Case 4 - Hearing Flashcards
What components make up the external ear?
Pinna and external auditory meatus
What is the function of the external ear?
Direct sound waves into auditory canal
What does the middle ear include?
Tympanic membrane and chain of ossicles (malleus, incus, stapes)
List the functions of the middle ear
Protection
Amplification
Conduction
Transduction - converts energy
What does the inner ear include?
Bony labyrinth: 3 semi-circular canals, cochlear
Membraneous labyrinth: vestibule, cochlear duct, 3 semi-circular ducts
What fluid does the bony and membraneous labyrinth contain? Give the characteristics of the fluid
Bony labyrinth - perilymph, low K+
Membraneous labyrinth - endolymph, high K+/ low Na+
Where is the tensor tympani and stapedius muscles found and what is their role?
Tensor tympani - around tympanic membrane
Stapedius - around stapes
They contract when the sound is very loud to reduce sound conduction (limits vibration) = attenuation reflex (or ‘acoustic’ reflex). Only lasts for around 40 mins before fatigue
What innervates the tensor tympani and stapedius?
Tensor tympani = trigeminal V3
Stapedius = facial nerve
What is the role of the pharyngotympanic tube?
Connects nasopharynx to the middle ear - equalises pressure on either side of the tympanic membrane
What does the vestibule contain and what is its role?
Utricle and saccule - have equilbirium receptors called maculae to measure static / linear acceleration
Describe the function of the 2 otolith organs
Saccule: cochlear duct empties here, responsible for vertical acceleration (i.e. sensation of going up and down, like a lift)
Utricle: where semi-circular ducts empty into, responsible for horizontal acceleration (i.e. sensation of forward and back, like a car)
What makes otolith organs so sensitive to changes in acceleration?
Otoconia (top layer) with striola resting on the otolithic membrane - formed by calcium carbonate crystals. These add weight to increase sensitivity to gravity.
Think of it as a very wobbly layer
What is the role of the semi-circular canals?
Forms the dynamic system - detects angular acceleration
How do semi-circular canals respond to movement and detect positioning of the head in space?
Have dilated ends called ampullae which contain crista (sense organs). The ampulla have hair cells covered by cupula (a gelatinous mass).
Turning head = moves the fluid layer which displaces the cupula. This stains the stereocilia within it:
- Deflection towards kinocilium = depolarises and increases firing in afferent nerves
- Deflection away from kinocilium = hyperpolarises and decreased firing
List the 3 semi-circular canals and what movement of the head they detect
Anterior SCC= nodding of head
Posterior SCC= head to shoulder
Lateral SCC = rotation of head
What neurotransmitter is released when there is deflection towards kinocilium?
Glutamate - through mechanically gated ion channels
What are the functions of the vestibular system?
- Detects forces generated by movement, translates into balance
- Provides information about the position of head in space
- Allows other CNS structures to rapidly compensate for changes
What is the kinocilium?
The tallest cilium (vestibular hair cells have 1)
How is sound amplified?
Lever action of ossicles
Concentration of sound waves from the large tympanic membrane onto the small oval window (20:1)
Describe how sound waves lead to a relay of information to the CNS (physiology of hearing)
1: sound waves travel through external auditory meatus to the tympanic membrane
2: Movement of TM displaces the ossicles, and the membrane vibrates in resonance to sound waves - vibrates the malleus, incus then stapes
3: Movement at stapes at oval window produces pressure waves in perilymph of scala vestibuli
4: Depending on frequency/ amplitude of sound waves, they can vibrate the basilar membrane: location of distortion varies with sound frequency (high frequency = short wavelength, vibrates membrane nearer oval window) and louder sound = higher vibration
5: vibration of basilar membrane causes sterocilia to beat against the tectorial membrane and alters K+ conductance:
- Towards kinocilium = depolarised (increased K+ conductance)
- Away from kinocilium= hyperpolarised (decreased K+ conductance)
6: depolarised hair cells cause voltage-gated Ca2+ channels to open = glutamate release
7: activates spiral ganglion, relays info to cochlear branch of CN VIII (relays info to CNS)
Describe the auditory pathway from the cochlear branch of CN VIII to the cortex
1: cochlear branch travels through the petrous part of temporal bone (via internal auditory meatus with CN VII) to reach pons-medullary junction
2: Synapses at cochlear nuclei
3: Information is sent off to superior olivary nucleus of pons
4: information to inferior colliculi of midbrain
5: axons continue upwards in the lateral lemniscus to the medial geniculate nucleus of the thalamus
6: projection fibres deliver information to superior temporal gyrus (area in primary auditory cortex) - high frequency sounds activate a different area to low frequency
** At each step, axons decussate so 2 tracts from each nucleus going up
Why is a lesion in the brainstem unlikely to cause hearing loss?
The decussation and cross-talk between nuclei aids in bilateral processing and localisation of hearing, i.e. still will have info from the other side
What is the role of the inferior colliculi?
Unconscious motor responses to acoustic stimuli, i.e. turning your head when you hear a noise
What areas are located nearby to the primary auditory cortex?
Brocas area - control muscles of speech
Wernickes area - speech comprehension
Secondary auditory cortex (association area)
Who can benefit from a cochlear implant?
Post-lingually deafened adults: already developed speech so can use hearing aids to optimise hearing
Pre-lingually deaf children: implant before age 4, ideally by age 12 months (or hearing aids given before 3 months old - encourages auditory inputs)
Why would a cochlear implant not be beneficial for an pre-lingually deafened 6 year old?
After the period of neuroplasticity, the cochlear implant would not improve hearing or speech
What is the vestibulo-ocular reflex?
When eye movements counter head movements, so permit the gaze to remain fixed on a particular point. They compensate for the head movement to produce a stable image, preventing retinal slip
What are the phases to VOR?
Semi-circular canals in one ear sense rotation of head
Slow phase: eyes slowly rotate in opposite direction
Fast phase: eyes rapidly reset to centre of gaze
Describe what would happen to the eyes (inc. extra ocular muscles) if the head rotated left (VOR)
Eyes rotate right:
- Left eye = stimulation of oculomotor nuclei to contract the MR, inhibits abducens nuclei to relax LR
- Right eye = contract LR and relax MR
Essentially the contralateral LR and ipsilateral MR contracts (to the direction of movement)
What is nystagmus?
Element of VOR involving alternate slow eye movement with rapid saccadic movement
What controls the slow and fast phases of the VOR?
Slow phase = vestibulo-ocular pathway
Fast phase = cerebral cortex
What part of VOR may be absent/ present in a patient in a coma?
Slow phase is still present, but fast phase isn’t as it is controlled by higher cortical centres