recap

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45.4 recap

Vertebrates have image-forming eyes in which a lens focuses incoming light onto the photosensitive retina. Opsin molecules are responsible for light sensitivity in all animals, and in mammals opsins are contained in receptor cells called rods and cones. Slight differences in the opsin molecules result in differences in spectral sensitivity, which is the basis for color vision. Rods and cones do not fire action potentials, but their graded membrane potentials alter neurotransmitter release onto retinal bipolar cells. The bipolar cells in turn influence ganglion cells that do fire action potentials in their axons, which travel in the optic nerves to the brain. Two other types of retinal cells, horizontal and amacrine cells, sharpen contrast and adjust sensitivity of the retina, respectively.

learning outcomes

You should be able to:

  • Trace the steps for how a photon of light is transduced into a change in membrane potential.

  • Compare the properties and locations of rod and cone cells in the retina.

  • Describe the structural implications for movement detection and visual acuity.

  • Explain ganglion cell receptive fields in terms of the lateral processing afforded by the horizontal and amacrine cells.

Question 1

Explain why a compound eye is better at detecting movement than at forming images.

Ommatidia are excellent at detecting movement as moving objects in a visual field switch neighboring ommatidia “on” and “off.” However, since each ommatidium is receiving light from only a small but discrete portion of the total visual field, only a pixilated (low-resolution) image can be formed.

Question 2

How do photons of light change the receptor potential of rod cells?

When photons excite rhodopsin, its conformation changes and that activates a G protein that activates a phosphodiesterase that converts cGMP to GMP. The decrease in cGMP levels causes the cGMP gated Na+ channels to close, reducing the dark current. Na+ is also pumped out of the proximal end of the photoreceptor cell and its membrane potential hyperpolarizes.

Question 3

Explain how rods and the various cones differ in their spectral sensitivity.

Rods and the various cones have different spectral sensitivities because they have slightly different opsins. The structure of the opsin determines which wavelengths of light a cell will absorb and how it will activate its associated 11-cis-retinal.

Question 4

If you are looking straight ahead and someone holds up a colored card on your right or left side, why can’t you tell what the color is?

You cannot discriminate color in the periphery of your visual field because the cone cells are concentrated in the fovea, which receives light only from the center of the visual field. Rods do not distinguish color and are more abundant in the peripheral retina.

Question 5

Give two reasons why your vision is impaired when you come from bright outside light into a dimly lit room.

Two reasons why vision is impaired when you come from a brightly to a dimly lit environment are that bright light bleaches many rhodopsin molecules, and the amacrine cells have to readjust the range of brightness sensitivity of the retina.

Knowing the path of information from sensory receptor cells to the CNS still does not tell us how that information is processed by the brain. What does the eye tell the brain, for example, in response to a pattern of light falling on the retina? In Chapter 46 you will learn how the mammalian brain reassembles sensory information into our perception of the world.