Physiology of hearing and vestibular systems Flashcards
What is the function of the Pinna?
Catches sound waves and relays them into the eardrum
What are sound molecules?
Disturbances of air molecules set up by vibrations in air
Where do the sound waves travel after the pinna?
The auditory canal to the tympanic membrane
What are the three bones of the middle ear?
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
What are the muscles of the middle ear?
- Tensor timpani
- Stapedius
What are the three functions of the middle ear?
- damning
- amplification
- impedance matching
What are the three parts of the ear?
Outer, middle and inner ear
What are the parts of the outer ear?
Pinna, auditory canal and tympanic membrane
What are the parts of the inner ear?
- Cochlea (hearing)
- Vestibule and semicircular canal (balance)
How does the tympanic membrane perform its damping functions?
- tympanic membrane is a resonator so it vibrates at the same frequency as the sound waves on its surface
- its attachment to Malleus, Incus and Stapes also allows it to be a critically dampening
- reflex to protect the inner ear where get contraction of muscles attached to ossicles which is triggered before you speak but is often too slow for rapid onset loud noises
How does the tympanic membrane perform its amplification functions?
- sound pressure which reaches oval window is increased by:
1) the lever action
2) area of tympanic membrane is far greater than oval window
How does the tympanic membrane perform its impedance matching functions?
Amplification allows for the same vibration frequency to be produced in liquid even though it is harder to vibrate liquid
What are the three chambers of the cochlear?
Scala vestibuli - rich in Na+
scala Tympani - rich in Na+
Scala media - rich in K+
What is the vascular layer of cochlea called?
stria vascularis
Describe the nerves in the cochlea
spiral ganglion leads to cochlear nerves which lead to temporal cortex of the brain
What is the function of the cochlea?
Transforms mechanical deformations into electrical signals
How are mechanical deformations transformed into electrical nerve signals?
- vibrations of oval window compress the fluid of inner ear and set up vibrations in basilar membrane
- which are then absorbed by deformations of flexible round window
- basilar membrane contains inner and outer hair cells which set up electrical signals
- vertical vibrations of basilar membrane are converted to lateral motion of hair cells
Which two cochlear chambers are continuous?
scala vestibuli and scala tympani
Which hairs are more important for which function?
afferent fibres go to inner hair cells (main transducers)
efferent fibres go to outer hair cells (modulators of sensitivity of inner hair cell)
How do each hair cell create electrical signals?
- hair cells are linked laterally
- between hair cells are ion channels which can be pulled open
- allowing potassium to enter hair cell and depolarise cell
- setting up AP in VIII nerve
- K+ is allowed back into endolymph via stria vascularis
What are the two characteristics of sound waves?
sound frequency (pitch) Sound amplitude (volume)
What units do we measure sound in?
Decibel
What is the calculation of decibels?
1 dB = 10 x Log10 actual sound intensity / reference sound intensity
How does the cochlea code for frequency differences?
- different frequencies will cause different hair cells along the basilar membrane to be maximally stimulated
- High frequency will maximally stimulate at oval window
- Low frequency will maximally stimulate near helicotrema
How is amplitude coded for?
frequency of the AP of hair cells
Why must decibels be a logarithmic scale?
- each 10 db means a sound is twice as loud
How can directionality of sound be detected?
- above 3000 Hz can tell which ear the amplitude is greater
- below 3000 Hz can tell by the delay in sound to one ear
Where is the auditory cortex of the brain located?
- superior portion of temporal lobe
- within sylvian fissure
Describe the structure of the auditory cortex?
- high and low freq sounds registered at different ends
- topographical map of basilar membrane (each column repeated several times for different locations of sound)
- association areas are close to the cortex
What are the association areas of the auditory cortex?
- Wernickes area (comprehension of speech)
- de Brocas area (production of speech)
What symptoms would damage to wernickes area have?
- speak jargon
- wernickes aphasia
What happens with damage to de Brocas area?
patient knows what they want to say they just can’t get it out
What are the different types of deafness?
- Conductive
- Sensorineural
What is conductive deafness?
impaired sound conduction from pinna to the inner ear
What causes conductive deafness?
- Ear wax
- perforated eardrum
- fluid in middle ear from infection
- damaged ossicles
What is sensorineural deafness?
damage to the hair cells or neural pathways
What causes sensorineural deafness?
- loud sounds cause hair cells to be sheared and cant be regenerated
- basilar membrane can rip with loud sounds
- antibiotics can enter hair cell channels and disrupt their function and cause degeneration
- tumours in auditory nerves and vascular damage to medulla
How can audiometry distinguish between types of deafness?
- detects thresholds for hearing at different frequencies
- nerve deafness results in defects at higher frequency reception
What body processes require input from the vestibular system (balance)
- many reflexes
- many eye movements
- setting background posture against which we move
- progress of movement at any given time
Which parts of the inner ear are used in balance?
semi-circular canals (3 canals on each side) and vestibules (utricle and saccule)
- all detect acceleration
Describe the structure of the semicircular canals
- base of semicircular canal swelling called ampulla
- ampulla contains cristae ampullaris (gelatinous ridge)
- cupula is gelatinous mass extending from ridge containing processes of hair cell
- hair cells contain motile cilia and one non-motile cilia called kinocilium
Describe how balance is sensed?
- when head rotates fluid within semicircular canals moves displacing the cupula
- when hairs bend toward the kinocilium they increase discharge in the nerve
- when they bend away they reduce AP firing
- one initial rotation hairs are bent toward increasing firing
- when the head stops rotating the hairs are bent back the other way decreasing discharge
- canal for each plane is (X, Y & Z) on each side
- therefore signals from right and left in all planes is sent to the brain
Describe the structure of the utricle and saccule?
- base of both there is a macula (which is otolithic)
- utricle macula is horizontal
- saccule macula is vertical
- above the hair cells is a gelatinous layer in which otoliths are embedded
What are otoliths?
calcium carbonate crystals
How do utricles and saccules contribute to sensing balance?
- measure linear acceleration so dependant on gravity or movement
- if hairs move toward kinocilium increase nerve discharge
- if move away from kinocilium decrease imoulse generation
Where does the vestibular nucleus of the medulla connect to?
1) cerebellum
2) spinal cord (via vestibulospinal tract)
3) reticular formation
4) equilibrium cortex in sylvian fissure
What is the function of the cerebellum connection?
cerebellum used in
- balance
- setting background against which predicted rapid movements can take place
What is the function of the spinal cord connection?
- muscles involved in maintenance of equilibrium
What is the function of the reticular formation connection?
causing general arousal and increasing conduction speed and sensitivity in all motor nerve pools
What is the function of the equilibrium cortex connection?
important in conscious sensations of equilibrium and vestibule ocular reflex and nystagmus
What is nystagmus?
Pursuit - pursuit of movement of the eye during head rotation where the eye fixates on a single object
Saccade - then flicks back to it’s original when limit of eyes rotation has been reached
What part of the vestibular system controls nystagmus?
Semicircular canals
What is the pathology behind motion sickness?
- there is conflicting vestibular system signals
- e.g. if reading in a car one signal is horizontal acceleration and one signals that you are stationary
