Audition Flashcards
Describe the destruction of the ear
Outer - pinna (tympanic membrane) Middle - ossicles, eustachian tube (oval window) Inner - cochlear, vestibular apparatus
What are the first stages of the auditory pathway?
- tympanic membrane
- ossicles
- oval membrane
- fluid in the cochlea
- sensory neurones
What are the 3 ossicles?
What is their function?
malleus, incus, stapes
amplify mechanical wave x20
What is the function of the Eustachian tube?
- allows secretions of middle ear to empty
- allow equalisation of air pressure
Which patients are more sucesptible to middle ear infections?
children - short eustachian tube –> reflux
patients with cleft palate
What are the 3 functions if the attenuation reflex?
Functions:
- adaptation
- protection
- help discern high frequency sounds
What is the attenuation reflex?
What muscles are involved?
Loud sound –> muscle contraction –> conduction in middle ear is reduced
- tensor tympani muscle = pulls handle of malleus medially, tensing the tympanic membrane
- stapedius muscle = pulls stapes posteriorly and tilts base in the oval window, tightening the annular ligament and reducing the oscillatory range, also can prevent movement of stapes
What are the 3 functions of the attenuation reflex?
adaptation (to loud sounds)
protection (prevent tympanic rupture)
help discern high frequency sounds (minor)
What are the 3 tubes of the inner ear?
Where is the organ of corti and the sensory cells?
- scala vestibuli
- scala media (organ of corti, sensory cells)
- scala tympani
Describe how the inner ear carries out its function
- stapes pushes against oval window
- fluid moves in opposite directions in two tubes as vestibule and tympani communicate
- basilar membrane vibrates up and down
- this moves the organ of corti
Describe the response of the basilar membrane to sound
High frequency sounds –> base is narrow and stiff
Low frequency sounds –> apex is wide and floppy
Different frequencies produce maximum amplitude at different places along length of basilar membrane
Describe the structure and function of the organ of Corti
- moves when the basilar membrane moves as it is sitting on it
- tectorial membrane slides on stereocilia of the inner and outer hair cells to produce movements which are translated to action potentials
How does the movement of hair cells produce action potentials?
- mechanically gated potassium channels linked the stereocilia together
- when the hairs move apart the channels open, when the hairs move together the channels close
- ## allowing/disallowing K+ to depolarise the hair cell
What happens following entry of K+ into the hair cell?
- depolarisation of the cell
- opens voltage gated potassium cells
- Ca2+ entry
- vesicles containing excitatory GLUTAMATE fuse with membrane
- NT release onto spiral ganglion neurite
What is the fluid in the organ of corti called?
endolymph
What are the structural and functional differences between inner and outer hair cells?
Outer hair cells - modulatory signals in signal, integration - one neurone for several cells - convergent signals Inner hair cells - conscious sensory input - many neurones per cell - divergent signal
What is meant by amplification by outer hair cells?
- motor proteins expand/contract in outer hair cells
- contraction –> steriocilia tug tectorial membrane –> moves inner hair cilia
This acts of amplify low level sound
What is the hair cell receptor potential a reflection of?
sound pressure waves
- graphs look identical
What does sound intensity change?
- number of cells firing
- firing rate of these cells
What is tonotopy?
- the organisation of sound frequencies
- different frequencies preserves throughout tracts, from basilar membrane, to auditory nerve, to cochlear nucleus
- high freqeuncies detected at the base of the basilar membrane
- low frequencies detected at the apex of the basilar membrane
What phase locking?
- allows frequency discrimination
- neurones fire in synchrony with phase of a stimulus
- doesn’t work at high frequencies as AP is a single length
Is phase locking, tonotopy or both, involved in the identification of the following:
- very low frequency sounds
- intermediate frequency sounds
- high frequency sounds
- very low frequency sounds –> phase locking
- intermediate frequency sounds –> both
- high frequency sounds –> tonotopy
How is sounds localisation achieved?
- interaural delay allows us to identify where the sound comes from
- the same sound hits the ipsilateral cortex earlier than the other ear
- inner ear bisons go on ipsi and contralateral auditory nerve
- contralateral is longer - delay
- presence of sound shadows produces this difference
Describe the cochlear nerve and auditory pathway
cochlea –> spiral ganglion –> ventral/dorsal cochlear nucleus –> inferior colliculus –> medial genticulate nucleus –> auditory cortex
- some cross and synapse in the superior olive
Where is the auditory cortex located?
- lateral temporal lobe
- tonotopic organisation
- 500Hz –> 16,000Hz
What are the other projection of the auditory pathway?
- inferior colliculus = MGN and superior colliculus: integration of auditory and visual signals
- brainstem neurones = innervates outer hair cells, regulates sound amplification
- auditory cortex = MGN and inferior colliculus
What are the cortical connections with the auditory cortex? What are they responsible for?
- interconnections between regions
- Broca’s area, Wernicke’s area, primary motor area, angular gyrus, visual areas
- learn to speak and interpret written language
Give examples of conductive deafness
- obstruction of the auditory canal
- otosclerosis (sclerotic degeneration between 3x bones)
- ruptured eardrum
- middle ear infection
- head trauma
What are the genetic causes of nerve deafness?
- > 40 greens, 300 syndromes with related hearing loss
- recessive, dominant or X linked –> structure or metabolism of the inner ear
- some genetic causes give rise to a late onset hearing loss
What are the acquired causes of nerve deafness associated with noise?
- cochlear damage
- permanent or temporary
- environmental or occupational noise
- acoustic trauma
What are the disease that cause acquired nerve deafness?
- inflammation
- DMII
- iodine deficiency, hypothyroidism
- tumours
- meningitis
viruses e.g. AIDS, mumps, measles, herpes zoster oticus - trauma
- stroke (transient/permanent)
What are the some of the iatrogenic causes of acquired nerve deafness?
Ototoxic and neurotoxic drugs
- aminoglycosides (partial recovery) - antibiotic, mitochondrial mutation makes you more susceptible
- methotrexate (not recovered)
- various other medication - reversible hearing loss
What perinatal conditions can cause acquired nerve deafness?
- premature birth
- foetal alcohol syndrome
- syphilis
What is the clinical application of amplification by outer hair cells?>
Tinnitus
- dysfunction of outer hair cell s
- motor neurones contract unnecessarily
- tugs on tectorial membrane
- inner hair cells detect movement
