Lecture 8- Physiology of hearing and balance Flashcards
Components of the outer ear
Pinna (auricle), ear canal, tympanic membrane
Directs sound to tympanic membrane ( eardrum)
Components of middle ear
Ossicular level system
Malleus (hammer), incus (anvil), stapes (stirrup), as well as tensor timpani and stapedius muscles
Transmits vibrations from the tympanic membrane to the inner ear
Components of the inner ear
Cochlea- hearing
Vestibule + semicircular canals- balance
conversion of mechanical vibrations to electrical signals in cochlear nerve
Middle ear functions- Critical damping
Tympanic membrane vibrates at same frequency as sound waves on its surface-attached to 3 bones and 2 muscles (attached to ossicles)
Activated before speaking or on hearing loud sounds (pre-programmed/ reflexive)
Middle ear functions- amplification
Related to impedance matching- sound pressure arriving at oval window increaed due to lever action of anvil and hammer and because area of tympanic membrane >oval window- force multiplied by 35%
Middle ear functions- impedance matching
Inner ear membranes operate in a fluid medium and so amplification needed to vibrate them- fluid has greater impedance than air which makes it difficult to vibrate
Weak vibrations of large area of eardrum in air magnified by ossicles/concentrated into strong vibrations over oval window to displace fluid
Matches impedance in air (outer) vs fluid (inner) environments
What are the 3 chambers of the cochlea?
Scala vestibuli Scala timpani ( continuous and contain perilymph, rich in Na+) Scala media- contains endolymph, rich in K+
Inner ear functions
Conversion of mechanical signal arriving at cochlea via oval window into electrical signals to register in auditory cortex as sounds
Mechanical vibrations move basilar membran, converted to electrical by hair cells imbedded in it. (3/4 rows of outer HC, 1 inner)
Where do afferent and efferent fibres of the cochlear nerve go?
90% of afferent come from IHC (as they are main transducers of mechanical vibration into nerve impulses)
Most efferent go to OHC, elucidating function as modulators of sensitivity of IHCs
How is tectorial membrane and basal lamina organised and how does it impact function?
They are arranged so vertical vibrations of basilar membrane are converted to lateral motion of hair cells.
As hairs vibrate laterally, filmaents linking adjacent HCs open ion channels, allowing K+ to enter and depolarise hair cells.
APs set up in VIII CN, potassium enters hair cell and is recycled into endolymph via stria vascularis.
Where is the human auditory complex located?
Superior portion of temporal lobe, embedded within sylvian fissure.
Contains map of frequencies- high and low at different ends
What shape is the primary auditory cortex?
Effectively a topographical columnar map of the basilar membrane, each of which repeated several times (diff sounds picked up on diff locations)
Assc. areas found close to prim AC, associating sounds with other somatosensory information
Brocas vs Wernicke’s area
Wernickes- cmoprehension of speech- gibbberish, fluent jargon or Wernicke’s aphasia)
Brocas- production of speech sounds- De Broca’s/ non-fluent aphasia
Sensorineural vs conductive deafness
Impaired sound conduction from pinna to inner ear (conductive)
Damage to hair cells/ neural pathways (sensorineural)
Conductive deafness- causes
ear wax in ear canal, perforated eardrum (common in sport diving and with v. loud sounds), fluid in middle ear w/ infection or damaged ossicles (disease/loud sounds)
