Module 1 The Nervous System

Lesson 1.1.6

1.1.6 page 8

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Homeostatic Imbalance in Hearing

In today’s world, health care workers are very concerned that your generation will experience increased numbers of hearing loss as it ages. Deafness can be defined as one of two main types: conduction deafness or perception deafness.

deafness: any type of hearing loss, whether it be the inability to detect certain frequencies, the inability to detect very soft sounds, or the inability to detect any sound at all

conduction deafness: loss of hearing that results when the structures of the ear required to transmit and amplify vibrations (tympanum, ossicles, and oval window) are unable to do so; involves a malfunction in the conduction apparatus

perception (nerve) deafness: loss of hearing that results when the structures of the organ of Corti are unable to initiate nerve impulse.

To understand the types and causes of deafness, read the section on pages 423 – 424 and make summary notes or a chart on hearing loss for your course folder. Pay particular attention to Table 12.3 on p. 423.

Self-Check

Answer the next two questions that deal with hearing loss. Check your answers.

What is meant by the term “deaf”?

State the parts of the ear that have been damaged or are non-functional in someone with
1. perception (nerve) deafness
2. conduction deafness

Check your work.

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Maintaining Balance

semicircular canals: three tubes that are situated at right angles to one another; contain mechanoreceptors that detect head and body rotation; responsible for dynamic balance, or rotational equilibrium (rotational equilibrium dynamic balance), which is the balance that is established in response to the head and body being moved or rotated

utricle and saccule: tiny chambers in the inner ear that contain hair cells which respond to changes in head position with respect to gravity and movement in one direction; responsible for static balance, or gravitational equilibrium

otoliths: tiny particles of calcium carbonate found in the utricle and saccule that contact the hair cells of these structures and stimulate them

dynamic balance (rotational equilibrium): balance resulting when the head and body are moved or rotated

static balance (gravitational equilibrium): balance resulting from changes in the position or the movement of the head in one direction; usually in response to gravity

vestibule: fluid-filled area of the inner ear located between the semicircular canals and the cochlea; contains the utricle and saccule

The ear has an important function in your ability to maintain balance. The inner ear has three additional structures known as the semicircular canals, the utricle and the saccule that are responsible for maintaining balance. 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 dynamic equilibrium or rotational equilibrium. That queasy feeling you experienced when you pulled ahead of the truck slipping backwards on the icy hill, or that similar 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 static equilibrium or gravitational equilibrium. Some impulses from the vestibular apparatus 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!

To understand how the body maintains balance, choose one of the following two options. You may summarize the information in any format you choose and store this information in your course folder.

Read page 424.

Watch and Listen

Watch the following videos
Hearing and Balance

Utricle and Saccule

Self-Check

After reading the text and/or watching the video segments, test your understanding of the concepts by completing the following questions. File your work in your course folder. If you do not understand a particular concept, consult your instructor.

Describe the structures of the inner ear that are involved in maintaining balance.
Explain how the structures of the inner ear allow for dynamic (rotational) equilibrium.
Explain how static (gravitational) equilibrium is attained.

Check your work.

Self-Check Answers

There are three structures of the inner ear involved in maintaining balance. These structures include the three fluid filled semicircular canals, situated at right angles to each other, and just below them, the fluid filled utricle and saccule, which contain tiny rock-like granules of calcium carbonate called otoliths.
The balance established while moving the head forward and backward is called dynamic (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, stick 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.
A person who is lying down or not moving 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 down, hanging upside down, etc. When an individual is moving around, calcium carbonate crystals in the utricle and saccule (otoliths) shift position and contact the stereocilia of these structures. This contact results in nerve impulses being transmitted to the brain, indicating 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 onc