what is sound and sound frequency?
-Sound is a pressure wave, comprising successive cycles of compression and rarefication of air molecules.
-Sound frequency = number of cycles per second (Hz)
-Humans can hear frequencies of 20-20,000 Hz
These are travelling waves, so if you’re at a fixed point in space, the x-axis could equivalently be time.
Speed ~340m/s in air;
what is pitch and loudness?
- Perceptually, pitch is determined by pressure frequency
- Perceptually, loudness is determined by pressure intensity (amplitude)
what is the role of peripheral auditory system
transmits sound waves to where they are transduced
Pinna = sound filter and funnel
Ossicles = 3 middle ear bones
Cochlea = fluid-filled bony structure containing receptor neurons
The outer ear funnels sound waves toward the tympanic membrane (ear drum) and also filters sounds in a direction-dependent manner.
The transmission sequence for “air conduction
The transmission sequence for “air conduction”:
Sound waves => vibration of tympanic membrane => ossicle movement => vibration of oval window
=> movement of cochlear fluid and basilar membrane => neuronal response in hair cells within cochlea
what happens in the middle ear?
mechanical amplification of sound pressure
Middle ear ensures efficient transfer of sound energy from air (outer ear) to fluid (inner ear).
middle ear is air filled
Explain the concept of force=pressure/surface area in transmission of sound
1) Pressure = Force / Surface area.
Given constant force, the smaller surface area of oval window compared to tympanic membrane => greater pressure.
Relative size of tympanic membrane & oval window gives a pressure amplification.
Role of ossicles
The ossicles act as a mechanical lever (increased force & reduced displacement)
convert vibration of tympanic membrane (air interface) to vibrations of oval window (fluid interface). Smallest bones in human body. Associated with the smallest muscles in the body.
Overall, middle ear achieves 20x amplification in PRESSURE.
3) Attenuation response – loud sounds cause contraction of tensor tympani and stapedius muscles – reduces size of vibrations of oval window. This takes time to activate
The inner ear
– a coiled, fluid filled structure
Compression forces oval window inward, bending basilar membrane down.
Most sounds comprise a range of frequencies, leading to complex vibrations in the basilar membrane
Spatially localised parts of the basilar membrane move in response to specific frequencies.
Difference between base and apex?
Apex is broad and floppy=> Sensitive to low frequencies. (low pitch)
Base is narrow and stiff => Sensitive to high frequencies.(high pitch)
what is the fluid in scala vestibuli and scala tympani?
Fluid in scala vestibuli and scala tympani (these are connected and visible on the previous slide)
High [Na+], [low K+]
What is this similar to? standard extracellular fluid
what is the fluid that fills the Scala media?
- Fluid in scala media. - Unusual - extremely high [K+].
Electrical potential in the endolymph is +80 mV relative to the perilymph
The auditory receptor cells (hair cells) have a resting membrane potential of ~ -60 mV
gradient from outside to inside the hair cells is > +120 mV
This is a strong electrochemical gradient for K+ ions to move into hair cells
where the Auditory receptor cells (hair cells) and Dendrites of spiral ganglion cells receive inputs from hair cells
- Hair cells are the non-spiking sensory receptors.
- Spiral ganglion cells are spiking cells, which receive inputs from the hair cells.
- Movement of the basilar membrane change membrane potential in hair cells.
which hair cells in involved in sound transduction?
inner hair cells
Each hair cell has dozens of ~parallel stereocilia, whose tips are linked by special proteins.
Note that the stereocilia are arranged in height order.
These hairs sit in endolymph (the fluid that fills the scala media
outer hair cell
There are three rows of “outer” hair cells – these are referred to as the cochlear amplifier – they can change length, and in doing so change the position of the tectorial membrane. This changes how much the inner hair cells respond. Beyond the scope of this unit.
what connect adjacent stereocilia?
Tip link proteins connect adjacent stereocilia.
Deflection of the stereocilia leads to cation channel opening.
which causes influx of K+. Due to the potential difference between endolymph and intracellular fluid.Stereocilia are surrounded by endolymph, an extracellular fluid unique because of its high [K+].
properties of hair cells
- These hair cells are very small, unmyelinated, do not have axons, they have graded potentials. The amount of neurotransmitter release is dependent on membrane potential.
- they have resting membrane potential of -60 mV
Deflection of the bundle towards longer and shorter stereocillia leads to?
- longer stereocilia opens channels, allowing K+ influx.
- shorter stereocilia closes channels.
- Deflection towards largest stereocilium
- K+ influx(!)
- Membrane depolarisation(!)
- opening of voltage-gated Ca2+ channels
- Ca2+ mediated neurotransmitter release - most likely glutamate.
- Hair cells are non-spiking, but action potentials occur in the spiral ganglion neuron
Spiral ganglion cells are frequency tuned
- Frequency tuning arises from frequency-specific movement of basilar membrane
- ~90% of spiral ganglion cells receive inputs from inner hair cells
~10 spiral ganglion cells innervate each inner hair cell
Many outer hair cells synapse onto a single spiral ganglion cell
what effects the number of action potential produced in ganglion cell?
AP rate in spiral ganglion cells depends on sound frequency (pitch) and intensity (loudness).
- sound intensity
- sound frequency ( after 1600 hz the number of AP produced reduces )
Specific regions of the basilar membrane vibrate in response to specific frequencies
Therefore, adjacent hair cells have adjacent characteristic frequencies.
different neurons that have the cell body in spiral ganglion are activated in response to certain sound frequencies.
This tonotopy is preserved in the spiral ganglion, cochlear nucleus and primary auditory cortex.
Interaural time delay (ITD)
Sounds coming from in front reach ears at the same time.
Sounds coming from the right reaches right ear ~0.6 ms earlier.
(Sound travels at ~340m/s in air)
Interaural level difference (ILD)
Sounds coming from in front have same intensity in each ear.
Sounds coming from the right are quieter in the left ear.
(Sound is attenuated by the head)
Cochlear nerve responses are “phase locked” at low frequencies
Action potentials occur at the same point (phase) in each cycle for a given frequency.
APs can still be phase locked, even if they do not occur on every cycle.
Responses to low frequency sounds are phase-locked
=> action potentials occur at the same point of each cycle.
=> constant offset between spiking in left and right auditory nerves.
Delay lines use different axonal conduction delays to match temporal offset.
does phase lock occur at high frequency?
At high frequencies (5 kHz+), responses are no longer phase locked.
what helps to distinguish the direction of where the sound is coming from?
Conduction velocities are typically 10-100 m/s in myelinated axons.
Different length axons can introduce different delays.