Auditory System Flashcards
What is the tympanic membrane?
- Part of outer ear/external
- Boundary between outer & middle ear
What makes up the outer ear/external?
- Auricle/pinna
- Auditory canal
- Tympanic membrane
Where does the auditory canal end?
Ends at tympanic membrane
What is within the middle ear?
Ossicles: Malleus, Incus, Stapes
What makes up the inner ear?
- Cochlea
- Labyrinth
- Vestibule (utricle & saccule)
What occurs because the auricle & auditory canal are funnel shaped?
- Direct sound inward toward tympanic membrane
- Filters sound waves as it directs them into the auditory canal
As sound waves press on tympanic membrane what does this create?
Chain reaction of vibration within ossicles (malleus, incus, stapes)
Within the middle ear describe the chain reaction after movement of tympanic membrane
Movement of tympanic membrane moves malleus which moves incus which moves stapes
Where is stapes attached?
To cochlear entrance at oval window
Arrangement of malleus, incus and stapes forms what?
- Lever
- Tympanic membrane on long arm & oval windows on short arm
How is sound waves amplified in the middle ear?
- Vibration of ossicles
- Causes stapes to push in and out of oval window
In response to loud sounds what does the tensor tympani muscle do and what is its innervation?
- Dampens movements of malleus
- Innervated by CN V
In response to loud sounds what does the stapedius muscle do and what is its innervation?
- Dampens movements of stapes
- Innervated by CN VII
Describe the general pathway that sound travels starting at auricle/ pinna
Auricle/ pinna through the external acoustic meatus to the tympanic membrane which sends vibration to ossicles to malleus to incus to stapes (taps on oval window)
What structure makes up the inner ear?
Cochlea
The cochlear duct is in center and what is it filled with?
Endolymph
The vestibular & tympanic ducts within the cochlea are filled with?
Perilymph
Where do vibrations reach the inner ear at?
Cochlea
Once the vibration reach the cochlea what happens?
- Fluid filled vibrations sent to Scala vestibuli (part of cochlea)
- Sent down and around the cochlea (through Scala vestibuli) and back out through the Scala tympani
- This pressure is released at the round window
Vibrations also get sent down the Cochlear Duct (Scala Media) which causes what?
- Basilar Membrane to vibrate
- Stereocilia & Kinocilia (rigid in tectoral membrane) move towards each other which excites
Once the sterocilia and kinocilla are excited in the inner ear what happens?
- Excite primary sensory neurons that send action potentials down the cochlear division of CN VII
- Vibrations become a neural impulse
Where are cell bodies of the sensory neurons?
In spiral ganglia (bipolar neurons that connects to the cochlear nerve)
Describe the scala vestibuli inner ear pathway
Scala vestibuli to down and around through cochlea to Scala tympanic back out, releasing pressure through round window
Describe the Cochlear Duct (Scala Media)
Cochlear duct (scala media) causes the basilar membrane to shift so hair cells excite and action potential are sent down axons of spiral ganglia to cochlear division of CN VIII
How are sounds converted to neural signals
- Sound waves strike eardrum
- Ossicles move, causing vibration of membrane at opening of upper chamber
- Movement of fluid to upper chamber
- Vibration of basilar membrane and attached hair cells
- Hairs bend because the tips are embedded in the immobile tectorial membrane. Hair cells depolarize
- Cochlear nerve endings activated
What are the 3 auditory functions in the CNS?
- Oriented head and eyes toward sounds
- Increases activity level throughout CNS
- Provides conscious awareness and recognition of sounds
How is sound centrally processed?
- Cochlear nerve (CNVIII) heads towards brainstem carrying the sound info
- Runs through internal acoustic meatus with CN VII (fascial N)
- Synapse on dorsal & ventral cochlear nuclei at pontomedullary junction
Hearing pathways ascend through brainstem bilaterally to?
- Reticular formation
- Inferior colliculi
- Superior Olivary Nuclear Complex
- Medial geniculate nuclei
- Auditory cortex
What is the function of the reticular formation in hearing pathways?
- Activate entire CNS
- Loud noise wakes you up
What is the function of inferior colliculi in the hearing pathway?
- Integrates auditory info from both ears to detect sound location
- Convey to superior colliculus to move head towards sound
Where do fibers from the dorsal cochlear nucleus ascend?
Contralateral lateral leminiscus (ascending auditory pathway)
- In pons & up to midbrain
- Cross at pontine tegmentum
Where does the dorsal cochlear nucleus pathway terminate?
- Terminate: Inferior colliculus
- Projections to superior colliculus
- Tectospinal reflex
Where do fibers from the ventral cochlear nucleus project and synapse?
- Fibers project and synapse bilaterally in reticular formation and superior olivary nuclear complex of pons
- Fibers ascend bilaterally in lateral lemnscius to reach inferior colliculi (projections to superior colliculus)
What is the function of ventral cochlear nucleus when fibers synapse in reticular formation and superior olivary nuclear complex?
- Localize sounds horizontally in space
- Auditory fibers crossing from trapezoid body (white matter)
What is the function of efferent pathways of the auditory system?
Modulate sensitivity of hair cells in response to sounds of varying intensities
What are the reflex pathways from ventral cochlear nuclei?
- Fascial and trigeminal motor nuclei to contract stapedius and tensor tympani muscles
- Dampens response of middle ear to loud sounds
Unilateral hearing loss is a (BLANK) issue
Peripheral
Injury to what structures could cause unilateral hearing loss?
- External auditory canal
- Middle ear
- Cochlea
- CN VIII
- Cochlear nuclei
Conductive hearing loss is due to damage where?
- Bone
- Abnormalities of external auditory canal or middle ear
Sensorineural hearing loss is due to what?
- Nerve/ neural
- Disorders of cochlea or CN VIII
How can hearing be tested?
- Finger rubbing
- Whispering
- Ticking watch
- Tuning fork
If there is a lesion proximal to cochlear nuclei what can be some symptoms?
- Auditory information is conveyed bilaterally
- Won’t result in full hearing loss
- May result in auditory hallucination or difficulty processing information
What are some causes of Sensorineural hearing loss? And where would a lesion occur?
- Cochlea or CN 8
- Causes: exposure to loud sounds, meningitis, ototoxic drugs, head trauma, viral infections, aging, tumors (acoustic neuroma)
What is conductive hearing loss? and where would damage occur to cause this?
- Transmission of vibration is blocked in the outer or middle ear
- External auditory canal or middle ear
What is a cause of conductive hearing loss?
Earwax build up
When examining Cochlear Division how can you assess hearing?
- Assess hearing first
- Finger rubbing (normal response is hearing the same on both sides)
What is Weber Test?
What would be the result for:
Normal?
Sensorineural?
Conductive hearing loss?
- Weber Test: Tuning fork is placed on vertex of skull in midline
- Normal: Hear the same on both sides
- Sensorineural: tone is quieter on affected side
- Conductive hearing loss: Tone is louder on affected side
What is the most common cause of Acoustic Neuroma and where does it develop?
- Most common cerebellopontine angle tumor
- Develops where CN VIII enters internal auditory meatus
What are some early signs of Acoustic Neuroma?
- Unilateral hearing loss
- Tinnitus (ringing in ear)
- Unsteadiness
What are some later signs of Acoustic Neuroma?
- Fascial nerve involvement
- Trigeminal nerve involvement
