2: Vestibular System Flashcards
Describe the structure of the vestibular apparatus of the inner ear.
MEMBRANOUS LABYRINTH
- Saccule (attached to cochlear duct)
- Utricle (attached to semicircular ducts)
- Semicircular ducts
- –Ampulla: contains cristae
BONY LABYRINTH
- Vestibule (has utricle and saccule)
- Semicircular canals (contain semicircular ducts)
Explain how movement is transduced to an electrical signal.
Baseline: small amount of K influx from slightly open channels, activated by motor protein tightening tip filament & opening K channels in response to low intracellular Ca
Movement TO kinocilium (tallest) -> opening K channels -> K influx -> rapid depolarization -> opening of Ca channels -> influx of Ca -> NT RELEASE from base of hair cell (-> AP in nerve terminal contacting hair cell) + K flow out base of cell into perilymph -> repolarization
Movement AWAY from kinocilium -> closed K chanels -> hyperpolarization
Describe the central projections of vestibular afferents and the particular functional roles of central vestibular structures.
Vestibular ganglion (Scarpa’s ganglion) -> axons -> vestibular nerve -> internal acoustic meatus -> VESTIBULAR NUCLEI in dorsolateral aspect of rostral medulla/caudal pons
Also -> flocconodular lobe (vestibulocerebellum)
Discuss the importance of the vestibulo-ocular reflex, the central connections involved in the reflex, and the regulation of this reflex by the vestibulocerebellum.
Reflex stabilization of the eyes during head movements
Connections: vestibular nuclei -> MLF -> CN III, IV, VI
Vestibulocerebellum regulates this reflex to adapt, especially in cases of damage to inner ear, so that eyes can continue to move with the head
Explain the role of connections of the vestibular system with the spinal cord.
Medial and lateral vestibulospinal tracts
–Regulate body muscle tone in response to vestibular stimuli (tip right, extensor muscles on right tighten)
Describe the mechanisms and importance of nystagmus.
Nystagmus: a to-and-fro movement of the eyes; normal when spinning
—Head moves R, eyes move L at same rate (vestibulo-ocular reflex); snap R to a new target when end of normal range is reached, then track L again
Occurs with destruction of left inner ear or vestibular nerve
- –Good inner ear has baseline firing, while damaged inner ear has none, so feels like you’re spinning
- –Lasts a few hours to days, then cerebellum compensates
Where is endolymph manufactured? Where is it resorbed?
M: STRIA VASCULARIS of the cochlear duct
–Very high K fluid
R: endolymphatic sac near sigmoid sinus in skull, conveyed by endolymphatic duct in vestibular aqueduct
–Block endolymphatic duct -> increased pressure -> hearing loss, vertigo
Describe the structure of the receptive organs of the inner ear.
CRISTAE
- Located within ampullae of semicircular ducts
- All hair cells oriented ONE direction
- —Ducts are organized in 3 planes
- Contain cupula (gelatinous mass) protruding into ampulla
MACULAE
- Located within saccule and utricle
- Small patches of hair cells with curved 3D orientation, in virtually every plane of the head
- –Striola = separates regions of hair cells with different polarity
- Hair cells attached to wall embedded in a gel with otoliths (CaCO3 crystals)
What fluid fills the bony labyrinth?
Perilymph - very similar to CSF
Communicates with CSF through COCHLEAR AQUEDUCT
Describe the structure of the hair cells of the inner ear.
Hair cell apex in endolymph, base in membrane/perilymph
Stereocilia on apical surface
–Tip links: link shaft of one stereocilium with K channel on top of neighbor
Explain the particular sensitivities of the maculae and christae.
MACULAE
- -Otoliths increase density of the gel - allow maculae to respond to gravity
- -Detects linear acceleration and position of head in relation to gravity
- —If head accelerates in plane of organ, crystals lag behind, bend hair cells
- -Rapidly adapting, so not very good at detecting gravity
CRISTAE
- -Movement of head -> fluid stays behind -> currents within the endolymph of the semicircular canals deflect cupula -> bend hair cells
- -Detect angular movements (pitch, yaw, roll) via cortical integration of excitation/inhibition in all three planes
What happens with destruction of one inner ear?
Nothing! (once the person has adapted and gets over vertigo/nystagmus)
Each inner ear detects a full range of movements, so canals are complementary
Vestibulocerebellar compensation is very important
Explain the role of connections of the vestibular system with the brainstem.
Reticular formation (brainstem): produces autonomic responses (nausea, vomiting, flushing, sweating) in response to intense vestibular stimulation/motion sickness
Explain the role of connections of the vestibular system with the thalamus.
Relay to part of the sensory cortex
Allow for perception of movement
-Produces vertigo with abnormalities in vestibular system
Explain the role of connections of the vestibular system with the cerebellum.
Mostly flocculonodular lobe and fastigial nucleus
Most important to adapting to changing conditions
- -Assure eyes remain stable with head movement even if inner ear is damaged
- -Adjusts vestibulocular reflex to fit with reality
- -Adapts you to new eyeglasses, loss of inner ear hair cells with age, etc.
