L19 - Hearing and Vestibular Flashcards
(19 cards)
Describe aspects of sound (what it is, frequency, pitch, amplitude)
- pressure waves which cause vibration of molecules: alternating patterns of compression and rarefaction
- frequency of oscillation gives it the pitch: high pitched sounds have a high frequency of oscillations per second (measured in Hz)
- aplitude (height) of pressure wave gives the volume of the tone (measured in dB)
- we don’t hear equally well at all frequencies: hear best ~1-4KHz; outside this range, amplitude needs to be greater
describe certain important decibels and loudness
- 0 dB is the threshold of hearing (at optimal tones of between 1 to 4 KHz)
- 60dB is normal noise
- around 100dB is when you start to get damage if listening at this level for longer amounts of time
- 130-140 dB is when you get immediate permanent hearing damage
describe the human ear anatomy/key structures (diagram on slides)
- the outer and middle ear are filled with air
- the inner ear (cochlea) is filled with fluid
- sound waves enter the ear canals
- cause vibrations of the tympanic membrane
- signal is then amplidfied by the ossicles in the middle ear: the malleus (hammer), incus (anvil), and stapes (stirrup)
- the pressure wave is then transmitted to the oval window
- this causes vibrations within the cochlea
describe the anatomy of the cochlea
Scala vestibuli (contains perilymph):
- connected to the oval window
- contains perilymph (like ECF high Na+, low K+)
Scala tympani (contains perilymph):
- connected to the SV by the helicotrema
- connected to the round window
- contains perilymph
Cochlea duct sits between SV and ST
- is filled with endolymph (high conc. K+, low Na+)
Bascilar membrane separates CD from ST
Organ of Corti sits on the basilar membrane
The receptors cells in the Organ of Corti are hair cells
Hair cells have stereocilia that are embedded in the tectorial membrane
what are the functions of the cochela
- pressure wave from the oval window travels through the Scala vestibuli and is transferred to the cochlea duct
- the basilar membrane vibrates and deforms
- hair cells within the Organ of Corti vibrate against the tectorial membrane
- mechanoreceptors on the stereocilia are opened or closed by this movement
- changes in membrane potential can activate afferent nerves
what does the deflection of stereocilia result in?
Deflection towards longest stereocilia opens mechanically gated (MG) ion channels
- more deflection = more K+ movement into cell
- more K+ into cell = greater depolarisation
Deflection away from longest stereocilia closes MG K+ channels.
- allows for rapid depolarisation
- sound wave vibration of basilar membrane causes rapid oscillation of stereocilia against pectoral membrane -> rapid oscillations in impulses propagating down auditory axons
how is tone determined?
- the basilar membrane widens as it extends though cochlea (which narrows over the same distance)
- end nearest oval window: ‘base’, high pitched tones
- end furtherest from oval window; ‘apex’, low tones
what does tonotopic organisation mean
- tones map to temporal lobe based on origin of signal along bascilar membrane
- lower pitch = more anterior
what four aspects do we need to determine sound qualities?
- Pitch/tone (frequency): locaion of sinal to afferent neuron on bascilar membrane. mapped to location in temporal lobe
- intensity (loudness): number of APs per second in auditory nerve fibres. magnitude of deflection of stereocilia
- duration: signalled by duration of the afferent discharge caused by the stimulus
- direction: indicated by time difference in activation of receptors in each ear. intensity differences in each ear.
what is deafness a result of
Raised threshold to sound stimuli
Possible causes:
- impaired sound transmission through outer or middle ear (conduction deafness): can be due to blockage or infection
- damage to receptors or neural pathways (sensorineural deafness): can be due to exposure to sound noises, some antibiotics, tumour, meningitis etc.
describe the vestibular system (what it does, what information it provides, reflexes)
- detects acceleration of the head (not velocity)
- provides information for proprioception and conscious awareness of movement/acceleration (linear and rotational) of the head and body
- provides information to assist with maintenance of balance and posture (signals more quickly than visual system)
- generates reflexes to compensate for head movement and the perception of motion in space (eg. vestibuloocular reflexes for stabilising images on the retina despite head movements
describe the vestibular apparatus (where its located, composed of what parts)
located within the inner ear
membranous fluid-filled tubes composed of 2 parts:
Semicircular canals (3 per ear)
- contain ampullae
- detect rotational acceleration
Otolith organs (2 per ear)
- utricle and saccule
- detect linear acceleration
- detect changes in head position relative to direction of gravitational force
describe the semicircular canals (diagrams on slides)
- detect rotational/angular acceleration
- alignes in 3 axes: anterior (nodding ‘yes’), lateral (shaking ‘no’), posterior (tilting head to side)
- mostly stimulated in preferred plane
- work in pairs, ones set in each ear
- contain endolymph (high conc K+, low Na+)
- each contain one ampulla
describe the role of the ampulla (diagram on slides)
- is widened area at the end of each semicircular canal (each of the 6 semicircular canals has one)
contains cupula: - gelatinous wedge
- hair cells embedded in cupola (stereocilia)
- not affected by gravity due to physical properties of gel and endolymph
- when head moves, the ampulla moves with it as attached to semicircular canal, cupola pushes against the stationary endolymph
- head rotation-> movement of the ampulla -> deflection of cupola -> activation of hair cells
describe vestibular transduction
- mechanoreceptors on the stereocilia are opened or closed by movement
- some neurotransmitter is always released from the hair cells at reat (allows for signalling in 2 directions up or down)
- deflection towards the longest stereocilia increases AP frequency
- deflection may from the longest stereocilia decreases AP fequency
- AP frequency in afferent nerves is related to: direction of bending, force bending (rotation speed)
- the discharge pattern from each ampullae from corresponding canals on each side of the head will be opposite
describe the otolith organs
Utricle and Saccule
- provide information about linear acceleration and changes in head position relative to gravity
- hair cells are embedded in gelatinous mass with tiny calcium carbonate crystals (otoconia) embedded on top, heavier than endolymph so sink in the direction of gravity
- distortion of otolith organs opens mechanically gated channels on stereocilia of hair cells
Utricle (approx horizontal when standing):
- hair cells orientated vertically
- respond when head moved away from horizontal and to linear accelerations in the horizontal plane
Saccule (approx vertical when standing):
- Hari cells oriented horizontally
- response when moving from lying to standing and vertical accelerations - jumping
describe the central vestibular pathways
- axons from the vestibular apparatus project to the vestibular nuclei in brainstem (reflex responses to movement of the head)
- to cerebellum and spinal cord (maintenance of balance)
- occulumotos nuclei (near superior colliculus) - stabilisation of the eyes
- vestibular cortex (close to primary auditory CTX) - awareness of head position
what are three vestibular disorders (cause and symptoms)
Vertigo:
- cause: diseases affecting vestibular structures or afferent nerves
- symptoms: illusion of movement, dizziness, nausea
Motion sickness:
- cause: mismatch in visual and vestibule information
- symptoms: dizziness, and neusea. can be improved by looking to horizon
“bed spins”:
- cause: alcohol consumption
- symptoms: ethanol gets into cupula, lowers density of gelatinous structure and causes it to ‘float’ in the endolymph, opening the MG channels, this causes depolarisation and a perception of movement
descrie ototoxic drugs
over 600 medication sat known to be harmful to hair cells, can result in temporary or permanent hearing loss and disorders of balance.
amino glycoside antibiotics (eg. gentamicin, streptomycin, kanamycin) are amongst the worse.
May be able to improve with ‘vestibular rehabilitaiton’. however this is irreversible
