Flashcards in L24: Vestibular Function Deck (15):
Vestibular system helps maintain posture & helps us see by maintaining head orientation even w/o visual input. What are some limits to this system?
this system doesn't work as well in too slow or too fast movements; also this system declines in older people.
What are the 2 impt vestibular reflexes?
1) vestibuloocular -enables us to see steady images
2) vestibulospinal -maintains postural integrity
What are the 2 components that determine output of vestibular system?
1) fluid dynamics of canal
2) neural computation in brainstem
Both the utricle and the sacculus are in the inner ear. What do they detect? What about the ampulla? What does the ampulla detect?
Utricle and sacculus are inertia guidance detectors. Utricle is impt in detecting tilt and linear acceleration in the horizontal plane while the sacculus detects tilt and linear acceleration in the vertical plane. BOth stimulate hair cells to detect motion and orientation.
Ampulla is a canal accelerometer.
There are hair cells in ampulla, in utricle, and in saccululus. Describe how the hair cells are polarized in these 3 parts.
in the ampulla, hair cells are in the same direction
in the sacculus, hair cells are pointing away from each other (these hair cells with opposite polarity are separated by a striola)
in the utricle, hair cells are pointing at each other (these hair cells with opposite polarity are separated by a striola).
*Utricle are oriented horizontally while saccules are oriented vertically.
Describe some of the features of the hair cells in ears.
Hair cells are made of stereocilia (immotile cilia), kinocilium, and a primary afferent. The directional force on hair cells will determine if hair cell is inhibited or excited.
There are hair cells in ampulla, utricle and sacculus of the ear. Are they stimulated the same way?
No. Stimulating hair cells in ampulla is different from stimulating the hair cells in the utricle and sacculus.
Explain how hair cells in the utricle & sacculus are stimulated.
-cilia of hair cells project into the gelatinous otolithic membrane
-calcium carbonate crystals called otoconia provide weight to the gelatinous otolithic membrane
*The otoconia-containing organs of the vestibular system signal tilt and linear acceleration by virtue of their crystals being denser than surrounding tissue.
in upright postion -> no force on these cells
tilt head back --> force of gravity on cilia in direction of kinocilia = depolarization so hair cells are activated b/c need to move/accelerate head forward. (THIS IS A LEARNED PROCESS).
tilt head forward --> hyperpolarization (cilia is in a direction away from kinocilia)
In order to detect acceleration in 3 degrees of rotation, we need to carry info in both sides of head. Therefore, left and right horizontal canals work together b/c they are in the same plane. What about the posterior and anterior canals?
The L anterior canal will work with the R posterior canal, forming a functional pair. The R anterior canal will work with the L posterior canal because that is how they lie on the same planes.
In each instance, rotation in a particular direction will yield excitation of hair cells in one canal of a functional pair & inhibiting the other.
Explain how hair cells in ampulla are stimulated.
-semicircular canals are filled with fluids
-when head turns right, fluid will move left
-fluid will put stress on cupola (a gelatinous membrane)
-cilia of hair cells project into cupola; all oriented in the same direction toward kinocilium (depolarization)
so if you turn your head to right, fluid will move left in right ear, activating hair cells, while these hair cells are being inhibited in the left ear.
Explain in more details the vestibulocochlear reflex using the example of the head turning left.
head turning left
-left ear is activated (CN 8)
-right ear is inhibited
-axons from the left vestibular nuclei will cross over to excite right abducens nerve
-axons from the left vestibular nuclei will also excite left oculomotor nucleus to activate left medial rectus muscle
*overall result --> eyes to rotate right
so both eyes move in opposite direction of the head turn.
T/F: CN 8 maintains resting discharge even when there's no head motion.
What is the normal rxn when the head moves right?
when head moves right, eyes move left, and then there's a quick right nystagmus to reset. So when eyes move left and can't move any furthere, there's a fast reset saccade back (right nystagmus).
In an abnormal situation, when there's no head rotation, but there's no activity coming from left canal, how will the brain interpret this?
When there's no activity from left canal, it's almost as if the left canal is inhibited, which means the right canal is excited. The brain will interpret this as the head moving to the right even tho it's stationary. So the eyes will move left slowly, followed by a quick right nystagmus to reset.