Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity. The semicircular ducts work in pairs to detect head movements angular acceleration. A turn of the head excites the receptors in one ampulla and inhibits receptors in the ampulla on the other side.
Then press PLAY to watch the reaction to head movement. Begin by pressing "expand" to show details from the horizontal semicircular ducts on both sides of the head. Beneath the ampullae are new details, which highlight the orientation of the stereocilia in both cristae and their outputs. The kinocilia are oriented in the direction of the ampullae ampullo fugal within the ducts on both sides. The two sides are mirror images.
There is a constant low level of ionic influx into the body of the hair cells, so there is a steady-state receptor potential and a spontaneous low-level discharge of afferent activity. These neutral neurophysiological properties are shown in graphs below each ampulla. By pressing the "play" button you will see an animation of this. A constant low level of spontaneous activity keeps all the muscles slightly and equally contracted, causing the eyes to look straight ahead.
When the head turns, inertia causes the fluid to move more slowly than the head, generating relative fluid motion in the semicircular duct in the opposite direction of the head turn. This moving fluid, shown by arrows in the lumens of the semicircular duct, bends the hair cells on both sides of the head. Because the two sides are mirror images, the stereocilia are bent toward their kinocilium on one side and away from their kinocilium on the other side.
Shearing of the stereocilia toward the kinocilium causes a depolarization of the receptor potential and an increase in afferent action potentials. There is an opposite effect on the other side — a decrease in afferent activity. These counteracting bilateral changes in afferent activity affect the vestibular and occulomotor nuclei.
The ampullo fugal movement of fluid on the patient's right reader's left causes an increase in afferent activity shown in green for "go" in the inset. This has a positive effect on the right medial and superior vestibular nuclei, which in turn stimulate the ipsilateral occulomotor and contralateral abducens nuclei.
There are exactly opposite effects on the other side shown in red for "stop" in the inset. The result of these combined counteracting effects is a smooth movement of the eyes toward the left, keeping the visual field stable as the head turns. Press "expand" to see the utricle at the top of Figure These two similar organs lie against the walls of the inner ear between the semicircular ducts and the cochlea.
The receptors, called maculae meaning "spot" , are patches of hair cells topped by small, calcium carbonate crystals called otoconia. The saccule and utricle lie at 90 degrees to each other. Thus, with any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer.
This bending of the cilia produces afferent activity going through the VIIIth nerve to the brainstem. Activate Figure The utricle is most sensitive to tilt when the head is upright. The saccule is most sensitive to tilt when the head is horizontal.
Unlike the semicircular ducts, the kinocilia of hair cells in the maculae are NOT oriented in a consistent direction. The kinocilia point toward in the utricle or away from in the saccule a middle line called the striola. The striola is shown as a dashed line in Figure Because hair cells are oriented in different directions, tilts in any direction will activate some afferents.
Then press PLAY to watch the reactions to head movement. The vestibulo-occular reflex VOR controls eye movements to stabilize images during head movements. As the head moves in one direction, the eyes reflexively move in the other direction. The action of the VOR can be seen by moving your head from side to side. The image you see is stable, despite the head movement. But as you increase the speed of oscillatory head movements, you can get to a rate of angular velocity where the VOR is no longer effective, and you will see the visual image start to shift.
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Optokinetic nystagmus. Vestibulo-ocular reflex and visual-vestibular interaction. The vestibular system is a sensory system that is responsible for providing our brain with information about motion, head position, and spatial orientation; it also is involved with motor functions that allow us to keep our balance, stabilize our head and body during movement, and maintain posture.
Thus, the vestibular system is essential for normal movement and equilibrium. Vestibular sensations begin in the inner ear in the vestibular labyrinth, a series of interconnected chambers that are continuous with the cochlea.
The most recognizable components of the vestibular labyrinth are the semicircular canals. These consist of three tubes, positioned approximately at right angles to one another, that are each situated in a plane in which the head can rotate. This design allows each of the canals to detect one of the following head movements: nodding up and down, shaking side to side, or tilting left and right.
These movements of the head around an axis are referred to as rotational acceleration, and can be contrasted with linear acceleration, which involves movement forward or backward. The semicircular canals are filled with a fluid called endolymph, which is similar in composition to the intracellular fluid found within neurons. When the head is rotated, it causes the movement of endolymph through the canal that corresponds to the plane of the movement.
The endolymph in that semicircular canal flows into an expansion of the canal called the ampulla. Within the ampulla is a sensory organ called the crista ampullaris that contains hair cells , the sensory receptors of the vestibular system.
Hair cells get their name because there is a collection of small "hairs" called stereocilia extending from the top of each cell. Hair cell stereocilia have fine fibers, known as tip links, that run between their tips; tip links are also attached to ion channels.
When the stereocilia of hair cells are moved, the tip links pull associated ion channels open for a fraction of a millisecond. This is long enough to allow ions to rush through the ion channels to cause depolarization of the hair cells.
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