Chapter 14 Animal Receptor Organs

Chapter 14Chapter 14

The Human Eye

Objectives• Students should be able to:

– State the relationship between receptors, the central nervous system and the effectors

– Describe the gross structure of the eye as seen in front view and in horizontal section

– State the principal functions of component parts of the eye in producing a focussed image of near and distant objects on the retina

– Describe the pupil reflex in response to bright and dim light

Sense organs

• Sense organs are receptors that receive stimuli from the environment and inform your body of any changes in the environment

14.2 The Human Eye

Upper eyelid

pupil

cornea Lower eyelid

iris

Conjunctiva (covers sclera)

Tear gland

eyelashes

Cornea• Transparent layer continuous with sclera

(white portion of the eye)• Specialised form of conjunctiva• Refracts or bend light rays into the eye

Conjunctiva• Transparent membrane• Secretes mucus, thus helping to keep

the front of the eyeball moist

Iris

• A circular sheet of muscles

• Controls the amount of light entering the eye

Pupil

• A hole in the centre of the iris

• Allows light to enter the eye

Eyelids• Protect the cornea from mechanical damage

Eyelashes• Help to shield the eye from dust particles

Tear gland• Secretes tears to wash away dust particles• Secretes tears to keep the cornea moist for

atmospheric oxygen to dissolve• Secretes tears to lubricate conjunctiva,

helping to reduce friction when the eyelids move

Chapter 14 The Human EyeStructure and Function of the Eye (Concept Map)April 11, 2023

Vertical Section of The Human Eye

April 11, 2023 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 9

Attaches lens to ciliary body


suspensory ligament


Controls amount of light enteringthe eye


iris

suspensory ligament


Refracts light rays;keeps eyeball firm


aqueoushumour

iris

suspensory ligament


Allows light to enter the eye


pupil

aqueoushumour

iris

suspensory ligament


Refracts light rays into pupil


cornea

pupil

aqueoushumour

iris

suspensory ligament


lens

Focuses light rays onto retina


cornea

pupil

aqueoushumour

iris

suspensory ligament


ciliary body

Contains ciliary muscles which control curvature or thickness of the lens


lens

cornea

pupil

aqueoushumour

iris

suspensory ligament


viteroushumour

Refracts light rays;keeps eyeball firm


ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament


Controls movement of eyeball


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

rectusmuscles


Protects against mechanical injury


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

sclera

rectusmuscles


Pigmented black toprevent internal reflection of light; blood vessels bring oxygen and nutrients to the eyeball


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

choroid

sclera

rectusmuscles


Region where images are sharply focused


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

fovea(yellow spot)

choroid

sclera

rectusmuscles


Transmit nerve impulsesfrom eye to brain

optic nerve


fovea(yellow spot)

choroid

sclera

rectusmuscles

viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament


Region over optic nerve; no photoreceptors, so insensitive to light

blind spot


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

optic nerve

fovea(yellow spot)

choroid

sclera

rectusmuscles


Light-sensitive layer containingcones responsible for colour vision in bright light and rods responsible for vision in dim light

retina


viteroushumour

ciliary body

lens

cornea

pupil

aqueoushumour

iris

suspensory ligament

blind spot

optic nerve

fovea(yellow spot)

choroid

sclera

rectusmuscles


Controlling the entry of light into the eye

• The pupil of the eye controls the amount of light that enters the eye

• Size of the eye is controlled by 2 sets of involuntary muscles in the iris– One set arranged radially– One set arranged in a circle around the

pupil

Chapter 14 The Human EyeAction of Iris in Bright LightApril 11, 2023


Action of Iris in Bright Light

Circular muscles of the iris contract1


Radial muscles of the iris relax


2



Pupil constricts and this reduces the amount of light entering the eye


Radial muscles of the iris relax2


3

Chapter 14 The Human EyeAction of Iris in Dim LightApril 11, 2023


Radial muscles of iris will contract

Action of Iris in Dim Light

Radial muscles of the iris contract1




Circular muscles of the iris relax2

Pupil dilates and this increases the amount of light entering the eye




Circular muscles of the iris relax2

3

• High light intensity, Circular muscles contract, radial muscles relax

• Pupil becomes smaller

• Low light intensity, Circular muscles relax, radial muscles contract

• Pupil becomes bigger

What is the pupil reflex?

• It is a reflex action (recall ‘Nervous System)

• Pupil changes in size as a result of changes in light intensity

• Pupil usually becomes larger when the surrounding light intensity is low, and smaller when the light intensity is high

Stimulus (change in light intensity)

Receptor (retina)

Sensory neurone in optic nerve

Brain

Motor neurone

Effector (iris)

14.3 How do we see?

• Light falling on object is reflected. Reflected rays fall on the eye.

• These rays are refracted through the cornea and the aqueous humour (watery fluid) onto the lens.

• The lens causes further refraction and the rays converge to a focus on the retina

Continue….• The image on the retina stimulates

either the rods or the cones depending on the intensity of the light.

• Image formed can be:– Upside down (inverted)– Back to front (reversed)– Smaller in size than the object being

observed (diminished)

Continue….• Impulses produced by light falling on the

rods and cones are transmitted via the optic nerve to the rear of the cerebrum.

• The brain interprets these impulses

Chapter 14 The Human EyeHow Do We See?April 11, 2023

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1

How Do We See?

objectcornea

lens

aqueoushumour

Light rays are refracted through the cornea and the aqueous humour onto the lens


1

2

How Do We See?

objectcornea

lens

aqueoushumour

The lens causes further refraction



1

2

3

How Do We See?

objectcornea

lens

aqueoushumour

retina

Rays converge to focus on retina




1

2

3

4

How Do We See?

objectcornea

lens

aqueoushumour

retina

image

The image on the retina stimulates either the rods or the cones, depending on the intensity of the light





1

2

3

4

How Do We See?

objectcornea

lens

aqueoushumour

retina

image

The image formed is • inverted• laterally inverted• diminished in size

The image on the retina stimulates either the rods or the cones, depending on the intensity of the light




Focusing (Accommodation)• This is the adjustment of the lens of the

eye so that clear images of objects at different distances are formed on the retina

Chapter 14 The Human EyeHow Eyes Focus on Distant ObjectsApril 11, 2023


Parallel light rays from distant object enters eyes

Vertical section of the eye

1

How Eyes Focus on Distant Objects


Ciliary muscles relax

Vertical section of the eye Enlarged portion of eye(front view)

1

2 2



Suspensory ligaments become taut


1

2 2

3 3



Lens becomes thinner


1

2 2

3 34

4



Focal length increasesVertical section of the eye Enlarged portion of eye

(front view)

1

2 2

3 34

5

4




Light rays from distant object focused on retina

1

2 2

3 34

5

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Photoreceptors stimulated

7

1

2 2

3 34

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Nerve impulses produced by optic nerves

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2 2

3 34

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Transmission of nerves to brain


789

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2 2

3 34

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Brain interprets impulses


789

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2 2

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Person sees the distant object


789

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3 34

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• Far object, circular muscles of the ciliary body relax• Suspensory ligaments are pulled tight, causing the lens to become

flatter and less convex• Image sharply focused onto retina

Chapter 14 The Human EyeHow Eyes Focus on Near Objects

April 11, 2023


Vertical section of the eye

Diverging light rays from near object enters eyes

1

How Eyes Focus on Near Objects



Ciliary muscles contract

22

1




Suspensory ligaments become slacken

3

22

3

1


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Lens becomes thicker

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3

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Focal length decreases

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22

3

4

5

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Light rays from near object focused on retina

3

22

3

4

5

61 4





3

22

3

4

5

61

7

4


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3

4

5

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3

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Person sees the near object

3

22

3

46

1

5

78

9

10

11


4

• Near object, circular muscles of the ciliary body contracts• Suspensory ligaments releases the pull on the lens• Elastic nature of the lens causes it to become thicker and more convex• Image focused sharply on the retina• After a certain distant called near point, the lens cannot adjust anymore.

The image formed will then be blur



Pupil Reflex

A reflex action in which the pupil changes size as a result of changes in light intensity.



Pupil Reflex

In bright light


In bright light• Circular muscles in iris contract.• Radial muscles relax.• Pupil becomes smaller, hence less light enters the eye


Pupil Reflex



Pupil Reflex


In dim light


In dim light• Radial muscles in iris contract .• Circular muscles in iris relax.• Pupil enlarges, hence more light enters the eye


Pupil Reflex




Pupil Reflex Focusing


The adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina.






Distant object (7 metres or more)







• Ciliary muscles relax• Suspensory ligament become taut• Lens becomes thinner and less convex








Near objectIn dim light• Radial muscles in iris contract .• Circular muscles in iris relax.• Pupil enlarges, hence more light enters the eye




Near object• Ciliary muscles contract• Suspensory ligaments slacken• Lens becomes thicker and more convex





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Chapter 14 Animal Receptor Organs