Vestibulo-occular Reflex, Nystagmus, and Caloric Testing
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 VOR is only effective up to a speed of about 50o/sec. 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. The VOR would occur in the dark, because the eyes move due to angular acceleration of the head.
The inset in Figure10.2 that appears when you press the "play" button shows the CNS connections involved in the VOR. This is a three-neuron circuit. One neuron is in Scarpa's (the vestibular) ganglion; one neuron is in a vestibular nucleus; and one neuron is in an extraoccular motor nucleus.
Figure 10.4 |
A variant of the VOR, called caloric nystagmus, is used as a test of the vestibular system. If the ear is irrigated with a fluid having a temperature different than the body (either warmer or cooler), a thermal gradient will be conduced across the small space of the middle ear.
Figure 10.4 shows a caloric response. Here, cold water is put in the right ear. About 20 ml is injected over about 30 s. The cold water cools the tympanic membrane, which cools the air in the middle ear, and finally the endolymph. This primarily affects the horizontal semicircular canal because it is close to the middle ear space.
Cooling somehow hyperpolarizes the hair cells, causing the eyes to drift slowly to the right as if the head was moving to the left. When the eyes have moved as far to the side as they can go, there is a quick resetting movement in the opposite direction. This cycle of slow and fast eye-movements is called a nystagmus. Nystagmus is labeled by the direction of the fast component. Figure 10.4 is an illustration of a left-beating nystagmus. Caloric vestibular testing is quantified by the magnitude and direction of the nystagmus. A useful mnemonic is COWS, meaning "cold-opposite warm-same". That is, irrigation of one ear with cold water produces a nystagmus away from the irrigated ear, while warm water produces a nystagmus toward the same ear. The normal response in a caloric vestibular test is symmetric and opposite responses in both ears. Weakness of the caloric response (eyes not moving when warm or cold water is flushed through one ear), or a spontaneous nystagmus (constantly moving eyes, as if the head was spinning when it is stable), indicates vestibular lesions.
- Question 1
- A
- B
- C
- D
- E
Linear acceleration is transduced in the:
A. Cupula
B. Cristae
C. Maculae
D. Organ of Corti
E. Scarpa's ganglion
Linear acceleration is transduced in the:
A. Cupula This answer is INCORRECT.
The cupula is a gelatinous vane that bends when fluid within the semicircular ducts moves because of angular rotation.
B. Cristae
C. Maculae
D. Organ of Corti
E. Scarpa's ganglion
Linear acceleration is transduced in the:
A. Cupula
B. Cristae This answer is INCORRECT.
The cristae respond to angular acceleration (rotation of the head).
C. Maculae
D. Organ of Corti
E. Scarpa's ganglion
Linear acceleration is transduced in the:
A. Cupula
B. Cristae
C. Maculae This answer is CORRECT!
The maculae are more sensitive to linear, not angular acceleration.
D. Organ of Corti
E. Scarpa's ganglion
Linear acceleration is transduced in the:
A. Cupula
B. Cristae
C. Maculae
D. Organ of Corti This answer is INCORRECT.
The organ of Corti senses sound.
E. Scarpa's ganglion
Linear acceleration is transduced in the:
A. Cupula
B. Cristae
C. Maculae
D. Organ of Corti
E. Scarpa's ganglion This answer is INCORRECT.
This answer is incorrect, though Scarpa's ganglion contains the cell bodies of the vestibular neurons that respond to linear and angular acceleration.
- Question 2
- A
- B
- C
- D
Which sensory receptor is most sensitive to angular acceleration?
A. Crista
B. Utricle
C. Saccule
D. Organ of Corti
Which sensory receptor is most sensitive to angular acceleration?
A. Crista This answer is CORRECT!
B. Utricle
C. Saccule
D. Organ of Corti
Which sensory receptor is most sensitive to angular acceleration?
A. Crista
B. Utricle This answer is INCORRECT.
Utricle is not the best answer. The utricle is most sensitive to gravity - changes in head position from an upright position.
C. Saccule
D. Organ of Corti
Which sensory receptor is most sensitive to angular acceleration?
A. Crista
B. Utricle
C. Saccule This answer is INCORRECT.
Saccule is not the best answer. THe saccule is most sensitive to gravity - changes in head position from a recumbent position.
D. Organ of Corti
Which sensory receptor is most sensitive to angular acceleration?
A. Crista
B. Utricle
C. Saccule
D. Organ of Corti This answer is INCORRECT.
The organ of Corti senses sound.
- Question 3
- A
- B
- C
- D
- E
Which of the following is correct regarding vestibular nuclei?
A. There are six
B. They are located beneath the floor of the third ventricle
C. They receive afferent fibers from the spinal ganglion
D. They project only to the cerebellum
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus.
Which of the following is correct regarding vestibular nuclei?
A. There are six This answer is INCORRECT.
THere are FOUR vestibular nuclei.
B. They are located beneath the floor of the third ventricle
C. They receive afferent fibers from the spinal ganglion
D. They project only to the cerebellum
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus.
Which of the following is correct regarding vestibular nuclei?
A. There are six
B. They are located beneath the floor of the third ventricle This answer is INCORRECT.
They are in the floor of the fourth ventricle.
C. They receive afferent fibers from the spinal ganglion
D. They project only to the cerebellum
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus.
Which of the following is correct regarding vestibular nuclei?
A. There are six
B. They are located beneath the floor of the third ventricle
C. They receive afferent fibers from the spinal ganglion This answer is INCORRECT.
They receive input from Scarpa's ganglion, not the spinal ganglion.
D. They project only to the cerebellum
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus.
Which of the following is correct regarding vestibular nuclei?
A. There are six
B. They are located beneath the floor of the third ventricle
C. They receive afferent fibers from the spinal ganglion
D. They project only to the cerebellum This answer is INCORRECT.
They have several projections.
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus.
Which of the following is correct regarding vestibular nuclei?
A. There are six
B. They are located beneath the floor of the third ventricle
C. They receive afferent fibers from the spinal ganglion
D. They project only to the cerebellum
E. They are connected to the 3rd, 4th, and 6th cranial nerve nuclei by the medial longitudinal fasciculus. This answer is CORRECT!
- Question 4
- A
- B
- C
- D
- E
Meniere's disease is associated with
A. loss of vestibular hair cells
B. damage to the vestibular nuclei
C. damage to cranial nerve VIII
D. damage to Scarpa's ganglion
E. excess endolymph in the membranous labyrinth
Meniere's disease is associated with
A. loss of vestibular hair cells This answer is INCORRECT.
Damage due to the loss of vestibular hair cells will impair vestibular function. However, the defining feature of Meniere's disease is swelling of the membranous labyrinth.
B. damage to the vestibular nuclei
C. damage to cranial nerve VIII
D. damage to Scarpa's ganglion
E. excess endolymph in the membranous labyrinth
Meniere's disease is associated with
A. loss of vestibular hair cells
B. damage to the vestibular nuclei This answer is INCORRECT.
Damage to the vestibular nuclei will impair vestibular function. However, the defining feature of Meniere's disease is swelling of the membranous labyrinth.
C. damage to cranial nerve VIII
D. damage to Scarpa's ganglion
E. excess endolymph in the membranous labyrinth
Meniere's disease is associated with
A. loss of vestibular hair cells
B. damage to the vestibular nuclei
C. damage to cranial nerve VIII This answer is INCORRECT.
Damage to cranial nerve VIII will impair vestibular function. However, the defining feature of Meniere's disease is swelling of the membranous labyrinth.
D. damage to Scarpa's ganglion
E. excess endolymph in the membranous labyrinth
Meniere's disease is associated with
A. loss of vestibular hair cells
B. damage to the vestibular nuclei
C. damage to cranial nerve VIII
D. damage to Scarpa's ganglion This answer is INCORRECT.
Damage to Scarpa's ganglion will impair vestibular function. However, the defining feature of Meniere's disease is swelling of the membranous labyrinth.
E. excess endolymph in the membranous labyrinth
Meniere's disease is associated with
A. loss of vestibular hair cells
B. damage to the vestibular nuclei
C. damage to cranial nerve VIII
D. damage to Scarpa's ganglion
E. excess endolymph in the membranous labyrinth This answer is CORRECT!
