COM S4 Flashcards
Identify photoreceptor cells as those containing light sensitive pigments and explain that these cells convert light images into electrochemical signals that the brain can interpret
• Photoreceptor cells detect and respond to stimulus of light
• Human eye found in retina – thin sheets of cells at back of eye
• Two types Rods and cones
Modified nerve cells (neurones)
Contain light sensitive pigments and convert light images into electrochemical signals that the brain can interpret
Describe the differences in distribution of the photoreceptor cells in the human eye
• The human eye has 125 million rods and 6.5 million cones
Distribution:
• Cones
More densely concentrated in central fovea & retina – each cone cell connects to one nerve cell
Fovea = small section of the macula at the back of the eye
Spread across the retina in groups
• Rods
Not located in fovea or macula
3-4 times more numerous than cone cells
More dense at edges of the retina
Describe function of the photoreceptor cells in the human eye (cones)
• Cones
Require more/bright light than rods to be stimulated
Used for day vision, colour vision and visual tasks requiring visual activity (e.g. reading small print) + visual acuity
Absorb light and change structure results in action potential light changed into electrochemical message that is transferred via optic nerve to brain
Describe function of the photoreceptor cells in the human eye (rods)
• Rods
More sensitive to light than cones but do not distinguish colours
Function best in dim light
Use for night vision
Allow us to detect shape, movement and to discriminate between light and dark shades
Describe the structure of the photoreceptor cells in the human eye
• Cones
Conical
• Rods
Narrower, longer and straighter
• Both
Contain visual pigments in stacks of disk shaped membranes at one end of the cell
Other end connects to nerve cell – two types of nerve cells in the retina (bipolar cells & ganglion cells)
The nature of photoreceptor cells in mammals
• Rods and cones
• Depending on number of cones may be sensitive to a range of colours
• Humans and primates full colour vision
• Nocturnal animals = higher proportion of rods than cones
• Rats don’t have cones – only see black and white
• Rods and cones in mammals
Ciliary photoreceptors
Ciliary membrane expanded and thrown into deep folds (look like discs)
The nature of photoreceptor cells in insects
• Compound eyes thousands of photoreceptor cells (ommatidium)
• Ommatidia (pl) each ommatidium (s) consists of:
Corneal lens can repair itself
Crystalline cone
o Daytime: reflects light into rhabdom (made of fibres) that respond to light intensity
o Night: becomes a tract (not cone) - info from one ommatidium (rhabdom) transferred to next rhabdom
Photoreceptors
• Focus on movement NOT visual acuity
• Visual acuity x
o Accommodation DOES NOT OCCUR: can’t change shape of lens
o Don’t have big enough brain to interpret
• Photoreceptors absorb certain colours from incoming light & make nerve impulses similar to vertebrates
Some insects able to distinguish colours
The nature of photoreceptor cells in insects “honey bee”:
E.g. Honey bee
– 4/8 of the visual cells in each ommatidium respond best to yellow-green light
– 2/8 respond to blue light
– Remaining 2/8 respond to UV light
• Rhabdomeric photoreceptors?
Increased surface areas – fine membranous bristles
Involve activation of enzyme phospholipase
The nature of photoreceptor cells in one other animal
• Invertebrates (e.g. flatworms)
- Invertebrates (e.g. flatworms)
- Simple light receptors
- Patches of photoreceptors in concave cup used to distinguish light from dark
- Walls of cup contain pigments (Ocelli) prohibits light penetration from 3 sites
Comparing the photoreceptor cells of animals
• Rods and cones in mammals different from those in insects and invertebrates
• Occur in different structures & work using different pathways
Mammal: Retinal rods and cones
Insect: Ommatidia
Invertebrates (Flatworm): Ocelli in light sensitive cup
• Flatworm different from insect and mammal
No image formed – just sensation of light and dark from different directions
• Photosensitive (nerve) ganglion cells in mammals similar in insects and
Outline the role of rhodopsin in rods
• Rods contain photosensitive pigment = rhodopsin
• Highly sensitive to light
• Rods specialised for night vision
• Sensitive to blue-green light
• Allows us to see shades of black, grey and white
• When light hits rhodopsin splits into two parts: opsin and vitamin A
Produces activity in the nerve cell
• Bright light rhodopsin broken down faster than can be manufactured
• Dim light production able to keep pace with rate of breakdown
Identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light
- Cones 3 different photosensitive molecules – colour pigments
- Contain retinal molecule
- Each retinal molecules linked to one of three different opsins known as photopsins/iodopsins
- Individual cone contains only one of three types of photopsins
- Each type absorbs light in particular range of wavelengths: red, green or blue
- So… There are 3 types of cones, each sensitive to red, green or blue light
Explain that colour blindness in humans’ results from the lack of one or more of the colour-sensitive pigments in the cones
• Colour blindness in humans results from lack of one or more of the photopsins in the cones
• Most common = red-green colour blindness
Cannot distinguish red from green lack either the red or green photopsin
Condition called dichromatism
• Some lack blue-sensitive cones blue cannot be distinguished
• Monochromatism single-colour vision person has only one type of cone
• Colour weakness fewer cones of one type than normal difficult to distinguish that colour
Occurrence of colour vision in animals and its relation to colour communication?
Use of colour for communication only effective if animal receiving message has colour vision
Mammals with poor colour vision, such as dogs, depend more on their senses of smell and hearing for communication
