pathway of bone on

8/9/2019 Pathway of Bone on

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Exp. 1 Pathways of soundconduction

Yu Yanqin, PhDZhejiang University, School of

Medicine


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[Purpose]

To learn how to use a tuning fork to

generate sound;

To understand the function of theauditory organ;

To understand the pathways of soundconduction.


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[Principle]

Types of Hearing

losses

Conductive hearing losses Sensorineural hearing

losses

Impairment of the cochlea

Impairment of the auditory

center


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[Principle]

Air conduction tests

Bone conduction tests


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Tuning fork tests

The Weber and Rinne tuning fork tests

distinguish between conductive and

sensorineural hearing losses.


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The Rinne test

In the Rinne test, air and bone conductiontests are compared.

In normal hearing, tones sound louder by air

conduction than by bone conduction. In conductive hearing loss, however, the bone-

conduction stimulus is perceived as louder.

In sensorineural hearing loss both air and boneconduction sounds are diminished, but the airconduction sound is perceived as louder.


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The Weber test

During the Weber test , the stem of a vibrating

tuning fork is placed on the head in the midline.

If the tone is perceived in the affected ear, thisindicates a unilateral conductive hearing loss.

In the case of unilateral sensorineural hearing

loss, the tone is heard in the unaffected earinstead.


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[Experimental method &procedure]

1. For the tuning-fork tests, the examiner,

using a rubber reflex hammer or his or

her elbow, strikes one tine strongly

enough to produce a sound clearly

perceived by the examiner at 30 cm.


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2. The Rinne test:

The stem of the tuning fork is placed on the mastoid

and the subject is asked to indicate when she or he

stops hearing the sound.

The fork is then held 2.5 cm from the pinna, and the

patient is asked if she or he still hears the sound.

If the sound is still audible, air conduction is greaterthan bone conduction (AC > BC in normal ear); if not,

BC > AC.


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Figure 1. Proper positioning of tuning forktines for air conduction testing.


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3. Use a cotton ball to block the external

auditory canal (to mimic conductive

hearing loss); repeat the Rinne test.


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The Weber test:

4. The Weber test (Figure 2) may be performed usinga 256 or a 512 Hz tuning fork.

During this test, the stem of a vibrating tuning fork isplaced on the head in the midline.

Ask the patient, "Do you hear this better in the right orthe left ear?" If the patient hesitates, then the Webertest shows that sound is not being referred to one ear.

If the tone is perceived in the affected ear, thisindicates a unilateral conductive hearing loss.

In the case of unilateral sensorineural hearing loss, thetone is heard in the unaffected ear instead.


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5. Proper recording of the Rinne testshould be "AC > BC" or "BC > AC" for

each ear; for the Weber test, "Weber R" or

"Weber L" or "Weber not referred.


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Figure 2. Midline positioning of the tuning forkfor the Weber test.


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[Discussion]

Compare air conduction with boneconduction.

Why does an accurate recording of yourvoice sound different from what you hearwhen you are actually speaking?

What are the mechanisms of conductivehearing loss and sensorineural hearingloss?


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Exp. 2 Measurement of visualacuity


Medicine


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[Purpose]

To learn how to measure visual acuity;

To understand the function of therefractive system of the eye.


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[Principle]


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[Principle]

Visual acuity (VA) is the spatial resolving

capacity of the visual system.


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Normal VA can discriminate one minute ofvisual arc which is bigger than or equal to thediameter of a cone cell in retina.

A standard eye chart

When viewed from a distance of 5 metres, theletter E on the 10th line occupies 5 minutes ofvisual arc in the observer's eye (Figure 1).

[Principle]


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Figure 1. The letter E on the 10th line occupies 5 minutes of visualarc. Each of the 3 black horizontal lines and 2 white gaps is thesame size and comprises 1 minute of visual arc (for a total of 5minutes.) A person with 1.0 vision should be able to resolve thisangle.


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The test should be given in good light. The eye chart should be placed upright on a wall, and

the testing distance should be measured exactly (5metres).

The person taking the test should stand at the testingdistance.

An occluder is used to cover one of the patient's eyes

for monocular testing, ie: to facilitate testing one eye ata time.

The lowest line that can be read correctly is the VA forthat eye (Figure 2).


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Figure 2. The eye chart (E-Game)


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[Discussion]

Explain the difference between emmetropia

and ametropia and their causes.

Outline the principle of measurement of visual

acuity.


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Exp. 3 Measurement of visualfield


Medicine


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[Purpose]

To learn how to measure visual field;

To understand the function of the

refractive system of the eye and the

retina.


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[Principle]

The area in space that may be visualized bythe eye is known as the visual field.

The size of your visual field is the furthest you

can see to the side when looking straightahead.

A visual field test is a method of measuring an

individuals entire scope of vision. Plotting the visual field is important for many

disorders, particularly disorders of the opticnerve and brain.


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1. Familiar yourself with thestructure of the arcperimeter and the working

principle of the instrument.


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2. While the subject is seated comfortably, and one

eye is covered. Ask him/her to look straight ahead at a

central target directly in front.

Instruct him/her to tell you when he/she becomes

aware of a visual pointer to one side, within his

peripheral field.

This process produces a map of the visual field.


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3. Change to different colors, such as red,green and blue; repeat the visual fieldtest.


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4. Record anddraw the mapsof the visualfields fordifferent colors

(Figure 2).


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Figure 2. Measurement ofvisual field

One eye/one color/one person


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[Discussion]

How does the visual field change with

glaucoma?

Are there any differences when you use

different colors for the visual field test?

pathway of bone on

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