Auditory Pathway Flashcards
Which bone is the external acoustic meatus located within?
Petrous part of the temporal bone.
Where is the petrous part of the temporal bone?
In between the sphenoid and occipital bones.
Which nerve passes through the external acoustic meatus?
Facial (VII) nerve.
Which nerve terminates in the external acoustic meatus?
Vestibulocochlear (VIII) nerve.
What does the external ear consist of?
Air.
-sound waves pass through
How does sounds travel through the middle ear?
Sound waves travel through air»_space; mechanical energy (e.g. ossicles).
How does sound travel through the inner ear?
Mechanical energy»_space; electrical energy (nerves).
What is the process of sound passing into the ear?
Sounds waves >> tympanic membrane >> mechanical energy (ossicles) >> movement of fluid (cochlear) >> movement of hair cells >> electrical energy (nerves)
How long is the external acoustic meatus?
2-3 cm.
What is another name for the auditory tube?
Eustachian tube.
What is the function of the auditory tube?
Enable air to pass from nasopharynx to middle ear.
-maintains equilibrium either side of tympanic membrane (e.g. opens when you swallow)
What does the external ear consist of?
Auricle & external acoustic meatus.
-ends at tympanic membrane
Where is the middle ear?
Air-filled cavity between tympanic membrane and the cochlea.
What structure opens into the middle ear?
Auditory tube.
Which part of the ear contains the ossicles?
The middle ear.
Which 3 bones makes up the ossicles?
- Malleus
- Incus
- Stapes
Why is the middle ear a high risk space for infections? (3)
- Connected to nasopharynx
- Traversed by chorda tympani and facial canal
- Connected to mastoid air cells (may spread to mid cranial fossa)
Why is there a risk of thrombosis in the middle ear?
Internal jugular vein lies inferior.
Why is there a link to pulsatile tinnitus in the middle ear?
Internal carotid artery lies anterior.
What are the cavities of the petrous part of the temporal bone? (3)
- Vestibule
- Semicircular canal
- Cochlea
Which part of the ear contains the cavities of the petrous part of the temporal bone?
The inner ear.
What do the cavities of the inner ear contain?
Perilymph.
What is the composition of perilymph?
Like extracellular fluid.
-high Na, low K
What is the cochlear duct?
A membranous sac running through the cochlea.
What does the cochlear duct contain?
Endolymph.
What is the composition of endolymph?
Like intracellular fluid.
-low Na, high K
What does the cochlea duct separate the cochlea into?
2 chambers;
- scala vestibule
- scala tympani
Which chamber is above the cochlear duct?
Scala vestibule.
Which chamber is below the cochlear duct?
Scala tympani.
Where are scala vestibule and scala tympani continuous?
Helicotrema.
-apex of the cochlea
What is the process of perilymph movement in the cochlea when sound is heard?
Stapes vibrations on the oval window >> hydraulic pressure in perilymph >> pass through scala vestibule to helicotrema >> scala tympani >> round window
What is the round window?
Outlet at the end of the cochlea that allows fluid to move and distort the cochlear duct.
What forms the roof of the cochlear duct?
Vestibular membrane.
What forms the floor of the cochlear duct?
Basement/basilar membrane.
What is the effect of perilymph movement on the cochlear duct?
Perilymph presses on vestibular membrane
» distorts endolymph
» presses on basilar membrane
What is the auditory receptor?
Spiral organ of Corti.
Where is the spiral organ of corti located?
On the basilar membrane.
What does the spiral organ contain?
Hair cells embedded into the tectorial membrane.
What is the spiral organ stimulated by?
Deformation of the cochlear duct by surrounding perilymph.
How does hair cell stimulation lead to an electrical signal?
Hair cell movement >> ion channels open >> influx of ions >> depolarisation >> axon fired >> runs through cochlear part of vestibulocochlear nerve
Where are 1* auditory fibres?
Axons and cell bodies are located in the spiral ganglion.
|»_space; form cochlear nerve
Where do 1* auditory fibres enter the brainstem?
At the cerebellopontine angle.
Where do 1* auditory fibres synapse with 2* neruons?
In dorsal and ventral cochlear nuclei.
What are auditory centres?
Regions in the brainstem that auditory information moves into.
What are 2 features of the auditory pathway?
- Polysynaptic (lots of nuclei)
- Bilateral (receives inputs at different loudness/timing from each each)
What is the advantage of the auditory pathway being bilateral?
Sound can be localised.
-know where it’s coming from
Name 4 auditory centres in the brainstem.
- Medial geniculate nucelus (thalamus)
- Inferior colliculus
- Superior olivary nucleus
- Cochlear nuclei
What is the inferior colliculus involved with?
Sight and hearing.
What is the pathway of ascending auditory information from the cochlea to the cerebral cortex?
Cochlea >> spiral ganglion >> cochlear nuclei >> sup olivary nuclei >> inferior colliculi >> medial geniculate nuclei >> Heschl's gyrus
Where is Heschl’s gyrus?
Superior gyrus of temporal lobe.
-in the cerebral cortex
Where does information cross over the midline?
- Trapezoid body
- Inferior colliculi (midbrain)
What does bilateral mean?
Both sides receive information from both ears.
What does tonotopic mean?
Different regions respond maximally to sounds of different pitches.
Which area of the auditory pathway is tonotopic?
Basilar membrane.
-hair cells respond to different pitches
What pitch of sound does the proximal basilar membrane respond to?
High pitch.
-more stiff
Where does high pitch sound project?
From proximal basilar membrane to deep Heschl’s gyrus.
What pitch of sound does the distal (apex) basilar membrane respond to?
Low pitch.
-less stiff
Where does low pitch sound project?
From the apex of the basilar membrane to ant/lat Heschl’s gyrus.
What is the function of the association area?
Integrates information from different sensory receptors and relates them to memory.
Where are the 1* auditory cortex located?
Heschl’s gyrus.
Where are the 2* auditory areas located?
In the left hemisphere.
What are the 2* auditory areas? (2)
- Broca’s area
- Wernicke’s area
Where is Broca’s area located?
Frontal lobe.
What is the function of Broca’s area?
Motor/production of words.
What does damage to Broca’s area lead to?
Expressive (non-fluent) aphasia.
-halted, fragmented speech
Where is Wernicke’s area located?
Temporal lobe.
What is the function of Wernicke’s area?
Sensory/understanding of words.
What does damage to Wernicke’s area lead to?
Receptive (fluent) aphasia.
-fluent speech but nonsense/difficult understanding
What is aphasia?
Impaired ability to understand/produce speech.
Which artery supplies Broca’s and Wernicke’s areas?
Middle cerebral artery.
What are the descending auditory pathway comprised of?
Motor information passing down.
What is the pathway of descending motor information from the cerebral cortex to the cochlea?
Heschl's gyrus >>med geniculate nucleus >> inferior colliculus >> suuperior olivary nucleus >> cochlear nuclei >> cochlea
Where does some information go from the inferior colliculus in the descending auditory pathway?
Cranial nerves III and VI.
-reflex head and eye movements
Where does some information so from the superior olivary nucleus in the descending auditory pathway?
To stapedius (via CN VII) and tensor tympani (via CN V3).
What is stapedius?
A muscle attached to stapes.
What is tensor tympani?
A muscle that causes tension in the tympanic membrane.
What is the function of stapedius and tensor tympani?
Dampen down sound to prevent damage to hair cells.
What is the effect of a unilateral lesion?
Virtually no effect on hearing (pathway is bilateral).
-ability to localised sound may be impaired
What must be affected if a patient is deaf?
Peripheral part of auditory pathway (up to brainstem).
What are the 2 types of deafness?
- Conductive deafness
- Sensorineural deafness
What is conductive deafness?
Defect of sound transmission up to spiral ganglion.
What is sensorineural deafness?
Defect in function of spiral ganglion or cochlear nerve.
What does Rinne’s test compare?
Air conduction and bone conduction.
How is air conduction tested in Rinne’s test?
Tuning fork at external acoustic meatus
» increased amplification
»ossicle/tympanic membrane
» cochlea
How is bone conduction tested in Rinne’s test?
Tuning fork on mastoid process
» little amplification
» cochlea
When is sound loudest in Rinne’s test if a patient has conductive hearing loss?
Bone conduction is louder.
-no note observed at external acoustic meatus
When is sound loudest in Rinne’s test if a patient has sensorineural deafness?
Air conduction is louder.
-both air and bone conduction are reduced equally
What is Weber’s test?
Tuning fork on midline of forehead.
|»_space; vibration to cochlea
What is activated in Weber’s test?
Spiral organ (without tympanic membrane/ossicle involvement).
What is Weber’s test used to test for?
Unilateral hearing loss.
- sound heard louder in damaged ear if conductive loss (not inhibited by auditory masking)
- sound heard louder in normal ear if sensorineural loss (needs to be amplified)
