10. Sound conduction and transduction Flashcards
What is the external auditory meatus?
Ear canal
What is the ear drum also known as?
Tympanic membrane
What does the middle ear comprise?
- Tympanic membrane
- Malleus
- Incus
- Stapes
Signals are transferred from the cochlear to the central pathways via which nerve?
Vestibulocochlear nerve (VIII)
What is sound?
- Sound causes a periodic change in air pressure, thus consists of compressed and rarefied air
- Occur at 343m/sec
- Frequency: number of compressed or rarefied patches of air that pass our ears
What frequencies is the human ear sensitive to?
20 - 20,000 Hz
What is the intensity of sound?
- The difference in pressure between the compressed and rarefied air regions
- Determines the loudness of sound that we perceive
Describe the passage of sound from the outside to the cochlea?
- Pinna (outer ear) collects sound and channels it down the external auditory meatus
- Entrance to ear - 2.5cm inside the skul
- Tympanic membrane vibrates
- The 3 bones (ossicles) transfer the movement of the ear drum to the fluid filled cochlea
- Hair cells in the cochlea can depolarise and hyperpolarise to transfer frequency as a neural signal
What is the eustachian tube?
- A tube that links the nasopharynx to the middle ear
- It is part of the middle ear
- Equalises pressure between middle ear and nasal cavity
What is the oval window?
- Membrane-covered opening between the middle ear and the vestibule of the inner ear
- Behind the stapes bone
What does the inner ear comprise?
• Cochlea • 3 fluid-filled chambes - scala vestibule - scala media - scala tympani
What is the function of the ossicles?
- Amplify the sound pressure
- Important as the fluid in the inner ear resists movement
- Makes the pressure bigger at the oval window compared to the tympanic membrane (small SA of OW also helps)
What membrane separates the scala vestibule and the scala media?
- The Reissner’s membrane
* Sound causes pressure difference either side of this membrane, separating the 2 fluids
What membrane separates the scala media and the scala tympani?
The Basilar membrane
What is the fluid filling the chambers of the inner ear called?
- Perilymph (CSF like - low k, high Na)
* Endolymph (high K, low Na)
Describe the Basilar membrane and how it carries out its function
- Wider at the apex than the base by x5
- More flexible at the apex and stiffer at the base
- Movement of the stapes causes the endolymph to flow in the cochlea => travelling wave in the membrane
- Distance of the wave depends on the frequency
- Different locations of the membrane are maximally deformed at different frequencies
What is the function round window?
- Window with a membrane between the middle and inner ear
- When there is pressure at the OW, perilymph is pushed into the scala vestibule
- Pressure travels to the scala vestibule, through the helicotrema and back down the scala tympani
- Fluid pressure has nowhere to go - RW bulges to allow for pressure
What stereocilia?
Inner and outer sensory hair cells on top of the basilar membrane
What is the function of the stereocilia?
- Amplify and improve the clarity of sound
* Extent of movement depends on frequency
What is the difference between the inner and outer hair cells?
Inner • 3,500 • Primary sensory cells • Generate APs in the auditory nerves • Stimulated by the fluid movements • 95% of afferent projections from here
Outer
• 20,000
• Become short on depolarisation
• Become long on hyperpolarisation
• Increased the amplitude and clarity of sounds
• More efferent projections connected here
Describe the pathway of a signal from the cochlea to the auditory cortex?
- E - Eighth nerve (vestibulocochlear)
- C - Cochlear nuclei
- O - superior Olivary nucleus
- L - Lateral Leminiscus
- I - Inferior Colliculus
- M - Medial geniculate body
- A - Auditory Cortex
What is tonotopy?
- The spatial arrangement of where sounds of different frequency are processed in the brain
- Different regions of the basilar membrane vibrate at different frequencies due to variations in thickness and width
- Nerves that transmit information from different regions of the basilar membrane therefore encode frequency tonotopically
- This tonotopy then projects through the vestibulocochlear nerve and is present throughout the auditory nuclei
- Low frequencies transmitted ventrally, high frequencies dorsally
How does neural firing compare at low, mid and high frequencies?
• Low frequency - phase locking: action potentials firing at times corresponding to a peak in the sound pressure waveform
• Mid frequency - phase locking and tonotopy
• High frequency - tonotopy
- different neurones fire on successive cycles
What is the interaural time difference?
- The difference in arrival time of a sound between two ears
- Important in the localisation of sounds
- Detected by neurones in the brainstem
How can some people with hearing loss still detect sound?
- Direct transmission through the cochlea
- Bone conduction
- Clinically important - allows us to detect where a problem is
Where is there a problem if bone conduction is better than air conduction?
Anywhere between the pinna and the cochlea
What are the main causes of hearing loss?
- Loud traumatic sounds
- 200 genetic conditions
- Infections e.g. meningitis
- Drugs
- Ageing
What is the decibel scale?
- Measures the intensity of sound
* Logarithmic scale using base 10
What are the possible causes of conductive hearing loss?
- Cerumen (ear wax)
- Infections (otitis)
- Tumours
- Fluid accumulation in the middle ear
- Perforated tympanic membrane
- Otosclerosis - abnormal bone growth
- Barotrauma
What does the Organ of Corti include?
Basilar and tectorial membranes, hair cells and supporting cells
Describe the stereocilia
• Bend towards the tallest stereocilia towards the tallest stereocilium changes the internal voltage of the cell
- mechano-transduction
• Connected by filamentous linkages - tip links
• Tip links share their location with ion channels
• Disruption of tip links abolishes mechano-transduction - loud noises, takes 12 hours to recover
What happens when efferent fibres are activated to outer hair cells?
- Frequency selectivity and sensitivity is enhanced
- Bodies shorten and elongate when internal voltage is changed - electromobility
- Happens at rate of 80 kHz
- Due to reorientation of the protein ‘prestin’
What happens at the spiral ganglion?
- Hair cells (mostly inner) form synapses with sensory neurones here (aka cochlear ganglion)
- NTs constantly released at rest, but the rate changes in response to a change of the presynaptic voltage, due to MT ion channel gating
- Each ganglion cell responds best to stimulation at a particular frequency
What is sensorineural hearing loss?
Problem with sensory apparatus of the inner ear or the vestibulocochlear nerve (retrocochlear hearing loss)
What are the causes of sensorineural hearing loss?
- Loud noises
- Ménière’s disease - excess fluid in the cochlea
- Many genetic mutations affect the Organ of Corti
- Aminoglycoside antibiotics are toxic for hair cells - Streptomycin
- Congenital diseases
- Ageing (presbycusis)
How do cochlear implants restore hearing?
- Hearing loss is primarily due to the loss of hair cells
- These do not regenerate in mammals
- A cochlear implant involves an elongated coil inserted into the cochlea with pairs of electrodes
- This bypasses the dead cells and stimulate the nerve fibres directly
- They detect sound, break it down into constituent frequencies, then send signals directly to the auditory nerve via antennas
- The pairs of electrodes correspond to single frequencies
What does the dorsal cochlear nucleus do?
- Locates sounds in the vertical plane
- Detect and differently affect sounds coming from different directions due to their asymmetrical shape - spectral cues
- Only animals with the dorsal cochlear nucleus can do this
What are T-stellate cells?
- Encode sound frequency and intensity of narrowband stimuli
- Their tonotopic array represents sounds’ sprectra
- Part of cochlear nucleus
What are Bushy cells?
- Produce more sharply but less temporally precise versions of the cochlear nerve fibres
- Provide the resolution required to encode the relative time of arrival of inputs to the two ears
- Part of the cochlear nucleus
What does the superior olivary complex (SOC) do?
• Compares the bilateral activity of the cochlear nuclei
• The medial superior olive computes the interaural time difference
• The lateral superior olive detects differences in intensities between the two ears
- interaural level difference is computed to localise sounds in the horizontal plane
- neurone inhibited by opposite sounds
Which hair cells do Superior Olivary Complex neurones feed back to?
- Neurones from the Medial SO => inner hair cells bilaterally
- Neurones from the Lateral SO => outer hair cells ipsilaterally
What happens at the inferior colliculus?
- Responses from different frequencies merge here
- The more we ascend towards the cortex, the more neurones become responsive to complex sounds
- Information about sound location in the IC - precedence effect
What happens at the superior colliculus?
- Auditory and visual maps merge
- Neurones 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
What happens in the auditory cortex?
- Neurones respond to complex sounds
- The primary auditory cortex is located in the superior bank of the temporal lobe
- This is the central area of the AC and it is tonotopically mapped
