Chapter 50 Flashcards
What are the layers of the retina? (remember your pneumonic)
- Inner limiting membrane
- Layer of optic nerve fibers
- Ganglion
- Inner plexiform
- Inner nuclear
- Outer Plixiform
- outer nucear
- Rods and cones
- Pigmented epithelium
What are the four functional segments of the rods and cones?
- Outer segment
- Innter segment
- Nucleus
- Synaptic body
What is contained in the outer segment?
- Light sensitive photochemical (rhodopsin in rods)
- layered discs
What is contained within the inner segment?
Cytoplasmic organelles:
-mainly mitochondria
What is the major purpose of the pigmented layer of the retinal epithelium?
-prevents light reflection which increases visual acuity
What two compounds together make rhodopsin?
Scotopsin and retinal
What is the affect of light exposure to rhodopsin?
-The cis-retinal is converted to all trans-retinal and then separates from scotopsin
What causes night blindness?
-Vitamin A defficiency
How does a lack of Vitamin A cause night blindness?
-Limits the amount of retinal and therefore rhodopsin that can be synthesized and then decreases low light (purple) vision.
Where is vitamin A stored and how can a deficiency be restored?
- the liver
- IV vitamin A
What is the membrane polarization of Rods in the light? How about when it is dark?
- Light: hyperpolarized
- Dark: Depolarized
What is the flow of effects when light striked a rhodopsin molecule?
Rhodopsin exposed to light–> Activated rhodopsin (all trans-retinal)–> activates transducin–> activates cGMP phosphodiesterase–> decreases cGMP–> closure of cGMP gated Na channels–> Decreases Na influx–> hyperpolarization
Is the polariazation state of the rod dependant on the amount of light striking it?
- Yes
- The more light, the more hyperpolarized.
What enzyme converts activated rhodopsin back to rhodopsin and reverses the whole excitatory process?
-Rhodopsin kinase
Are rods or cones more sensitive?
Rods (30-300 times)
What is the light adaptation?
- When moving from dark to light:
- the rods will be exposed to light and rapidly convert their retinal to vitamin A ehich reduces the concentration of photosensitive pigments
What is the dark adaptation?
When moving from light to dark:
-retinal and opsins are coverted back to light sensitive pigments (rhodopsin) and Vitamin A is converted back into retinal (to form rhodopsin)
Which adapts more quickly to light and dark? Which achieves the greatest sensitivity to light?
- The cones adapt quickly but briefly
- The rods achieve a greater sensitivity (the tortise wins the race)
What cones are stimulated by what color?
- blue pigment cones= blue light
- Red pigment cones= red light
- Green pigment cones= green light
White light= all 3 equally
What does the overlap in cone sensitivity allow?
-allows for the detection of mixed color
What is protonope?
- lack of or loss of RED cones
- shortened visual spectrum at the long wavelengths
What is the name for green color blindness?
- Deuteranope
- normal visual spectral length, missing green component
What is the inheritance pattern of color blindness?
- X linked recessive
- so mostly in males
Which are the only retinal neurons that transmit visual signals via action potentials?
Ganlgion cells through the optic nerve.
What are the 5 main type of neural cells located within the retina?
- Photoreceptive cells (rods and cones)
- Horizontal cells
- Bipolar cells
- Amacrine cells
- Ganglion cells
Where are horzontal cells and where do they transmit?
- outer plexiform layer
- transmit signals from the rods and cones to the bipolar cells
What is the location and function of bipolar cells?
- Transmit signals from the rods and cones to the ganglion cells and the amicrine cells
- between the outer and inner plexiform layer
What is the major function of the amicrine cells?
-lateral inhibition
What do ganglion cells do?
-transmit signals from the retina to the brain
What is the output of horzontal cells?
- always inhibtory
- provide lateral inhibition
How are amicrine cells directionally senstive?
-They respond to a spot of movement in a specific direction across the retina
What are some characteristics of X ganglion cells?
- most numerous
- small fields so fine detail
- resposnible for color vision
What are some characteristics of Y ganglion cells?
- largest and highest velocity
- wide spread retinal areas
What is the main purpose of the Y ganglion cells?
-to alert the brain to a new visual stimulus and gives clues as to where to move the eyes
Why are ganglion cells located laterally to the sopt of light inhibited? (off)
-the “on” ganglion cell laterally inhibits those around it, increasing contrast
What type of bipolar cells transmit a direct, excitatory signal?
-depolarizing bipolar cells
Why type of bipolar cells transmit a lateral inhibitory signal?
-hyperpolarizing bipolar cells
How do the ganglion cells start to distinguish color?
-some cone colors excite the depolarizing ganglion cells while other inhibit it (hyperpolarizing ganglion cells). (comon with red and green)
