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Flashcards in Vestibular System Deck (16):

Recognize and name the components of the vestibular system

Two sensor types reside within the inner ear:

•2 pairs of otolith organs that detect linear acceleration (changes in velocity) and the direction of the gravity vector
o Utricle
o Saccule

•3 pairs of semicircular canals that detect head rotation (angular acceleration – simultaneous change in velocity & direction)
o Anterior
o Posterior
o Horizontal
o All at 90° to each other
o Situated in the inner ear at a 45° angle


Describe the hair cell

Hair cells mechanically convert motion stimuli into electrical signals.
• Stereocilia of a hair cell bundle have graduated lengths
• One single, longest one on one side of the bundle = kinocilium
• Each steriocilium is connected to the next one by a hair-like fiber called a tip link


Describe hair cell physiology

•Hair cell’s bundle bends in the direction of the kinocilium → pulls on the tip link → mechanically opens K+ channels at the tip of each stereocilium
•K+ rushes into the cell → hair cell depolarizes → opens Ca2+ channels at the base of the hair cell
•Ca2+ influx → release NT → afferent nerve fires action potentials at a higher frequency

•Bending the hair cell bundle to the side away from the kinocilium brings the tips of the cilia closer together
•Tip links relax → closure of potassium channels → hair cell hyperpolarizes.


What are three vestibular reflexes?

Vestibulo-ocular reflex (VOR)
Vestibulo-colic reflex (VCR)
Vestibulo-spinal reflex (VSR)


Describe the Vestibulo-spinal reflex (VSR)

Assist balance and provide a framework for coordinated motion of the limbs during ambulation.
*In response to gravity, the VSR provides continuous muscular tone that acts as a priming framework for muscle action.
*Righting reflexes: when the upright human is pushed to one side, the limbs on half the body tend to extend, while those on the opposite side tend to flex. Serves to rebalance the body


Describe the Vestibulo-colic reflex (VCR)

Helps maintain head stability and upright stance of the head. The vestibular system senses the direction of gravity and initiates motor reflexes in the muscles controlling the head and neck that serve to right the head


Describe the Vestibulo-ocular reflex (VOR)

Allows the eyes to maintain focus on a point of interest during head movement.
*As the inner ear senses a head rotation in any direction, a signal is sent to MLF to the abducent, trochlear and oculomotor nuclei.
*These cause the eyes to move in precisely the opposite direction of the head movement, so that the net effect is that the eye remains steady in space.
*Fastest reflex in the body!


What is the Macula?

The saccule and urticle contain a disc-like, thickened membrane, called the macula, which lies along one wall of each sac. The macula contains the hair cells that serve as the sensory apparatus


What is the Otoconoia?

Otoconoia (sticky calcium carbonate crystals with a greater density than the surrounding fluid) adhere to the macula. They contribute additional mass to the load on the hair cell cilia, and this allows the structure to sense the changing orientation of the hair cell with respect to gravity.


The Urticle

The urtricular macula lies horizontally on the floor of the vestibule
o The urticular hair bundles project vertically
o Tilting the head from horizontal causes shear between the hair cells and the overlying otolithic membrane
o Informs the brain of the tilting of the head with respect to gravity (horizontally from side to side) and of linear acceleration in the horizontal plane (accelerating in a vehicle).


The Saccule

The saccular macula hangs vertically on the lateral wall of the vestibule
o Saccular hair bundles project horizontally
o Vertical displacement of the head results in shear caused by the relative inertia of the otolithic membrane.
o Informs the brain of the direction of gravity and of linear acceleration in the vertical direction (accelerating and decelerating in an elevator)


Axis of polarity of hair cells: Urticle vs Saccule

The axis of polarity of the hair cells in each otolith organ is not uniform. In both, the hair cells form two broad patches.
o Urticle: the cilia of hair cells in each patch are all roughly orientated toward a curving central stripe, known as the striola. Optimizes the range of signals that can excite the urticle
o Saccule: the cilia of hair cells in each patch are all roughly oriented away from the striola


Semicircular Canals

Horizontal, Anterior, Posterior

There are three semicircular canals in each ear. The six semi-circular canals function as pairs and detect angular acceleration.

At one end of each canal, there is a swelling known as an ampulla. Within each ampulla there is a specialized ridge of sensory epithelium called crista that contains hair cells. The hair cells arising from the crista project into a gelatinous mass called the cupula. The cupula is suspended in fluid and bisects the bulge of the ampulla.

Within each crista, the hair cells all have the same axis of polarity; the kinocilium is uniformly found on the same side of each hair cell bundle. If the cupula is deflected, all of the hair cells in the given crista will respond identically.


Horizontal Canal

Hair cells oriented with kinocilia facing the side of the ampulla nearest the utricle

Stimulated by ampullopetal fluid movement. Fluid flows along the limb of the semicircular canal toward the ampulla and urticle


Anterior and Posterior Canals

Kinocilia face the side of the ampulla that opens into the arm of the semicircular canal and away from the utricle

Stimulated by ampullofugal fluid movement

Fluid flowing away from the urticle and ampulla


What happens within the Semicircular Canals when you turn your head?

Head moves right:
•Fluid in the right horizontal canal moves left relative to the cupula due to inertia
•Hair cells in the right horizontal canal crista are depolarized
•Cupula deflected in the ampullopetal direction
•Increased afferent firing rate of nerves serving the right horizontal canal crista
•Meanwhile, in the left ampulla hair cells are hyperpolarized and afferent action potential frequency decreases.

Head rotation stops:
Inertia will cause endolymph to continue moving for a brief time. Cupula will once again deflected, but in a direction opposite to that at the beginning of the head rotation.