The Retina

• 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

• 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

•

• 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.

Rhodopsin

Action Potential Generation

• 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

Light and Dark Adaptation

• 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.

Convergence

• 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 and Cones

• Rods out number cones

• There are about 120 million rod cells

• There are about 6 million cone cells

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Distribution of Rods and Cones

Cones

• 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

Cones

• 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

Cones• 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

The Trichromatic TheoryRed sensitive

conesGreen sensitive

conesBlue Sensitive

conesColour

perceived by brain

√ √ √ White

√ Red

√ √ Orange

√ √ Yellow

√ Green

√ Blue

√ √ Magenta

black

The Trichromatic Theory

• 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

Generation of action potential in Cone Cells

• 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.

Acuity

• 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

The Blind Spot

• 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 cone-

shaped

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