• WALT
• That the retina contains millions of light sensitive cells
• That there are two types of light sensitive cell
• How an action potential is generated

• The retina is a layer at the back of the eye which is sensitive to light
• It contains two types of light sensitive cells
• The rods
• The cones
• They connect to bipolar neurone which connect to ganglion cells.
• The fibres of the ganglion cells form the optic nerve

• Microscopic structure of the retina

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• Structure of a single rod cell
False colour scanning EM of cone and rod cells

• Each rod possesses up to a thousand vesicles in its outer segment containing rhodopsin.
• Rhodopsin is made up of the protein opsin and retinal (a derivative of vitamin A).
• Retinal normally exists in its cis isomer form, but light causes it be become converted to its trans isomer form.
• This change initiates reactions which lead to the splitting of rhodopsin into opsin and retinal bleaching .

• The free opsin acts as an enzyme which sets of a series of reactions that leads to the hyperpolarisiation of the rod cell membrane
• It becomes MORE NEGATIVE
• This generates an action potential is the connecting neurone cells

• Rhodopsin is very sensitive to light
• In bright light it is broken down faster than it can be resynthesised
• It is “bleached” for most of the time
• We are light adapted
• If we go into a dark room we can’t see much until the rhodopsin is resynthesised and we become dark adapted.

• Several rod cells connect with a single bipolar neurone. This increases the ability of the brain to detect a small amount of light
• This is called convergence and it allow for weak stimuli to be amplified giving rods a greater sensitivity

• Rods out number cones
• There are about 120 million rod cells
• There are about 6 million cone cells

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• Each cone is connected to its own bipolar cell
• This means that the eye is able to distinguish between two or more separate stimuli
• This is called visual acuity – the amount of detail we see

• Vision in bright or moderate light is brought about by the functioning of the cone cells.
• Cone cells provide vision in colour as well as black and white
• They enable you to see in fine detail – visual acuity

• Cone cells contain a pigment called iodopsin and retinal
• There are three types of iodopsin but only one type is found in each cone cell
– 10% red cones
– 45% blue cones
– 45% green cones
• Each type of iodopsin absorbs most strongly in a particular part of the visual spectrum

Colour Vision -
Colour vision is due to the presence of 3 kinds of cone cells detecting 3 kinds of primary colours ( trichromatic theory ): red, blue and green

Red sensitive cones
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Green sensitive cones
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Blue Sensitive cones
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Colour perceived by brain
White
Red
Orange
Yellow
Green
Blue
Magenta black

• Very few people cannot distinguish colours at all
• Most colour blind people have abnormal colour vision
• Some males have inherited their in ablilty to distinguish between reds and greens
• The genes for red and green iodopsin are found on the X chromosome

Absorption spectra for three kinds of cones in the retina

• Test for colour blindness

• A similar process occurs in cone cells except that the pigment is iodopsin .
• It is less sensitive to light and so a greater intensity is required to cause its breakdown and so initiate a nerve impulse.

• Cone cells are found throughout the retina
• They are far more concentrated at a point called the fovea
• As each cone cell synapses with one bipolar cell this allow us to see in detail
• The fovea is the area of greatest visual acuity

• There is a point on the retina where there are no rods or cones
• This is the blind spot- it is where the optic nerve leaves the retina

Difference between rods and cones
Rods Cones

Difference between rods and cones
Rods Cones
Outer segment is rod-shaped Outer segment is coneshaped

Rods Cones
Outer segment is rod-shaped Outer segment is cone-shaped
More numerous than cones Few in number than rods
Distributed more or less even over the retina
More concentrated in & around yellow spot
None found at the yellow spot Most numerous at yellow spot
Give poor visual acuity because many rods share a single neuron to brain
Give good visual acuity because each cone has its own neurone connected to the brain
Sensitive to low light intensity; therefore for night vision
Sensitive to high light intensity, therefore for day vision
Not sensitive to colour vision Sensitive for colour vision
Contain rhodopsin in one form Contain iodopsin in 3 forms