibration of sound waves to the cochlea
- Occurs in infections such as otiti, tumours and cerumen
List the common causes of conductive hearing loss
- In children, fluid accumulation is common
- Wax
- Otitis media
- Perforated tympanic membrane
- Abnormal growth of a bone (otosclerosis) which obstructs the ear canal
- Barotrauma (temporary)
What happens following moton of the stapes?
- Generation of a pressure difference between the two liquid filled chambers of the cochlea
- This in turn causes vibration of the basilar membrane
Describe the location and anatomical structure of the organ of corti
- On the basilar membrane, inside the scala media
- Contains hair cells (more outer than inner, these hair cells synapse with nerve endings)
What is the function and structure of the basillar membrane?
- Elastic, with heterogenous mechanical properties
- Vibrates at different positions in response to different frequencies
- Breaks complex sounds down by distributing the energy of each component frequency along its length
- Hair cells (sensory receptors) are along the whole length of the basilar membrane - tonotopic map
- Basilar membrane is narrow and tough
What is the function of hair cells?
- Sensory receptors of the inner ear
- Hair bundles are deflected by motion of the basilar membrane
- 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)
What are the tip links of sterocilia?
- Connect sterocilia
- Work as small springs stretched by stereocilia sliding
- Tip links share their location with ion channels
- Their disruption abolishes mechanotransduction
- Response currents are the result of opening of ion channels activated by stretching tip links
How is it known that the hair bundle is not passive?
- The hair bundle complies with the direction of the stimulus
- Measured stiffness more negative when the channels open
List the 4 aspects of the active process
- Amplification (a particular segment of a living basilar membrane vibrates more than a dead basilar membrane)
- Frequency tuning (dead basilar membrane produces a broad response, and is not tuned for a specific frequency, while living selectively amplifies frequencies)
- Compressive nonlinearity (motion of the basilar membrane is augmented, amplification diminishes with increasing stimulus intensity)
- Spontaneous otoacoustic emission (70% of noral ears emit pure tones when in a quiet environment)
What are the two types of hair cells?
- Inner hair cells (95% connect to these, 3500 per cochlea)
- Outer hair cells (5% connect to these, 11000 per cochlea)
What is electromotility?
- Outer hair cells shorten and elongate based upon their internal voltage
- Due to reorientation of the protein prestin
How is information transmitted to the cochlear nucleus?
- Hair cells synapse with the sensory neurones in the cochlear ganglion (spiral ganglion)
- Each ganglion responds best at a particular frequency
- Tonotropic map is present
What is sensorineural hearing loss?
- This is when the problem is rooted in the sensory apparatus of the inner ear or in the vestibulocochlear nerve (retrocochlear hearing loss)
- Most widespread type
List the causes of sensorineural hearing loss
- Loud noises
- Genetic mutations affecting the organ of corti
- Aminoglycoside antibiotics (toxic for hair cells)
- Congenital diseases (rubella, toxoplasmosis)
- Acoustic neuroma (tumour)
- Ageing
Describe the use of cochear implants
- Hair cells do not regenerate in mammals
- One solution is to bypass the dead cells to 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
- A elongated coli is insterted into the cochlea with pairs of electrodes corresponding to single frequencies
How are nerve fibres arranged in the ventral cochlear nucleus?
- Tonotopically
- Low frequencies ventrally
- High frequencies dorsally
How are sounds located in the verticle plane?
- Performed by the dorsal cochlae nucleus
- Sounds of high frequencies produce intensity differences between the two ears
- The ears detect and affect differently sounds coming from different directions
- Due to their asymmetrical shape (this is called spectral cues)
What is the function of the superior olivary complex?
- Compares bilateral activity of the cochlear nuclei
- Medial superior olive computes interaural time difference
- Lateral superior olive detects differences in intensity between the two ears. Localises sounds in the horizontal plane
- Interaural level difference is computed to localise sounds in the horizontal plane
How is information that arrives ipsilaterally to the lateral superior olive transmitted?
- Exitation arrives ipsilaterally at the same time as inhibition from the contralateral side
- This is carried out via large axons with large synapses (calyces of Held) which are smaller and conduct more slowly
How does the superior olivary complex send feedback?
- Feedback is sent to the hair cells
- Increases representation of signals in noise and protects it from damage by loud cells
- Used to balance responses from the two ears and reduce sensitivity of the cochlea
When is sensorineural hearing loss caused by malfunctioning of the auditory pathway?
- Demyelination - loss of myelin (due to inflammation or viral), most common in MS
- Blast injuries (distruption in the balance between inhibition and excitation)
What is the function of the inferior colliculus?
- All ascending auditory pathways converge here
- central nucleus, dorsal cortex and external cortex
- Only central nucleus is tonotopically organised
- More we ascend towards the cortex, the more neurons responsive to compex sounds
- Precedence effect
What happens in the superior colliculus?
- Auditory and visual maps merge
- Neurons are tuned to respond to stimuli with specific sound directions
- Autitory map created is fundamental for reflexes in orienting the head and eyes to acoustic stimuli
What happens in the primary auditory cortex?
- Neurons respond to complex sounds
- Primary auditory cortex is in the superior bank of the temporal lobe, it is tonotopically mapped, as well as mapped for loudness, rate and frequency
- Can be trained in dyslexia and brain repair
- Related to gaze control
Describe the decibel scale of sound.
- Logarithmic scale of sound - used to measure sound level
- Multiply this by ten and that is the decibel scale of sound
What is the precedence effect?
- Localisation of sound
- Comparison of the first detection of the sound and the echo to determine the exact location
- Your brain filters out all of the sounds that are not necessary to localise the sound
- Filters out sounds as an echo if there is longer than 30-50 milliseconds delay
Describe the pathway of auditory information
- Ventral cochlear nucleus in the medulla
- Dorsal cochlear nucleus
- Superior olivary complex in the pons
- Inferior colliculus in the midbrain
- Superior colliculus (via the medial gemiculate body)
- Auditory cortex in the cerebrum
What is the function of the superior auditory cortex?
- Superior auditory cortex identifies a what and where stream in the auditory system.
- In the visual pathway this is clearly defined
What is the middle ear?
- The portion of the ear that is between the eardrum and the entry to the cochlear canal
- Consists of the handle, malleus and incus (ossicles) to the oval window
What is the inner ear?
- Semicircular canals
- Cochlea
- Vestibular and auditary nerve
Describe the mechanisms of sound transduction from the middle ear to the cochlea
- Kinocilia are embedded in the tectorial membrane that lies in the scala media
- Upward movement of the basilar membrane displaces stereocilia away from modiolus (part of the cochlea), K+ channels open → K+ enters from endolymph → hair cell depolarises
- Depolarisation opens Ca++ channels in body of hair cell
Glutamate released from base depolarises axon of spiral ganglion cell → action potential - Downward movement displaces stereocilia towards modiolus → K+ channels close → hair cell hyperpolarises
- Highly sensitive – response to threshold sound requires 0.3 nm deflection
- Depends on maintenance of endolymph at +80 mV by stria vascularis
- Nerves synapsing with hair cells are dendrites of the spiral ganglionic cells (bipolar)
- The cell bodies of these spiral ganglionic cells lie in the spiral ganglion with their axons collating to create the cochlear nerve
Compare the effect of low and high frequencies on the basilar membrane
- High frequencies vibrate basilar membrane nearer to the base
- Low frequencies vibrate membrane nearer to apex
Describe frequency
The number of wave cycles per second
List contents of the outer ear
- Canal to tympanic membrane
- External ear
List the three chambers of the cochlea
- Scala vestibuli (perilymph)
- Scala media (endolymph)
- Scala tympani (perilymph)
Describe the structure of the three chambers of the cochlea
- Scala vestibuli and tympani are continuous with one another at the helictrema
- Reisseners membrane (vestibular membrane) separates vestibuli and media
- Basilar membrane separates the media and tympani
Describe the mechanism of amplification
- Large tympanic membrane to the smaller oval window thereby increasing S.A so increased pressure and so the waves created are greater
- Ossicles (tiny middle ear bones) use leverage to increase oval window pressure as well thereby increasing amplitude and volume etc.
- Amplify by 30dB
Describe the mechanism of protection of the middle ear
- Muscles in the ear that contract to alter the amount of amplifcation
- Tensor tympani (V3) causes rigidity of the ossicles so decreased amplification
- Stapedius (VII) stabilises the stapes bone so increased rigidity
Describe the mechanism of sound transduction from the middle ear to the cochlea
- Sound funneled into the external acoustic meatus
- Vibrations made on the tympanic membrane that causes vibrations in the ossicles (malleus, incus and stapes)
- Stapes creates vibrations on the oval window that creates pressure waves in the scala vestibuli perilymph (within the cochlea)
- Waves travel to the apex in the scala vestibuli then back through the scala tympani to the round window
- This disturbs the vestibular membrane and basilar membrane
- The stereocilia on the hair cells within the organ of Corti on the basilar membrane will now move due to the perilymph waves
Describe the relationship between the sterocilia and kinocilia
- The kinocilium is the longest sterocilium
- Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity.
