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

cornea Lower eyelid

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

• A circular sheet of muscles

• Controls the amount of light entering the eye

• 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

Attaches lens to ciliary body

suspensory ligament

Controls amount of light enteringthe eye

suspensory ligament

Refracts light rays;keeps eyeball firm

aqueoushumour

suspensory ligament

Allows light to enter the eye

aqueoushumour

suspensory ligament

Refracts light rays into pupil

cornea

aqueoushumour

suspensory ligament

Focuses light rays onto retina

cornea

aqueoushumour

suspensory ligament

ciliary body

Contains ciliary muscles which control curvature or thickness of the lens

cornea

aqueoushumour

suspensory ligament

viteroushumour

Refracts light rays;keeps eyeball firm

ciliary body

cornea

aqueoushumour

suspensory ligament

Controls movement of eyeball

viteroushumour

ciliary body

cornea

aqueoushumour

suspensory ligament

rectusmuscles

Protects against mechanical injury

viteroushumour

ciliary body

cornea

aqueoushumour

suspensory ligament

sclera

rectusmuscles

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

viteroushumour

ciliary body

cornea

aqueoushumour

suspensory ligament

choroid

sclera

rectusmuscles

Region where images are sharply focused

viteroushumour

ciliary body

cornea

aqueoushumour

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

cornea

aqueoushumour

suspensory ligament

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

blind spot

viteroushumour

ciliary body

cornea

aqueoushumour

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

cornea

aqueoushumour

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

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

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

Radial muscles of the iris relax2

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

• 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

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

How Do We See?

objectcornea

aqueoushumour

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

How Do We See?

objectcornea

aqueoushumour

The lens causes further refraction

How Do We See?

objectcornea

aqueoushumour

retina

Rays converge to focus on retina

How Do We See?

objectcornea

aqueoushumour

retina

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

How Do We See?

objectcornea

aqueoushumour

retina

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

How Eyes Focus on Distant Objects

Ciliary muscles relax

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

Suspensory ligaments become taut

Lens becomes thinner

Focal length increasesVertical section of the eye Enlarged portion of eye

(front view)

Light rays from distant object focused on retina

Photoreceptors stimulated

Nerve impulses produced by optic nerves

Transmission of nerves to brain

Brain interprets impulses

Person sees the distant object

• 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

How Eyes Focus on Near Objects

Ciliary muscles contract

Suspensory ligaments become slacken

Lens becomes thicker

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Focal length decreases

April 11, 2023

Light rays from near object focused on retina

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Person sees the near object

• 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

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

Chapter 14 Animal Receptor Organs

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