Download PHOTORECEPTORS - rods and cones

PHOTORECEPTORS: RODS AND CONES.
The retina contains 2 types of light sensitive cell – rods and cones. They are called transducers i.e. they
convert the external energy of light stimuli into electrical impulses.
RODS
CONES
Rods provide images in black and white only.
Cones provide detail in colour.
Greater sensitivity for low light intensity (night
vision)
There are 3 types sensitive to red, green and blue
light.
Images lack acuity (detail).
They only work under high light intensity.
Rods are absent at the fovea (yellow spot) of the
retina – but are packed tightly at the edges of the
retina.
Good visual acuity (detail).
There are 120 million rods in the retina.
Cones are concentrated at the fovea (yellow spot) of
the retina – the centre of the retina. This is where
the sharpest colour vision is.
There are 6 million cones in the retina.
Colour blindness.
Colour blindness is a reduced ability to distinguish between certain colours. The most common type of colour
blindness is red-green colour vision deficiency that affects around one in twenty males; the condition is sex-linked and
is due to an abnormal gene on the X-chromosome. This affects males more as they have only one X chromosome so
even as the gene is recessive they will inherit the characteristic.
Blind spot
This is where the optic nerve enters the retina and there are no rods and cones.