This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Equilibrium (Balance)

The sense organs for balance are located near the inner ear, protected by the same hard bones of the skull that surround the cochlea. When a person suffers inner ear damage, the sense of balance is often affected, because these sense organs are in close proximity. However, they are separate systems.

Where are sense organs for equilibrium? What is the vestibular apparatus?

Structures used by the sense of equilibrium are located near the cochlea and the bones of the middle ear.

This diagram shows the relationship of the ossicles (bones of the middle ear) to the vestibular apparatus: the organs for the sense of equilibrium.

The vestibular system consists of two main parts with slightly different functions: the semicircular canals (the loops) and two chambers under the canals: the saccule and the utricle. The loops (semicircular canals) and sacs (saccule and utricle) are collectively called the nonauditory labyrinth or just the labyrinth. The entire labyrinth is filled with a fluid called the endolymph, the same fluid found in the cochlea.

What is the specific stimulus for the semicircular canals? How do they work?

The semicircular canals are rotary motion detectors. When you turn your head in any direction, you move the fluid in the canals (the endolymph). As the endolymph moves in the semicircular canals, the liquid stimulates hair-like cells located in chambers at the base of the canals. So the sense of balance depends ultimately on mechanoreceptors, like the sense of hearing.

Pierre Flourens, a French anatomist, discovered the vestibular system. He believed it was part of the auditory system. He was disappointed in 1824 when he removed the vestibular system in pigeons and found their hearing unaffected. However, the birds showed other peculiar reactions. They could no longer maintain normal head and body position.

Birds with their vestibular systems removed

The vestibular apparatus also includes another system, besides the semi-circular canals: two structures called the sacs, the saccule and utricle. These two chambers are linear motion detectors, which means they detect acceleration and deceleration: movement that can be in a straight line. Within the saccule and utricle are crystals called otoliths (ear stones) surrounding modified hair cells. When the body accelerates or decelerates, the hairs bend under the weight of these crystals, much as a weight on the end of a spring would bob back and forth if you accelerated or braked a car. Again it is modified hair cells that generate electrical potentials leading ultimately to nerve impulses sent to the brain.

What do the otoliths look like under a microscope? What happens to the otoliths in old age and what sort of accident can this produce?

The otolith crystals look splendid in an image from an electron microscope, like finely cut gems. Unfortunately, as time passes, the endolymph dissolves the otolith crystals. By age 70, some of the crystals are gone. Fragments of the remaining otoliths may dislodge suddenly and tumble down on the hairs of the saccule and utricle, creating sudden imbalance or vertigo. This can cause people in their 70s or older to fall down suddenly, sometimes causing broken hips.

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