Auditory and vestibular pathways : hearing and balance

Question 1

What are some issues with teh auditory system?

Answer 1

Over 9 million people in UK have a
hearing impairment
Two children born each day
Lack of regeneration and therapies

Question 2

What is sound?

Answer 2

What is sound?
Audible variations in air pressure
Frequency of sound: Number of compressed air patches (cycles) that pass the ears each second. Expressed in units called hertz (Hz).
We can perceive changes in air pressure over a range of 20 - 20,0000 Hz.

Cycle : Distance between successive compressed patches

Question 3

What is pitch and intensity?

Answer 3

• Pitch : High pitch = high frequency; low frequency= low pitch
• Intensity : High intensity louder than low intensity
• 1 octave = doubling frequency
• Speed of sound is constant ~ 343 m/sec

Question 4

What is the structure of the auditory system?

Answer 4

• External, middle and inner ear
• Outer ear : Pinna - captures sound- sound localisation
• Middle ear : ossicles - transmit mechanical energy from sound through ear
• Inner ear : cochlea and labryinth
• Mechanical energy> fluid waves> action potentials

Question 5

What are the stages of the auditory pathway?

Answer 5

Sound wave causes :
- Tympanic membrane to move
- Moves the ossicles
- Moves membranse at the oval window
- Motion at oval window moves cochlear fluid
- Movement of fluid causes sensory neuron response

Question 6

What is the direction of movement in the external ear?

Answer 6

• From pinna to tympanic membrane (Ear drum)

Question 7

What is the direction of movement in the middle ear?

Answer 7

• Tympanic membrane and ossicles
• Movements of footplate transmit sound vibrations to the fluids of the cochlear (inner ear)
• Sound amplification by ossicles ::
• More energy required to move fluid than air
• Ossicles have lever action that amplifies the pressure 20 x
• Now enough pressure to move fluid
• Focuses energy on the oval window

Question 8

Describe the middle ear and its importance

Answer 8

• Ear drum (tympanic membrane) :
• The appearance and mobility of the drum are important for diagnosing middle ear disease and ifection
• Tympanic membrane is well supplied with blood vessels and sensory nerve fibres that make it acutely sensitive to pain

Question 9

Describe the inner ear?

Answer 9

• The cochlea
• Scala vestibicular and tympani- contain perilymph (fluid)
• Scala media : contains endolymph (fluid)

Question 10

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What is the anatomy of the cochlea?

Answer 10

• The organ of Corti (auditory epithelium)
• Rests on the flexible basilar membrane
• Inner and outer hair cells - sensory
• Hair cells held rigidly
• Spiral ganglion nerve cells

Question 11

What is the movement of fluid in the inner ear?

Answer 11

Movement at oval window displaces fluid - Bends basilar membrane at base - Travelling wave moves along basillar membrane.
- Endolymph movement bends basilar membrane near base, wave moves towards apex
- Wave in the BM at the same frequency as the sound wave
- Point of maximal displacement depends on the frequency of sound
- BM acts as a frequency analyser

Question 12

Describe the anatomy of the inner ear

Answer 12

• Basilar membrane is wider at the apex
• Stiffer at the base
• Max amplitude of vibration is acheived at different positions on BM = Tonotopy = topographical frequency mapping organisation

Question 13

Describe the components of the inner ear

Answer 13

• Cochlea : site of mechano-sensory signal transduction (air pressure waves from middle ear send waves down basilar membrane)
• Consits of a set of membranes, suspended in fluid
• The organ of Corti = composed of a set of structures located on the upper surface of the Basilar membrane
• Basilar membrane is frequency tuned
• These properties allow the cochlea to decompose complex sounds into their component frequencies

Question 14

How do pressure waves encode a neural signal?

Answer 14

• Hair cells convert pressure waves into APS
• Hair cells synapse on afferent sensory neurones whose cell bodies lie in the cochlear (spiral) ganglion. Hair cells are not neurones as they lack axons
• Inner hair cells :
• 10-20 ganglion cell innervates each inner hair cell
• Feeds into the auditory nerve (CN VIII)
• Has efferent innervation from higher centres
• Sound encoding - signal transduction
  Outer hair cells :
• 1 ganglion cell innervates many outer hair cells
• Motor proteins ( for sound amplification)
• Feeds into the auditory nerve (CN VIII)
• Has efferent innervation from higher centres
• Sound amplification
• Vibration causes a shearing force across the hair cells
• Hair cells move forward and backward

Question 15

How does transduction occur in the sterocillia?

Answer 15

• By means of mechanically gated ion channels
• Ion channels open when the tip links joining the stereocilia are broken
• The entry of K+ depolarises cell
• This opens voltage gated Ca 2+ channels
• Leads to release of neurotransmitter from the vesicles
• Diffuses into the synaptic cleft

Question 16

What are the central auditory processes?

Answer 16

Central auditory processes
Receptors of primary neurones are distributed to hair cells — detect
vibrations in the endolymph
Cell bodies of primary neurones are in spiral ganglion of cochlear
Central processes of primary sensory neurones pass in auditory nerve fibres to the dorsal and ventral cochlear nuclei (in pons)

Dorsal and ventral cochlear nuclei contain cell bodies of 2 ary neurones
* Project to both sides of the
brainstem and auditory cortex
e.g. from ventral cochlear:
* Processes pass to superior olive on both
sides of brain
- Processes then pass to inferior colliculus (midbrain)
-Then to medial geniculate nucleus (thalamus)
-Then to the auditory cortex (temporal lobe)

Question 17

What are some hearing disorders?

Answer 17

Hearing Disorders
Reduction of hearing or deafness might be conductive or sensorineural in origin

Conductive hearing loss = Hearing loss associated with the conductive portion of the hearing pathway
Sensorineural hearing loss = Hearing loss associated with the neural portion of the hearing pathway
Can include cochlear, brainstem, thalamic or cortical
problems
* Hearing disorders can also include tinnitus (sound
sensation in absence of sound) / auditory hallucination.
* Tinnitus occurs in Méniere’s Disease, URTI, or
following exposure to loud sounds and can be
conductive (tensor tympani / stapedius myoclonus) or
sensineural in origin.
* Auditory hallucination may suggest cortical / thalamic
lesion.

Question 18

What 3 inputs do we use for balance?

Answer 18

1. Visual
2. Proprioceptive
3. Vestibular

Question 19

What is Romber’s test and sign?

Answer 19

We remain stable if we have 2 out of 3 inputs
If we lose 2 inputs we become unstable
Romberg’s test relies on this:
Patient closes eyes
Patient sways/falls in positive test
Patient remains steady in a negative test

Question 20

What is the labryinth in the auditory system?

Answer 20

• Vestibule = lies between cochlea and semi-circular canals
• Oval and round windows on the lateral wall
• Contains utricle and saccule which have equilibrium receptors called maculae
• Respomd and report gravitational and changes in head position
• Saccule :
• continuous with the cochlear duct anteriorly
• Utricle :
• continuous with the semi-circular ducts posteriorly

Question 21

What are the static and dynamic parts of the vestibular system?

Answer 21

• Dynamic part = semicircular canals and crista
• Semi-circular canals - act via nedial-longitudinal fasiculus
• Static part - Utricle and Saccule - the Maculae - acts via vestibulospinal pathway
• The vestibular system also uses hair cells to transduce movements
• sensitive to different kinds of movement because of the specialised structure in which the hair cells reside

Question 22

What is the function of maculae (utricle and saccule) ?

Answer 22

• Detects force of gravity and tilts of the head
• Maculae provide information of head position relative to trunk and senses linear acceleration e.g. walking, driving, falling
• Utriclar macula = detects horizontal acceleration (walking, driving)
• Saccular maculaa= detects vertical acceleration (e.g. falling)
• Purpose of each structure = to transmit mechanical energy, derived from the head movement, to its hair cells

Question 23

auditory

What is maculae?

Answer 23

Maculae
The utricle and saccule both have a sensory epithelium — called a macula
(contains equilibrium receptors)

• Hair cells in maculae project into a
  gelatinous cap
• Calcium carbonate crystals (otoliths)
  encrust surface of gelatinous cap
• When head angle changes (or
  accelerates) force is exerted onto
  otoliths
  Gelatinous cap moves & deflects hair cells

-Bending towards kinocilium = depolarising, excitatory receptor potential
Bending away kinocilium =
hyperpolarisation, inhibitory receptor
potential

Movements are transduced by hair cells in maculae when hair bundles are deflected
Note: transduction mechanism of vestibular hair cells similar to auditory hair cells

• Hair cells orientated vertically in saccule and horizontally in utricle
• Due to different orientation, tilts in any direction will activate some afferents
• Utricle most sensitive to tilt when head is upright
• Saccule - most sensitive to tilt when head = horizontal
• CNS considers info simultaneously from both otgans to interpret head movement

Question 24

What are the dynamic semicircular canals in the vestibular system?

Answer 24

• Detects head rotation/turning movements
• Semicircular canals convert rotational motion of head/body into neural impulses
• Canals are arranged in a mutually perpendicular
  manner (superior, lateral, posterior) to cover 3
  planes of motion
• Signalling from a canal can be increased or
  decreased depending on direction of movement
• Act to move eyes and neck in response to head
  movement (vestibulo-occular reflex)

Vestibular system
Semicircular canals are arranged mutually perpendicular to each other and
therefore cover 3 planes of motion and detect angular acceleration :
Superior canal plane
Posterior canal plane
Superior - ‘YES’ / Sagittal movement (nodding)
Lateral - ‘NO’ [Axial movement (shaking)
Posterior - ‘MAYBE’ / Coronal movement (tilting side to side)

Question 25

What is the anatomy of the semicircular canals?

Answer 25

Bulge along the canal

Hair cells clustered within sheet of cell
— the crista
Cilia project into gelatinous cupula
* Within the ampulla
ALL hair cells in an ampulla orientated
in same direction (excited/inhibited
together)
Semi-circular canals filled with
endoplymph (same as scala media of
cochlear)
Cilia bend when canal is suddenly
rotated
Endolymph exerts force on cilia
Direction of bend either excited or
inhibits hair cells
- The three semi-circualr canals work together to sense all possible head rotations

Question 26

What are the central vestibular pathways ?

Answer 26

• Coordinate and integrate information about head and body movement to control motor output of head, eye and body positions
• Though its numerous connections the vestibular nerve (system) maintains equilivrium by adjusting body position when loss of balance is threatened

Question 27

What are some vestibular system disorders?

Answer 27

• Vestibular system disorders cause a range of symptoms
• Vertigo (Sense of falling./ imbalance/dizziness toward side of lesion)
• Meniere’s disorder - including nauseam imbalance
• Motion sickness (VS can trigger nausea)
• Nystagmus - eye movement in the absence of angular acceleration
• Caloric reflex test