The Retina Flashcards
Perception LEc
What is the Purkinje Tree?
There are tiny little blood vessels that branch all over the surface of the retina and provide it with food and nourishment.
It branches out from the optic disk or blind spot.
Diabetic retinopathy
swelling and abnormal growth of capillaries, insufficient oxygen supply to the retina, may cause bleeding into the vitreous. Treated by using a laser to seal off blood vessels.
Detached retina
Detached retina: the retina separates from the pigment epithelium, scotoma (blind spot) in the area that detaches. Caused by glaucoma or by injury. Use laser, heat, or cold probe to cause scar tissue to form and hold the retina in place.
Macular degeneration
Macular degeneration: degeneration of the part of the retina called the macula that includes the fovea and surrounding region. Leads to loss of central vision. The most common form is age related.
Retinitis pigmentosa
Retinitis pigmentosa: a hereditary disease, initially affects rods and peripheral vision, then gradually starts to affect cones and eventually leads to complete blindness.
Which are the only cells that have axons?
ganglion cells- which axons reach a rather long distance into the brain- that you get action potentials.
*Horizontal and amacrine cells don’t even have proper axons
* Rather these neurons respond with graded potentials. If the resulting current is large enough at their synapses then they release neurotransmitter.
What are the typical properties of parallel pathways?
1. Physiologically/functionally distinct. Rods give you very high sensitivity so you can see at night. But if rods were all you had, you would be in trouble. A very small percentage of people, called rod monochromats, have no cones. They see a wash of brightness during daylight conditions and are functionally blind at high light levels. So both the rods and the cones are needed.
2. Anatomically distinct. Rods and cones have different shapes. For many parallel pathways, neurons in the separate streams are separated from one another, e.g., localized in different layers. This is not true of rod vs cones, but is true of many other parallel pathways.
3. Complete coverage (or nearly complete coverage) of the visual field by each stream. There are no rods in central fovea but otherwise the rods cover entire visual field. Likewise, the cone mosaic covers the entire visual field.
4. Recombine: parallel streams often (but not always) remerge eventually. Rod-cone pathways recombine at the ganglion cells. We need the different sensitivities of the rods and cones at the very front-end of the visual system but once the light has been transduced (with proper sensitivity) into a neural signal, there’s no need to keep the separate processing.
Parasol dendritic trees are big, they combine from many _____.
Midget trees are small. Midget ganglion cells receive synapses from only ____ bipolar cell each which in turn receive synapses from only ____ cone.
cones
one
one
In the periphery, the dendritic fields of both (midget and parasol) ganglion cell types get ______.
It’s easy to distinguish by:
2.
(1) where you are in the retina, (2) how big is the tree.
Information Processing in the Retina
The retina performs five important jobs:
1.
2.
3.
4.
5.
- transduction
- data compression
- light adaptation
- spatial filtering
- wavelength encoding
What are the photopigment molecules in rods called?
Rhodopsin- opsin and vitamin A
What is the state of isomerization?
When a quantum of light is absorbed by a molecule of rhodopsin, it changes the chemical state of the photopigment. The two parts of the molecule split.
The isomerization sets off a biochemical chain reaction that eventually leads to an electrical current flowing in the rod. If the current is big enough, this neural signal is transmitted to the bipolar cells in the retina because neurotransmitter is released at the rod-bipolar synapse. Absorbing a single photon of light in a rod is enough to evoke a regular/reliable photocurrent.
What is the minimum amount of photons that a human can detect?
Single photons
What is bleaching?
When the rhodopsin pigment changes color when exposed to light.
How is data compressed in the retina?
There are roughly a hundred million light receptors in each eye, but only half a million ganglion cells.
in order to take the original image on the photoreceptors, and send it to the more central portions of the brain, a great deal of compression takes place. A large part of the compression is accomplished by having blurry vision in the periphery
