The Eye Flashcards
What are rods
Only detect presence/absence of light so detect black and white
What are cones
Detect colour, sensitive to light of dif wavelengths
-three dif types, red/green/blue sensitive
Role of the retina
Contains large number photoreceptor cells which when exposed to light stimulates the sensory neurones in the optic nerve
What is the fovea
Area of the retina where there are lots of photoreceptors
Where are rods and cones found
Rods found in peripheral parts of the retina, cones packed together in the centre
What do rods contain
Light sensitive pigment rhodopsin made of retinal and opsin
What are bipolar neurones
Connect photoreceptors to the optic nerve which takes impulses to the brain, have one dendrite and one axon with cell body between these two structures
How is impulse transmitted from photoreceptors
Light enters eye, hits photoreceptors and is absorbed by light-sensitive pigments
-light bleached the pigments causing chemical change, triggers action potential in bipolar neurone
-results in stimulation of neurone of ganglion cell (near inner surface of retina), nerve impulse then generated in optic nerve and info goes to brain for processing
What are bipolar neurones connected to
One cone cell, or many rod cells
What happens when it’s dark
Rods aren’t stimulated
-sodium ions pumped out of cell using active transport but diffuse back into cell through open voltage gated sodium channels which remain open in dark
-makes inside of cell only slightly negative compared to outside so cell membrane depolarised due to influx of sodium ions
-triggers release of glutamate from rod cell
-these NTs inhibit the bipolar neurone so can’t fire an AP so no AP in optic nerve and no info goes to brain
What happens when it’s light
-rod cells stimulated, light energy causes rhodopsin to break apart into retinal and opsin
-opsin binds to voltage gated sodium channels of presynaptic membrane of rod cells causing them to close
-sodium ions actively transported out of cell but can’t diffuse back in, more sodium ions outside cell so membrane hyperpolarised, more negative inside than outside as no influx of sodium ions
-when rod cell hyperpolarised it stops releasing glutamate so no inhibition of bipolar neurone
-bipolar neurone no longer inhibited and it depolarises, if change in pd reaches the threshold an AP is transmitted to the brain via optic nerve
Why are rod cells highly sensitive to light intensity
-rhodopsin doesn’t take a lot of energy to break down
-signals from several rod cells act together which increases likelihood of action potential firing along bipolar neurone
