X-Sheet 11 The Eye and Ear

Life Sciences X-Sheets

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X-Sheet 11

The Eye and Ear

Animal Responses to the Environment: The Eye and Ear


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Terminology & definitions Photoreceptors: Specialized receptors to receive the stimulus of light and convert it to an impulse. Photoreceptors in the retina of the eye are called rod and cone cells. Refraction: To bend light – refraction takes place when light passes through a lens that is bent by a convex [ ( ) ] shape or a concave [ ) ( ] shape. Stereoscopic vision: Also known as binocular vision - to see with two eyes, where each eye will produce a slightly different image of the same object and allows us to judge distance, depth and size of an object. Accommodation: The ability to change the focal length of the object by changing the convex shape of the lens to assist with focussing on a near or distant object. Myopia: Short-sightedness caused by a cornea that is too rounded so the image falls short of the retina. Hypermetropia: Long-sightedness caused by a lens that cannot become rounded enough to refract light so the image falls behind the retina. Mechanoreceptors: The Organs of Corti are receptors located in the cochlea of the ear, which are stimulated by sound waves and convert the sound waves into impulses. Ossicles: Three little bones called the hammer, anvil and stirrup located in the middle ear and that function to amplify sound. Otis media: inflammation and infection of the middle ear which causes pressure on the eardrum. X-planation / Key Concepts / Diagram The Human Eye:



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Path of Light:

Light rays pass from an object to the eye, through the transparent convex cornea, aqueous humour, the biconvex lens and vitreous humour.

As the light rays pass through the curved surfaces of the cornea and the lens, light is refracted (bent).

The lens refracts the light rays and forms an inverted (upside-down) image on the retina, bringing the image into focus by making fine adjustments.

The rod and cone cells (photoreceptors) are stimulated by the light rays and convert the stimulus into impulses.

These impulses are transmitted along the optic nerve across the optic chiasma (cross-over) so that impulses enter the lower visual centres on opposite sides of the mid-brain at the occipital lobes.

The upright images are interpreted for size, shape and colour of the object that was seen.

Accommodation

Binocular vision means to see with TWO eyes (bi = two). The image from the left eye is always slightly different to the image from the right eye. The two images join in the brain (occipital lobes) and results in stereoscopic vision, which allows us to judge distance, depth and size of objects. The eyes can change the convex curve of the lens and therefore the focal length. This process is termed accommodation. Near vision (round lens) - When viewing an object at a distance of less than 6 metres:

The ciliary muscles contract, causing the ciliary body to move closer to the lens.

This causes the tension on the suspensory ligaments to slacken.

This results in tension on the lens being released.

The lens becomes more convex and rounded, increasing the refractive power of the lens.

The focal length decreases, bringing the object into focus onto the yellow spot of the retina.

Distant vision (long lens) - When viewing an object at a distance of more than 6 metres:

At rest, the eyes are set for distant vision. (When people daydream, they have a faraway look because their eyes are set for distant vision.)



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The ciliary muscles relax, causing the ciliary body to pull back from the lens.

This causes the tension on the suspensory ligaments to increase, so they become taut (pull tight).

This results in the lens pulling to a longer, thinner shape (less convex), decreasing the refractive power of the lens.

The focal length increases, bringing the object into focus onto the yellow spot of the retina.

Pupillary mechanism (reflex)

The pupillary mechanism is a reflex action regulated by the Autonomic Nervous System, to prevent excess light from passing into the eye at one time. Excess light will cause damage to the retina and the photoreceptors (rod and cone cells). The iris controls the amount of light that enters the eye by controlling the size of the pupil. The circular and radial muscle fibres in the iris regulate the size of the pupil.



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Visual defects: Short-sightedness This is also called myopia or nearsightedness. It is a refractive defect where the image focuses in front of the retina because the cornea is too rounded. Distant objects are seen as blurred. Myopia may be genetic or overstraining of the yes over a long period. Glasses and contact lenses that are concave [) (] are prescribed to reduce refraction. Refractive surgery may be an option, where the cornea is reshaped to flatten it and so decrease refraction. Long-sightedness This is also called hypermetropia or farsightedness and a refractive defect where the image focuses behind the retina. The person will not be able to see objects when they are close by, as the images are blurred. This condition is caused by the following:

An eyeball that is too short (genetic) – corrected using convex lenses [( )].

When the lens cannot become round enough during accommodation – corrected using convex lenses [( )].

A cornea that is too flat: Refractive surgery is performed in extreme cases.

Correction of visual defects



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Astigmatism: This is an optical defect that results in blurred vision caused by an irregular curvature of the cornea or the lens so the eye has different focal points that occur in different planes. Glasses and hard contact lenses correct the irregular focal points. Cataracts: This is the clouding of the lens when the lens cortex liquefies to form a milky white fluid. Cataracts must be removed surgically. The Human Ear

The ears are the sense organs for hearing with mechanoreceptors in the cochlea of the ear that are stimulated by sound waves, which are converted to impulses. The impulses are transmitted via sensory neurons to the auditory centre in the cerebral cortex of the brain where they are interpreted. The ears are also the organs for balance and equilibrium. These impulses are transmitted via sensory neurons to the cerebellum where they are interpreted to ensure balance and equilibrium.

The Human Ear Path of Sound Sound waves move from the vibrating source (for example, a person talking or a car driving past) in horizontal waves. Humans hear sounds with a vibration frequency of between 16 and 20 000 Hz.

Sound waves are collected by the pinna and passed down the external auditory canal.

The vibrations reach the eardrums (tympanic membranes).

The eardrum vibrates according to the frequency of the sound waves.



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The vibrations are transmitted to the three ossicles (the hammer, anvil and stirrup) in the middle ear, to amplify the vibrations.

The stirrup passes the vibration through the oval window, into the inner ear.

The oval window vibrates and causes wave movements in the liquid of the perilymph.

The wave movements are transferred to the endolymph inside the cochlea.

The hair cells of the organs of Corti (the mechanoreceptors) brush or bend against the membranes, which converts the mechanical stimulus of the sound wave into an impulse.

The impulse is passed through the auditory nerve to the auditory centre in the cerebral cortex of the brain.

The sensation of sound is perceived and interpreted by the cerebral cortex.

Excess vibrations are passed out through the round window, to prevent sound pressure and echoes.

Balance and equilibrium: is controlled by the semi-circular canals, the utriculus and the sacculus that contain receptors. The receptors will convert the stimuli of balance, euqilibrium and pressure into impulses and send them to the cerebellum. Hearing defects Middle ear infection: also termed otitis media and refers to an inflammation of the middle ear. When the middle ear becomes infected by bacteria, there is extreme pain as pressure builds up behind the eardrum. The pressure is caused by pus, which collects in the middle ear cavity. The Eustachian tube becomes blocked so there is a lack of the ability to equalise the pressure on both sides of the eardrum. In some cases, the eardrum may burst and pus drains out of the ear. Antibiotics are generally prescribed. Severe scarring of the eardrum can affect the person’s hearing. Deafness: The terms ‘hearing impairment’, ‘hard of hearing’ or ‘deafness’ mean that the person has a loss of the ability to detect sounds either partially or completely. Deafness can be as a result of:

Sensorineural hearing loss

Noise-induced hearing loss

Genetic hearing loss

Diseases that could cause hearing loss or impairment o Measles o Meningitis o Autoimmune disease o Mumps o Foetal alcohol syndrome o Syphilis – the foetus is infected by the infected mother

Physical trauma Hearing aids and cochlear implants: A hearing aid is an apparatus that is worn in or behind a person’s ear. The apparatus amplifies sound, so that the person is able to hear better. A cochlear implant is a surgically implanted electronic device that stimulates the auditory nerves with an electronic field, inside the cochlea Link between hearing defects and speech disorders:



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When babies and toddlers are unable to hear properly or their hearing is impaired in any way, it will affect the development of speech. Humans learn to talk by mimicking the sounds that they hear. When a person cannot hear the sounds of the words, it is difficult to learn to talk. The sound vibrations made in the throat of a talking person are mimicked. Deaf people succeed in mastering spoken language to varying degrees, but the sounds that are made are different to that of hearing people. To overcome this challenge, a language of signing with the hands has been developed to assist deaf people to communicate effectively. X-ample Questions QUESTION 1: (Taken from Bios best 2008 Grade 12 Biology) The the diagram below that represents a section through part of the human eye. 1.1. Supply labels for the parts numbered 1, 3 and 5. (3) 1.2. Supply the number and name the part that controls the amount of light that

enters the eye. (2) 1.3. Mention the changes that the part names in Question 3.2 will undergo when

exposed to bright light. (4) 1.4. List one function for each of parts 4 and 7. (2) 1.5. Supply the number and the name of the part that is responsible for the

accommodation of the eye. (2) 1.6. Explain the changes that will take place when the part named in Question 3.5 views an object closer that 6m. (5)



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QUESTION 2: (Taken from DoE May/June 2008 Paper 2) Study the following diagram of the human ear and answer the questions that follow.

2.1 Identify parts B and G. (2) 2.2 State the function of each of parts C and E. (2) 2.3 Explain:

a) Why you can often clear a buzzing/humming in the ear by swallowing (3) b) Why the membrane labelled F is much larger than membrane C (2)

2.4 A dog has lost part A in an accident. Part A is replaced with a stiff, non-elastic, solid plastic structure. a) Is the plastic structure as effective as the original ear of the dog? (1) b) Explain your answer to QUESTION 4.4 (a). (2)

2.5 Explain how part D and the retina of the eye function in similar ways (4) 2.6 Give ONE reason for part D being spirally shaped. (2) X-ercise Various possible options are provided as answers to the following questions. Choose the correct answer and write only the letter (A – D) next to the question number. 1.1. The tough, non-elastic tissue covering the outer portion of the eyeball is the ….

A choroid B sclera C conjunctiva D iris



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1.2. The shape of the lens in the human eye may be altered by the contraction or

relaxing of the….

A optic nerve B muscles of the iris C muscles of the ciliary body D pupil

1.3. The sensation of sight in human beings originates in the ….

A yellow spot B optic nerve C cerebrum D retina

1.4. The following are part of the human eye:

1 cornea 2 lens 3 retina 4 iris 5 choroid Which parts are respectively concerned with/in:

Adjusting the focus?

Preventing internal reflection?

Recording changes in light intensity?

A 2, 4 and 3 B 3, 4 and 1 C 2, 5 and 4 D 4, 1 and 5

1.5. When the tention of the suspensory ligaments in the human eye is slackened,

the….

A lens becomes less convex B eye is focused for distant vision C pupil enlarges D lens bulges

1.6. When the pupil of the human eye constricts, the receptors and effectors are

respectively the….

A fovea centralis and ciliary muscle B pupil and ciliary muscle C pupil and radial muscles of the iris D fovea centralis and the circular muscles of the iris

1.7. In accommodation of the human eye, the….

A pupil enlarges B ciliary muscles are involved C suspensory ligaments are always in a state of tention D circular muscles of the iris contract



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1.8. Images of objects less than 6m from the eye are clearly focussed onto the retina, when the…. A ciliary muscles contract B pupil widens C muscles of the iris contract D curvature of the lens decreases

1.9. The optical disc where the fibres of the optic nerve leaves the eyeball, is the….

A yellow spot B iris C blind spot D retina

1.10. Which of the following occurs when you look up from reading a book to look at a

distant mountain on a clear, sunny day? The….

A radial muscles of the iris contract B the pupil becomes more dilated C the ciliary muscles contract D the lens becomes thicker and rounder

Answers to the X-ercise Questions: 1.1. B

1.2. C

1.3. C

1.4. C

1.5. D

1.6. D

1.7. B

1.8. A

1.9. C

1.10. C

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THE HUMAN EYE & EAR

Checklist

Make sure you …

Describe the structure and state the functions of the parts of the human eye.

State what is meant by binocular vision.

Describe the changes that occur in the human eye for each of the following: o Accommodation o Pupil reflex/pupillary mechanism

Describe each of the following visual defects using diagrams, and state how each visual defect is treated: o Short-sightedness o Long-sightedness o Astigmatism o Cataracts

Describe the structure and state the functions of the different parts of the human ear.

Describe the functioning of the human ear in:

Hearing (include the role of the organ of Corti, without details of its structure)

Balance (include the role of maculae and cristae, without details of their structure)

Describe the cause and state the treatment of the following hearing defects:

Middle ear infection (treatment using grommets)

Deafness (treatment using hearing aids and cochlear implants)

Test Yourself

Question 1

Which one of the following does not perform a protective function in the eye?

A sclera

B conjunctiva

C eyelid

D vitreous humour

Question 2

Which one of the following will not occur when a bright light is shone into a person’s eyes?

A lens flattens and becomes less convex

B pupil becomes smaller

C circular muscle of iris contracts

D longitudinal muscles of iris lengthen

Question 3

Which sequence below is correct with regard to the passage of light through the eye to form an image on the retina?

A cornea → conjunctiva → lens → vitreous humour → retina

B conjunctiva → cornea → lens → aqueous humour → retina

C conjunctiva → cornea → lens → vitreous humour → retina

D cornea → lens → vitreous humour → aqueous humour → retina

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Question 4

All of the following could result in hearing loss except…

A rupture of the tympanic membrane.

B tightening of the ligaments linking the ossicles to one another.

C damage to the microscopic hairs within the cochlea.

D no fluid movement within the semi-circular canals of the inner ear.

Question 5

Which one of the following contains cells that are sensitive to light?

A. retina

B cornea

C optic nerve

D lens

Question 6

The function of the Eustacian tube in the ear is to

A bring food and oxygen to the middle ear

B remove waste products from the middle ear

C let air equalize pressure on either side of the tympanum

D remove excess perilymph from the inner ear

Question 7

The sacculus and the utricculus give information about the

A position and movement of the head

B condition of the muscles

C position of the head

D condition of the joints

Question 8

The shape of the lens in the human eye may be altered by contraction or relaxation of the

A optic nerve

B muscles of the ciliary body

C muscles of the iris

D pupil

Question 9

A light stimulus is converted into a nervous impulse in the

A blind spot

B iris

C retina

D optic nerve

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Exam Questions

Question 1

1.1 Which one of the following parts of the eye is most important in refracting light from a distant object?

A pupil

B lens

C retina

D cornea

1.2 Static balance (gravity) is sensed by the

A semicircular canals

B cochlea

C organ of Corti

D maculae

1.3 The part of the ear where a sound wave is changed into a vibration is called the

A organ of Corti

B tympanic membrane

C ossicles

D pinna

1.4 Which one of the following parts of the body are most involved in helping a blind-folded man standing on one leg to keep his balance?

A brain, muscles, organ of Corti

B brain, cochlea, maculae

C muscles, semicircular canals

D brain, cochlea, semicircular canals

1.5 The shape of the lens in the human eye may be altered by contraction or relaxation of the…

A optic nerve

B muscles of the ciliary body

C muscles of the iris

D pupil

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Question 2

Study the diagram that shows the anterior and longitudinal section of the human eye and answer the questions that follow.

2.1 Identify parts numbered 1 – 3 (3)

2.2 Give the function of structure numbered 3. (1)

2.3 Name the condition responsible for:

a) the size of structure 1 in the diagram

b) he shape of the lens in the diagram (2)

A person in a darkened room is asked to cover one eye. A dim electric bulb, positioned at varying distances from the person (not all measurements were at different distances), is switched on at one-minute intervals for a period of 10 seconds. During this period the diameter of the pupil of the eye is measured. The results obtained are shown in the table below.

Time intervals in

minutes

1 2 3 4 5 6 7 8 9

Diameter of pupil in

mm

2 4 5 5 8 7 3 1 6

2.4 Provide an appropriate title for this table. (2)

2.5 Which structure in the eye controls the size of the pupil? (1)

2.6 Plot a line graph of the results in the table above. (8)

2.7 Between which two measurements did the following change in the diameter of the pupil occur: largest decrease? (1)

2.8 Why did the diameter of the pupil remain the same during the third and the fourth time intervals? (2)

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Question 3

Study Diagrams I and II that illustrate the lens and parts of one layer of the human eye, as well as the graph below, and answer the questions that follow.

3.1 Identify parts A and B. (2)

3.2 Which Diagram (I or II) shows part of the eye…

(i) where the ciliary muscles are contracted (1)

(ii) under dim light conditions (1)

3.3 Explain your answer in QUESTION.3. 2 (i). (2)

3.4 Which letter on the graph indicates each of the following:

(i) The eye looking at a nearby stationary object (1)

(ii) The eye looking at an object moving towards the viewer (1)

3.5 Explain the significance of the elastic nature of the lens. (3)

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Question 4

The diagram below illustrates the structure of the human ear.

4.1 Label structures:

B - ____________________________ C - _____________________________ (2)

4.2 Make an X on the diagram to indicate the region where fluid pressure waves cause tiny cilia to bend for the sensation of hearing. (1)

4.3 What type of secretion is formed in part A and why? (2)

4.4 Explain the function of part F in the process of hearing. (2)

4.5 Identify passage D and state its function. (3)

4.6 Of what substance is the part labelled G made? (1)

Question 5

A mother is concerned about her 1 year old son. He cries constantly while holding his ear; he is unsteady on his feet, he has a blocked nose; high temperature and small yellow stains on his pillow. The mother thinks he has ottis media

5.1 What is ottis media? (2)

5.2 Give FOUR reasons why you would support the mother’s diagnosis. (4)

5.3 What can the mother do to alleviate his discomfort until he can see a doctor? (1)

5.4 Give TWO reasons why it is important to see a medical doctor if the ear infection continues. (2)

The doctor suggests a procedure called a myringotomy, a surgical procedure that can help treat the infection. The diagram below illustrates the myringotomy

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5.6 What do you think a myringotomy is?

5.7 What do you think is the purpose of the ‘tube’?

Question 6

Study the representation of a cochlear implant below and answer the questions that follow

6.1 What is a cochlea implant? (2)

6.2 List the parts comprising the external part of the cochlear implant. (2)

6.3 What is the function of the receiver? (2)

Question 7

Describe how hearing and balance occurs in the human ear. (17)

Additional Question

Question 1

Read the passage below regarding the eye and answer the questions that follow.

‘Sometimes the lens of the eye becomes clouded - a disease called a cataract. This causes blurred vision and eventually blindness. The problem usually occurs in old age and is particularly common in developing countries. In India, 5.5 million people are unable to see because they have cataracts. The cure is simple - remove the clouded lens and fit the patient with glasses. However, there are few eye hospitals in India and most patients cannot afford the treatment.

One solution to this problem has been the setting up of Eye Camps, where a tent or local building is turned into a clinic. A temporary operating theatre is set up and patients are examined and treated. Surgeons may operate on up to 200 patients in a day, removing a diseased lens in five minutes. Ten days later the bandages are removed and the patient is fitted with glasses - their sight restored.’

1.1 Explain how a cataract causes blurred vision and blindness. (2)

1.2 State two reasons why India still has a large percentage of its population suffering from cataracts. (2)

1.3 Explain why glasses are still needed by the patients after an operation to remove their cataracts. (2)

1.4 While it is possible for light rays to reach the retina following a cataract operation, one of the functions of the lens is not replaced by the removal of the diseased lens and the fitting of glasses. What important function is being referred to here and why is it necessary? (2)

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Answers

Test Yourself 1. D 2. A 3. C 4. B 5. A 6. C 7. A 8. B 9. C

Exam Questions

Question 1

1.1 D 1.2 D 1.3 B 1.4 C 1.5 B

Question 2

2.1 1 = pupil

2 = iris

3 = ciliary body (3)

2.2 contracts or relaxes to focus the image using the lens (1) 2.3 a) amount of light present (1)

b) distance of object from eye / focus (1)

2.4 Table showing the diameter of the pupil after increasing time intervals. (2) 2.5 iris (1) 2.6 (GRAPH BELOW)

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9

GRAPH SHOWING THE DIAMTER OF THE PUPIL(MM) AT INCREASING LIGHT INTERVALS

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2.7 betw 6 and 7 min (no units, no marks) (1) 2.8 Light source was the same distance from the person (2)

Question 3

3.1 A: Ciliary body/Ciliary muscle

B: Suspensory ligament

3.2 (i) Diagram 2

(ii) Diagram 1

3.3 Suspensory ligament slackens

3.4 (i) D

(ii) F

3.5 Can change its shape

to focus image onto the yellow spot

irrespective of the distance from the eye OR for near or distant vision OR

for accommodation

Question 4

4.1 B = eardrum / tympanum / tympanum

C = round window (2)

4.2 X drawn on cochlea (1)

4.3 wax traps dust and insects (2)

4.4 transmits nerve impulses to the (auditory) part of the brain (2)

Eustachian tube

4.5 equalise pressure between middle √ ear and on other side of ear drum etc (3)

4.6 bone (1)

Question 5

5.1 middle ear infection

5.2 pain, fever, unbalanced, puss, blocked nose

5.3 pain relief –tablets, drops etc

5.4 He might need antibiotics to to help clear up the infection, as the infection could lead to hearing loss

5.5 A procedure that involves a small cut ion the tympanic membrane to drain the fluid out of the middle ear is inserted in the hole

5.6 Unblocks the Eustachian tube by allowing air in and allows the fluid to seep out to help with the infection

Question 6

6.1 It is an electrical device that is surgically implanted under the skin to help people with severe or total deafness

6.2 Microphone and speech transmitter and

6.3 Picks up sound from the environment and processes it to electrical signals

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Question 7

Hearing

Sound waves are directed into the auditory canal by the pinna

The sound waves make the tympanic membrane vibrate and the vibrations are passed on to the ossicles in the middle ear

The ossicles make the oval window vibrate and this causes pressure waves to be set up in the inner ear

These vibrations also cause the organ of Corti to be stimulated and it generates impulses which are sent to the cerebrum along the auditory nerve

The cerebrum interprets the impulses as sound

Balance

Sudden changes in speed and direction causes the endolymph within the semicircular canals to move.

The movement of the fluid stimulates the cristae in the ampullae – situated at the base of the semi circular canal

When the direction of the head changes, gravitational pull stimulates maculae – in the sacculus and utriculus.

Within the cristae and maculae the stimuli are converted to impulses

These impulses are sent to the brain by the vestibular branch of the auditory nerve.

(any 17)

Additional Question

Question 1

1.1 too little/no light passes through the lens

Thus less or no light to form an image on the retina etc (2)

1.2 poverty thus cannot afford operation

Too few clinics where they live (2)

1.3 to take over the role of the removed lens by bending the light rays (refraction)

To form an image on the retina (2)

1.4 the ability of the lens too change shape / become more convex or rounded (accommodation) To allow the person to see close up (2)

Documents

X-Sheet 11 The Eye and Ear