Audiology 1- Anatomy and Physiology Flashcards
Non-acoustic function of the outer ear
-Protects tympanic membrane from direct assault and injury
-Self-cleansing action- jaw action, hairs and production of wax
Acoustic characteristics and function of outer ear:
-collects sound and funnels it down to the ear drum, giving a 5dB boost
- generates resonance which increases the sound pressure in the tympanic membrane
-aids sound localisation by changing frequency spectrum of the incoming sound, depending on the direction of the source
what makes up the outer ear?
-pinna
- external auditory meatus (ear canal)
What does the pinna do?
collects sound waves and funnels them down the ear canal to the tympanic membrane (ear drum)
what is ear canal resonance?
Everything has a natural frequency of vibration that is its resonance frequency
- The ear canal has a resonant frequency of 2-5000hz, the sound waves are boosted at the ear drum at these frequencies by 15-20db making our hearing most sensitive between 2000 and 5000 hz
what is the role of the outer ear in sound localisation?
- collecting and filtering sound waves, which provides cues about the sound source’s location
- intensity and timing differences at the two ears
There is a reduction in the level of higher frequency (3000-6000Hz) that reaches the tympanic membrane when the sound source is behind the ear helping to differentiate if sounds come from the front or back
Describe the tympanic membrane (ear drum)
- 8-10mm across and made up of a thin fibrous layer covered by skin on the outside and mucosa on the inside
- the fibrous layer extends throughout the pars tensa but not the pars flaccida
- vibrates back and forth with incoming compressions and rarefactions of the sound waves
Describe the middle ear
- air filled chamber between the ear canal and the inner ear
- connected to the naso-pharynx via the Eustachian tube
- the auditory ossicles (three bones of the middle ear) from a chain to connect the tympanic membrane with the oval window of the cochlea
What makes up the auditory ossicles (middle ear bones)?
malleus
incus
stapes
What is the function of the middle ear?
efficiently transmit sound through to the cochlea.
If we didn’t have it most of the sound coming into the cochlea the sound would be reflected away.
Sound is collected over a relatively large area of the eardrum and focused down to a much smaller area, the stapes footplate so the pressure is greater and helps push the sound through to the cochlea
How is sound energy efficiently transfered to the cochlea increased?
- surface area difference, the ratio between the eardrum and oval window
-lever action of ossicles (middle ear bones)
describe the lever action of the middle ear bones
When the eardrum vibrates, the ossicles (middle ear bones) move, transmitting the vibrations through the middle ear.
Because the malleus is longer than the incus, it provides a mechanical advantage, amplifying the force of the vibrations.
This amplification increases the pressure of the sound waves before they reach the inner ear. The eardrum has a much larger surface area than the oval window.
The ossicles effectively concentrate the force of the vibration onto the smaller oval window, further increasing the pressure
Functions of the eustachian tube
- Main function is to aerate the middle ear so that the pressure in the middle ear cavity remains the same as atmospheric pressure
- The ET remains closed except whilst swallowing or yawning, when the ET opens briefly to allow air to enter the middle ear (replacing air that has diffused into the mucous lining) keeping the middle ear pressure the same as atmospheric pressure
- Fluid can build up in the middle ear cavity can cause hearing loss. A second function of the ET is to drain any middle ear fluids.
Describe what the inner ear does
- converts sound waves into neural code that is transmitted to the cochlea that we recognise as auditory information/ sound.
- Its highly frequency selective, can hear lots of small changes in frequency
- can cope with a huge range of sound pressures e.g. from a pin drop to a fighter jet
describe the anatomy of the cochlea
The shape of the cochlea resembles that of a snail shell with 2¾ turns.
The central conical core of the cochlea is called the modiolus. The outer wall of the modiolus forms the inner wall of a canal which spirals the full 2¾ turns around the central core.
A thin shelf of bone, called the osseous spiral lamina projects from the modiolus and partially divides this canal into two parts along its entire length.
The basilar membrane connects this shelf of bone to the outer wall of the bony cochlea and, thus, separates the canal into two main passages, the scala vestibuli and scala tympani. The SV and ST are joined via a communicating opening between them at the apex called the helicotrema.
Running between the scala vestibuli and scala tympani is the cochlear duct (called the scala media in cross section).
The basilar membrane forms the lower boundary of the scala media, the upper boundary is called the vestibular or Reissner’s membrane.
Thus, the cochlea is a long, coiled, fluid-filled tube (about 33 mm in humans) that is divided along most of its length into three sections: the scala vestibuli, scala media and scala tympani.
When coiled the cochlea is approximately 9 mm in diameter at its base and 6 mm in height.
Within the modiolus is seen a bundle of nerve fibres called the spiral ganglion. The central processes of spiral ganglion neurons form the cochlear nerve.
Sitting on the basilar membrane is the auditory receptor organ, the organ of Corti.
Describe how sound is transmitted to the cochlea
- The stapes pushes against the oval window of the cochlea
- causes fluid in the cochlea to vibrate at the same frequency as the sound and sets up a travelling wave in the basilar membrane which peaks at the frequency it is tuned to
- pressure relieved by the bulging round window
what are travelling waves
- The cochlea converts acoustic signals into the neural code that conveys auditory information to the brain.
- Sound energy travels along the basilar membrane in the form of a travelling wave.
- The travelling wave peaks at the part of the BM which is tuned to the stimulus frequency and then dies away.
How is intensity information coded in the auditory nerve?
- As the amplitude of a sound increases, the firing rate increases (up to saturation) and more fibres carry the signal.
- Some auditory nerve fibres have a high spontaneous firing rate and these primarily carry low level intensity sounds.
- Other fibres have a low spontaneous rate and code high level intensity sounds.
How is information transmitted from the cochlea to the brain
- 30,000 nerve fibres form the auditory nerve.
- The nerve fibres carrying sound signals lead to different parts of the auditory cortex (in the Sylvian fissure) depending on the frequencies they carry.
The high tones terminate deep within the Sylvian fissure while the low tones end near the outer surface. - in the auditory pathway, messages are carried from the cochlea, and each relay nucleus does a specific work of decoding and integration.
what do the inner hair cells do?
the hearing
when the stereocilia are bent the hair cells depolarise and a signal is sent along the auditory neurones and the auditory pathway out to the cochlea, you hear a sound
what do the outer hair cells do?
around the peak of the travelling wave, the outer hair cells physically expand and contract so they amplify our ability to hear quiet sounds, give a bigger peak. They will stiffen for loud sounds.
They also make a fine tuning between frequencies so we can hear a very small change in frequency and suppresses a loud sound
what is the basilar membrane?
helps with frequency selectivity
narrow and stiff at the basal end and broad and slack the other end.
It sets off a travelling wave in the cochlea that moves along the basilar membrane and peaks at the point that it is tuned to that incoming frequency and it dies away.
Runs throughout the length of the cochlea.
How is frequency information coded in the auditory nerve?
- different sets of auditory nerve fibres, elicit different auditory sensations in the auditory cortex
the frequency to which a fibre is most sensitive is referred to as the characteristic frequency (CF)
the characteristic frequency of a fibre is dependent on the location along the basilar membrane to which it is attached.
