Vertebrates 19 - Vision Flashcards
Median eyes
pineal or parietal organs. Not image forming, just sense brightness and the duration of light. Connect to pineal gland
Pineal glad
Produces melatonin most when dark. In mammals it reads from the retina instead of pineal eyes.
Melatonin
Affects circadian rhythms. Metamorphosis in amphibians, sexual cycles (with change in seasons), skin pigment changes in amphibians
Image forming eyes
Real eyes. Durr.
Sclera
Outer layer, white part. Holds everything else in.
Coroid layer
Blood vessels, pigmented epithelia. Pigment can absorb or reflect light.
Tapetum lucidum
Reflective layer in coroid layer made of guanine crystals. Reflects light so it can activate photoreceptors better, but image is a bit less clear. Important for nocturnal animals.
Fovea
Highest concentration of photoreceptors
Optic disk
Blind spot where optic nerve is. Can see images here.
Vitreous humour
Supports lens and retina.
Aqueous humour
B/w cornea and lens.
Cornea
Allows light to pass through
Lens
Focuses light onto the retina.
Ciliary body
Affects size/shape of the lens to focus the light.
Iris
Control amount of light entering eye through pupil.
Pupil shapes
Goats/sheep - horizontal; snakes, cats - vertical
Eyelid
Skin for protecting the eye
Nictitating membrane
“third eyelid”, covers most of eye. Can be under the eyelid (cat) or without eyelid (shark).
Retina
Pigmented layer. Photoreceptors go to bipolar cells to ganglion to nerve. A few other things to integrate info before the brain.
Retinal pathway
Optic nerve to the optic chiasm where R and L cross and split. Processed in the visual cortex on the opposite side from eye. Thalamus is relay center to occipital lobe or superior colliculus
Superior colliculus
Coordinates motor movements of eyes. Connected to the thalamus.
Photoreceptors in the eye
Rods and cones. Have cell bodies to produce proteins etc, and the outer segment which is important in sensing light. Contain second layer of membrane called membrane disks
Membrane disks.
In the outer segment. Contain rhodopsins: protein part (opsin) and retinal (Vit A derivative)
Rods
high sensitivity, low acuity, 400-600nm
Cones
low sensitivity, high acuity (detail). Usually sensitive to only one wavelength.
Cone types
Depend on the rhodopsin (specifically the opsin, retinal is same) Blue (450nm), Green (525nm), Red (650nm), some UV (360nm)
Tetrachromats
Have 4 cones. Birds and lizards
Dichromats
Have 2 cones. Most mammals. Lost 2 rhodopsins to reach this.
Trichromats
Have 3 cones. Include primates, humans. Gained a 3rd after ancestor lost 2.
Evolution of mammals and vision
First mammals were nocturnal etc. Colour helped gain access to new food etc.
Phototransduction in Dark
Increase cGMP, opens Na channel (dark current channel), flow in. Depolarize the cell, release glutamate to transmit signal
Phototransduction in Light conditions
Retinal converted from cis to trans, elongates and changes shape of rhodopsin, activates transducin, activates phosphodiesterase, makes cGMP to GMP, Na channel doesn’t stay open, repolarizes membrane.
Compare Phototransduction in light and dark
Dark: depolarized, Na open, inactive rhodopsin, glutamate released. Light: Na closed, active rhodopsin, hyperpolarized
Terrestrial eyes
Tear glands for moisture, cornea and lens involved in refracting light, thin lens, eyelids (1 or more), some have nictitating membrane, fast pupillary response (more changes), variable number of rods/cones, tapetum lucidum in some
Aquatic eyes
No tear glands, Cornea does not refract (water does), lens does, round lens, most have no eyelids, some have nictitating membrane, slow pupillary response (generally constant), mostly rods (less light), tapetum lucidum in some
Eyelid
Helps protect the eye as well as help get moisture across the eye.
Eyes on side
Little overlap, but greater field behind. Defense in prey species (rabbits, shore birds, chameleons)
Binocular vision
Both in front: greater binocular vision, less periphery. Predators: raptors, cheetahs etc
Depth perception
Image to one eye is a little larger than the other, brain understands this as a difference in distance.
