Neurology of Hearing and Balance Flashcards
Hearing is also known as
audition
Main divisions of the ear
- peripheral auditory system
outer, midd, inner ear and CN 8 - central auditory system
brain & brainstem
Peripheral auditory system of ear
outer ear (pinna)
middle ear
inner ear
cranial nerve VIII
Central auditory system of ear
brain
brainstem
Middle ear
- acoustic energy hits tympanic membrane
- TM begins to vibrate which indicates an energy change (acoustic to mechanical energy)
- mechanical energy transmitted through ossicles (malleus, incus, stapes)
- footplate of stapes rocks in/out of oval window
Inner ear
rocking of stapes creates waves in the cochlear fluids
- energy change: mechanical energy changed into hydraulic energy
Waves disrupt hair cells in organ of corti
- 3rd energy change: hydraulic energy to electrochemical energy
Nerve conduction
CN VIII = Vestibulocochlear
- cochlear branch of this nerve connects into hair cells of organ of corti
- this nerve conducts electrochemical impulse to the brainstem
Brainstem organization
- CN VIII inputs into the brainstem’s cochlear nuclear complex (CNC) or CN
- Cochlear nucleus lies where the pons and medulla meet at cerebellopontine angle
- vestibular & cochlear branches of CN VIII diverge here
Cochlear Nucleus
an area of specialized cells for auditory information
What does cochlear nucleus do
processes incoming auditory signals by
- differentiating frequencies
- timing information
which is critical for sound localization
Brainstem organization: Lateral Lemniscus
a prominent fiber bundle/tract in the midbrain that plays a crucial role in auditory pathway
INFO GOES ALONG THIS TRACT
Stapedius reflex
CN VII (FACIAL) triggers stapedius to contract in response to loud noise - causes stapes to move out of oval window
sound is dampened (45 secs)
stapedius reflex is triggered
bilaterally
stapedius reflexo is used to
- diagnose sensorineural hearing loss and acoustic neuroma
- help locate lesion lower brainstem
- Facial nerve branches both ears, absence stapediala reflux on one side can indicate a lesion in facial nerve
inferior colliculi
auditory center of the midbrain
- maintains tonotopic organization
- important for localization of sound/pitch/discrimination
inferior colliculi regulates
acoustic startle reflex
(sudden movement when unexpected sound occurs)
Diencephalon organization
Medial Geniculate Body
Medial Geniculate Body
the auditory center of the thalamus
- relay station (auditory tracts to auditory parts of cerebral cortex)
lateral geniculate body in the
thalamus, is the visual center
cerebral cortex organization
PAC = primary auditory cortex
primary auditory cortex (PAC)
central auditory pathway ends here
- found on superior temporal gyrus
- area is tonotopically organized
- perceives & discriminates sound
Main divisions of balance
Peripheral vestibular system
- semicircular canals
- cranial nerve VIII
Central vestibular system
- brain
- brainstem
Peripheral vestibular system of balance
- semicircular canals
- cranial nerve VIII
Central vestibular system of balance
- brain
- brainstem
Semicircular canals
3 fluid-filled canals that correspond to our 3D world
- anterior canal
- posterior canal
- horizontal canal
anterior canal
coronal plane of space
posterior canal
sagittal plane of space
horizontal canal
transverse plane of space
Ampulla
- swelling in semicircular canal
- Crista structures inside each
cristas
- inside each ampulla
- have hair cells similar to cochlea
- hair cells sensitive to body movements in different planes of space
Utricle and Saccule
within macula are hair cells that have calcium carbonite embedded in gelatinous membrane
displacement of crystals impact
balance via impact on cerebellum and eyes
generation of neural impulse
- with body movement, fluids move in semicircular canals
- if sterocilia bend towards kinocilia, hair cell depolarizes & signal goes to brain
- if bends away = inhibition (no signal sent)
Vestibular nuclei - Cerebellum
- Vestibular nuclei projects fibers to cerebellum
- cerebellum coordinates motor movement
- these connections facilitate coordinated movements necessary to preserve body’s balance
Vestibular Nuclei - Eye movements
Vestibulo-ocular reflex
- keeps eyes fixed/stable when moving head
- visual field and retinal image
Vestibular related reflexes
- Vestibulo-Ocular Reflex
- Vestibulospinal Reflex
- Vestibulocollic Reflex
- Acoustic Startle Reflex
- Stapedius Reflex
Vestibulospinal Reflex
- maintain balance, posture, & stability
- via activating skeletal muscles
(bending over and lifting one leg to balance)
Vestibulocollic Reflex
stabilizes head in space when head moves, command is to move head in opposite direction of the current head motion
Acoustic Startle Reflex
sudden movement when an unexpected sound occurs
Stapedius Reflex
AKA acoustic reflex
- bilateral response triggered by CN VII
- in response to intense noise
- causes stiffening of ossicular chain, pulls stapes away from oval window, dampens sound for 45 seconds
