Vision: The Eye

6. Explain the steps in phototransduction in a rod, including its resting state and what happens when a photon is absorbed.

Answer: At rest, a rod photoreceptor contains high levels of cGMP in its outer segment. The cGMP binds to cGMP-gated cation channels, keeping them open, thus maintaining a depolarized state. When a photon is absorbed, cGMP levels decrease, cGMP dissociates from the channels, and the cGMP-gated channels close. This reduces the flow of Na+ and Ca2+ into the cell. However, K+ channels remain open in the presence of light, so positive charge flows out of the cell more rapidly than it flows in, leading to hyperpolarization.

4. What is the role of horizontal cells?

Answer: Horizontal cells have cell bodies in the inner nuclear layer and processes that laterally connect photoreceptor cells and bipolar cells. It is thought that they regulate the amount of transmitter released from the photoreceptors to act on bipolar cells. This is important for luminance, or the ability to detect contrast over a wide range of light intensities.

15. Which statement about color vision is false?

a. Because there are several different color types of cones, they are more sensitive to low (dim) illumination levels than the population of rods is.

11. The death of retinal cells in retinitis pigmentosa is most likely caused by

b. apoptosis.

4. During accommodation, the shape of the _______ is changed by the _______ in order to see objects accurately at varying distances.

b. lens; ciliary muscle

1. The iris

b. regulates the amount of light entering the eye.

10. What observations led Kuffler to define two types of retinal ganglion cells: OFF-center and ON-center? Explain how this receptive field organization is useful in detecting luminance contrast and changes in light intensity.

Answer: Kuffler discovered that each ganglion cell responds to stimulation in its receptive field by increasing or decreasing its firing rate. ON-center and OFF-center cells responded oppositely to the presence or absence of light. These receptive fields overlap in the retina, allowing for a more complex detection of contrast and changes in light intensity.

7. Why is light adaptation in the retina so important, and what does it involve?

Answer: Light adaptation occurs when photoreceptors decrease their sensitivity as illumination increases. This prevents the receptors from saturating, and it extends the range of light intensities that they can respond to.

5. What are some differences between photoreceptors and other sensory cells?

Answer: Photoreceptors exhibit a graded change in membrane potential in response to light instead of action potential. They also maintain a resting membrane potential around -40 mV, and they hyperpolarize in the presence of light.

2. Describe the five types of retinal neurons. Which cells are in the outer nuclear layer?

Answer: Photoreceptors, or rods and cones, have an outer segment adjacent to the pigment epithelium and cell bodies in the outer nuclear layer. Photoreceptor cells synapse on bipolar cells, which in turn synapse on ganglion cells. The processes of horizontal cells allow for lateral interactions between photoreceptors and bipolar cells. The processes of amacrine cells are postsynaptic to bipolar cell terminals and presynaptic to the dendrites of ganglion cells. Amacrine cells can be classified into different subclasses, based on their function.

8. What are the advantages of animals having both rods and cones, rather than just one type of photoreceptor?

Answer: Rods are very sensitive to light and thus facilitate vision in low light. Cones have very high spatial resolution (providing visual acuity) and different photopigments (allowing animals to see in color). Having both rods and cones allows animals to see and process a great variety of visual stimuli in different environments and across a broad range of light conditions.

9. What is the evidence that human color vision is trichromatic?

Answer: Studies show that any color stimulus can be duplicated by a second stimulus composed of three superimposed light sources of short, medium, and long wavelengths (as long as the intensity of the light sources can be independently adjusted).

3. Is the retina part of the central nervous system? Explain.

Answer: The retina is part of the central nervous system because it is formed via outpocketing of the diencephalon during development. The retina contains a complex neuronal circuitry, allowing it to convert the graded electrical activity of the photoreceptors into action potentials that travel along the axons of the optic nerve.

1. Are there more rods or cones in the retina? In the fovea?

Answer: There are many more rods (about 90 million) than cones (about 4.5 million) in the retina. However, cone density increases dramatically in the fovea, with the foveola being completely rod-free.

20. A genetically engineered macaque has had the gene that codes for M pigment knocked out. Which vision impairment should the knockout macaque have?

Deuteranopia

3. On its way to the retina, light passes through tissues and fluids in which order?

a. Cornea, aqueous humor, lens, vitreous humor, retina

21. A man with dichromatic vision would have the most trouble with which task?

a. Determining when a red light turns green

20. Which statement about the retina's operation across different levels of ambient light is true?

a. For a given level of ambient light, an ON-center ganglion cell responds proportionately to a small spot of light over an intensity range of about one log unit. b. Via adaptational mechanisms, ON-center ganglion cells can dynamically encode brightness levels in their ON-center over a range of 6 log units of ambient light levels. c. Ganglion cells generally do not report absolute light intensities, but rather encode relative intensity differences between center and surround. d. Interactions within the inner plexiform layer play an important role in modulating the photic sensitivity of ganglion cells. e. All of the above Answer: e

27. How would the firing of an ON-center ganglion cell respond as a light moved from the edge of the receptor field to the center of the receptive field?

a. It would increase.

18. Which feature is responsible for the superior acuity of the fovea?

a. Lack of retinal blood vessels

7. What is the most direct path that light information travels on its way to the optic nerve?

a. Photoreceptor cell; bipolar cell; ganglion cell; optic nerve

6. Which sequence represents the most direct pathway for the transmission of visual information from the eye to the brain?

a. Photoreceptor → bipolar cell → ganglion cell → brain

13. By which mechanism are rod signals transmitted in conditions of low light?

a. Rod bipolar cells synapse on amacrine cells, which in turn synapse on cone bipolar cells.

For an OFF-center ganglion cell, which stimulus on the cell's receptive field would cause the highest rate of action potential firing? (In the figure, black fill indicates darkness, and white fill indicates light in the receptive field.)

a. a

5. Age-related macular degeneration, or AMD,

a. is rising in incidence in the United States. b. can be detected by means of the Amsler grid. c. can result from gradual loss of the pigment epithelium (dry AMD). d. can be treated by laser-induced phototoxicity (wet AMD). e. All of the above Answer: e

28. In which scenario would an ON-center ganglion cell fire the most robustly?

b. A light filling the center of the receptive field

8. Which mechanism is not a component of the phototransduction signaling cascade initiated by light falling on a rod?

b. Activation of protein kinase A

14. Which of the following correctly matches rods and cones with their properties?

b. Rods: high sensitivity to light; cones: high spatial resolution

8.Which of the following statements about the location photoreceptors in the outermost layer of the retina is true.

b. The proximity to the pigment epithelium allows nourishment for the photoreceptor cells and recycling of photopigment.

22. Why are males more likely than females to have red-green color deficiencies?

b. The red and green pigment genes are located on the X chromosome.

25. What is the major anatomical difference between ON-center and OFF-center bipolar cells that explains their selective response to light increments?

b. They have different glutamate receptor types.

13. The reason that rods do not contribute to photopic vision is

b. their response is saturated.

2. Our underwater vision is poor because

b. under water there is no longer a refractive index difference between the cornea and the surrounding media.

16. What accounts for the fact that rods do not contribute to vision in daylight?

c. All membrane channels are closed due to saturation in daylight.

23. How would an OFF-center neuron's firing rate change when a light was turned on, turned off, and then turned on again?

c. Decrease, increase, decrease

26. A ganglion cell's firing rate is proportional to what property of light?

c. Intensity

5. Which statement about the optic disc is false?

c. It contains a small depression known as the fovea.

30. Due to a genetic modification, a mouse has no horizontal cells in its retinas. What impact will this have on the mouse's vision?

c. Reduced ability to determine relative stimulus intensity

11. If a person had a mutation in the gene that codes for interphotoreceptor retinoid binding protein, what would be the most likely downstream effects?

c. Retinal could not be transported to and from the outer segment for the retinoid cycle.

19. Which property best differentiates the types of cones?

c. The photopigments they contain

10. Which statement regarding photoreceptor cells is true?

c. Unlike typical neurons, cation channels are open at rest, allowing the influx of sodium and calcium.

9. The mechanism that accounts for light-induced hyperpolarization of photoreceptors is

c. a rapid fall in the concentration of cGMP, leading to closure of Na+ / Ca2+ channels.

4. The fovea

c. lies at the center of the macula lutea.

12. The main reason that rods are more sensitive to light than cones is that

c. the rod transduction mechanism provides greater signal amplification.

9. Applying a drug that increases cGMP levels in photoreceptors would cause which effect on a photoreceptor's response to a photon of light?

d. An attenuated hyperpolarization from normal

The illusion shown, in which dark rectangles are placed within a pattern of light and dark bars, is best explained by which statement?

d. Brightness percepts are generated on a statistical basis as a means of contending with the inherent ambiguity of luminance.

3. Why is myopia in humans thought to be more common now than it was in ancient times?

d. Early humans did not engage in the kinds of activities (e.g., reading and writing from an early age, watching television) characteristic of modern life.

2. An optometrist discovers that her patient has poor drainage of his aqueous humor, and a test confirms high intraocular pressure. These symptoms suggest which condition?

d. Glaucoma

29. Which cell type is thought to be responsible for the antagonistic surround of ganglion cells?

d. Horizontal

17. Which statement about center-surround receptive fields in the retina is false?

d. Light in the center of an off-center cell will increase its firing rate.

14. Which statement about the distributions of rods and cones is true?

d. The density of cones is 200-fold higher in the fovea than in the most eccentric retinal regions.

17. Which symptom would you expect a person with damage to the fovea to experience?

d. Trouble reading

7. The two main functions of the retinal pigment epithelium are _______ and _______.

d. phagocytosis of shed outer segments; regeneration of the photoreceptor photopigments

1. Which eye structure is paired with an appropriate characteristic?

e. Ciliary body: produces fluid to fill the front of the eye

15. Which statement about rod and cone convergence in the retina is true?

e. Convergence allows rods to pool signals, generating larger responses in bipolar cells.

16. You measure changes in membrane potential in an ON-center bipolar cell that is exposed to light in the center of its receptive field. What response would you expect to see?

e. Depolarization due to decreased release of glutamate by the photoreceptor cell

24. A photoreceptor cell is exposed to a flash of light. How does the membrane potential of this cell and its corresponding ON-center bipolar and ganglion cells change?

e. Hyperpolarize, depolarize, depolarize

10. Which mechanism decreases sensitivity in a photoreceptor as levels of illumination increase?

e. Light adaptation

6. A 68-year-old man notices that the boxes of his crossword puzzle look wavy and blurry for the word he is working on, but the boxes in the periphery of his focus remain clear. This symptom suggests that the man has which condition?

e. Macular degeneration

12. Mesopic vision is most likely to occur in which of the the following scenarios?

e. Walking outdoors at the break of dawn