Receptors

Module 2: Life Processes

Life Sciences
Matric Syllabus
Mind Action Series: Life Sciences
Textbook and Workbook

• 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) Receptors

Part Structure Function
Sclera • 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

Choroid • 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 during
body 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

Pigmented • A pigmented layer of cuboidal cells • Pigment absorbs excessive rays of
layer 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 colours
receptors) • 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 retina
lutea) • Contains MANY CONES.
2.2b) Receptors

Lens • 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 provides
cavity 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 provides
cavity fluid, vitreous humour. pressure within the eye, holding
the retina firmly against the
chorid.
• Prevents eyeball from collapsing.
2.2b) 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) 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.

2.2b) Receptors

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

Receptors

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Receptors