Vestibular System Transduction Flashcards
What are the vestibular sensory organs in the mammal?
- Utricle and saccule and three ampullae of the semicircular canal
- The vestibular appartus consists of a series of fluid-filled sacs and ducts
Where are these sensory organs in the brain?
- Enclosed in the connective tissue of the membranous labyrinth
- The labyrinth is encased by the bony labyrinth with the petrous portion of the temporal bone
The otolith organs detect what kind of movement? The semicircular canal organs?
- Otolith - detect linear acceleration
* Semicircular canal - angular accelerations
The two horizontal canals act together to form what signal?
• Horizontal rotation of the head
The contralateral pairs of anterior and posterior canals signal what?
• Read rotation vertically
What is the maculae?
- The sensory epithelia of both otolith organs
- The utricle lies at the floor of the vestibule while the saccule hangs vertically on the laterall wall of the vesibule
- Together they act as static receptors and signal postural changes
- The otoconia give additional mass and inertia to the jelly-like load on the hair cell bundle and act to reflect changing orientation of the hair bundle with respect to gravity
Endolymph is a special body fluid why?
- High extracellular potassium, low extracellular sodium
* The opposite of blood or other extracellular fluids
How does the vestibular hair cell depolarize?
- Mechano-transduction
* The stereocilia move and that physically pulls open a NSC channel
What is the kinocilium?
• The tallest cilium on the apical surface of the vestibular hair cell
What environments are the vestibular hair cells surrounded by?
- Apical part = endolympth
- Base of hair cells = perilymph
- The stereocilia are on the apical side
Afferent nerves from the vestibular system have what kind of tone at rest?
- They have a tone is the important part
- There is a resting AP frequency
- Depolarization of the hair cell increases the frequency of AP by NT release
- Hyperpolarization decreases NT release and decreases the AP of afferent vestibular nerves
How does hyperpolarization of the vestibular hair cell happen?
- If the cilia are pulled against the cell’s polarity it hyperpolarizes the cell
- The polarity of the cell is in the direction of the kinocilium (from shortest to tallest cilia)
The saccular hair bundles project what way and what does that mean?
- Horizontally, allowing them to detect displacement vertically
- Riding an elevator
Utricle hairs project how and sense what?
- Project vertically and detect head tilt forward and back
* Position of head relative to gravity
What is the orientation of the cilia in the anterior, posterior and horizontal semicircular canals?
- Anterior +posterio = away from utricle
- Horizontal = toward utricle
- The cupula is a gel-like substance that houses the projecting hair cell cilia
- Thus, when endolympth moves around the cupola will deflect all cilia in the same direction
- The cupola lives in the crista and the crista is specific for each ampula (each semicircular canal has its own ampula)
What is the consequence of inertia on the cupola?
- Acceleration (angular rotation) through the semicircular canal will deflect endolymph one way (opposite of head movement) at first
- Any endolymph movement deflects the cupola
- If the rotation is constant velocity the cupola will go back to neutral as endolymph catches up
- When you stop the movement it sloshes the other way, again deflecting the cupola
What is so important about the two sides of the vestibular system acting together?
- The brain takes both information sets into account and doesn’t think it’s normal unless there are both information sets present
- Vestibular nerve dysfunction manifests as a sense of rotation because there isn’t the balance of both sides creating an overall picture
Trace the vestibular afferents to the cortex
• Vestibular afferent nerves have cell bodies in the Scarpa’s ganglion
• Project to the ipsilateral vestibular nuclei located at the border of the caudal pons and rostral medulla
• There are four major vestibular nuclei
○ Superior, lateral, medial, inferior
• There is a large mix of afferent information at each of the nuclei
• Visual and auditory system also contribute
• There is also a large variance in the directionality of the second order neurons to the cortex
• SOME primary vestibular afferent that project directly to the flocculo-nodular lobe of the cerebellum
Where do the semicircular canals project?
- Mainly to the medial and superior vestibular nuclei
- These cells give rise to the medial longitudinal fasciculus that carries information to the oculomotor nuclei
- Combined with mononeurons to neck musculature the longitudinal fasciculus tract becomes the medial vestibulospinal tract
- Coordination of eye movements and head movements
Where do utricles send afferents?
- Lateral vestibular nucleus
- Medial and inferior nucleus too
- Lateral nucleus also receives spinal and cerebellar input
- Axons from this nucleus make up the descending lateral vestibulospinal tract
- Activation of fibers in this tract will lead to excitation of extensor muscles of the limbs and so participate in the postural reflexes associated with utricular sensation of gravity
Where do the saccule, utricle and semicircular canals all project to?
- Inferior vestibular nucleus
- Also recieves spinal and cerebellar input and in turn projects to the vermal cerebellum
- Sends output to the reticular formation and other brainstem centers too
