10. Sound Conduction and Transduction Flashcards
What is sound?
- Transverse wave
- Consist of compressed and rarefied air
- Characterised based on frequency/pitch
- Loudness depends on the amplitude of the wave
- Measured in decibels based on a logarithmic scale
- The large range in sound intensities makes measuring sound difficult to manage, which is why we measure sound LEVELS (decibels)
Components of the outer ear
- The pinna= wing allows for the elevation of sound
- External auditory meatus- a cone at the proximal part of the canal which focuses the noise and increases the pressure at the tympanic membrane
Components of the middle ear
- The ossicles are the smallest bones in the body (MALLEUS, INCUS, STAPES)
- Air filled tympanic cavity
- Tympanic membrane- vibrates due to air waves, ossicles improve signal
- Fluid filled cochlea
The middle ear extends from the cochlea to the tympanic membrane
How are the pressure of vibrations increased at the tympanic membrane
Increas in pressure of vibrations is achieved by TWO methods:
- Focusing vibrations from the large surface area of the membrane to the smaller surface area of oval window
- The incus has a flexible joint with the stapes, the ossicles use leverage to increase the force of the oval winow
Action of the ossicles
The ossicles convert the movement of the tympanic membrane to the movement of the foot plate
(Trying to induce a pressure wave in the fluid of the cochlear)
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.
The impedance measures of reluctance of a system in receiving energy from a source. The frequency at which the impedance of the system is minimal is called the resonant frequency.
Malleus and Incus are relics of evolution (reptilian jaw). Their position can be adjusted by the tensor tympanic muscle and stapedius muscles to control the tension of the tympanic membrane.
Why cant the tympanic membrane translate directly to the cochlear fluid?
99% of the energy of the sound wave would just bounce of the interface due to impedence- measures the reluctance of a system in recieving energy from a source
The frequency at which the impedence of a system is minimal is called the resonant frequency
TWO muscles that control the movements of the ossicles:
- Tensor Tympani
- Stapedius
counteract and reduce the movement of the ossicles as part of the AUDITORY REFLEX
i.e work when hearing a loud noise and when talking/ chewing so you dont hear internally generated noises
Hyperacusis
Painful sensitivity to low intensity sounds- can occur in conditions that lead to flaccid paralysis of auditory reflex muscles
How is the cochlea connected to the ossicles
Through the stapes
The stapes and the footplate vibrate the OVAL WINDOW- part of the membrane of the cochlea
The round window below this is a ‘pressure release’ window (round window moves out as pressure is placed on fluid to equalise it)
THREE compartments to the inner ear
- Scala Vestibuli (perilymph fluid)
- Scala Tympani (perilymph fluid)
- Scala Media (contains endolymph fluid)
The helicotrema connects the scalas vestibuli and tympani
It allows fluid mixing
Basilar membrane
The end destination of the conducted sound waves - The basilar membrane is an elastic structureof heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies.
Different parts of the basilar membrane are sensitive to different frequencies - this property is used to generate a place code or tonotopic map
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
Organ of Corti
The sense organ of the cohlea of the inner ear which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Lies above the basilar membrane and beneath the tectorial membrane
Consists of inner hair cells and outer hair cells- snese deflection in the basilar membrane
Inner Hair cells function
- Found on their own
- Send connections back to the brain (afferents)
- They have stereocilia which move in response to the movement of endolymph
- Inner hair cells do not make contact with the tectorial membrane
- 95% of the afferent projections (sensory axons that carry signals from the cochlea towards the brain) project from inner hair cells.
- Inner hair cells provide sensory transductions
Outer hair cells funtion
- Found in groups of THREE
- These are in contact with the tectorial membrane
- recieve efferent connections
- They are Electromotile- they expand and contract
- By expanding or contracting they can amplify the amount of vibration
- Damage to the hair cells results in SENSORINEURAL hearing loss
- Also responsible for otoacoustic emissions- noises the ear makes itself (could be cause of tinnitus)
- Most of the efferent projections (from the brain to cochlea) connect to outer hair cells.
Formation of the auditory nerve
Via grouping of the outer and inner hair cells- cell bodies are found in the spiral ganglion
Hair cells mode of action
Just as is needed for all action potential generation the endolymph (where stercocilia are) and perilymph (where base is) have different ion concentrations (seperated by teh stria vascularis):
- Endolymph- high K+/ low Na+
- Perilymph- high Na+/ low K+
Simplified:
Upwards movement of the basilar membrane displaces the sterecilia towards the centre of the cochlea: K+ channels OPEN and the hair cell depolarises
Downwards movement of the BM displaces the stereocilia the other way: K+ channels CLOSE and there is hyperpolarisation
Central auditory pathway
Fibres of the Organ of Corti project out of the cochlea via the auditory VESTIBULOCOCHLEAR nerve (C8)
at this level the projection is ipsilateral (after this point everything is BILATERAL i.e if you have a central defecit the problem will be on both sides)
This pathway then goes to the superior olivary nucleus
ALL the ascending pathwyas converge on the inferior colliculus which is involved in reflex associations e.g turning towards a loud noise
lateral inhibition also occurs in this pathway
How do the hair cells work as sensory receptors in the inner ear?
- 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 (we can use a glass probe) changes the internal voltage of the cell, ultimately producing an electric signal that travels towards the brain. This is called Mechano-transduction (MT).
- Stereocilia are connected by filamentous linkages called tip links. They work as small springs stretched by stereocilia’s sliding
- The opening of MT ion channels in response to an external stimulus, relaxes the tip link and, in turn, the whole hair bundle.
- Imagine pulling on a door, free to open, with an elastic string attached to the handle. As you pull, the stiffness of the string would appear to droponce the door opens.
- Unlike a door, a healthy hair bundle actively complies with the direction of the stimulus: the measured stiffness becomes negative!
- The hair bundle has the capacity to do work. This points to the existence of an active processin hair cells.
Explain the active process of hair cells.
The need for an active amplification: large portion of energy is lost in the viscous damping effects of the cochlear liquids. 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:
Explain how electromobility works.
When the efferent fibres are activated, frequency selectivity and sensitivity is enhanced. 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 electromobility and can happen at a rate of 80 kHz. It is due to the reorientation of the protein prestin.
The spontaneous back and forth movement might be the cause of otoacoustic emissions and possibly of other aspects of the active process. Caveat: otoacoustic emissions in animals without OHCs.
Describe the superior olivary complex.
The superior olivary complex compares the bilateral activity of the 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
- Bushy cells carry information about the timing of inputs at every cycle. 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). 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.
The Superior Olivary Complex neurons send feedback 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.
