Week 4 Flashcards
Middle ear
Ossicles
Malleus, incus, stapes
Amplification system, amplifies the signals that reach the tympanic membrane
Tensor tympani and stapedius muscle
Provide protection
Muscles provide attenuation reflex, if noise is loud they contract and reduce transfer signal from tympanic membrane to inner ear
The inner ear cochlea
Drains into 3 compartments
Scala vestibuli
Scala media: auditory organ, organ of cortical
Scala tympani
Function of the inner ear
Stapes pushed by air movement into the oval window
Moves liquid into basilar membrane. Endolymph
Basilar membrane made to vibrate, liquid vibrates
Response of the basilar membrane to sound
Apex: wide and floppy
Base: narrow and stiff
High frequency sounds, vibrates to base of basilar membrane
Low frequency: only vibrates at apex doesn’t reach base
Frequency producing maximum amplitude, different frequency vibration on different path of membrane
Organ of corti
Lies on basilar membrane, so it time membrane moves OOC moves too
Tectorial membrane over it
Sensory neurones: hair cells in organ of corti
-embedded in tectorial membrane attached to side of bony structure
Each time basilar membrane moves tectorial membrane moves up tugs on hair cells-> provides signal transduction
Hair cell receptor potentials
Changes and movements in liquid- signals elicit hair cell potential
Mechanically gated K+ channels
Depolarisation of hair cells
Stereocilia
Mechanically gated K+ channels
When moved K+ channels open allowing K influx into cells
Moved opposite direction influx stops channels closed
Influx K+, opens voltage gated Ca2+ channels on hair cells influx Ca2+, release excitatory neurotransmitters into synaptic cleft, spiral ganglion neurite, glutamate
The auditory nerve
Hearing acuity depends on function inner hair cells
Little signal to the brain from outer hair cells
Inner hair cell and (small bit outer hair cells)-> spiral ganglion cells—> auditory nerve
Amplification by outer hair cells
Sound intensity is low, amplification by ossicles
2nd amplification by outer hair cells
-motor proteins make wall of cells less contractable pull tectorial membrane further amplifying movement, increasing signal reaching inner hair cells
characteristic frequency
Specific to every hair cell
Tonotopy
How different neurones sensitive to different frequencies
Flexibility basilar membrane varies apex to base
Sensitivity to frequency that can move membrane at that segment- characteristic frequency
Cochlear nerve and auditory pathways
Both ears send signals to both sides of brain at same time
Spiral ganglion, auditory nerve, dorsal and ventral cochlear nucleus, decussate superior olive, ascend lateral lemniscus, to inferior colliculus, to MGN, to auditory cortex
The auditory cortex
Top lateral temporal lobe
Primary auditory cortex - sensitive higher frequencies
Secondary auditory cortex
Phase locking
Only possible at relatively low frequency sounds, phase locking has a limit
Peaks ands troughs always coincide with AP response via sensory neurones activated by particular sound
Neurones refractory period- limit on how fast can generate new action potentials so phase locking doesn’t work at higher frequency sounds
Frequency identification
How we identify pitch of noises
Very low frequency- phase locking
Intermediate frequency- phase locking and tonotopy
High frequency- tonotopy
Sound intensity
Loud or soft noise
When increases deflection of basilar membrane increases
Number of hair cells firing higher. Organ of corti
Firing rate is higher, deflections larger in amplitude
Sound localisation interaural delay
In superior olive
Parallel pathways both ears
Time lag, sound travels slowly by the time sound reaches the ear slight time delay, signal not completely simultaneous
Delay is perceived in superior olive allows us to localise the signal
Sound localisation interaural intensity difference
Identification of where sound comes from
Sound intensity difference, sound shadows behind object that’s receiving sound waves
Loss hearing in one ear- lose ability to localise sound because only get signal from one ear, lose interaural delay and interaural intensity difference
Other projections of the auditory pathway
Inferior colliculus- MGN and superior colliculus: integration of auditory and visual signals
Brainstem neurones- outer hair cells
Auditory cortex- MGN and inferior colliculus
Whole mechanism hearing
Depends on air ability to move tympanic membrane ossicles and oval window
If anything obstructs this become deaf
Auditory canal- obstructed ear wax, small objects
Ear infections can perforate tympanic membrane
Clinical aspects
Deafness- conduction of mechanical wave
-obstruction of auditory canal
-otosclerosis - sclerosis ossicles movement restricted- comes with age
- ruptured eardrum
-middle ear infection
-head trauma
Deafness- nerve deafness
Genetic ->40 genes ~300 syndromes with related hearing loss
-Recessive, dominant or X linked genetic mutations- structure or metabolism of the inner ear
-Some genetic causes give rise to a late onset hearing loss
Congential:
-congenital rubella syndrome
-human cytomegalovirus HCMV
-toxoplasmosis
-hypoplastic auditory nerves or abnormalities of cochlea
Presbycusis: normal progressive age related loss hearing acuity or sensitivity:
-1:3 significant hearing loss by age 65
-1:2 significant hearing loss by age 75
-not preventable or reversible
Acquired nerve deafness
Noise:
-cochlear damage
-permanent or temporary
-environmental or occupational noise
-acoustic trauma
Diseases:
-inflammatory
-diabetes mellitus
-iodine deficiency/hypothyrodism
-tumours
-meningitis
-viral infections (AIDS, mumps, measles, herpes zoster oticus)
-trauma
-stroke
Ototoxic and neurotoxic drugs
-aminoglycosides, partial recovery. Rare mitochondrial mutation m.1555A>G can increase an individuals susceptibility to the ototoxic effect of aminoglycosides
-methotrexate, not recovered, treatment of autoimmune induced inflammatory HL
-various other medications- reversible HL
