9 - Sound conduction and transduction Flashcards
State the main causes of hearing loss
- loud traumatic noises
- genetic conditions
- infections (meningitis or rubella)
- drugs (for sever heart conditions and chemotherapy)
- ageing
What is the average hearing range in humans?
20 - 20,000 kHz
Define pitch
the perception of frequency
the quality of sound dependent on the frequency of vibration of the waves producing it.
Define timbre
(the character or quality of a musical sound or voice as distinct from its pitch and intensity)
what distinguishes two sounds at the same frequency and intensity
Why do we use the decibel scale of sound level?
range of sound is extremely large - this is a more manageable scale using the logarithmic function
Give a brief overview of the pathway of sound/vibration from air to hair cells
The ear detects sound waves in the air and, via a series of mechanical couplings, projects the stimuli onto the hair cells - the sensory receptor of the internal ear.
What is the cochlea?
snail shaped organ filled with liquid
‘the organ of hearing’
part of the inner ear that converts mechanical energy (vibrations) into nerve impulses sent to the brain
What is located on the cochlea that is essential in the perception of sound?
Describe their organisation
hair cells
4 rows - 1 row of inner hair cells and 2 rows of outer hair cells
State the 3 main parts of the mammalian ear?
the outer ear; middle ear; and inner ear
Describe the structure and function of the outer ear
The outer ear includes the pinna (visible on the side of the head) and ear canal.
its function is to funnel sound waves into the ear canal towards the tympanic membrane (eardrum).
Describe the structure and function middle ear
The middle ear is between the tympanic membrane and stapes, which is inserted in the oval window.
The middle ear is an air‐filled cavity found within the temporal bone of the skull. It is lined with a mucosa continuous with the Eustachian tube and houses the three middle ear ossicles that are suspended within it.
The middle ear is connected to the back of the mouth via the Eustachian tube.
3 ossicles: The malleus (hammer), incus (anvil) and stapes (stirrup) form a chain between the tympanic membrane and the oval window that transmits sound waves from the outer to inner ear. The footplate of the stapes connects to a membrane stretched across the oval window; this vibrates with ossicular movement passing the sound waves to the inner ear.
Describe the structure and function inner ear
The inner ear is involved with balance as well as hearing. Within the cochlea of the inner ear, the sound waves are converted into electrochemical signals to be passed to the brain.
The inner ear converts the sound waves to electrical signals.
What is the function of the ossicles of the middle ear?
to facilitate transmission of vibrations from tympanic membrane to the cochlear
TO MATCH THE IMPEDANCE
Define what is meant by impedance
a measure of the reluctance of a system in receiving energy from a source.
Define resonant frequency
The frequency at which the impedance of the system is minimal
Why is it importance for the ossicles to match impedance?
the more different the impedance levels of the two mediums, the less efficient transmission is (more is reflected)
NOTE: the ossicles provide a middle ground between the air and cochlea, which have very different impedances
What is meant by ‘conductive hearing loss’?
all forms offering loss involving the outer and middle ear
when the ear is not capable of transmitting the vibration of sound waves onto the cochlea
Give some example of causes of conductive hearing loss
- fluid accumulation in the ear (children)
- wax
- perforated tympanic membrane
- otosclerosis (abnormal bone growth)
- barotrauma (temporary)
- tumour
- otitis media
- congenital malformations
What is the function of the first 2 ossicles? (the malleus and incus)
malleus and incus - (position can be adjusted by tensor tympanic muscle and stapedius muscles) to control the tension of the tympanic membrane (thought to be how how the cochlea is protected from loud sounds)
What is the function of the stapes (3rd ossicle)
the motion of the stapes generates a difference in the pressure between the two liquid-filled chambers of the cochlea, which in turns causes the vibration of the basilar membrane
Where are hair cells located?
on top of the basilar membrane
What is the organ of Corti?
the basilar and tectorial membranes, hair cells and supporting cells
How are hair cells stimulated from the organ of Corti?
vibration of the basilar and tectorial membranes (due to pressure differences between the 2 chambers) stimulates the hair cells
Fluid movement through scala vestibuli, (around the helicotrema,) and back through the scala tympani differentially moves the basilar membrane on which the organ of Corti and its hair cells reside.
Describe the properties of the basilar membrane
an elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies
Describe how the basillar membrane works as a frequency analyser
How does this relate to hair cells?
breaks complex sounds down by distributing the energy of each component frequency along its length.
Sensory receptors are needed along the whole length of the membrane to detect all frequencies: these receptors are the hair cells.
How are hair cells stimulated?
The motion of the basilar membrane deflects the hair bundles of the hair cells, that act as sensors.
This is through a process called Mechano-transduction
What are tip links?
What does the stretching of them induce?
filamentous linkages that connect stereocilia
They work as small springs stretched by the stereocilia’s sliding.
stretching induces response currents by causing the opening of Ca+ gated ion channels
Describe the change in stiffness if a hair bundle when MT (mechano-transduction) channels open
measured stiffness becomes negative since it cimplies with the direction of the stimulus
What are the 2 main components of the inner ear
the bony labyrinth and membranous labyrinth.
Bony labyrinth – consists of a series of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and three semi-circular canals. All these structures are lined internally with periosteum and contain a fluid called perilymph.
Membranous labyrinth – lies within the bony labyrinth. It consists of the cochlear duct, semi-circular ducts, utricle and the saccule. The membranous labyrinth is filled with fluid called endolymph.
What is the name of the fluid that can be found in the body labyrinth?
perilymph
Why is there a need for active amplification (contribute to ears sensitivity)?
a large portion of energy is lost in the viscous damping effects of the cochlear liquids
What are the 4 aspects of the active process?
amplification
frequency tuning
compressive nonlinearity
spontaeneous otoacoustic emission
Describe the role of Inner and Outer Hair cells in the transmission of signals from the brain to the cochlea and vice versa
95% of afferent projections project from IHCs. IHCs provide sensory transduction.
Most of the efferent projections (from the brain to the cochlea) connect to OHCs.
What is meant y the ‘electromobility’ of hair cells?
‘dancing’ outer hair cells
During activation of efferent fibres are, the cell body’s of OHCs shorten and elongate when their internal voltage is changed.
It is due to the reorientation of the protein prestin.
Describe the pathway of the spiral ganglion in afferent signal from hair cells to the brain
- Hair cells (mostly IHCs) form synapses with sensory neurons in the cochlear ganglion (spiral ganglion).
NOTE that each IHC transmits info to multiple spinal ganglion - Neurotransmitters are continuously released at rest, but the rate changes in response to a change of the presynaptic voltage (as a result of MT ion channel gating). - Each ganglion cell responds best to stimulations at a particular frequency.
- The tonotopic (sound-location) map begins
Describe the pathway of the nerve fibres of afferent signals from the spiral ganglion to the brain
- The axons in the cochlear nerve transmit the information to the cochlear nucleus.
- Each axon is responsive to a single frequency. Some low threshold and high rate of spontaneous activity, some high threshold but less spontaneous activity.
How are the frequency and periodicity of the stimulus encoded for in afferent nerve fibres from the spiral ganglion to the brain?
- The tonotopic map plus the firing rate of nerve fibres encodes the stimulus frequency.
- The firing pattern of a cochlear nerve encodes the periodicity of the stimulus.
What does phase-locking mean?
a response is phase-locked once it happens at the same point of the cycle, every cycle
What is meant by Sensorineural Hearing Loss
when the problem is rooted in the sensory apparatus of the Inner ear or in the vestibulocochlear nerve (retrocochlear hearing loss)
hair cells are killed off
This is the most widespread type of hearing loss by a large margin.
Give some causes of Sensorineural Hearing Loss
- Loud noises, headphones at high volume can cause temporary or permanent hearing loss
- Many genetics mutations affect the Organ of Corti
- Aminoglycoside antibiotics are toxic for hair cells
- Congenital diseases (rubella, toxoplasmosis)
- Ageing (presbycusis)
Hearing loss is primarily due to ______
the loss of hair cells
NOTE: there is no regeneration in mammals
How do Cochlear Implants work?
the dead cells and bypasses and the nerve fibres are stimulated 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.
Where do the afferent nerve fibres from the cochlear ganglion convey information to?
How are the neurones arranged here?
to the cochlear nucleus of the brain stem where different kinds of neurons are arranged tonotopically (low frequencies ventrally, high frequencies dorsally).
What are spectral cues?
- question needs editing
helps to localise sound sources in the vertical plane
Sounds of high frequencies (info in dorsal cochlear nuclei) produce intensity differences between the two ears.
The ears detect and affect differently sounds coming from different directions due to their asymmetrical shape.
Only animals with the dorsal cochlear nucleus can make use of these
Only animals with the _______ can make use of spectral cues
dorsal cochlear nuclei
dorsal cochlear nucleus (high frequency) locate sounds in the vertical plane
Describe the different neurons in the cochlear nucleus
T-STELLATE CELLS
They encode sound frequency and intensity of narrowband stimuli.
BUSHY CELLS
Bushy cells produce more sharply but less temporally precise versions of the cochlear nerve fibres.They provide the resolution required to encode the relative time of arrival of inputs to the two ears.
Name the 2 parts of the superior olivary complex
MEDIAL SUPERIOR OLIVE
LATERAL SUPERIOR OLIVE
What is the function of the superior olivary complex
compares the bilateral activity of the cochlear nuclei
What role does the medial superior olive have (in comparing the bilateral activity of the cochlear nuclei)?
- the interaural time difference is computed: sounds are first detected at the nearest ear before they reach the other one.
- Bushy cells carry information about the timing of inputs at every cycle.
- A map of interaural delay can be formed due to delay lines (birds).
What role does the lateral superior olive have (in comparing the bilateral activity of the cochlear nuclei)?
- detects differences in intensity between the two ears
- Neurons are excited by sounds arising from the ear in the same side (ipsilaterally), while they are inhibited by opposite sounds (contralaterally). Interaural level difference is computed to localise sounds in the horizontal plane.
How is feedback from the SOC (superior olivary complex) send back to the hair cells?
- Neurons from the medial superior olive - IHCs bilaterally.
- Neurons from the lateral superior olive - OHCs ipsilaterally.
The feedback is used to balance the responses from the two ears, but also to reduce the sensitivity of the cochlea.
What happens at the INFERIOR COLLICULUS?
Here responses from different frequencies merge. In the IC all ascending pathways converge.
In the IC many neurones carry information about sound location.
What happens at the SUPERIOR COLLICULUS?
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.
What happens in the auditory complex?
What are the 2 pathways?
neurones respond to complex sound
2 pathways:
PRIMARY AUDITORY CORTEX A1
SUPERIOR AUDITORY CORTEX
Where is the PRIMARY AUDITORY CORTEX A1 located? What is mapped in this area?
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.
What happens at the SUPERIOR AUDITORY CORTEX?
We can identify a “What” and “Where” stream in the auditory system (primates).
Name the two liquid-filled chambers of the cochlea
scala vestibuli
scala tympani
What is the difference between conductive and sensorineural hearing loss?
conductive - involves the outer and middle ear
sensorineural - involves the inner ear or vestibulocochlear nerve
