Hearing and balance Flashcards
Describe what is sound?
- sound waves are pressure waves with alternating peaks of compressed air and valleys
- distinguished by amplitude (dB) and frequency (Hz)
- converted to APs in cochlea
Describe the external auditory canal
- S shaped (boosts 3kHz sounds-speech)
- elastic cartilage portion runs posterosuperior
- inner bony portion runs anteroinferior
What is the reason for the shape of the pinna?
- differentiation of location of sounds
* on ground vs in tree
Describe the tympanic membrane
- angled
- connective tissue
- skin externally
- mucous membrane lined internally
- attached by fibrocartilaginous ring to temporal bone
- concavity at centre
Describe the middle ear
- air-filled
- mucous membrane lined
- in the temporal bone
- 2 parts: tympanic cavity and epitympanic recess
What is the roof of the middle ear?
tegmental (temporal bone) wall -middle cranial fossa
What is the floor of the middle ear?
jugular wall
• tympanic nerve from CNIX
What is the lateral wall of the middle ear?
membranous wall
• tympanic membrane
• temporal bone
What is the posterior wall of the middle ear?
mastoid wall
• chord tympani
What is the anterior wall of the middle ear?
carotid wall
• auditory tube
What is the medial wall of the middle ear?
labyrinthine wall
Describe the mastoid area
- posterior to epitympanuc recess
- mastoid antrum: cavity continuous with air filled spaces
- separated from middle ear by thin tegmen tympani
- mucous membrane continuous from middle ear to mastoid antrum (infection spreads)
Describe the pharyngotympanic tube
- connects ME to nasopharynx
- equalizes pressure
- extends from anterior wall of ME, anteriorly, medially and inferiorly to nasopharynx posterior to inferior meatus
- 2 parts: bony (lateral 1/3) and cartilaginous part
Describe the ossicles
- synovial joints (subject to osteoarthritis with age)
- from tympanic membrane to oval window
- muscles contract to dampen prolonged noises
List the muscles of the middle ear?
- tensor tympani
* stapedius
Tensor tympani
O: pharyngotympanic tube, sphenoid, bony canal
I: upper handle of malleus
N: mandibular branch CNV
F: reduce vibrations in loud noises
Stapedius
O: attached to inside of pyramidal eminence
I: neck of stapes
N: branch of facial nerve
F: prevents excess oscillation in loud noises
How well is the sound transferred in middle ear?
- ossicles transmit to oval window
* >60% of sound energy is successfully transferred to cochlea
Describe the bony labyrinth
- vestibule
- semicircular canals
- cochlea
- lined with periosteum and contain perilymph
Describe the membranous labyrinth
Suspended in perilymph
Membranous structures filled with endolymph:
• semicircular ducts
• cochlear duct
• utricule and sacule
List the organs of balance
- semicircular ducts
- utricle
- saccule
Describe the cochlea
- located centrally in bony labyrinth, dividing it into 2 canals: Scala vestibuli and Scala tympani
- connected via lamina modiolis
- spiral ligament
- vestibular membrane
- basilar membrane
- spiral organ
spiral ligament
thickened outer wall against bony cochlea
vestibular membrane
separates endolymph in cochlear duct from perilymph in scala vestibuli
basilar membrane
separates endolymph in cochlear duct form perilymph in scala tympani
spiral organ
- organ of hearing
- rests on basilar membrane
- projects into cochlear duct
Describe the transmission of sound from oval window and on?
Oval window moves medially
- > creates wave in perilymph of Scala vestibuli
- > wave through cochlea causing and outwards bulging of secondary tympanic membrane at round window
- > causes basilar membrane to vibrate
- > receptor cells in spiral organ stimulated
- > receptor cells transmit to brain through CN
Modiolus
• core spongy bone and contains cavity (spiral ganglion) that contains nerve cell bodies of sensory afferents
Where are the low frequency tones? and high?
- low frequency are at apex
* high at base
Describe the organ of corti
- lies on basilar membrane within cochlear duct
- made of sensory receptors: inner and outer hair cells
- stereo cilia project up from hair cells and are embedded in tectorial membrane
- bending of stereocilia
- outer hair cells control sensitivity if inner hair cells: modulate basilar membrane movement and act as amplifiers
Describe the auditory pathway
• first order neurons in CNVIII
- > enter brainstem at pontomedullary junction
- > synapse on dorsal and ventral cochlear nuclei
Dorsal cochlear nucleus
- 2nd order fibres cross midline and join lateral lemniscus, the major ascending auditory pathway -> synapse at inferior colliculus
- 3rd order neurons -> synapse medial geniculate bodies in thalamus
- 4th order neurons run to primary auditory cortex in superior temporal gyrus of temporal lobe
Ventral cochlear nucleus
- 2nd order fibres remain ipsilateral and cross midline -> run to superior olivary nuclei in pons (sound localisation)
- 3rd order neurons join lateral lemniscus -> inferior colliculus -> thalamus -> cortex
What is the consequence of unilateral damage above the cochlear nuclei?
- does NOT cause deafness
- problems with localizing sounds and separating sound from background noise
- each lateral lemniscus contains info from both ears
What is the result of damage to CNVIII?
deafness in that ear
What is conductive hearing loss?
- sound through outer or middle ear interrupted
- wax, ruptured eardrum, otitis media, arthritis of ossicles
- hearing aids boost sounds to counter reduced conduction
What is sensorineural hearing loss?
- cochlea (hair cells, auditory nerve or auditory cortex damaged
- lost high frequencies: old age and loud noise
- head injury, hereditary, stroke, drugs
- cochlear implant works IF CNVIII is intact
List the hearing loss screening tests
- Weber
* Rinne
What is the Weber test?
- differentiate conductive from sensorineural loss
- compares hearing in both ears (should be equal)
- bang fork and press on bone -> compare one to the other
What is the Rinne test?
• finds conductive loss ONLY
• bang the fork and hold on mastoid until patient can’t hear it anymore, then quickly place it in front of ear
-> if can hear it back again, then conductive loss
What are the organs of balance?
- utricle (largest sac)
- 3 semicircular canals
- saccule (smaller sac)
What are the sensory receptors of balance?
- macula of utricle: centrifugal and vertical acceleration
- macula of saccule: linear acceleration
- crista: movement in any direction
What is the pathway for balance?
semicircular canals, saccule and utricule -> vestibular nuclei
a-> neocortex (perceived orientation)
b-> spinal cord & cerebellum (postural control)
c-> oculomotor system (eye movements
d-> parabrachial nuclei check congruency with visual, proprioceptive inputs (motion sickness if not congruent)
What is the name of the spot that controls motion sickness?
area postrema
Describe the semicircular ducts
- crista in ampulla contain hair cells and supporting cells
- crista is small elevation covered by cupula
- cupula is mass of gelatinous material
What happens in semicircular ducts during head movement?
semicircular ducts and hair cells move
- > hair bundles bend
- > receptor potential
- > nerve impulses
- > vestibular branch of CN
Describe the utricle and saccule anatomy
- macula has sensory hairs in gelatinous otolithic membrane containing otoconia or otoliths
- otolithic membrane denser than endolymph (flops when head moves)
- otoliths move downward with gravity, stimulating hair cells (tells us which way is up)
- macula responds to force of gravity
- saccule=vertical motion
- utricule = horizontal motion
What do the vestibular nuclei have connections with?
- spinal cord extrapyramidal and vestibulospinal tracts
- motor nuclei of CNIII, IV, and VI
- cerebellum - vestibulocerebellar tracts
- vestibular neocortex of insulating and parietal lobes via thalamus (consciousness of movement)
- autonomic or visceral nuclei in reticular formation (maintaining BP)
What is the vestibule-ocular reflex?
- generates eye movements to compensate for head movement
- fix gaze on object while head moves
- to maintain balance and orientation
- semicircular canals -> vestibular nuclei -> 2nd order neurons -> motor nuclei of CNIII, IV and VI via MLF
- vestibulocerebellum adapts (inhibits) this reflex
What is vertigo?
Disturbance in vestibular pathway: • physiological • peripheral -inner ear disease (eg. BPPV) • central (brainstem, cerebellum) • altered sensory input
