5.8 Flashcards
Retina contains light-sensitive cells:
rods and cones
Rods and cones are
receptor cells
a receptor cell is able to
detect changes in the environment and converts sensory information into a form that can be transmitted to the CNS
The light has to travel through several layers of neurones to reach the
light-sensitive rods and cones
Underneath the rods and cones is a layer of cells containing the pigment, melanin which
absorbs any light that is not absorbed by the rods and cones
(Rods) outer segment contains many vesicles of the light-sensitive pigment
rhodopsin
when light hits a rod cell it causes the rhodopsin to
split into opsin and retinal
(rods) when rhodopsin splits it is a process known as
bleaching
Bleaching results in the permeability of the rod cell to sodium ions to be
altered
Rod cells set up a
generator potential
(ROD GP) if this is enough to overcome the threshold value, an AP is generated in the sensory neurone which carries the impulse along the
optic nerve to the brain
after, the rhodopsin is regenerated, which requires energy from
ATP
Rod cells are extremely sensitive to low light intensities, so they are most useful for
vision in dim light
rod cells cannot distinguish between different wavelengths, so they produce black and white images only, known as
monochromatic vision
several rods synapse with just one sensory neurone - means that rod cells have
low visual acuity
Grouping of rod cells together also increases their sensitivity known as
summation
if each of the several rod cells results in a tiny amount of neurotransmitter being released into the synaptic cleft, these small amounts ‘add together’ so that there is enough to set up an
action potential in the sensory neurone
there are many more rod cells in the retina than
cones
rods are found evenly spread across the retina, although there are very few at the
fovea
cones are similar in structure to rods, but contain a pigment of
iodopsin
(Cones) 3 different kinds of iodopsin each sensitive to
red, green or blue light
(cones) different colours are perceived depending on the
proportion of the different types of cone that are stimulated
(CONES) e.g. if red and green cones are stimulated equally, the brain interprets the colour as yellow. this is called the
trichromatic theory of colour vision
cone cells are not as sensitive to light as rods, so they only work in
high light intensity
there are fewer cones than rods in the
retina
most of these cone cells are concentrated at the
fovea
the fovea is the part of the retina where the centre of the image is focused when you
look straight at an object
most cones have an individual connection to a sensory neurone, meaning that
cones have much greater visual acuity than rods
