Control L18 auditory pathways Flashcards
What parts of the skull are related to the ear’s location?
externally:
squamous part of temporal bone
external auditory meatus (canal)
mastoid process
internally:
squamous part of temporal bone
petrous part of temporal bone
internal auditory meatus (canal)
What cranial nerves travel through the internal auditory meatus?
CN VII and VIII
What is the function of the external ear?
auricle receives sound waves and funnels them to the external acoustic canal
what is the function of the middle ear?
Transformation of sound waves into mechanical waves/vibrations
what is the function of the inner ear?
conversion of mechanical waves into electrical waves/signals
what is the anatomical name for the eardrum?
the tympanic membrane
what is the middle ear?
an air filled space in the petrous part of the temporal bone. seperated from external ear by tympanic membrane
what bones are found in the middle ear?
the ossicles - malleous, incus and stapes (closer to outer –> closer to inner ear)
how does the middle ear communicate with the nasopharynx?
the pharyngotympanic tube aka eustachian tube
why are middle ear infections more common in infants?
as the pharyngotympanic tube is positioned quite flat so infection spreads easier than in an adult who has a more angled pharyngotympanic tube where gravity prevents infection moving up into the ear easily
what is the role of the pharyngotympanic tube?
equalise pressure between internal environment and atmospheric pressure
why is the middle ear a high risk space for infections/spread of infection?
has close relationships with other surrounding structures:
- with nasopharnyx via pharnygotympanic tube
- with cranial cavity (specifically middle cranial fossa) via mastoid air cells
- with IJV which lies inferiorly so risk of thrombosis
- internal carotid artery which lies anteriorly and can lead to pulsatile tinitis
- facial muscles and tongue via facial nerve as middle ear is transversed by chorda tympani and facial canal.
- parotid gland via lesser petrosal nerve
- inner ear via the oval window
what are the two special sense organs in the inner ear?
the vestibular system
cochlear system and cochlear ducts
what is the role of the vestibular system?
maintain balance and equilibrium
what is the vestibular system formed of?
vestibule
semicircular canal
semicicrular ducts
what nerve supplies the vestibular system?
vestibular nerve
how is the cochlear organised?
tonotopically - highest frequency sounds/pitch received at base and gets lower towards apex
what nerve supplies the cochlear system?
cochlear nerve
what nerve does the cochlear and vestibular nerve develop from?
CN VIII - vestibulocochlear
what is the function of the cochlear system?
hearing
what are the fluid filled cavities in the inner ear called?
membranous (inner) labyrinth filled with endolymph
bony (outer) labyrinth filled with perilymph
where are the fluid filled cavities of the inner ear found?
within the petrouts part of the temporal bone
How do sound waves –> mechanical waves –> electrical signal?
sound waves hit tympanic membrane causing ossicles to vibrate. The vibration of the stapes bone hits the oval window which causes fluid waves in the perilymph of the bony cochlear deforming the endolymph inside the cochlear duct of the inner ear. The cochlear duct contains the spiral organ which converts the waves into electrical signals.
what separates the bony cochlear into two chambers?
cochlear ducts
what are the two chambers of the bony cochlear?
scala vestibuli (superior)
scala tympani (inferior)
what is found inside the cochlear duct?
endolymph
spiral organ (of corti)
where do the two bony chambers become continuous?
at the apex of the cochlear via the helicotrema (a narrow slit)
what is the spiral organ of corti? How does it convert fluid waves/pressure into an electrical signal?
a structure within the cochlear duct that sits above the basilar membrane and contains hair cells with tips embedded into the tectorial membrane. It moves with the fluid waves so can detect deformation of the endolymph in cochlear duct by the perilymph in the surrounding bony chambers. It converts fluid pressure into electrical signals via the cochlear nerve.
in what cochlear cavity is the cochlear duct found?
in the membranous labyrinth
what are properties of the auditory pathway?
polysynaptic
tonotopically organised
bilateral (so sound can be localised)
compares input of timing and loudness of sound
What is the pathway of signals from the inner ear to the auditory cortex?
- primary neruons via the cochlear nerve (formed of axons of bipolar neurons in spiral ganglion) project to the dorsal and ventral cochlear nuclei in the rostral medulla.
- Fibres synapse here and then travel via secondary neurons bilaterally to the superior olivary nuclei at the level of the mid pons.
- superior olivary nucleus sends bilateral ascending fibres to the inferior colliculus as the lateral lemniscus
- inferior colliculi receive fibres in the lateral lemniscus and exchanges info between right and left via the inferior colliculus commissure
- auditory information sent to medial geniculate nucleus via the brachium of the inferior colliculus
- information sent from MGN to primary auditory cortex via auditory/acoustic radiations
where does CN VIII enter the brainstem?
at the cerebellopontine angle
what is the trapezoid body?
the fibres of the secondary neurons of the auditory pathway which travel to the contralateral side
what part of the brainstem are the inferior colliculi found?
dorsal surface in the tectum of midbrain
what part of the brainstem is the medial geniculate nuclei found?
dorsal surface and are projections of the thalamus
what connects the inferior colliculus to the medial geniculate nucleus>
brachium of inferior colliculus
where is the primary auditory cortex found?
superior temporal gyrus aka Heschl’s gyrus
what pitch sounds are heard at which part of the cochlear?
apex = low pitch
base = high pitch
pitch increases from apex –> base
how is pitch of sound distributed in the auditory cortex?
low frequency = antereolateral part
high frequency = posteromedial part
which hemisphere house the secondary auditory areas specialised for linguistic functions?
left
what are the two secondary auditory areas specialised for linguistic functions
expressive speech area aka Broca’s area
receptive speech area aka Wernicke’s area
what is the role of Broca’s area?
production and fluency of words
what is the role of Wernicke’s area?
understanding of words
How is speech affected by Broca’s aphasia?
It becomes non-fluent but understands the words
How is speech affected by Wernicke’s aphasia?
Speech is still fluent but lack of understanding of what they’re saying/what they’re saying doesn’t make sense
what artery supplies the associated auditory areas (Broca’s and Wernicke’s)?
MCA
what are the two auditory reflexes?
protection from loud noises - via CN V3 and VIII contracting the tensor tympani muscle and stapedius muscle respectively preventing damage
movement in response to sound - via CN III, IV and VI moving head and eyes
what is the smallest bone in the body?
stapes
where are the cell bodies of the hair cell axons?
in the spiral ganglion
What is the difference between the inferior colliculi and the inferior colliculus commissure?
inferior colliculus = collection of cell bodies
inferior colliculus commissure = bundle of axons which signal between the two inferior colliculi
