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Lecture 9: Vestibular System
Transcript of Lecture 9: Vestibular System
What is the vestibular system?
Direction: allows you to know if, and in what direction, you are moving
Orientation: Allows you to know which way is up
Most sensitive to head movements
Filled with the same fluids that fill the cochlea (i.e. paralymph and endolymph)
Vestibular labyrinth: 3 semicircular ducts
How does the vestibular system work?
Fluid in vestibular labyrinth causes deflection of hair cells
Hair cells (same as those found in cochlea) are deflected in response to head movement
Hair cells can be deflected in either direction, resulting in either depolarization or hyperpolarization
Present in utricle, saccule, and semicircular ducts
Bending back of the stereocilia results in an increased firing rate
Bending forward of the stereocilia results in reduced firing rate
At rest (i.e. when there is no head movement) SOME of the potassium channels on these stereocilia are open -- “baseline” ion flow
When the head is moved to the right the rate of ion flow increases
When the head is moved to the left the rate decreases
Both of these changes result in a change in the firing rate of the cells, and that signal is relayed to the brain by the vestibulocochlear nerve.
Utricle and Saccule
Detect linear acceleration
Utricle contains ~30,000 hair cells
Saccule contains ~16,000
The utricle and saccule (which make up the “body” of the vestibular labyrinth) detect linear accelerations or changes in the the velocity of the head
For example, when you speed up in a car, or when you’re slowing down when you apply the breaks
These are also the structures that tell you you’re going up or down in an elevator
Hair cells here
When the head moves, the utricle (or saccule) move as well, because they are held in place by the bony structure surrounding the labyrinth
The otolithic membrane lags behind, due to inertia, and shifts relative to the underlying macula
Hair cells have stereocilia that extend into the otolithic membrane. When the otolithic membrane shifts relative to the macula due to inertia, a neuronal signal is generated by the opening and closing of hair cell tip links.
When the head is tilted to the right the cell is depolarized, which leads to an increased firing rate
When the head is tilted to the left the cell is hyperpolarized
For any head movement some cells are depolarized, and some will be hyperpolarized
Hair cells are arranged in such a way that they will respond differently to different movements
Arrows represent direction of movement that will result in maximal firing rates for individual hair cells
At striola, the direction of maximal firing switches 180º
Detect angular acceleration
Anterior: side to side
Posterior: front to back
Filled with endolymph
~8 mm in diameter
Detection of angular acceleration due to lag of endolymph relative to bony structure of vestibular system
Flow of endolymph is measured by sensory structure near opening of semicircular canal: “ampulla”
Angular acceleration in a clockwise direction bends the cupula to the left, which will cause the hair cells to bend forward and hyperpolarize
Counterclockwise movement will result in the cupula bending to the right, which will cause the hair cells to bend back and depolarize
Only shows the action of one of the three semicircular canals, so movement at any angle can be detected and relayed to the brain
Only shows the action on one side of the headOn the other side, there will be a corresponding and opposite response
Vestibular nuclei are able to integrate the signals from their respective vestibular labyrinths to determine the direction of head movement
Because of the angles of orientation of the semicircular canals, the way signals are interpreted by the vestibular nuclei are somewhat counterintuitive. The horizontal semicircular canals make up a functional pair (not counterintuitive). However, the left anterior semicircular canal makes a functional pair with the right posterior semicircular canal. Conversely, the left posterior semicircular canal makes a functional pair with the right anterior semicircular canal.
Test of vestibular function