Vision Flashcards
Describe general photoreceptors.
What are the two specific types?
Receptor cells in the retina that transduce light into action potentials
Rods and cones
What portion of the EM spectrum can the human eye detect?
380-760
Describe Rods
receptors that respond to low intensity light, such as at night
Cones
Photoreceptor that is sensitive to different light intensities. Responsible for color vision and high acuity vision. Daytime vision
What is the retina, where is it located?
The retina is neural tissue that contains photoreceptors. It is located on the inner surface of the posterior eye
Where is the fovea?
Function?
The fovea is located in the central region of the retina. It is made specifically of cones and is responsible for mediating most acute vision.
Optic Disk
Location of exit point from the retina for fibers of ganglion cells that make up optic nerve. Forms a blind spot that we dont perceive
What are photopigments?
special molecules imbedded in the membrane of rods and cones that is responsible for transduction of visual info.
What is the photopigment in rods?
rhodopsin.
What is the photopigment in cones?
three classes of cone opsins: the basis of color vision
What happens when light activates a rhodopsin molecule?
Retinaldehyde dissociates from opsin revealing an enzymatic site on the opsin molecule. Combines rapidly with many molecules of G protein transducin…. long cascade of events. Ultimately results in change in membrane potential that results in glutamate release
describe rod/cone photoreceptors at resting potential
They steadily release glutamate. Light always hyper-polarizes the photoreceptors
Bipolar cells response to glutamate differ. Why?
They have different receptors. Glutamate depolarizes one group of bipolar cells but hyper-polarizes another. Differ by receptive field.
How does light close sodium channels in photoreceptors?
Light causes rhodopsin and RETINAL to unbind. This allows opsin to combine rapidly with the G protein transducin. Transducin acts through an enzyme, phosphodiesterase (PDE) to turn cGMP into 5’GMP. cGMP holds sodium channels open. Therefore, as transducin and PDE turn cGMP into 5’ GMP, sodium channels close. This results in hyperpolarization
What is the relationship between light, glutamate, and off-center bipolar cells?
Off-Center bipolar cells are excited when light is turned off. It receives more glutamate which depolarizes the off center cells.
What is the relationship between on-center bipolar cells, light, and glutamate?
Turning on a light in the center of on-center bipolar cells excites them. They receive less glutamate which otherwise inhibits the on-center cell
Bipolar cells release glutamate that always excites the ganglionic cells. How are on-center/off-center ganglionic cells stimulated?
On-Center bipolar cells are stimulated in response to light. Their stimulation leads to the stimulation of on-center ganglionic cells.
Off-Center bipolar cells are stimulated when light is turned off. Their stimulation results in depolarization of off-center ganglionic cells
What contacts do the horizontal cells make in the retina?
Do they have action potentials?
photoreceptors and bipolar cells.
No, only ganglionic cells have action potentials.
What contacts to amacrine cells have?
They are in contact with bipolar and ganglionic cells.
FYI: All cell types except ganglionic generate graded potentials. (ganglionic fire APs)
describe the scotopic visual system.
The scotopic system works in dim light and involves rods. There is highly convergent processing. Very little differentiation between wavelengths of light (all cats are gray at night). High convergence means that many rods converge onto few bipolar cells which converge to one ganglion cell. peripheral vision
Describe the photopic system
The photopic system requires more light and involves more detailed processing. Differentiates wavelengths, color vision. High acuity, Low convergence.
Describe how sensory info gets from ganglion to brain. Where does the info go?
Optic nerve splits at the optic chiasm. The nerve from each eye sends fibers to both sides of the brain. After the optic chiasm, optic nerves are referred to as the optic tract. Most axons of the optic tract terminate at the lateral geniculate nucleus (visual part of thalamus). What about the superior colliculus and area V1?
Describe small ganglion cells.
They have input from single cones. They project to the parvocellular layer of the LGN and discriminate color.
What are large ganglion cells?
They are cells that receive input from many cones. They project onto the magnocellular layer of the LGN which can not discriminate color.
