10. Vestibular System: Structure and Function
Part 2 of 2

Lincoln Gray, Ph.D.
.

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 50^o/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 Caloric Testing. Pressto watch the reactions to caloric testing.

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.

Back | Table of Contents | Home Page | Index of Terms | Next