Neuro 3 - special senses Flashcards
Auditory system qualities
Mechanical to electrical transduction
Very rapid, works at extremes of physiology
Very small movements transduced
- 70% of over 60s have hearing impairment
- 840 babies born each year with bilateral impairment
What is sound?
Noise generated, then compress and rarefy air as sound wave
Amplitude (peak to trough distance) - loudness - decibels - Db
Frequency (no. of cycles passing point in time) - pitch - Hertz - Hz
But sound is very complex, use Fourier analysis to study tones in a complex sound
- peripheral auditory system is doing this, breaking down complex sounds and encoding to nervous system
Audiometric curve of human hearing
Total area inside curve = area of auditory perception, around 20Hz - 20kHz, 0Db - 120Db
Bottom line = threshold for hearing, lowest intensity (differs at different frequencies)
Top line = level of feeling and pain, sounds too intense and damaging to auditory system
Conversation area in middle, so speak at the most sensitive frequencies
Anatomy of human ear
OUTER - pinna, to funnel sound along external auditory canal
(divided by tympanic membrane)
MIDDLE - ossicles, eustachian tube
INNER - cochlea coil, vestibular system
Outer + middle = conductive part
Inner = sensory-neural part, for transduction
Function of middle ear
Impedence matching:
- airwaves cause tympanic membrane to vibrate
- lever action of ossicle (moving through air)
- stapes moves against oval window, to move fluid
More energy needed to move fluid than air
Hence why oval window very small, energy concentrated to very small area, so 20x increase in pressure
- necessary to move fluid, otherwise not much energy would transfer
Ossicular reflex in middle ear
To protect against intense sounds (= attenuation reflex), reduce damage to ear
2 muscles, stapedius and tensor tympani are attached to ossicles and the bony part of ear
- if muscles tense, bones can’t move freely, stiffens lever action, so decreases energy conduction
BUT
- must be continuous loud sounds, ineffective for impulse noise
- doesn’t work at very high frequency sounds - though can help discrimination of low, so can understand high pitch
Cochlea spiral structure
35mm long when uncoiled
Structure formed by 10 weeks gestation, functional by 20 weeks
Compartmentalised into series of chambers
Chambers of cochlea
Three chambers in a section:
Scala vestibuli + scala tympani - perilymph
Scala media - endolymph, + organs of hearing
Compartmentalisation is essential to keep fluid apart
Scala media contents
Stria vascularis – maintains ion composition of fluid (Na+/K+)
Tectorial membrane – gelatinous membrane overlying organ of Corti
Organ of Corti:
- on flexible basilar membrane
- inner and outer sensory hair cells
- spiral ganglion nerve cells
- associated supporting, non-sensory cells to hold hairs rigidly
Cochlear fluids
PERILYMPH - in scala vestibuli and scala tympani - resembles CSF - to bathe cell bodies of organ of Corti - 0mV potential ENDOLYMPH - in scala media - resembled intracellular fluid - sealed in tight compartment - to bathe surface of organ of Corti - maintained by stria vascularis - +80mV potential
Properties of inner and outer hair cells
Stereocilia (hair-like) projecting from apical surface
Mechano to electrical transduction
Transduction channels in hair bundles
Outward K+ channels in basolateral membrane
Sensitive to damage and disease
Properties of inner hair cells
~3500, one row Flask shaped Hair bundle flat Afferent innervation mainly 10-20 afferent terminals per cell Inward Ca2+ channels
Properties of outer hair cells
~1200, three rows
Columnar shape
Hair bundle in V or W shape
Mainly efferent innervation
(small afferent – one neurone to many cells)
Prestin motor protein in lateral membranes
When sound enters the ear…
Along external auditory canal Vibrates tympanic membrane Moves ossicles in lever action Stapes displaces fluid in scala vestibuli Travelling wave Displace basilar membrane up and down Sensory hair cells contact tectorial membrane, so stereocilia displace membrane -> Travelling wave
Stereocilia response to movement
Arranged in rows, with tallest at back
Tip links between ends of tall and short stereocilia
Movement of the cilia opens ion channels, so K+ rush in – driving force for K+ from endolymph into cell 135mV
- sensitive to very small movements
⇒ depolarisation of hair cell
Voltage gated Ca2+ open, Ca2+ in
Neurotransmitter release, increased firing of nerve cell
After stimulus gone, return to upright position
Phase locking
At rest – some trickle of transducer current (K+ movement)
Positive stimulus – push towards tallest cilia - increase current up to a maximum point
Negative stimulus – push towards smallest cilia – switch off current
Activity in hair cell -> receptor potential -> nerve activity
- only encodes 1-4kHz, we can hear up to 20, so other mechanism also
Tonotropy
For hearing 4+ kHz
Maximum displacement of basilar membrane occurs at different positions – regional variations in fibrous structure
Depends on frequency of sound
In low freq – displacement close to apex
In high freq – displacement close to base of basilar membrane
Tonotopic map in brain so can understand, auditory nerves end at different points on cochlear nucleus (isofrequency bands)
PASSIVE MECHANISM
OHCs role in hearing
Cochlear amplifier - amplification and tuning of IHC stimuli
Tonotopy is passive
Must be active component, otherwise would dissipate and lose energy by end of basilar membrane
Only occurs in live cochlea, by OHCs
- drugs can reduce this
How do OHCs amplify
Depolarisation -> activates prestin (motor protein) -> cells shorten rapidly
Rigidly held cells contract, so amplify basilar membrane movements
Gain (size change) modulated by efferent system
Otoacoustic emissions
Sounds produced by ear
Send click sound in, hear click back
Neural encoding of loudness in IHCs
Louder sounds, larger stimulus
IHCs have numerous (10-20) synaptic contacts with different nerve types:
High spontaneous rate fibres – easily excited at low sound levels, soon saturated
Medium spontaneous rate fibres – excited at medium sound levels, will saturate
Low spontaneous rate fibres – not excited until sound loud, saturate slowly – good for detecting changes in sound at very high sound levels
- sound encoded by activation of populations of neurones
Cochlea to brain main pathway
Cell bodies of afferent nerve terminals in spiral ganglion
Ventral cochlear nucleus
Superior olive – ipsi + contralateral input from both ears, important for localisation of sound. Then carried parallel bilaterally.
Inferior colliculus
Medial geniculate nucleus
Auditory cortex
Function of vestibular system
Report on position and movement of head
Give sense of balance and equilibrium
Coordination and posture adjustment, with cerebellum
- disorders – feels like vertigo - nausea, disequilibrium, nystagmus
Anatomy of vestibular labyrinth
Three semicircular canals – crista
- sensitive to head rotation and angular acceleration
- have sensory hair cells
Otolithic organs – saccule and utricle
- sensitive to gravity, linear acceleration
- have sensory hair cells
Hair cells the same, though for different functions, due to structures they sit in
Vestibular hair cells
Kinocilium are tallest cilia
Type I and II
Afferent and efferent innervation
Transduction similar to IHCs
Transduction and encoding in vestibular hair cells
Positive direction – towards tallest (kinocilium) – depolarisation – excitation
Negative direction – away from kinocilium – hyperpolarisation – inhibition
But, high resting rate of nerve fibres, firing even when hairs vertical, so high sensitivity to direction
Stimulation of macula
Macula = sensory epithelium of the otolithic organs
- saccule and utricle
Otoliths are crystals of calcium carbonate, lie over:
Gelatinous cap, coating hair cells
When head moves, otoliths (high density) lean
Moves gelatinous cap
Deflects stereocilia
Transduction
Positive direction is head to chest
Negative is head back
Hair cell orientation in vestibular apparatus
Many hair cells oriented in different directions:
Saccule and utricle have positive direction facing outwards and inwards respectively
- and organ is curved, so get wide range, have all cell orientations necessary
Semicircular canals also are at 90degrees to each other, so can respond to rotation in any given direction
- hair cells oriented one way in each ampulla
- adaptation after 15-30s as fluid catches up
Peripheral vestibular pathologies
Kinetosis (motion sickness)
Lesions/irritations – usually unilateral
Meniere’s disease – excessive fluid pressure
Benign paroxysmal positional vertigo – otoliths dislodged from utricle
When chronic, can compensate using visual stimuli, proprioceptors
Hearing loss
MILD – difficulty following speech in noisy situations, 25-39dB quietest sounds heard
MODERATE – difficulty following speech without hearing aid, 40-69dB quietest sounds heard
SEVERE – rely on lipreading, even with hearing aid, 70-94dB quietest sounds heard, signing maybe preferred language
PROFOUND – 95dB+ only, signing or lipreading
- conductive hearing loss (damage/blockage to outer or middle ear) or sensorineural hearing loss (damage/loss of sensory hair cells or neurones)
Conductive hearing loss
Damage/blockage to outer or middle ear
Outer – obstructions (eg wax), tympanic membrane rupture (from infection, foreign object, barotrauma)
Middle – otosclerosis (overgrowth of stapes so not proper lever action), glue ear (chronic fluid build up), otitis media (acute inflammation, redness, pain)
Sensorineural hearing defects causes
- drug induced
- genetic defects
- acoustic trauma to hair cells and neurones, acute and chronic
- presbycusis - age related
- infectious disease
- tinnitus
Drug induced sensorineural hearing loss
eg Aminoglycoside antibiotics
- ototoxic, so only used when necessary
LOSS OF OHCS
- dose dependent
- > partial deafness, poor frequency discrimination
- hearing aid helps intensity only, as lost cochlear amplification and tuning
progresses to… (with higher dose/longer treatment)
LOSS OF IHCS AND OHCS
- complete deafness
- need cochlear implant
Acoustic trauma -> hair cell defects
If mild, up to 90dB
- disruption of hair bundle, breakage of tip links
- may be reparable
- temporary threshold shift, eg after club night
Severe, more than 130dB (or quieter but sustained)
- complete loss of area of hair cells
- scar formation fills void
Due to:
- metabolic exhaustion
- physical disruption of hair bundle
- excitotoxicity, damage at synapse
- build up of toxic metabolites
- hole in reticular lamina
