6 – The Auditory Pathway Flashcards
Name cranial nerve 8
Vestibulocochlear nerve
What are the anatomical structures of the ear surrounded by?
The Squamous part of the temporal bone
What does squamous mean?
Flat
What is the petrous part of the temporal bone?
It is the rough part of the temporal bone
What is the part of the skull where sound first enters the ear is called…?
The external auditory meatus (canal )
What is the anatomical term for canal>
Meatus
Where is the external auditory meatus (canal) located?
In the auditory canal
Where is the internal auditory meatus (canal) located?
In the petrous part of the temporal bone
What is the significance o the internal auditory canal?
It is the area where the auditory nerve enters the skill
Name the divisions of the ear
- External ear
- Middle ear
- Inner ear
What is the function of the external ear?
To receive sound waves
It is the first point of entry of sound waves
What is the function of the middle ear?
Sound waves are converted into mechanical waves here
What is the function of the inner ear?
Mechanical waves are converted into electrical impulses here
Where does the external ear start?
At the auricle
What is the function of the auricle?
It funnels sound waves into external auditory canal
Describe the lateral aspect of the external ear
The lateral 1/3rd is cartilaginous
Describe the medial aspect of the external ear
The medial 2/3rds is bony
Where are sound waves funnel to from the external auditory canal?
They are funnelled towards the tympanic membrane
What is the tympanic membrane known as?
The eardurm
Where does the tympanic membrane emit sound waved too?
The middle ear
Where does the middle ear begin?
From the tympanic membrane
Describe the middle ear
It is an air filled space in the petrous part of temporal bone
What does the middle ear communicate with?
Communicates with nasopharynx via Eustachian (auditory tube)
How does the middle ear communicate with the nasopharynx?
via Eustachian (auditory tube)
What is another name for the Eustachian tube?
Pharyngotympanic tube or auditory tube
What does the middle ear contain?
Contains ossicles:
Name the 3 ossicles found in the middle ear
Malleus
Incus
Stapes
What is the function of the ossicles?
To vibrate to transmit sound
What is the function of the Eustachian (pharyngotympanic) tube?
Ventilation
Drains any fluid from the middle ear to the nasopharynx
Equalises pressure
Why are children more prone to getting a middle ear infection
In children the Eustachian tube is pretty much in line with the nasal cavity but as we age the tube becomes more vertical
So as a kid it is easier for infection to spread from one cavity to another
Why is the middle ear known as a high risk space?
- Connected to nasopharynx
- Connected to mastoid air cells
- Internal jugular vein lies inferior
- Internal carotid artery lies anterior
- Traversed by chorda tympani and facial canal
How does the fact the middle ear is connected to the nasopharynx make it a high risk space
Prone to infection
How does the fact the middle ear is connected to mastoid cells make it a high risk space
Means infection may spread to th middle cranial fossa
How does the fact the middle ear lies inferiorly to the internal jugular vein make it a high risk space
Increases the risk of thrombosis
How does the fact the middle ear lies anteriorly to the internal carotid artery make it a high risk space
Links to pulsatile tinnitus
How does the fact the middle ear is transferred by the chordates tympanic and facial canal make it a high risk space?
Increased risk of infection if the middle ear is infected the rest of the facial nerves can be affected
Where is the inner ear located?
Located in petrous part of temporal bone
What is the inner ear composed of?
Composed of 2 special sense organs :
- Vestibular system
- Cochlea
What is the vestibular system essential for?
For balance and equilibrium
What is then vestibular system made up of?
- Vestibule
2. Semicircular canals (SCC)
What is the cochlea system essential for?
Perceiving sound
Describe the cochlea
It is a snail shaped structure and each section of the snail can detect different frequencies
What can the cochlea detect?
Can detect frequencies ranging from 20 to 20,000 Hz
What frequency does the apex of the snail detect?
Very low frequencies
Which part of the cochlea detects higher frequencies?
The base of the snail structure
Which nerve supplies our vestibule and cochlea?
Cranial nerve 8 Vestibulocochlear nerve
Describe the inner ear
The petrous part of the temporal bone and it has a series of fluid filled cavities
Name the fluid filled cavities in the inner ear
The Bony (outer) labyrinth The Membranous (inner) labyrinth
What does the bony labyrinth of the inner ear contain?
Contains perilymph
What is perilymph?
Like Extracellular fluid
What does the Membranous labyrinth of the inner ear contain?
Contains endolymph
What is endolymph?
like intra cellular fluid
How many semicircular canals?
3:
Posterior
Lateral
Anterior
Describe how the auditory pathway organised
Tonotopically organised
Describe the auditory pathway
It is Polysynaptic and Bilateral
Where does the auditory pathway start?
In the cochlea to the cochlea nerve
Go through the auditory pathway
- Cochlea
- Cochlea nerve
- Auditory centres in the brainstem
- Auditory centres in the brain
What does the auditory pathway compare?
It compares 2 inputs:
- Timing
- Loudness
What is the benefit of the auditory pathway being bilateral?
So we can localise where the sound is coming from
Name the chambers the cochlear is split into
- Scala vestibuli (SV)
2. Scala tympani (ST)
Where are the 2 chambers of the cochlear continuous?
The two chambers are continuous at the apex of the cochlea through a narrow slit (helicotrema)
Describe the movement of fluid in the cochlea
As the fluid (perilymph) moves around the bony cochlea it deforms the fluid (endolymph), inside the cochlear duct.
What is at the roof of the cochlea duct?
Vestibular membrane
What is at the floor of the cochlea duct
Basilar membrane
What splits the cochlea into 2 chambers?
Cochlea duct
What does the cochlea duct contain?
A spiral organ of cortisones
What is the spiral organ stimulated by?
stimulated by the deformation of the cochlear duct by the perilymph in the surrounding Scala vestibuli (SV) and Scala tympani (ST)
What does the spiral organ of corti do?
This is the sensory organ that transmits sound information to the CNS
What is the spiral organ of corti connected to?
Cranial nerve VIII
What is found in the spiral organ or corti?
Hair cells
Where are hair cells found in the spiral organ of corti?
The tips are embedded into the tectorial membrane
What do the hair cells do in the spiral organ of corti?
What get convert pressure into electrical signals
What are the hair cells connected to?
Sensory neurones of the Vestibulocochlear nerve
Where are the cell bodies of the sensory neurones in the auditory pathway found?
In the spiral ganglion
Where does the Vestibulocochlear nerve renter the brain stem from?
At the cerebellopontine angle
What does the word nuclei describe in the CNS?
Collection of cell bodies
Name the nuclei soicifoc to the auditory pathway
The dorsal and ventral cochlear nuclei
After entering the brain stem what happens to the cochlear nerve fibres?
They synapse with their second order neurones in the dorsal and ventral cochlear nuclei
What is found just superior to the cochlear nuclei in the pons?
The superior olivary nucleus
What is found just superior to the superior Olivary nuclei in the level of the midbrain?
The inferior colliculus
What is found just superior to the inferior colliculus?
The medial geniculate nucleus of the thalamus
What connects the inferior colliculus to the medial geniculate nucleus?
The inferior brachium (arm )
Where is the primary auditory cortex found in the brain?
In the temporal lobe
What is the primary auditory cortex also called?
Heschl’s gyrus
Talk through the auditory pathway
- Cochlea
- Spiral ganglion
- Cochlear nuclei (via cochlear nerve)
- Fibres then synapse with the superior olivary nuclei
- Fibres travel to inferior colliculus via LATERAL LEMINISCUS
- Fibres travel to medial geniculate nucleus via INFERIOR BRACHIUM
- Heschl’s gyrus
To which superior olivary nucleus do fibres from the left ear travel to?
Both superior olivary nuclei
What does the apex of the cochlea detect?
Low pitch sounds
Where are low frequency sounds projected to in Heschl’s gyrus?
At the anterolateral end
Where are high frequency sounds projected to in Heschl’s gyrus?
At the posteriomedial end
Name the secondary auditory areas
- Brocha’s area
2. Wernicke’s area
Where is Broca’s area found?
Anterior to the primary auditory cortex
What is broca’s area Incharge of?
Motor production of language
Where is Wernicke’s area found?
Posterior to the primary auditory canal
What is Wernicke’s area I charge if?
Sensory understanding of language
What dies the middle cerebral artery supply?
The lateral aspect of the brain
Where does the spiral organ of cortisones sit?
Above the basilar membrane
What happens if we hear a very very loud noise?
Hint descending pathway
Our brain sends motor fibres down our descending auditory pathway to the stapedius (via CN VII)
Also sends signals to the tensor tympani (via CN V3)
