Wednesday, 19 September 2018 3.6 Organisms respond to changes in their internal and external environments 3.6.1.2 Eye Receptors • identify the pigments in rod and cone cells • explain how rod cells’ visual acuity, sensitivity to light and sensitivity to colour are accounted for by the presence of rhodopsin and connections to the optic nerve • explain how cone cells’ visual acuity, sensitivity to light and sensitivity to colour are accounted for by the presence iodopsin and connections to the optic nerve

The Eye

Photoreceptors These are found in the retina. There are two types Rods and Cones and they are arranged as shown: Outer ! Inner Light Pigmented Layer Ganglion Cells Rod To Optic Nerve Bipolar Neurones Cone

Light receptors Both rod and cone cells act as transducers by converting light energy into the electrical energy of a nerve impulse.

Rod cells Cannot distinguish different wavelengths of light and therefore produce images only in black and white. Rod cells are more numerous than cones.

Rod cells Many rod cells share a single sensory neurone. Rod cells can therefore respond to light of very low intensity. This is because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached.

Rod cells A number of rod cells are attached to a single bipolar cell (= retinal convergence), there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were attached to each bipolar cell. As a result, rod cells allow us to see in low light intensity (i.e. at night), although only in black and white.

Changes in the electrical potential of a receptor when stimulated by three separate stimuli. Only the third stimulus produces a generator potential high enough to trigger a nerve impulse.

A rod cell opsin Rhodopsin (pigment in rod cells broken down) LIGHT Signal from Bipolar cell

A rod cell As many rod cells are joined to the same bipolar cells, only a single impulse will be stimulated. This means that they cannot distinguish between the separate sources of light that stimulated them. 2 dots close together will appear as a single blob. Rod cells therefore have low visual acuity.

Cone cells Cone cells are of three different types, each responding to a different wavelength of light. Depending on the proportion of each type that is stimulated, we can perceive images in full colour. Each cone cell usually has its own bipolar cell connected to a sensory neurone. This means that often the generator potential is not exceeded. As a result, cone cells only respond to high light intensity and not to low light intensity.

Cone cells Cone cells contain a different pigment to rod cells (iodopsin). This requires a higher light intensity to be broken down and create a generator potential. As cone cells are attached to their own bipolar cell, if 2 adjacent cells are stimulated, the brain receives 2 separate impulses. Cone cells give very accurate vision, they have good visual acuity.

Greater numbers than cones Fewer numbers than rods Rod cells Cone Cells Rod-shaped Cone-shaped One type Red, green & blue types Greater numbers than cones Fewer numbers than rods Distributed more in the periphery Fewer at periphery, concentrated in fovea Poor acuity Good acuity High sensitivity Low sensitivity Rhodopsin pigment Iodopsin pigment Iodopsin pigment

Cone cells Light is focussed by the lens on a point known as the fovea. The fovea therefore receives the highest intensity of light. Therefore cone cells, but not rod cells, are found at the fovea. The concentration of cone cells diminishes further away from the fovea. At the peripheries of the retina, where light intensity is at its lowest, only the rod cells are found.

Wavelengths of light absorbed by different cones

Colour Blindness If you have normal vision you will see a figure seven in reddish brown dots. People with red-green colour blindness will not see the 7, why? These people lack red sensitive cones, but the green stimulated cones are stimulated by the red light, so all dots appear green

The Dress