How does the external ear influence sound?
- Auricle and auditory canal create a funnel shape
- Directs sound toward tympanic membrane
- Filters sound waves
- Tympanic membrane creates a chain reaction of vibration within the ossicles when sound waves are pressed against it
How does sound move through the middle ear?
- Sound waves create movement of tympanic membrane, tympanic membrane creates movement of the malleus, which creates movement of the incus, which creates movement of the stapes
- Sound is amplified by vibration of ossicles
- Stapes pushes in and out of the oval window
- Vibrations reach inner ear through oval window
How does the middle ear respond to really loud sounds?
- Tensor tympani muscle (innervated by CN V, V3) dampens movement of the malleus
- Stapedius muscle (innervated by CN VII) dampens movement of the stapes
What structures are within the inner ear?
- Cochlear duct (scala media) that is filled with endolymph
- Vestibular duct (scala vestibuli) that is filled with perilymph
- Tympanic duct (scala tympani) that is filled with perilymph
How does sound move through the inner ear via the vestibular duct?
- Vibration Reaches the inner ear at the cochlea
- Fluid filled vibrations sent to the vestibular duct within the cochlea
- Send down and around the cochlea via the vestibular duct and out through the tympanic duct
- Pressure from the vibration of fluid is released through the round window
How does sound move through the inner ear via the cochlear duct?
- Vibrations sent through the cochlear duct
- Causes the basilar membrane to vibrate
- Stereocilia and kinocilia in the tectoral membrane move toward each other and become excited
- This excites primary sensory neurons with cell bodies in the spiral ganglia that send action potentials down the cochlear division CN 8
- Vibration becomes a neural impulse
What are the auditory functions of the CNS?
- Orients head and eyes toward sound
- Increases activity level throughout CNS
- Provides conscious awareness and recognition of sounds
How is sound become centrally processed?
- Cochlear nerve (CN 8) heads toward the brainstem carrying the sound information
- Runs through internal acoustic meatus alongside CN 7
- Nerve synapses on dorsal and ventral cochlear nuclei at pontomedullary junction
Once sound information reaches cochlear nuclei, where do signals travel?
- Ascends bilaterally to different structures in CNS
- To reticular formation to activate the entire CNS (arousal) upon a very loud noise
- To inferior colliculi to integrate auditory info from both ears to detect sound location and conveys the signal to the superior colliculus to move your head toward the sound
- Superior olivary nuclear complex
- Medial geniculate nuclei for thalamic relay of info to the cortex
- Auditory cortex
Where do signals from the dorsal cochlear nuclei travel?
- Crosses over at the pontine tegmentum and ascend in the contralateral lateral lemniscus
- Terminates in the inferior colliculus
- Some fibers sent from inferior to superior colliculus for the tectospinal reflex
Where do signals from the ventral cochlear nuclei travel?
- Signals ascend bilaterally within the reticular formation and superior olivary nuclear formation complex of the pons to help localize sounds horizontally in space
- Signals then ascend bilaterally in the lateral lemniscus to reach inferior colliculi, signals will then travel to the superior colliculi
What is the name of the area of decussation for auditory information?
Trapezoid body
How do signals reach the cortex to determine the volume and meaning of sounds?
- Inferior colliculi send signals from the contralateral cochlear nuclei to the medial geniculate nucleus of the thalamus
- Signals travel through auditory thalamic radiations to the auditory cortex for conscious awareness of sound intensity
- Signals then travel to the auditory association cortex (near Wernicke’s area) to compare sounds to memories of sounds to categorize as language, music, or noise
What are the efferent (motor) pathways in the auditory system?
- Modulation of sensitivity of hair cells in response to sounds of varying intensities
- Reflex pathways from ventral cochlear nuclei sent to facial and trigeminal nuclei to turn on stapedius and tensor tympani muscles in order to dampen the response of the middle ear to loud sounds
Unilateral hearing loss can be caused by damaged to what peripheral structures?
- External auditory canal
- Middle ear
- Cochlea
- Cranial nerve 8, vestibulocochlear
- Cochlear nuclei
Conductive hearing loss can be caused by what?
Abnormalities of external auditory canal or middle ear
Sensorineural hearing loss can be caused by what?
Disorders of cochlea or cranial nerve 8
How can you test hearing?
- Finger rubbing
- Whispering
- Ticking watch
- Tuning fork
What can occur if there is a lesion in the pathway proximal to cochlear nuclei?
- Auditory information is conveyed bilaterally so damage wont result in full hearing loss
- Auditory hallucinations (hearing things that aren’t there
- Difficulty processing auditory information
What MOI(s) can cause sensorineural hearing loss via cochlea or CN 8 damage?
- Exposure to loud noises
- Meningitis
- Ototoxic drugs
- Head trauma
- Viral infections
- Aging
- Tumors
What MOI(s) can cause conductive hearing loss via external auditory canal or middle ear damage?
Transmission of vibration is blocked, typical by earwax build up
How can you test the cochlear division of cranial nerve 8?
- Weber test
- Tuning fork is struck then placed on vertex of the skull in midline
What to the results of the Weber test mean?
- Equal hearing bilaterally is normal
- If tone is quieter on the affected side sensorineural hearing loss is present
- If tone is louder on affected side conductive hearing loss is present
What is acoustic neuroma?
- AKA vestibular schwannoma
- Most common cerebellopontine angle tumor
- Grows where CN 8 enters the internal acoustic meatus
- Early signs and symptoms: unilateral hearing loss, tinnitus, unsteadiness
- Later signs: facial nerve involvement, trigeminal nerve involvment
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Course Introduction36
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Descending Motor System41
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Somatosensation Part 156
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Somatosensation Part 217
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Somatosensation Part 319
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Cerebellum68
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Basal Ganglia49
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Limbic System53
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Higher Order Cerebral Function56
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Reticular Formation32
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Visual Pathways63
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Vestibular System49
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Auditory System24
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Eye Movements and Pupillary Control33
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Cranial Nerves55
