Life SciencesMatric Syllabus

Mind Action Series: Life Sciences Textbook and Workbook

Module 2: Life Processes

2.2b) Receptors

• 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

The eye fits into a bony socket, situated anteriorly in the cranium. It is held in position by six extrinsic muscles.

2.2b) Receptors2.2b) Receptors

2.2b) Receptors2.2b) Receptors

Part Structure Function

Sclera • a tough, white, inelastic layer made of connective tissue

• covers the posterior 5/6 of the eye

• protects the internal parts 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, occupying the front 1/6 of the eye.

• It is more convex than the rest of the eyeball.

• Transparent - contains no blood vessels.

• It is covered by the conjunctiva (thin membrane)

• Allows light rays to pass through to the light-sensitive cells.

• Causes converging refraction of these light rays

• The conjunctiva has many pain receptors which enable the eyelids to close reflexively when foreign matter touches the surface of the eye.

2.2b) Receptors2.2b) Receptors

Part Structure Function

Choroid • a thin, dark layer containing pigments and blood vessels

• Pigment absorbs excessive rays of light preventing their reflection within the eye, which would cause blurring.

• Blood vessels supply nutrients and oxygen to the cells of the retina.

Cillary body

• It is an extension of the choroid. Contains circular cillary muscles.

• Cillary muscles control lens shape during accommodation.

Iris • Continuation of the choroid. It forms a circular coloured curtain with a hole, the pupil, in the centre.

• Contains two sets of involuntary muscles – circular and radial. They work antagonistically: as one contracts, the other relaxes.

• Controls the amount of light entering the eye by pupillary mechanism, a reflex action.

• In bright light, the circular iris muscle contracts, the radial muscles relax and the size of the pupil decreases to let less light in.

• In dim light, the radial muscles contract, the circular muscles relax and the size of the pupil increases to let more light in.

2.2b) Receptors2.2b) Receptors

Part Structure Function

Pigmented layer

• A pigmented layer of cuboidal cells bordering on the choroid.

• Pigment absorbs excessive rays of light.

Rod cells (photo-receptors)

• Long, thin cells• Found on edge of retina

• Respond to dim light and provide night vision (black and white).

• Provide peripheral vision.

Cone cells (photo-receptors)

• Fatter cells• Less cone cells than rod cells• Concentrated in yellow spot • Red, blue and green cones respond

to different wavelengths of colour.

• Respond to high-intensity light (bright light) and enable colours to be seen.

• Provide sharp vision.

Blind spot • No rods or cones where the optic nerve leaves the eyeball.

• Blood vessels enter and leave the eye here.

• Not sensitive to light stimuli

Yellow spot (macula lutea)

• Tiny spot near the centre of the retina

• Contains MANY CONES.

• Area of sharpest vision.

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Part Structure Function

Lens • a round, biconvex, flexible, transparent structure, enclosed in an elastic capsule

• Held in place behind the iris by the suspensory ligament, attached to the ciliary body

• Has no blood vessels.

• Can change shape to allow sharp, precise focussing of light rays on the retina

Anterior cavity

• Small cavity, filled with a watery fluid, aqueous humour.

• Aqueous humour provides nutrients and oxygen to the lens and the cornea, and carries away waste.

Posterior cavity

• Larger cavity, filled with a jelly-like fluid, vitreous humour.

• Vitreous humour provides pressure within the eye, holding the retina firmly against the chorid.

• Prevents eyeball from collapsing.2.2b) Receptors2.2b) Receptors

There are three stages involved in seeing:•Pathway of light rays and image formation•Stimulation of photoreceptors•Pathway of nerve impulses

Pathway 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) Receptors2.2b) Receptors

• 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.

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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.

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