The vestibular system is the part of the inner ear that helps us maintain our balance and sense of head orientation and movement. It’s as old as the cochlea in evolutionary time. The systems are highly similar. In both, hair cells transduce motion into nerve
signals, but the vestibular system senses much slower vibrations of just a few Hz in contrast to the cochlea’s 20 to 20,000 Hz.
The cochlea, on the right hand of the picture, gives us our sense of hearing. The three semicircular canals of the vestibular apparatus, on the left, are fluid-filled tunnels in the skullbone. Each semicircular canal contains hair cells in the bulge at its base, the ampulla. Because the semicircular canals on each side of the head are oriented roughly at right angles to each other, they signal head movement no matter how we move.
The utricle and saccule sense static head position. They contain particles of calcium carbonate that pull on hair cells. The hairs in the utricle are vertical, while those of the saccule are horizontal; thus the organ can tell you the angle of the head in geometric coordinates! If one of the bony particles, called otoconia, should slip out of position you may experience a form of dizziness when you bend over that is called benign paraoxysmal positional vertigo (sometimes the “paraoxysmal” is left out). A doctor will tell you how to bend your head to reposition the otoconia and return to normal or it may slip back on its own in a few months. You can find more about vertigo here.
Understanding the vestibular part of the inner ear helps to explain why we get so dizzy when we spin, a joy that diminishes with age. If you are a parent of toddlers, know that when you reach middle age they will be just the age to delight in carnival rides, and of course you will be invited to join them after a large, greasy lunch. A noble calling, parenthood.
We judge motion by comparing what the eyes see with what the vestibular organs sense about our body movement. If the vestibular apparatus says that we’re not moving, then any movement sensed by the eyes must be outside the body. (The brain discounts the appearance of movement caused by eye movement itself.) Then the brain tells the eyes just how to move to keep track of the moving object. If we’re moving and the world is moving, too, we need to keep track of the objects while adjusting for our body’s movement. The eyes and the semicircular canals have to work together on this. However, if you twirl around and set the fluid of one or two pairs of the semicircular canals into motion and then stop suddenly, you fool the brain into thinking the world is spinning. The reason is that the vestibular fluid is moving as though you’re moving, so the eyes are moved to track objects.
This eye movement, called nystagmus, makes the world seem to spin. You can produce the same effect by drinking alcohol, which lowers the specific gravity of the fluid of the semicircular canals and stimulates the vestibular hair cells as though you’re moving. Again the world spins. OR you can squirt ice water into the ear canal to produce convection in the semicircular canals. This produces a very powerful spinning sensation, a vigorous nystagmus, and–frequently–vomiting. We did this when I was a lab instructor in a medical school neuroscience course as a grad student. You have to keep a bowl near the poor medical student’s mouth. (Remember that the med student was a volunteer. I volunteered once for an anoxia experiment in physiology class and learned what sudden loss of consiousness is like. Brighter students know better than to volunteer.
Questions (Please answer one.)
1. Describe the roles of vision and the vestibular apparatus in a coordinated act in sports, such as a second baseman’s double play, a pole vault, or a turnaround jump shot. What are the differences and similarities in the roles of the two senses?
2. Our sense of balance involves another sense besides vision and vestibular function, called proprioception. Test your balance at this site or that one. Why do you think that three senses are necessary for balance?
3. Which of these illusions involve the vestibular system?