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A Powerpoint for Grade 12 Life Sciences / Biology students focussing on receptors and how they work, specifically the eye.

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  1. 1. Module 2: Life Processes Life Sciences Matric Syllabus Mind Action Series: Life Sciences Textbook and Workbook
  2. 2. 2.2b) Receptors
  3. 3. • All living organisms are able to detect changes in their environment and to respond to them. This is important as certain changes may harm the body.• As you know, the changes in the environment are called stimuli and are registered by receptors in the body.• There are many different receptors that detect a variety of stimuli such as light, sound, touch, pressure, pain and chemicals (taste and smell).• The two receptors in this section include the eye and ear. 2.2b) Receptors
  4. 4. The eye fits into a bony socket, situated anteriorly in the cranium. Itis held in position by six extrinsic muscles. 2.2b) Receptors
  5. 5. 2.2b) Receptors
  6. 6. Part Structure FunctionSclera • a tough, white, inelastic layer made • protects the internal parts of the of connective tissue eye • covers the posterior 5/6 of the eye • provides points of attachment for the six extrinsic eye muscles • maintains the shape of the eye (as it is inelastic)Cornea • continuation of the sclera, • Allows light rays to pass through to occupying the front 1/6 of the eye. the light-sensitive cells. • It is more convex than the rest of • Causes converging refraction of the eyeball. these light rays • Transparent - contains no blood • The conjunctiva has many pain vessels. receptors which enable the eyelids • It is covered by the conjunctiva to close reflexively when foreign (thin membrane) matter touches the surface of the eye. 2.2b) Receptors
  7. 7. Part Structure FunctionChoroid • a thin, dark layer containing • Pigment absorbs excessive rays of light pigments and blood vessels preventing their reflection within the eye, which would cause blurring. • Blood vessels supply nutrients and oxygen to the cells of the retina.Cillary • It is an extension of the • Cillary muscles control lens shape duringbody choroid. Contains circular accommodation. cillary muscles.Iris • Continuation of the choroid. • Controls the amount of light entering the eye It forms a circular coloured by pupillary mechanism, a reflex action. curtain with a hole, the • In bright light, the circular iris muscle pupil, in the centre. contracts, the radial muscles relax and the • Contains two sets of size of the pupil decreases to let less light in. involuntary muscles – • In dim light, the radial muscles contract, the circular and radial. They circular muscles relax and the size of the work antagonistically: as one pupil increases to let more light in. contracts, the other relaxes. 2.2b) Receptors
  8. 8. Part Structure FunctionPigmented • A pigmented layer of cuboidal cells • Pigment absorbs excessive rays oflayer bordering on the choroid. light.Rod cells • Long, thin cells • Respond to dim light and provide(photo- • Found on edge of retina night vision (black and white).receptors) • Provide peripheral vision.Cone cells • Fatter cells • Respond to high-intensity light(photo- • Less cone cells than rod cells (bright light) and enable coloursreceptors) • Concentrated in yellow spot to be seen. • Red, blue and green cones respond • Provide sharp vision. to different wavelengths of colour.Blind spot • No rods or cones where the optic • Not sensitive to light stimuli nerve leaves the eyeball. • Blood vessels enter and leave the eye here.Yellow spot • Tiny spot near the centre of the • Area of sharpest vision.(macula retinalutea) • Contains MANY CONES. 2.2b) Receptors
  9. 9. Part Structure FunctionLens • a round, biconvex, flexible, • Can change shape to allow sharp, transparent structure, enclosed in precise focussing of light rays on an elastic capsule the retina • Held in place behind the iris by the suspensory ligament, attached to the ciliary body • Has no blood vessels.Anterior • Small cavity, filled with a watery • Aqueous humour providescavity fluid, aqueous humour. nutrients and oxygen to the lens and the cornea, and carries away waste.Posterior • Larger cavity, filled with a jelly-like • Vitreous humour providescavity fluid, vitreous humour. pressure within the eye, holding the retina firmly against the chorid. • Prevents eyeball from collapsing. 2.2b) Receptors
  10. 10. There are three stages involved in seeing:• Pathway of light rays and image formation• Stimulation of photoreceptors• Pathway of nerve impulsesPathway of light rays and image formation• As light passes from the air into the eye, it moves successfully through the cornea, aqueous humour, pupil, biconvex lens, vitreous humour, and then through the entire thickness of the neural layer of the retina to stimulate the photoreceptors. 2.2b) Receptors
  11. 11. • When light passes from one substance to another of a different density, its rays are bent (refraction). During its passage, light is bent as it enters the cornea and on bending and leaving the lens. – The cornea is responsible for most light refraction, but it’s curvature is constant so it cannot change its refractive power. – The lens, however, is elastic and can change its curvature to allow fine, sharp focusing of the image.• The light rays converge to form a focused image on the yellow spot. The image will be smaller than the object, upside down and reversed from left to right. 2.2b) Receptors
  12. 12. Stimulation of the photoreceptors• In the retina, the rods and cones are stimulated by light.• In both photoreceptors, photo pigments are broken down by light energy.• This breaking down generates an electrical impulse in the photoreceptors.Pathway of nerve impulses• The impulses from the photoreceptors travel along two layers of neurons.• The axons of the ganglia neurons form the optic nerve, which leaves the eye at the blind spot and carries impulses to the cerebral cortex.• The impulses are interpreted as vision in the occipital lobe of the cerebral cortex. 2.2b) Receptors