C: Auditory pathways Flashcards
What are the 2 parts of the temporal bone?
1) Squamous part - flat part
2) Petrous part - incredibly hard bone
What is the name of the external ear which captures sound and funnels it into external acoustic meatus?
Pinna
Which 2 cranial nerves pass out of the cranial cavity through the internal acoustic meatus?
1) CN7 - facial nerve
2) CN8 - vestibulocochlear nerve
For what 5 reasons is the middle ear a high risk space?
1) Connected to the nasopharynx - prone to infection
2) Connected to mastoid air cells - infection may spread to middle cranial fossa (which lies just above temporal bone)
3) Internal jugular vein lies just beneath - thrombosis risk
4) Internal carotid artery lies anterior - link to pulsatile tinnitus
5) Transversed by chorda tympani and facial canal (that facial nerve runs in) - infection risk
Why does pulsatile tinnitus occur?
The internal carotid artery lies anterior to the middle ear
Pulsation of the internal carotid can cause ossicles to move slightly which leads to pulsatile tinnitus
The petrous part of the temporal bone contains what 3 cavities?
1) Vestibule
2) Semi circular canals (anterior, posterior and lateral)
3) Cochlear
The cavity of the petrous part of the temporal bone contains what fluid, what is the composition of this fluid?
Contains perilymph
Similar to extracellular fluid - contains high sodium and low potassium
What are the 4 main parts of the membranous labyrinth of the inner ear, which bony cavity does each lie within?>
1) Cochlear duct (within the cochlear)
2) Saccule (within vestibule)
3) Utricle (within vestibule)
4) 3 semi-circular ducts (within the semi circular canals)
What fluid is contained within the membranous labyrinth of the middle ear, what is its composition?
Endolymph - similar composition to intra-cellular fluid, low levels of sodium and high levels of potassium
The cochlear duct separates the bony cochlear into which 2 chambers?
1) Scala vestibuli
2) Scale tympani
At which point of the cochlear are the scala vestibuli and tympani continuous?
The apex of the cochlear - the helicotrema
What is the course of movement of fluid in the cochlear?
1) Hydraulic pressure in the perilymph created by the stapes bone pass to the apex of the cochlear via the scala vestibuli
2) Pressure passes through the helicotrema and descend via the scala tympani to the round window
3) As the fluid moves around the cochlear it deforms the fluid of the cochlear duct - the endolymph
What structures form the roof and the floor of the cochlear duct?
Roof = vestibular membrane Floor = basilar membrane
How is the spiral organ stimulated?
By the deformation of the cochlear duct by the perilymph in the surrounding scala vestibuli and scala tympani
How is an action potential generated in the primary auditory neurones?
Fluid in the scala vestibuli distorts the vestibular membrane which distorts the endolymph and in turn the basilar membrane
This distorts the cilia of the hair cells within the tectorial membrane which causes ion channels to open and action potentials to be fired in the primary auditory neurons
Axons of primary auditory neurons are carried in which nerve?
Cochlear part of CN8
Primary auditory fibres are what kind of neurone?
Bipolar neurons
Do the cell bodies of the primary auditory neurons lie closer to the cochlear or brain stem?
Closer to the cochlear within the spiral ganglion
Where do primary auditory neurons (ie CN8) enter the brainstem?
Cerebellopontine angle
Where do primary axons of the auditory pathway synapse with secondary neurons?
In the dorsal and ventral cochlear nuclei within the brain stem
What are the 2 important characteristics of the auditory pathway?
1) Polysynaptic - goes to various centres with lots of branching
2) Bilateral - info from one side ascends on both sides
Why is it important that the auditory pathway is bilateral?
Depending on position of the head and direction of sound, the 2 ears will receive the same sound but with slightly different timing and volume.
Because the auditory pathway is bilateral these 2 inputs are integrated and allows the brain to localise the sound - ie tell where the noise is coming from
What are the 4 auditory centres in the brainstem?
1) Cochlear nuclei (ventral and dorsal)
2) Superior olivary nucleus (just superior to olives)
3) Inferior colliculus
4) Medial geniculate nucleus of the thalamus
Where do the primary and secondary neurones in the auditory pathway synapse?
The dorsal and ventral cochlear nuclei in the caudal pons
Primary axon splits with one bit going to dorsal and one bit to ventral pons
At the level of which structures in the auditory pathway does info travel bilaterally?
From the superior olivary nucleus upwards
Via what structure do the secondary neurones in the auditory pathway cross the midline?
Trapezoid body
At what level in the auditory pathway is there further communication between the 2 sides of the pathway?
Between the inferior colliculi
What are the 7 structures in the ascending auditory pathway passed through?
1) Cochlea
2) Spiral ganglion
3) Cochlear nuclei (cerebellopontine angle)
4) Superior olivary nucleus
5) Inferior colliculus
6) Medial geniculate nucleus of thalamus
7) Heschl’s gyrus
In which region of the brain stem if the superior olivary nucleus located?
Pons
In which region of the brainstem are the inferior colliculi located?
Midbrain
What is the name of the nucleus within the thalamus involved in the auditory pathway?
Medial geniculate nucleus
Via what structures does auditory info travel from the superior olivary nucleus to the inferior colliculus?
Lateral lemniscus
Via what structure does auditory info travel from the inferior colliculi to medial geniculate nucleus?
Inferior brachium
Why does the base of the basilar membrane respond maximally to sounds of high pitch?
Stiff - requires high frequency vibrations to move it
Why does the apex of the basilar membrane respond maximally to sounds of low pitch?
More flexible - will vibrate in response to sounds of lower frequency
Where is the primary auditory cortex?
Heschl’s gyrus in superior temporal lobe - not a lot of it can be seen from lateral view - most is deep within the cortex
Sound information of low pitch will project to which part of heschl’s gyrus?
Anterolateral part
Sound information of high pitch projects to which part of heschl’s gyrus?
posteromedial (deeper) part
Where is the auditory association cortex?
Superior temporal gyrus
What is meant by the tonotopic representation within Heschl’s gyrus?
Sounds of different frequencies project to different parts of the gyrus
What are the 2 secondary auditory areas, in which hemisphere are they located?
Broca’s area and Wernicke’s area
Both located in the left hemisphere
Where is Broca’s area located?
Inferior part of frontal lobes just anterior to pre-central gyrus
Where is Wernicke’s area located?
Posterosuperior temporal lobe
What is Broca’s area responsible for?
Motor - production of words
What is Wernicke’s area responsible for?
Sensory - understanding of words
Descending neurons in the auditory pathway which reach the inferior colliculus may leave the brain stem to do what?
Reflex head and eye movement via CN3 and 6
Descending neurons in the auditory pathway which reach the superior olivary nucleus may leave the brain stem to do what, via what nerves?
To stapedius via CN7 and to tensor tympani via CNV3
Prevents damage during loud noise
What is the furthest structure that neurons in the descending part of the auditory pathway reach?
Can go all the way to cochlear (straight from cochlear nuclei)
Where in the brainstem are the cochlear nuclei located?
Cerebellopontine angle
