Structure and Function of the Semicircular Canals
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Questions and Answers

What is the primary function of the utricle in the otolith organs?

  • Sense vertical acceleration of the head
  • Sense the direction of gravity
  • Sense horizontal translation of the head and head tilt (correct)
  • Sense angular head rotation
  • What is the result of stereocilia deflection towards the kinocilia in a hair cell?

  • Death of the hair cell
  • Inhibition of the hair cell
  • Excitation of the hair cell (correct)
  • No change in the hair cell
  • What is the purpose of the calcium carbonate crystalline-structure material embedded in the gelatinous material of the otolith organs?

  • To provide a structure for the sensory hair cells to project into
  • To provide an inertial mass to the otolith organs (correct)
  • To increase the sensitivity of the sensory hair cells
  • To reduce the movement of the fluid in the otolith organs
  • What is the direction of fluid movement in the SCCs in response to angular head rotation?

    <p>In the direction of the angular head rotation</p> Signup and view all the answers

    What is the function of the cupula in the ampulla of the SCCs?

    <p>To provide a gelatinous barrier</p> Signup and view all the answers

    What is the result of the deflection of the hair cells in the otolith organs?

    <p>The brain receives information about the direction of head movement</p> Signup and view all the answers

    What is a primary symptom associated with vestibular problems, aside from oscillopsia?

    <p>Decreased somatosensation</p> Signup and view all the answers

    What is the role of the vestibular system during head motion in patients with vestibular hypofunction?

    <p>Generating an adequate compensatory eye velocity</p> Signup and view all the answers

    What is the primary diagnostic indicator used in identifying most peripheral and central vestibular lesions?

    <p>Nystagmus</p> Signup and view all the answers

    What is the direction of the nystagmus named by in individuals with a unilateral vestibular lesion?

    <p>The direction of the fast component</p> Signup and view all the answers

    What is the purpose of the Head Impulse Test (HIT) in the examination of eye movements?

    <p>To examine the VOR at high acceleration</p> Signup and view all the answers

    What is a result of a deficit in the VOR during head motion?

    <p>Motion of images on the fovea</p> Signup and view all the answers

    What is the result of excitation of the anterior SCC afferents on the eyes?

    <p>Rotation of both eyes in the direction opposite the angular head movement</p> Signup and view all the answers

    How do the SCCs work in relation to head movement?

    <p>In a coplanar fashion</p> Signup and view all the answers

    What is the term for the difference in firing rates between the two SCCs during head movement?

    <p>Push–Pull Mechanism</p> Signup and view all the answers

    What is the effect of simultaneous hyperpolarization of the opposite labyrinth during ipsilateral head rotation?

    <p>Spontaneous firing rate reduction of the opposite labyrinth</p> Signup and view all the answers

    What is the limitation of the contralateral vestibular afferents during rapid head rotations?

    <p>They cannot detect head rotation when the head velocity is greater than the inhibitory cutoff</p> Signup and view all the answers

    What is the duration of the signal generated by movement of the cupula?

    <p>It lasts only as long as the cupula is deflected</p> Signup and view all the answers

    What is the primary function of the fast component in the eye movement during nystagmus?

    <p>To reposition the eye to the center of the orbit</p> Signup and view all the answers

    In a patient with left beating nystagmus, what is the direction of the slow movement?

    <p>To the right</p> Signup and view all the answers

    What is the cause of spontaneous nystagmus at rest?

    <p>Asymmetry between the two vestibular systems</p> Signup and view all the answers

    What is the primary function of the Head Impulse Test?

    <p>To test the Vestibulo Ocular Reflex (VOR) at high acceleration</p> Signup and view all the answers

    What is the direction of the fast component in left beating nystagmus?

    <p>To the left</p> Signup and view all the answers

    What is the result of an acute unilateral insult on the vestibular system?

    <p>Spontaneous nystagmus at rest</p> Signup and view all the answers

    What is the primary function of the vestibulo-ocular reflex during changes in head position?

    <p>To maintain corrective eye movement during rapid head rotation</p> Signup and view all the answers

    What is the advantage of the head impulse test in patients with acute vertigo?

    <p>It can be performed quickly and repeated within a short time</p> Signup and view all the answers

    What is the typical response of the eyes during the head impulse test when the VOR is functioning normally?

    <p>The eyes move in the direction opposite to the head movement</p> Signup and view all the answers

    What is the purpose of the head-shaking induced nystagmus test?

    <p>To diagnose a unilateral peripheral vestibular defect</p> Signup and view all the answers

    What instruction is given to the patient during the head-shaking induced nystagmus test?

    <p>To close their eyes during the test</p> Signup and view all the answers

    What occurs when the VOR is not functioning normally during the head impulse test?

    <p>The eyes move off the target and a corrective saccade is made</p> Signup and view all the answers

    Study Notes

    Vestibular System

    • The brain detects head movement and direction through comparison of inputs between the two vestibular systems.
    • The Semi-Circular Canals (SCCs) work in coplanar fashion, with increased firing rate on one side and decreased firing rate on the other side during head rotation.

    Push-Pull Mechanism

    • The brain recognizes the difference in firing rates between the two SCCs and interprets movement.
    • A faulty interpretation can lead to difficulties with gaze stabilization, postural stability, and motion perception.

    Inhibitory Cutoff

    • When the head is rotated to the ipsilateral side, simultaneous hyperpolarization of the opposite labyrinth occurs.
    • The inhibition of the hair cells in the opposite labyrinth can only reduce the firing rate to zero, at which point the inhibition is cut off.

    Velocity Storage System

    • The signal generated by movement of the cupula is brief, lasting only as long as the cupula is deflected.
    • This reflex is helpful to maintain corrective eye position during any change in head position and to correct eye movement rapidly.

    Head Impulse Test

    • The test is performed by having the patient fixate on a near target and then manually rotating their head in an unpredictable direction using a small-amplitude, moderate-velocity, and high-acceleration angular impulse.
    • When the VOR is functioning normally, the eyes move in the direction opposite to the head movement and gaze remains on the target.

    Head-Shaking Induced Nystagmus Test

    • This test is useful in the diagnosis of a unilateral peripheral vestibular defect.
    • The patient is instructed to close their eyes, and the head is shaken in a side-to-side motion.

    Hair Cells

    • The kinocilia and stereocilia are mechanosensing cilia and organelles.
    • The direction of deflection of the hair cells tells the brain how the head is moving.
    • Deflection of the stereocilia toward the kinocilia leads to excitation (depolarization), and deflection away from the kinocilia leads to inhibition (hyperpolarization).

    Otolith Organs

    • The Utricle is responsible for horizontal translation of the head and head tilt.
    • The Saccule is responsible for vertical translation of the head.
    • Together, they sense linear acceleration and static tilt of the head with respect to the gravitational axis.

    Oscillopsia

    • Oscillopsia is the subjective experience of motion of objects in the visual environment that are known to be stationary.
    • It can occur with head movements in patients with vestibular hypofunction since the vestibular system is not generating an adequate compensatory eye velocity during the head motion.

    Examination of Eye Movements

    • The examination of eye movements is critical for defining and localizing vestibular pathology.
    • Key tests include observation for nystagmus, Head Impulse Test, Head-Shaking Induced Nystagmus test, positional testing, and Dynamic Visual Acuity (DVA) test.

    Observation for Nystagmus

    • Nystagmus is the primary diagnostic indicator used in identifying most peripheral and central vestibular lesions.
    • An involuntary eye movement, nystagmus due to a peripheral vestibular lesion is composed of both slow and fast components.
    • The direction of the nystagmus is named by the direction of the fast component.

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    Description

    Learn about the anatomy and function of the Semicircular Canals, including the cupula, sensory hair cells, and endolymph fluid. Understand how they respond to angular head rotation and play a crucial role in our sense of balance and movement.

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