SCN3230-ABED_1: Module 1 Lesson 3 - 6

Lesson 3 — Mechanoreception: The Ear

Homeostatic Imbalance in Hearing

Read pages 423 - 424

Health care workers are very concerned that your generation likely will experience increased hearing loss as it ages. Deafness is one of two main types: conduction deafness or perception deafness.

The term "deaf" refers to any type of minor or major hearing loss. In perception (nerve) deafness, any of the structures of the organ of Corti have been damaged, including the hair cells, the sensory neurons, the basilar membrane, the tectorial membrane, or the auditory nerve. In conduction deafness, structures such as the tympanum, any of the ossicles, or the connection between the ossicles and the oval window have been damaged. In addition, other conditions can impede the conduction and amplification of sound in the inner ear, including accumulations of wax in the auditory canal and against the tympanum.

Maintaining Balance

Read pages 424 - 425

The ears have an important function in maintaining balance. The inner ear has three structures known as the semicircular canals, the utricle, and the saccule that are responsible for maintaining balance. The three fluid-filled semicircular canals are situated at right angles to each other. Just below them, the fluid-filled utricle and saccule contain tiny rock-like granules of calcium carbonate called otoliths.

When you twirl on the dance floor, ride a rotating ride at the midway, or suffer through a rough boat ride, the semicircular canals are working overtime. This is called rotational equilibrium.

At the base of each of the semicircular canals is a jelly-like covering called a cupula. The tiny hairs of the hair cells in the semicircular canals, better known as stereocilia, protrude into the cupula. When the body rotates, the fluid inside the semicircular canals moves and bends the stereocilia. The hair cells receive this information and send it to the brain.

That queasy feeling you get when a fast-moving elevator comes to an abrupt stop is the result of nerve impulses from the utricle and saccule. This is called gravitational equilibrium.

A person who is not moving or lying still is said to be in a static position. The movement or lack of movement of the fluid in the inner ear tells the brain whether a person is standing, lying, hanging upside down, etc. When an individual is moving, calcium carbonate crystals in the utricle and saccule (otoliths) shift position and contact the stereocilia of these structures. This contact results in nerve impulses transmitted to the brain to indicate that the individual is changing positions.

When the person is in an established position, the crystals stop rolling around. The sensory neurons in the vestibule of the utricle and saccule do not send any impulses to the brain, thereby indicating that the individual is in a static position again.

Some impulses from the semicircular canals, utricles, and saccule travel to the spinal cord where body position can be adjusted by reflex action. Other impulses are sent to the cerebellum where other reflexive muscular coordination occurs. Further impulses move to higher centres in the cerebrum involved in the control of eye movement. Input from the eyes is very important in maintaining balance. To show this, try closing your eyes and standing on one leg!

Watch and Listen

Watch the following segments of the Biologix 07 series (6 minutes).

Try This - optional

You need a willing friend to help you with this exercise.

Have your friend sit in a swivel chair with his or her head in an upright position. Have the person raise his or her legs from the floor. Rotate the chair in a clockwise direction, but not too quickly. After 20 rotations, stop the chair and have your friend stand up. Be prepared to support your friend! Make observations.
Allow your friend to recover for about 5 minutes.
Rotate the chair with your friend sitting upright in it clockwise 20 times. Immediately after, rotate the chair counterclockwise 20 times. Be sure your friend holds his or her head straight. Stop the chair and have your friend stand up. What do you observe this time?
Allow recovery for about 5 minutes.
Have your friend sit in the swivel chair again, but this time have him or her tilt his or her head to the right. Rotate the chair clockwise 20 times. Ask your friend to stand and hold his or her head upright. What happens?
What evidence did you collect in your experiment that suggests that the fluid in the semicircular canals continues its movement even after you have stopped rotating the subject in the chair?
Describe how the semicircular canals detect changes in motion during a roller coaster ride.
Are the utricle and saccule involved in maintaining balance during a roller coaster ride?

Check Your Work.

You should have observed that, after the first rotation, your friend felt dizzy. This is because the hair cells in the semicircular canals were stimulated while rotating and the brain adapted to the movement. When the chair stopped rotating, the hair cells continued to be stimulated due to momentum. It took a while for the hair cells to stop sending signals to the brain.

In the second rotation, your friend rotated clockwise and then quickly rotated counterclockwise. When your friend stopped spinning, he or she likely felt less dizzy than in the first scenario. This is because the counterclockwise movement cancelled the clockwise motion.

In the third scenario, your friend likely feels as if he or she is falling. This is because tilting head shifted the utricle and saccule while spinning. Even when the head is upright, momentum causes the utricle and saccule to be stimulated continuously at a tilted angle, resulting in a feeling of falling.