Eye
Structure
The most important
structures to learn
the function of
are…
• Retina
• Lens
– Ciliary Muscles
• Iris
The Iris and Pupil
Circular
muscles
contracted,
Radial
Relaxed
Circular
muscles
relaxed,
radial
contracted
• The iris controls the
amount of light entering
the eye and able to reach
the retina
• High light intensities are
able to damage the retina
(rods).
• The iris consists of two
types of muscle, Circular
(parasympathetic) and
radial (sympathetic).
Forming an image
• Most refraction happens at cornea
• Lens makes fine adjustments. This is called
accommodation
• Visual association area perceives image right way up
Accommodation
•
•
•
•
•
Far object
Parallel rays
Little refraction needed
Ciliary muscles relax
More tension in suspensory
ligaments
• Lens pulled thinner
•
•
•
•
•
Near object
Diverging rays
Much refraction needed
Ciliary muscles contract
Tension lost in suspensory
ligaments
• Lens more spherical (natural
shape)
Eye Problems: myopia (near/short
sight)
• Parallel rays refracted
too much
• Image focussed in
front of retina
• Treated with diverging
(concave) lens
• Increases focal length
Hypermetropia (far/long sight)
• Diverging rays
refracted too little
• Image focussed
behind retina
• Treated with
converging (convex)
lens
• Decreases focal
length
Retina
• 3 layers
– Ganglion cells (sensory neurone)
– Bipolar neurones
– Rods & Cones
• Fovea has more cones
• Periphery has more rods
• Optic nerve head (blind spot) has neither
Rods
&
Cones
• Contain light sensitive
chemical (pigment)
• Convert light into
electrical impulse
Rods and cones
• Like neurones but…
• Membrane is depolarized
at rest
• Gated Na+ channels are
open
• (Inhibitory)
neurotransmitter is
constantly released
• Keeps bipolar neurone
hyperpolarized
Converting light into electrical impulse
•
•
•
•
•
•
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Light changes 11-cis-retinal into all trans-retinal
Rhodopsin → opsin + all trans-retinal
Opsin closes gated Na+ channels
Membrane is hyperpolarized
Inhibitory neurotransmitter release is reduced
Bipolar neurone is depolarized
If depolarization reaches threshold an action
potential is generated in the bipolar neurone
Adaptation
In the dark human eyes become “dark adapted” in the light they
become “light adapted”
• Dark adapted
– Large amounts of
rhodopsin, so…
– low levels of light can
break some down
but..,
– High levels of light
break down large
amounts of rhodopsin
(bleaching)
– Bright light hurts
• Light adapted
– Low levels of
rhodopsin
– Bright light is able to
break down some
but…
– Dim light does not
break down enough to
generate an action
potential
Summary of differences
Rods
Pigment is rhodopsin
Sensitive to dim light
Poor visual acuity
No colour vision
Share connections to
bipolar neurone
Spread around
periphery of retina
Cones
3 types of iodopsin
(pigment B, G, R)
Only sensitive to bright
light
High visual acuity
Colour vision
Single connection to
bipolar neurone
Mostly in fovea
RetinƏl convergence
• Cones have individual
connection to bipolar
neurone
• Several rods share a
bipolar neurone
Sensitivity
• Rhodopsin is more easily broken down than
iodopsin
• Retinal convergence
– In dim light only a small amount of pigment may be
broken down in an individual rod or cone
– This may not be enough alone to trigger an action
potential in a bipolar neurone
– However, the reduction in neurotransmitter release
from several rods at once may be enough to trigger
and AP in the bipolar neurone
Visual Acuity
• When an impulse
reaches the visual
sensory area from a
ganglion cell it can either
have come from one
cone or several rods. The
brain cannot tell which
• The area of retina
represented by several
rods is larger
• The image from rods is
less resolved, more blurry
(pixelated)
The Trichromatic theory of colour
vision
• 3 types of cone
• 3 types of iodopsin
– Pigments B, G, R
– Sensitive to different
wavelengths of light
• Different wavelengths
break down different
proportions of pigment
• Brain perceives colour
as relative stimulation
from each type of cone
• E.g. a mixture of
pigment G and R
breakdown looks yellow
Parallel processing
• 3 pathways along optic
nerve
– Colour
– Shape
– Movement
• Impulse sent to visual
sensory area
• Association area
processes inputs from
sensory area in light of
other inputs from memory
and other senses
Ageing
• Cataracts
– Lens becomes
cloudy/opaque
– Lens can be surgically
removed or replaced by
artificial lens
– Corrective lenses are
needed to allow focussing
on near/far objects
• Macular degeneration
– Loss of rods and cones
– Dry
• Cells die
• Untreatable
– Wet
• Blood vessels grow
through retina and burst
• Treatable