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?

In the direction of the angular head rotation Signup and view all the answers

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

To provide a gelatinous barrier Signup and view all the answers

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

The brain receives information about the direction of head movement Signup and view all the answers

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

Decreased somatosensation Signup and view all the answers

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

Generating an adequate compensatory eye velocity Signup and view all the answers

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

Nystagmus Signup and view all the answers

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

The direction of the fast component Signup and view all the answers

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

To examine the VOR at high acceleration Signup and view all the answers

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

Motion of images on the fovea Signup and view all the answers

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

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

How do the SCCs work in relation to head movement?

In a coplanar fashion Signup and view all the answers

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

Push–Pull Mechanism Signup and view all the answers

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

Spontaneous firing rate reduction of the opposite labyrinth Signup and view all the answers

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

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

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

It lasts only as long as the cupula is deflected Signup and view all the answers

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

To reposition the eye to the center of the orbit Signup and view all the answers

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

To the right Signup and view all the answers

What is the cause of spontaneous nystagmus at rest?

Asymmetry between the two vestibular systems Signup and view all the answers

What is the primary function of the Head Impulse Test?

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

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

To the left Signup and view all the answers

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

Spontaneous nystagmus at rest Signup and view all the answers

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

To maintain corrective eye movement during rapid head rotation Signup and view all the answers

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

It can be performed quickly and repeated within a short time 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?

The eyes move in the direction opposite to the head movement Signup and view all the answers

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

To diagnose a unilateral peripheral vestibular defect Signup and view all the answers

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

To close their eyes during the test Signup and view all the answers

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

The eyes move off the target and a corrective saccade is made 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.