Term
| What are the two overlapping systems of the retina? |
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Definition
| one system for low light levels (the rod system) and a second system for high light levels (the cone system) |
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Term
| To what four systems is output of the retina projected? |
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Definition
| One pathway leads from the retina to the hypothalamus, so that biological rhythms can be synchronized with day and night. A second pathway leads to the superior colliculi, to provide input for visually targeted eye movements, and a third pathway provides sensory input for the pupillary reflex. Finally, a major portion of the retinal output is directed to the lateral geniculate and striate cortex, for detailed analysis of form, motion and color. |
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Term
| What are the photoreceptor outersegments in the retina embedded in? |
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Definition
|
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Term
| What are the biochemical and metabolic roles of the pigmented epithelium? |
|
Definition
| it renews photopigments and phagocytoses photoreceptor disks, which turn over at a high rate. |
|
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Term
| Is glutamate in the vertical pathway of the retina excitatory or inhibitory |
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Definition
| Either or; unique to retina; usually only excitatory |
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Term
| What are the only cells within the vertical pathway to fire action potentials? |
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Definition
|
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Term
| What type of potentials are created by photo receptor cells and bipolar cells? |
|
Definition
| graded potentials only (receptor potentials and synaptic potentials), with neurotransmitter being released in proportion to membrane depolarization |
|
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Term
| What are the lateral inhibitory connections created by horizontal cells critical for? |
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Definition
| the circular, center-surround, antagonistic receptive fields of the bipolar and ganglion cells |
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Term
How does the ratio of rods to cones vary between the fovea and peripheral retina?
What does this make the peripheral retina more sensitive to? |
|
Definition
the peripheral retina contains a higher ratio of rods to cones
low light levels |
|
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Term
| Where is the retina thinnest? |
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Definition
|
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Term
| Are the photoreceptors of the retina depolarized or hyperpolarized in the presence of light? |
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Definition
|
|
Term
| What is meant by the term "dark current" in the retina? |
|
Definition
| in the dark, a continuous inward current, called the dark current (carried by Na+ and Ca2+) holds the photoreceptor membrane at a relatively depolarized membrane potential |
|
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Term
| What does the axon-like process of photoreceptor cells in the retina contain? |
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Definition
voltage-gated Ca2+ channels which open with depolarization, allowing Ca2+ influx and exocytosis of
neurotransmitter |
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Term
| How does phototransduction work? |
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Definition
| The mechanisms of phototransduction harness the energy of light to activate PDE.When the photoreceptor is stimulated with light, a phosphodiesterase (PDE) is activated. The activated PDE quickly degrades cGMP, removing the stimulus which holds the CNG channels open. As the CNG channels close, the membrane potential of the photoreceptor hyperpolarizes, due the presence of normal, neuronal leak channels. In strong light, the photoreceptor membrane potential resembles a typical neuron, assuming a negative membrane potential of around –65 to –70 mV. |
|
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Term
| What does opsin resemble closely in structure and function? |
|
Definition
| a metabotropic neurotransmitter receptor |
|
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Term
| How does the absorbance of light by retinal change its configuration? |
|
Definition
| from 11-cis- retinal to all-trans-retinal |
|
|
Term
| What is the name of the special G-protein in rods that stimulates PDE? |
|
Definition
|
|
Term
| What is the importance of the biochemical cascade involved in transduction in rod cells? |
|
Definition
|
|
Term
| What color change does retinal undergo after absorbing light? |
|
Definition
|
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Term
| What happens to retinal after it has absorbed light? |
|
Definition
| All-trans-retinal dissociates from the opsin, diffuses into the cytosol of the outer segment, and is transported into the pigmented epithelium. In the pigmented epithelium, retinal is converted back to the 11-cis form, and transported back into the outer segment, where it recombines with opsin. |
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Term
| What is responsible for light adaptation? |
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Definition
| Light induced closure of the CNG channels in the outer segment decreases the internal Ca2+ concentration |
|
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Term
| Why is adaptation important? |
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Definition
| it greatly expands the range over which photoreceptors can signal a change in light level |
|
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Term
| What other mechanisms contribute to light adaptation? |
|
Definition
Continuous bright light can cause bleaching of enough retinal to affect the light
response. Finally, the pupillary light reflex adjusts the amount of light allowed into the eye for stimulation of the retina. |
|
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Term
| What are the main differences between cone and rod photoreceptors? |
|
Definition
| Cone photoreceptors have three types of opsins, are better in bright light |
|
|
Term
| What is vision in low light called? |
|
Definition
|
|
Term
| In what level of illumination are both rods and cones active? |
|
Definition
|
|
Term
| What type of vision involves only cones? |
|
Definition
|
|
Term
| How are retinal ganglion cells best activated? |
|
Definition
| large, relative differences in brightness |
|
|
Term
| What is the critical factor in determining the rate of firing of retinal ganglion cells? |
|
Definition
| For ganglion cells, the ratio of intensities, and not the absolute intensity, is critical in determining their output (rate of action potential firing |
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Term
| Is diffuse white light an effective stimulus for a color-opponent ganglion cell? |
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Definition
|
|
Term
| How are visual accuity and sensitivity related? |
|
Definition
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