Term
| What are 4 primary functions of the Vestibular system? |
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Definition
1) Relay to motoneurons for posture to control balance
2) Relay to motoneurons for extraocular movements during head movements
3) "Sense" of space ("up" and "down")
4) Respiration and Circulation as we move and change posture. |
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Term
| Describe the Morphology of the Vestibular Apparatus |
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Definition
1) 2 fluid-filled sacs (Otolith Organs: Utriculus and Sacculus) and 3 channels (Semicircular canals) surrounded by "bony labyrinth).
2) Simple squamous epithelium of Utriculus and Sacculus (called "Membranous labyrinth") contains Endolymph (high K and low Na- +80mV) (Similar to Scala media)
3) Cavity between membranous and bony labyrinth contains Perilymph (similar to Scala vestibuli and Scala tympani) |
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Term
| How does sensory transduction occur in the Vestibular apparatus? |
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Definition
1) Hair cells (with stereocilia and Kinocilium) embedded in membranous labyrinth. Apical ends of hair cells are joined by tight junctions for fluid segregation (Endolymph bathes stereocilia and Perilymph bathes body of hair cells)
2) When hair cells bend towards Kinocilium, mechano-gated ion channels in stereocilia open, and pottasium rushes from the Endolymph to the Perilymph.
3) Hair cells depolarize and release glutamate on VIII afferents (opposite occurs if they bend away). Side to side bending has no effect.
3) Some channels are open at rest (Resting Discharge) |
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Term
| Explain the anatomical organization of hair cells within Semicircular canals and how it contributes to measuring angular acceleration during head movements. |
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Definition
1) Hair cells are confined to Ampulla (with "crista ampularis" sensory epithelium) of each Semicircular canal, with stereocilia embedded in Cupula.
2) In horizontal canals, Kinocilia point toward utricle (away from the major part of the canal and towards central end of ampula)
3) In vertical canals, Kinocilia face away from utricle (towards major part of canal)
4) As head turns to the left, fluid in horizontal canals appears to be moving "right," causing bending of stereocilia on the left toward the Kinocilia (increase firing), and those on the right away from the Kinocilia (decrease firing) |
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Term
| What information is provided by The Otolith Organs and not the Semicircular Canals? |
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Definition
Provided by Utricle (linear horizontal acc. like car speeding up) and Saccula (linear vertical acc. like elevator falling).
1) Static position of head in space (with respect to gravity)
2) Linear accelerations placed on head (falling or accelerating/stopping in car) |
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Term
| How does hair Cell organization differ in Otolith Organs from Semicircular Canals? |
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Definition
Otolith Organs (Saccule= vertical linear acc. and Utricle= horizontal linear acc.)
1) Stereocilia are embedded in otolithic membrane, which is filled with and made heavy by carbonate crystals called "otoconia" (higher specific gravity than surrounding fluid)
2) Hair cells exist in multiple orientations
Semicircular Canals
1) Stereocilia are embedded within Cupula
2) Hair cells within a canal face same direction. |
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Term
| Which Otolith organs code for horizontal and vertical linear acceleration, respectively? |
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Definition
1) Horizontal (car speeding up)= Utricular Macula
2) Vertical (elevator rising/falling)= Saccular Macula |
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Term
| What is the relevance of the "Striola"? |
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Definition
This is the point where the direction that the hair cells in the Ottolith organs face is abruptly reversed
** Remember, orientation is not changed in Semicircular canals ** |
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Term
| What are the primary CNS connections made by projecting Vestibular afferents? |
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Definition
1) Bilateral connections exist between nuclei on right and left brainstem
2) Cerrebellar inputs from Medial Cerebellum (Fastigial nucleus) and Vestibulocerebellum (Flocullus)
3) Project to CN nuclei for III, IV and VI
4) Project on spinal motoneurons through Vestibulospinal tracts (interneuron intermediates) and Reticulospinal tracts.
5) Project to S1 and SII via VPL thalamic nucleus
6) Control brainstem regions regulating respiration and circulation. |
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Term
| What are the 4 primary Vestibular nuclei in the brainstem? |
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Definition
Bilateral connectivity exists between two sides of brainstem.
Nuclei extend rostro-caudally from dorsal brainstem to pons and medulla.
1) Superior
2) Medial
3) Lateral (Deiter's)
4) Inferior (Descending) |
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Term
| Explain the pattern of excitation/inhibition from medial vestibular nuclei in the VOR. |
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Definition
1) All elements of oculomotor circuits that mediate VOR receive complementary antagonistic inputs from left and right vestibular nuclei (i.e. right IV nucleus gets excitation from left V. nucleus and inhibition from right)
- When head is stationary, this system is in balance, and IV does not fire. |
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Term
| What clinical manifestation follows damage to the left inner ear in terms of the VOR? |
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Definition
NYSTAGMUS (slow phase followed by quick phase)
1) Left IV motonuerons no longer receive ipsilateral inhibition from vestibular nucleus and lateral rectus muscle contracts (eye turns left)
2) Brain "thinks" head is rotating towards the right (because eye is moving to the left)
3) Compensatory VOR causes eye to move toward side of lesion (left), until it reaches the edge and "snaps back". |
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Term
| What is "Vestibular Caloric Testing" and how is it used? |
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Definition
COWS (Cold Other/Warm Same)
Responses should be equivalent for both sides!
1) Injection of warm or cold water into ear canal, causing movement of fluid in horizontal semicircular canal.
2) Warm causes fluid to rotate towards ampulla, resulting in stimulation of ipsilateral labyrinth and "quick phase" ipsilaterally followed by a contralateral "slow drift."
3) Cold causes fluid to rotate away from the Ampula, resulting in in an ipsilateral "quick phase" followed by contralateral "slow phase". |
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Term
| What "non-vestibular" inputs enter Vestibular Nuclei? |
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Definition
1) Proprioceptors including muscle spindles in neck (Group 1a fibers)
- need to tell between whole-body and neck-only movements
- Complementary inputs from vestibular receptors and neck cancel out during head movements (don't get dizzy just from moving head!) |
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