Vestibular System Transduction

Question 1

What are the vestibular sensory organs in the mammal?

Answer 1

• Utricle and saccule and three ampullae of the semicircular canal
  
  • The vestibular appartus consists of a series of fluid-filled sacs and ducts

Question 2

Where are these sensory organs in the brain?

Answer 2

• 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

Question 3

The otolith organs detect what kind of movement? The semicircular canal organs?

Answer 3

• Otolith - detect linear acceleration

* Semicircular canal - angular accelerations

Question 4

The two horizontal canals act together to form what signal?

Answer 4

• Horizontal rotation of the head

Question 5

The contralateral pairs of anterior and posterior canals signal what?

Answer 5

• Read rotation vertically

Question 6

What is the maculae?

Answer 6

• 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

Question 7

Endolymph is a special body fluid why?

Answer 7

• High extracellular potassium, low extracellular sodium

* The opposite of blood or other extracellular fluids

Question 8

How does the vestibular hair cell depolarize?

Answer 8

• Mechano-transduction

* The stereocilia move and that physically pulls open a NSC channel

Question 9

What is the kinocilium?

Answer 9

• The tallest cilium on the apical surface of the vestibular hair cell

Question 10

What environments are the vestibular hair cells surrounded by?

Answer 10

• Apical part = endolympth
  
  • Base of hair cells = perilymph
  • The stereocilia are on the apical side

Question 11

Afferent nerves from the vestibular system have what kind of tone at rest?

Answer 11

• 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

Question 12

How does hyperpolarization of the vestibular hair cell happen?

Answer 12

• 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)

Question 13

The saccular hair bundles project what way and what does that mean?

Answer 13

• Horizontally, allowing them to detect displacement vertically
  
  • Riding an elevator

Question 14

Utricle hairs project how and sense what?

Answer 14

• Project vertically and detect head tilt forward and back

* Position of head relative to gravity

Question 15

What is the orientation of the cilia in the anterior, posterior and horizontal semicircular canals?

Answer 15

• 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)

Question 16

What is the consequence of inertia on the cupola?

Answer 16

• 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

Question 17

What is so important about the two sides of the vestibular system acting together?

Answer 17

• 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

Question 18

Trace the vestibular afferents to the cortex

Answer 18

• 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

Question 19

Where do the semicircular canals project?

Answer 19

• 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

Question 20

Where do utricles send afferents?

Answer 20

• 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

Question 21

Where do the saccule, utricle and semicircular canals all project to?

Answer 21

• 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