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Vision

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In order for vision to
occur, light rays must form an image on the retina that stimulates the
photoreceptors (rods and cones) to send impulses along the optic nerve (2nd
cranial nerve) to the visual cortex of the brain, where the image is perceived
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10
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Night vision
In a dim light the pupils open wide, allowing light to reach the rods at the sides of
the retina
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Consequently, when a person moves from a bright light to a dim light, nothing
can be seen at first, but as rhodopsin gradually re-forms it becomes possible to
see again – not in colour, but in various shades of grey (k Box 10
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Dazzled by bright light
Box 10
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Vitamin A is one of the required
precursors for the formation of
rhodopsin, and is found in many
foods from both plants and
animals
...
It
takes time for the pupils to get smaller, the rhodopsin to bleach, and the eyes
to become adapted to bright light and colour
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The condition is often inherited
and people who are affected lack one or more of the three different kinds of
cone found in the retina
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Colourblindness can sometimes make it difficult for children to read and can
affect people’s career choices, so early assessment can be important for a
child’s self-esteem
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3
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10
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In order for the light rays to form a clear image on the retina, they
undergo four processes
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10
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1
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10
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2
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10
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The pupil
contains a sphincter (ring of muscle tissue) around the margin that contracts
in bright light, causing the pupil to become smaller
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16)
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16 Mydriasis is dilation of
the pupils, a common cause being
drugs of abuse, e
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amphetamines,
cocaine, LSD and mescaline
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g
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BRIGHT LIGHT – pupillary circular muscles contract DIM LIGHT – radial muscles contract
sclera iris pupil

pupillary circular muscles

radial muscles

Fig
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7 Pupil light reflex
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Accommodation (focusing the lens)
Accommodation is the ability of the eye to change its focus from distant to
near objects (and vice versa) by action of the ciliary muscles (k Fig
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6)
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A convex (outwardcurving) lens refracts (bends) light rays more and enables nearer objects to be
brought more precisely into focus on the retina (k Box 10
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4
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The nearer the object, the greater the degree of convergence; when
looking into the distance, the eyes diverge until parallel
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Convergence is one of several cues that
help us to discern depth (k Box 10
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Box 10
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9
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The
afferent pathway travels from the
retinal cells and optic nerve to the
visual cortex
...
The efferent pathway of
the oculomotor nerve (k Fig
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14)
arises in the midbrain and supplies
parasympathetic (k Fig
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20) fibres
to the eye, constricting the pupil and
altering the shape of the lens
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18 Extreme convergence
(cross-eyed viewing) occurs when
focusing on the nose
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3
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When the images are perceived,
the small differences between them produce a three-dimensional effect
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10
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Binocular vision enables people to see where objects are in relation
to their own bodies and is particularly important for skills like pouring fluids
into containers, throwing and catching, driving a car, crossing a road or
reaching out to touch objects
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This is
because the brain is unable to work out depth and speed of movement
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10
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Step 1: Light rays from this view stimulate the eyes to send
impulses to the brain
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Part of the information about the image comes from the left
visual field and part from the right visual field
...
Step 3: The two images are corrected in the brain to give a single image
with a 3D effect which is the correct way up
...
It is an X-shaped structure formed
by the two optic nerves as they partly cross-over on the pathway from the eyeballs to
the visual cortex in the brain (k Fig
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8)
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This allows visual images from both eyes to reach the visual
cortex where they are combined to form a single image
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19)
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3
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Myopia
Myopia (short-sightedness) occurs when the light rays entering the eye from distant
objects are focused in front of the retina rather than on it and cannot be corrected by
accommodation
...
This makes distant objects appear blurred but
those close to the eyes can be clearly seen
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20)
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19 Neurological test to light
When a torch is shone directly into one
eye for three seconds, both pupils
should constrict briskly to protect
the retina from damage to the high
intensity of light
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Dilation of the pupil on one side only is
also abnormal and may be a sign
of brain injury or other condition that
raises pressure within the cranium
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Light rays entering the eye from objects nearby are focused behind the retina rather
than on it and appear blurred, whereas distant objects can be seen clearly
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21)
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20 Myopia is often inherited
but regular eye checks can ensure
that corrective lenses are properly
fitted
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Astigmatism
A normal cornea is round, and evenly curved from side to side and top to bottom
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10
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4 Visual impairment
Visual impairment is sight loss that cannot be fully corrected by spectacles or contact
lenses
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Most visually-impaired people have
low vision, which has many causes including those mentioned below
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21 Presbyopia develops
because of gradual changes in the
structure of the lens
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People with
presbyopia will tend to squint or hold
reading materials at arm’s length
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22 Although cataracts can
develop at any age, senile cataract
is the most common cause of
blindness in the world
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10), and
ultimately transparency is lost
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10
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Surgical removal of the lens restores some sight, and useful
vision is obtained by replacing the cataract with an artificial lens
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22)
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opaque lens
pupil
iris
sclera

Fig
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9 Image of a cataract in the lens
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13)
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The two types of AMD are:
• dry (atrophic) AMD results from reduced blood supply to the macula and is
the most common and least serious cause of blindness in
the elderly
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10
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One treatment of this condition is
injection of growth factor inhibitors into the eyeball
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Without
prompt treatment, it will lead to blindness in the affected eye
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10
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Glaucoma
Glaucoma is too much fluid within the eyeball usually because aqueous
humour is unable to drain away, causing pressure within the eye, pain and
blurred vision (k Fig
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3)
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Blindness results if left untreated
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This roughening can lead to pain in the eyes, breakdown of the cornea of the
eyes, and eventual blindness
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23)
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It
originates in the part of the brain where eye movement is coordinated by the
extraocular muscles
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This includes exercises for the eye
muscles, wearing special glasses or surgery
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23 Trachoma is responsible
for the blindness or visual
impairment of about 1
...
Its
elimination is an objective of the
World Health Organization (WHO)

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