Questions: Vestibular System: Balance and Head Motion

Question 1

You sit blindfolded in a chair that starts spinning and then keeps turning at a constant speed for a minute. What happens to your sense of rotation?

• A. It stays strong for the whole minute
• B. It is strong at first and then fades even though you are still turning
• C. You feel nothing at the start but strong rotation after a minute
• D. You feel that you are moving in a straight line

Answer: (B)

When rotation starts, the fluid (endolymph) in the canals lags behind and bends the cupula, so hair cells signal turning. During constant rotation the fluid comes up to speed, the cupula returns to rest, and the signal fades. When the chair stops, the fluid keeps moving briefly, and you feel as if you are turning the other way.

Question 2

The semicircular canals are the main sensors of linear acceleration, such as speeding up in a car.

• T. True
• F. False

Answer: False

Linear acceleration and head tilt are detected by the otolith organs. Their hair cells are covered by a gel layer topped with calcium carbonate crystals (otoconia), which lag behind or slide when the head accelerates or tilts. The canals detect rotation.

Question 3

You can read text while shaking your head from side to side, but the text blurs when you hold your head still and shake the page at the same rate. Explain why.

Model answer: When the head moves, the semicircular canals detect the rotation and the vestibulo-ocular reflex quickly moves the eyes in the opposite direction by the same amount, keeping the image steady on the retina. When the page moves, the eyes must rely on visual tracking, which is slower and cannot keep up with rapid motion.

The vestibulo-ocular reflex is one of the fastest reflexes in the body because it uses a short pathway from the vestibular nerve through brainstem nuclei to the eye muscles. It works even in darkness.

Question 4

How does a vestibular hair cell signal motion in both directions?

• A. It fires only when bent and is silent otherwise
• B. It releases some transmitter at rest; bending toward the tallest stereocilium increases release and bending the other way decreases it
• C. Different hair cells use different neurotransmitters for each direction
• D. It changes color depending on the direction of bending

Answer: (B)

At rest, some mechanically gated channels are open, so the hair cell releases transmitter and its nerve fiber fires steadily. Bending toward the tallest stereocilium opens more channels, depolarizes the cell and raises firing. Bending away closes channels and lowers firing. This lets one cell encode both directions.