RECEPTOR THE HUMAN EYE

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

LOCATION In sockets (orbits) of skull moved by six muscles optic nerve connects it to brain

PROTECTION tear gland produces tears. Tears contain lysozyme which kills bacteria. blinking protects eye from injury

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

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.

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.

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.

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.

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.

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 = 3C Circular muscles Contract, pupil Constricts http://hsc.unm.edu/touch/datasets/datasets/animations/sluggish.shtml

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

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

Yellow Spot ‑ central part of retina ‑ Images normally focused on this spot - high concentration of photoreceptor cells ---- precise vision Blind Spot ‑ part of retina where optic nerve is attached ‑ no photoreceptor cells present here ‑ images falling here is not registered

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

ACCOMMODATION adjustment of the lens so that clear images of objects at different distances are focused on the retina

ciliary muscles contract tension on suspensory ligament slacken 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

Can you write down the steps when looking at far objects? http://www.bioplek.org/animaties/oog/accomodatie.html

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

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

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

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