Auditory Function and Balance

Question 1

Sound: explain frequency, pitch, intensity, volume and the decibel scale

Answer 1

Sound

• Transverse waves
  
  • Compressions and rarefactions
• Measure in decibels (dB) = logarithmic scale
  
  • Humans can hear 20-20,000 Hz

Frequency

• Number of compressed and rarefied cycles of air/ sec (Hz)
• We can hear sounds between 20 ad 20,000Hz

Pitch

• Perception of frequency

Intensity

• Amplitude of sound waves

Volume

• Perception of amplitude

Decibel scale

Log scale of loudness

Question 2

Summarise the anatomy of the external and middle ear

Answer 2

Summarise the anatomy of the ear, cochlea and labyrinth

• External ear
  
  • Pinna (auricle)
    
    • Elastic cartilage covered in skin
    • Catches sound waves and passes them along deeper into the ear
    • Gives an idea as to elevation
  • External acoustic meatus
    
    • = auditory canal
    • Focuses noise and increases pressure at tympanic membrane
  • membrane
• Middle ear (tympanic cavity) – hearing only
  
  • Tympanic membrane (ear drum)
    
    • Slightly cone-shaped membrane of connective tissue
  • Auditory ossicles:
    
    • Malleus
    • Incus
    • Stapeus
  • Air is continuous with eustachian tube
    
    • Links nasopharynx and middle ear
• Inner ear – hearing and equilibrium
• 2 layers
  
  • Bony labyrinth
    
    • Fluid-filled
    • 3 parts:
      
      • Vestibule
      • Semi-circular canals
      • cochlea
  • Membranous labyrinth
    
    • Continuous series of sacs and ducts that follows shape of bony labyrinth
    • Separated from bony walls by perilymph
• Cochlea – snail-shell shape
  
  • Connects with middle ear via stapes action on TM
• 3 chambers:
  
  • Scala vestibuli
  • Scala media
  • Scala tympani
    
    • Scala media contains endolymph
      
      • High K+
      • Low Na+
    • Scala vestibuli and scala tympani contain perilymph fluid
      
      • Low K+
      • High Na+
      • Fluid can move between these 2 chambers via hole called “helicotrema”
  • Chambers separated by sensitive membranes including basilar membrane:
    
    • Basilar membrane = stiff band of tissue that runs between scala media and scala tympani
      
      • Runs alongside the central perilymph-filled chamber
    • Sections of BM vibrate when sound hits them
• On top of basilar membrane = organ of corti
  
  • Made up of inner and outer hair cells and neurons
  • Lies on top of basilar membrane and beneath tectorial membrane
• Inner hair cells = found alone
  
  • Send afferents to brain
  • Have stereocilia:
    
    • Base in perilymph
    • respond to endolymph movement in scala media
  • Do not make contact with tectorial membrane
  • Around 3500 total
  • Function: transmit auditory stimulus
• Outer hair cells = found in groups of 3
  
  • In contact with tectorial membrane
  • Receive efferents
  • Vulnerable to damage
  • Function:
    
    • Expand and contract due to electromotility à able to amplify vibrations
    • Control of inner hair cells

Question 3

Summarize the anatomy of the inner ear

Answer 3

Inner ear – hearing and equilibrium

2 layers

• Bony labyrinth
  
  • Fluid-filled
  • 3 parts:
    
    • Vestibule
    • Semi-circular canals
    • cochlea
• Membranous labyrinth
  
  • Continuous series of sacs and ducts that follows shape of bony labyrinth
  • Separated from bony walls by perilymph

Question 4

Summarize the anatomy of the cochlea

Answer 4

• Cochlea – snail-shell shape
  
  • Connects with middle ear via stapes action on TM
• 3 chambers:
  
  • Scala vestibuli
  • Scala media
  • Scala tympani
    
    • Scala media contains endolymph
      
      • High K+
      • Low Na+
    • Scala vestibuli and scala tympani contain perilymph fluid
      
      • Low K+
      • High Na+
      • Fluid can move between these 2 chambers via hole called “helicotrema”
  • Chambers separated by sensitive membranes including basilar membrane:
    
    • Basilar membrane = stiff band of tissue that runs between scala media and scala tympani
      
      • Runs alongside the central perilymph-filled chamber
    • Sections of BM vibrate when sound hits them
• On top of basilar membrane = organ of corti
  
  • Made up of inner and outer hair cells and neurons
  • Lies on top of basilar membrane and beneath tectorial membrane

Question 5

Hair cell anatomy

Answer 5

• Inner hair cells = found alone
  
  • Send afferents to brain
  • Have stereocilia:
    
    • Base in perilymph
    • respond to endolymph movement in scala media
  • Do not make contact with tectorial membrane
  • Around 3500 total
  • Function: transmit auditory stimulus
• Outer hair cells = found in groups of 3
  
  • In contact with tectorial membrane
  • Receive efferents
  • Vulnerable to damage
  • Function:
    
    • Expand and contract due to electromotility à able to amplify vibrations
    • Control of inner hair cells

Question 6

Middle ear function: explain the mechanisms for amplification and protection of the middle ear

Answer 6

• Need to amplify pressure of sound waves in tympanic cavity so that they have enough energy to produce a pressure change in the fluid-filled inner ear
  
  • Need more energy because of impedence
• 2 methods:
  
  • Using auditory ossicles:
    
    • Lever action increases force at oval window
    • Stapeus vibrates against superior oval window to induce pressure wave in inner ear fluid
    • Round window (below oval window) acts as a pressure release window – moves outwards to equalise pressure when stapes pushes oval window into cochlear
  • Using tympanic membrane:
    
    • Focus vibrations from large SA of the TM to the smaller SA of the oval window
• Change in SA à pressure increase

Question 7

Sound transduction: explain the mechanisms of sound transduction from the middle ear to the cochlea

Answer 7

Central auditory pathway:

• Important in localising sound
• Afferent ganglion = spiral ganglion
  
  • Enters brain stem at level of 8^th CN
• Projects onto ipsilateral cochlear nuclei in the medulla
  
  • Beyond this everything is bilateral
• Traverses superior olivary nucleus
• ALL pathways converge at inferior colliculus in caudal midbrain
  
  • Bidirectional pathway, with afferents from inner hair cells and efferents going towards outer hair cells
• Continues to medial geniculate nucleus
• Continues to auditory cortex

Tonotopic mapping

• BM covered in >20,000 fibres
• Further down membrane à fibres get longer
  
  • At the base = short and stiff
    
    • Vibrates in response to high frequency sounds
  • End = long and loose
    
    • Vibrates in response to low frequency sounds

This generates either:

• Place code (phase locking?)
  
  • Very low frequency and mixed frequency sounds
• Tonotopic map
  
  • Very high frequency and mixed frequency sounds
  • Mirrored in higher levels of auditory processing
    ​

Auditory reflexes:

• Control of ossicles in amplification:
  
  • 2 muscles contract to reduce ossicle movement:
    
    • Tensor tympani
    • Stapedius
  • This movement = part of auditory reflex
    
    • Reduce vibration made by loud natural noises to protect ear
    • Also work during talking/ chewing so you don’t hear internally-generated noises so much

Question 8

Central auditory pathways: list the main structures in the central auditory pathways and their functions, explain tonotopic mapping, and identify the part of the pathway involved in auditory reflexes

Answer 8

part of the pathway involved in auditory reflexes

• Central auditory pathway:
  
  • Important in localising sound
  • Afferent ganglion = spiral ganglion
    
    • Enters brain stem at level of 8^th CN
  • Projects onto ipsilateral cochlear nuclei in the medulla
    
    • Beyond this everything is bilateral
  • Traverses superior olivary nucleus
  • ALL pathways converge at inferior colliculus in caudal midbrain
    
    • Bidirectional pathway, with afferents from inner hair cells and efferents going towards outer hair cells
  • Continues to medial geniculate nucleus
  • Continues to auditory cortex
• Tonotopic mapping
  
  • BM covered in >20,000 fibres
  • Further down membrane à fibres get longer
    
    • At the base = short and stiff
      
      • Vibrates in response to high frequency sounds
    • End = long and loose
      
      • Vibrates in response to low frequency sounds
• This generates either:
  
  • Place code (phase locking?)
    
    • Very low frequency and mixed frequency sounds
  • Tonotopic map
    
    • Very high frequency and mixed frequency sounds
    • Mirrored in higher levels of auditory processing
• Auditory reflexes:
  
  • Control of ossicles in amplification:
    
    • 2 muscles contract to reduce ossicle movement:
      
      • Tensor tympani
      • Stapedius
    • This movement = part of auditory reflex
      
      • Reduce vibration made by loud natural noises to protect ear
      • Also work during talking/ chewing so you don’t hear internally-generated noises so much

Question 9

Deafness: list the causes of conductive and sensorineural deafness

Answer 9

Conductive Hearing Loss:

• Due to diseases of the middle ear
  
  • Ossicle destruction
  • Ossicle joint stiffening
• Amplification system is eliminated à conductive hearing loss
• A heavily waxy ear can also block sound waves from the ear drum

Sensorineural Hearing Loss:

• When the cochlea or the cochlear nerve is damaged
• Signal transmitted to the auditory cortex is reduced/lost à sensorineural hearing loss

Conductive hearing loss

• Causes: wax/ otitis media/ otosclerosis of ossicles/ perforated tympanic membrane

Causes of pre-lingual hearing loss:

• Perinatal/ bacterial meningitis/ congenital infection (CMV, rubella, syphilis)/ head injury

Causes of hearing loss in adults:

• Presbyacusis = normal ageing
• Genetic/ ototoxicity (e.g. chemotherapy)/ infection/ tumours/ head injury

Question 10

Identify the functions of the vestibular system

Answer 10

Signals orientation in space

Reflex balance reactions to sudden instability of gait/ posture

• = vestibulospinal reflex
• Instability = “vestibular ataxia”

Preserve visual acuity during head movement

• = vestibulo-ocular reflex
  
  • Extraocular muscles supplied by superior and medial vestibular neurons
  • Abduction of eye:
    
    • Axons of medial vestibular nucleus cross midline
      
      • Project to contralateral abducens nucleus
        
        • à abduction
    • Axons from CNVI nerve excite contralateral oculomotor nucleus
      
      • à adduct other eye

BP and HR control during rapid up/down tilts

Synchronises respiration with body orientation

Provoke motion sickness

Question 11

List the types of signals derived from the vestibular system

Answer 11

• Components of vestibular system:

Otolith organs:

• Sense:
  
  • acceleration of head
  • strength and direction of gravity
• Components of otilith organs:
  
  • Saccule
    
    • Bed of sensory cells situated in inner ear
    • Detects linear accelerations and head tilts in the VERTICAL plane
      
      • Therefore is oriented in vertical plane too
  • Otolith hair cells
    
    • Have an overlaying layer of OTOCONIA
    • Project normal to the plane of the saccule
    • Have cilia:
      
      • One kinocilia per cell
      • The rest are stereocilia
    • Have directional sensitivities in all combinations of lateral and antero-posterior directions
    • Stimulated by INERTIAL RESISTANCE of the otoconial mass to linear head ACCELERATION
  • Utricle
    
    • Oriented horizontally with hair cells projecting vertically
• Semi-circular canals:
  
  • Sense angular rotation of head
  • Hair cells:
    
    • Project from ampullae
    • Unidirectionally oriented:
      
      • Head rotation ACCELERATION to right preferentially stimulates canals on right and vice versa
      • Head rotation DECELERATION preferentially stimulates canals on the opposite side
      • Each canal has a TONIC FIRING RATE so that when the head is still they are balanced out
  • Each canal is stimulated by rotation in its own plane
  • Loss of canal function à permanent partial impairment of sensitivity for movement in acceleratory direction of that canal

Vector sum of utricular and saccular stimulation patterns give signal of linear acceleration in all 3d directions

Question 12

Clinical vestibular disturbance: define dizziness, vertigo and oscillopsia; recognise how these conditions differ; explain the consequences of uni- and bilateral loss of vestibular function

Answer 12

• Vertigo
  
  • False perception of movement in space
  • Normal:
    
    • Vestibular projections go via hypothalamus à spatial cortex
  • Vertigo caused by a unilateral lesion
• Oscillopsia
  
  • Inability to stabilise eyes during head movement in bilateral vestibular lesions
    
    • Impairment of vestibulo-ocular reflex
  • Objects in visual field appear to oscillate
• Loss of vestibular function
  
  • Unilateral
    
    • à severe nausea and vomiting
    • Inability to stabilise eyes = “vestibular nystagmus”
      
      • The intact canal has an unopposed tonus
        
        • Eyes therefore driven to SIDE OF LESION as normal vestibulo-occular reflex
        • Drifting movement is detected by the brainstem
          
          • à intermittent “resetting” of eye position with fast saccades
          • à rapid involuntary eye movement
    • Vertigo:
      
      • Tonus of the intact canal à signal that head is rotating to intact side
        
        • à feeling of intense spinning
      • Causes:
        
        • Benign positional (lasts seconds)
        • Vertevrobasilar insuficiency/ migraine (lasts minutes)
  • Bilateral
    
    • Inability to stabilise eyes = “oscillopsia”
    • Mild gait ataxia