Co-ordinationPart 2

Sense Organs• Parts of the body that detect stimuli (receptors)

Definition: groups of receptor cells responding to specific stimuli: light, sound, touch, temp. and chemicals.

• Eye – vision• Ear – hearing and balance• Nose – smell• Tongue – taste• Skin – touch, temperature and pain

Eye• Eye is a sense organ.• Rod and cone cells in the retina are receptors. Sensitive to light.

• Protected by a bony socket in the skull (orbit). Completely surrounded by bone except for the front.

• Conjunctiva – a thin, transparent membrane, to protectTear glands keep this membrane moist and contain an enzyme lysozyme to kill bacteria.Blinking spreads the fluid across the eye

Eye (cont.)• Eyelids, eyebrows and eyelashes help stop dirt/dust landing on the

eye.

• Sclera protects inside part of the eye.

• Retina - (back of the eye)Receptors

receive lightSend electrical

impulses to brain

Along optic nerve

Brain creates image

Retina (cont.)• Fovea – tightly packed with receptor cells which focus an image when

we look straight at an object

• Blind spot – part of retina where the optic nerve leaves (NO RECEPTORS), light falling on this area won't send impulses to the brain (we can't see)

• Chloroid – absorbs the light passed through the retina

Rod and Cone receptors• Two kinds of receptors in the retina• Rod sensitive to dim light (not colour)• Cone distinguish between colours (bright light)3 types of Cone for red, blue and yellow.

Fovea contains almost mainly cone – meaning we can focus an image

Further out in retina rods, show less detailed images. In darkness we can see in black and white in less detail.

Rods ConesNumber in the retina 120 million 7 millionWhere concentrated Outer edges of the retina The foveaWork best in Dim light Bright light

Iris• Iris is circular piece of tissue in front of the lens. Coloured part• The gap is the pupil• Adjusts to allow more/less light through to the retina• Strong light = smaller pupil• Why?• To stop too much light damaging the retina

How does iris adjust size?• Iris contains muscles • Circular muscles around the pupil contract – makes pupil smaller (constrict)• Radial muscles around the edge contract – makes pupil larger (dilate)

• Iris reflex• Reflex action – we don't need to think about this action. Our brains do it

automatically

• What are the advantages of this reflex action?

Cornea - Refracts light - bends it as it enters the eyeIris - Controls how much light enters the pupilLens - Focuses light onto the retinaRetina - Contains the light receptorsOptic nerve - Carries impulses between the eye and the brain

Focusing light• The lens can change its shape to focus near and distant objects. This is called

accommodation.

• Accommodation – the change of the shape of the lens, in order to focus on objects at different distances.

• Position of object Ciliary muscles Suspensory ligaments Muscle tension on lens Lens shape• Near Contract Slackened Low Fat

• Distant Relax Stretched High Thin