1

RECEPTOR

THE HUMAN EYE

2

INSTRUCTIONAL OBJECTIVE

state the function of the different parts of the eye

describe how a focused image of near and distant object is produced on the retina

describe the pupil reflex in response to bright and dim light

3

LOCATION

In sockets (orbits) of skull moved by six muscles

optic nerve connects it to brain

4

PROTECTION

tear gland produces tears. Tears contain lysozyme which kills bacteria.

blinking protects eye from injury

5

6

Sclerotic layer thick wall (the 'white' of the eye) protective layer around eyeball Fn: Provides protection against mechanical

injury

Cornea ‑ transparent allows light

to enter Fn: bends (refracts) light

into pupil towards lens

7

Choroid layer Fn: pigmented black, absorbs light so as to

prevent reflection within the eye contains network of blood capillaries which

brings nutrients to the eye and also removes metabolic wastes.

8

Ciliary body Fn: contains ciliary muscles which alters the

shape of lens ‑ muscles attached to lens by suspensory

ligaments

Iris consists of 2 sets of muscles:

radial muscle and circular muscle-these two muscles work in opposite ways

Fn: Controls the amount of light falling on the retina.

9

How muscles of iris work?

When muscles contract, they become shorter.

Therefore, when radial muscles contract, they pull together and become shorter.

Therefore the size of the pupil widens or dilates.

When circular muscles contract, the circumference becomes smaller. This causes the size of pupil to be smaller or constrict.

10

Eye in Dim Light

Radial muscles contract, become short, pupil enlarges or dilates.

Circular muscles relax, becomes longer, circumference becomes bigger, pupil enlarges or dilates.

Did you notice the muscles work in opposite ways, ie. radial contract, circular relax.

11

Eye in Bright Light

Radial muscles relax, become longer, pupil enlarges or dilates.

Circular muscles contract, becomes short, circumference becomes smaller, pupil constricts.

Did you notice the muscles work in opposite ways, ie. radial relax, circular contract.

12

Animation on size of pupil

Notice the size of the pupil before and after shining the pen-light.

This formula will help you:IN BRIGHT LIGHT = 3CCircular muscles Contract, pupil Constricts

http://hsc.unm.edu/touch/datasets/datasets/animations/sluggish.shtml

13

Lens ‑ transparent, elastic and biconvex Attached to suspensory ligament The stretching of the suspensory ligaments causes it

to change shape. When looking at far objects, the lens becomes thin or

less convex. When looking at near objects, the lens becomes fat,

more convex. Fn: bends light rays and brings them to a focus on

retina

14

Retina

light sensitive layer, contains photoreceptors (light receptors)

Fn: when stimulated by light from the image, convert the light energy into electrical impulse.

photoreceptor cells connected to optic nerve which carries sensory impulses to brain

15

Yellow Spot ‑ central part of retina ‑ Images normally focused on this spot - high concentration of photoreceptor cells ----

precise visionBlind Spot

‑ part of retina where optic nerve is attached ‑ no photoreceptor cells present here ‑ images falling here is not registered

16

HOW DOES THE EYE WORK

Light falling on an object is reflected. Some of these light rays falls on the eye The light rays are refracted through the

cornea and aqueous humour onto the lens The lens bends the rays and cause it to focus

on the retina. Photoreceptors in retina stimulated. Impulse produced and transmitted via optic

nerve to brain

17

ACCOMMODATION adjustment of the lens so that clear images of

objects at different distances are focused on the retina

18

Near Objects

ciliary muscles contract

tension on suspensory ligament slacken

lens more convex ie.short and fat

light rays bent (refracted) more

Image on retina

Near object focused on retina

19

Can you write down the steps when looking at far objects?

http://www.bioplek.org/animaties/oog/accomodatie.html

20

PUPIL REFLEX

immediate response of the eye in controlling the size of the pupil due to changing lighting conditions.

http://www.bioplek.org/animaties/zenuwstelsel/pupilreflex.html

21

HOW THE PUPIL REFLEX WORKS Light enters pupil and falls on retina Photoreceptors stimulated Impulse produced Impulse carrying information about lighting condition (dim or

bright) is transmitted along sensory neurone in optic nerve Impulse reaches brain Transferred to relay neurone and then Motor neurone Motor impulse carried from brain to muscles of iris If dim light, circular muscles relax and radial muscles contract --

pupil dilates If bright light, circular muscles contract and radial muscles relax

pupil constrict

22

The value of having TWO eyes

Two eyes view the same object from two different positions.

This provides vision in 3-Dimensions, the ability to judge distance.

Survival value

23

THE END