Light Absorption Optic Nerve

A bright light stimulus results in a strong hyperpolarization.

opsin pass through several unstable intermediate stages. One of these stages is known as photoexcited rhodopsin because it triggers a cascade of reactions that result in the alteration of membrane potential that is the photoreceptor cell's response to light.

To get a better idea of how rhodopsin alters the membrane potential of a photoreceptor cell and how that photoreceptor cell signals that it has been stimulated by light, let's look at one type of vertebrate photoreceptor cell, the rod cell. Like other vertebrate photoreceptor cells, the rod cell is a modified neuron (Figure 45.13a). However, it does not produce action potentials, but instead releases a neurotransmitter that influences the membrane potentials of other neurons. Those neurons process signals from the photoreceptor cells to produce action potentials in their axons, which make up the optic nerve. The optic nerve transmits information about the visual world to the brain.

Each rod cell has an outer segment, an inner segment, and a synaptic terminal. The outer segment is highly specialized and contains a stack of discs of plasma membrane densely packed with rhodopsin. The function of the discs is to capture photons of light passing through the rod cell. The inner segment contains the cell nucleus and abundant mitochondria. The synaptic terminal is where the rod cell communicates with other neurons.

To see how a rod cell responds to light, we can penetrate a single rod cell with an electrode and record its membrane potential in the dark and in the light (see Figure 45.13a). From what we have learned about other types of sensory cells, we might expect stimulation of the rod cell by light to make its membrane potential less negative. But photoreceptor cells are atypical, and the opposite is true. When a rod cell is kept in the dark, it has a relatively depolarized resting potential in

45.13 A Rod Cell Responds to Light

(a) The vertebrate rod cell is a neuron modified for photosensitivity.The membranes of a rod cell's discs are densely packed with rhodopsin.

(b) The plasma membrane of a rod cell hyperpolarizes—becomes more negative—in response to a flash of light.

Time

A bright light stimulus results in a strong hyperpolarization.

comparison with other neurons. In fact, the plasma membrane of the rod cell is almost as permeable to Na+ ions as to K+ ions, and Na+ ions are continually entering the outer segment of the cell.

When a light is flashed on the dark-adapted rod cell, its membrane potential becomes more negative—it hyperpolarizes (Figure 45.13b). The rod cell changes its rate of neu-rotransmitter release as its membrane potential changes (since the rod cell hyperpolarizes, neurotransmitter release decreases).

How does the absorption of light by rhodopsin hyperpo-larize the rod cell? When rhodopsin is excited by light, it initiates a cascade of events. The photoexcited rhodopsin combines with and activates another protein, a G protein called transducin. Activated transducin in turn activates a phosphodiesterase, which converts cyclic GMP (cGMP) to GMP. This reaction plays a central role in phototransduction. In the dark, the cGMP in the outer segment binds to both cation channels, keeping them open and allowing Na+ and Ca2+ to enter the outer segment. As cGMP is converted to GMP, the sodium channels close, and the cell hyperpolarizes.

This mechanism may seem like a roundabout way of doing business, but its advantage is its enormous amplification ability. Each molecule of photoexcited rhodopsin can activate several hundred transducin molecules, thus activating a large number of phosphodiesterase molecules. The catalytic capacity of a molecule of phosphodiesterase is great: It can hy-drolyze more than 4,000 molecules of cGMP per second. The bottom line is that a single photon of light can cause a huge number of sodium channels to close (Figure 45.14).

Invertebrates have a variety of visual systems

Photoreceptors are incorporated into a variety of visual systems, from simple to complex. Flatworms obtain directional information about light from photoreceptor cells that are organized into eye cups. The eye cups are paired bilateral structures, each partly shielded from light by a layer of pigmented

Disc

Disc

Photoreceptor Rhodopsin Sodium Channels

45.14 Light Absorption Closes Sodium Channels The absorption of light by rhodopsin initiates a cascade of events resulting in the hyperpolarization of the rod cell.

cells lining the cup. The photoreceptors on the two sides of the animal are unequally stimulated unless the animal is facing directly toward or away from a light source. The flat-worm generally uses directional information from the eye cups to move away from light.

Retinula Compound Eyes Insect

Corneal lens

Crystalline cone

Pigment cell

Retinula cell

Corneal lens

Crystalline cone

Pigment cell

Arthropods have evolved compound eyes that provide them with information about patterns or images in the environment. Each compound eye consists of many optical units called ommatidia (singular, ommatidium) (Figure 45.15). The number of ommatidia in a compound eye varies from only a few in some ants, to 800 in fruit flies, to 10,000 in some dragonflies.

Each ommatidium has a lens structure that directs light onto photoreceptors. Flies, for example, have eight elongated photoreceptors in each ommatidium. The inner borders of the photoreceptors are covered with microvilli that contain rhodopsin and trap light. Axons from the photoreceptors communicate with the nervous system. Since each omma-tidium of a compound eye is directed at a slightly different part of the visual

>- Ommatidium ^^ only a ^^ or perhaps a bro-ken-up, image can be communicated from the compound eye to the CNS.

Retinula cell

Bundle of axons 45.15 Ommatidia: The Functional Units of Insect Eyes to brain (a) The micrograph shows the compound eye of a fruit

Basement fly (Drosophila). (b) The rhodopsin-containing retinula membrane cells are the photoreceptors in ommatidia.

Image-forming eyes evolved independently in vertebrates and cephalopods

Both vertebrates and cephalopod mollusks have evolved eyes with exceptional abilities to form images of the visual world. Like cameras, these eyes focus images on a surface that is sensitive to light (Figure 45.16). Considering that they evolved independently of each other, their high degree of similarity is remarkable.

The vertebrate eye is a spherical, fluid-filled structure bounded by a tough connective tissue layer called the sclera. At the front of the eye, the sclera forms the transparent cornea, through which light passes to enter the eye. Just inside the cornea is the pigmented iris, which gives the eye its color. The function of the iris is to control the amount of light that reaches the photoreceptor cells at the back of the eye, just as the diaphragm of a camera controls the amount of light reaching the film. The central opening of the iris is the pupil. The iris is under neuronal control. In bright light, the iris constricts, and the pupil is very small. As light levels fall, the iris relaxes, and the pupil enlarges.

Behind the iris is the crystalline protein lens, which makes fine adjustments in the focus of images on the photosensitive layer, the retina, at the back of the eye. The most sensitive area of the retina is called the fovea. The cornea and the fluids within the eye are mostly responsible for focusing light on the retina, but the lens allows the eye to accommodate—that is, to focus on objects at various locations in the near visual field. To focus a camera on objects close at hand, you adjust the distance between the lens and the film. Fishes, amphibians, and reptiles accommodate in a similar manner, moving the lenses of their eyes closer to or farther from their retinas. Mammals and birds use a different method: They alter the shape of the lens.

The lens is contained in a connective tissue sheath that tends to keep it in a spherical shape, but it is attached to suspensory ligaments that pull it into a flatter shape. Circular muscles called the ciliary muscles counteract the pull of the suspensory ligaments and permit the lens to round up. When the ciliary muscles are at rest, the flatter lens has the correct optical properties to focus distant images on the retina, but not close images. Contracting the ciliary muscles rounds up the lens, changing its light-bending properties to bring close images into focus (Figure 45.17). As we age, our lenses become less elastic, and we lose the ability to focus on objects close at hand without the help of corrective lenses. As a consequence, most adults over the age of 45 need the assistance of bifocal lenses or reading glasses to compensate " - 4517 for their lost ability to accommodate. focus

Retina Inverted

A camera's lens focuses an inverted image on the film in the same way the eye's lens focuses an image on the retina.

Ciliary muscle

Suspensory ligaments

Optic nerve

Blind spot

Fovea

Retina

Iris Sclera

Eye of human

A camera's lens focuses an inverted image on the film in the same way the eye's lens focuses an image on the retina.

Ciliary muscle

Suspensory ligaments

Optic nerve

Blind spot

Fovea

Retina

Iris Sclera

Eye of human

Optic nerve

Double layer of receptor cells

Eye of squid

The eye of the squid is very similar in structure to the vertebrate eye, but it evolved independently.

Optic nerve

Double layer of receptor cells

Eye of squid

|45.16 Eyes Like Cameras The lenses of cephalopod and vertebrate eyes focus images on layers of photoreceptor cells, just as a camera's lens focuses images on film.

The vertebrate retina receives and processes visual information

During embryonic development, neuronal tissue grows out from the brain to form the retina. In addition to a layer of photoreceptor cells, the retina includes four layers of cells that process visual information from the photoreceptors and produce an output signal that is transmitted to the brain via

Optic nerve

Optic nerve

Germinal Disc Birds
Staying in Focus Mammals and birds their eyes by changing the shape of the lens.

the optic nerve. The light-absorbing outer segments of the photoreceptor cells are all the way at the back of the retina. Light must pass through all the layers of retinal cells before being captured by rhodopsin. The outer segments are partly buried in a layer of pigmented epithelium that absorbs photons not captured by rhodopsin and prevents any backscat-tering of light that might decrease visual sharpness.

the photoreceptors of the retina. Until now we have referred to only one kind of photoreceptor cell, the rod cell. But there are two major kinds of vertebrate photoreceptors, both named for their shapes: rod cells and cone cells (Figure 45.18). A human retina has about 5 million cones and about 100 million rods. Rod cells are highly sensitive to light, so they are well suited for vision under dim light, are saturated by daylight conditions, and do not contribute to color vision. Cone cells are less sensitive to light, so they are better suited for daylight and color vision. Cones are also responsible for our sharpest vision. Even though there are many more rods than cones in human retinas, our foveas contain only cones.

Because cones have low sensitivity to light, they are of no use in dim light. At night our vision is not very sharp, and we see mostly in shades of gray. You may have trouble seeing a small object such as a keyhole at night when you are looking straight at it—that is, when its image is falling on your fovea. If you look a little to the side, so that the image falls on a rod-rich area of your retina, you can see the object better. Astronomers looking for faint objects in the sky learned this trick a long time ago. Animals that are nocturnal

Rod cells

Cone cells

(such as flying squirrels) have retinas containing a high percentage of rods and may have poor color vision. By contrast, some animals that are active only during the day (such as chipmunks) have mostly cones in their retinas.

The human retina has three kinds of cone cells, each containing slightly different isomers of opsin. These opsin molecules differ in the wavelengths of light they absorb best. Although the same 11-cis-retinal group is the light absorber in all three kinds of cones (see Figure 45.12), its molecular interactions with opsin determine the spectral sensitivity of the rhodopsin molecule as a whole. One isomer of opsin causes the retinal group to absorb short-wavelength light (e.g., violet and blue) most efficiently; the others result in absorption of middle wavelengths (e.g., green) long wavelengths (e.g., yellow and red) (Figure 45.19).

The density of rods and cones is not the same across the entire retina. In humans, light coming from the center of the visual field falls on the fovea, where the density of cone cells is highest. The human fovea has about 160,000 photorecep-tors per square millimeter. A hawk has about a million pho-toreceptors per square millimeter, making its vision sharper than ours. In addition, the hawk has two foveas in each eye: One receives light from straight ahead, while the other receives light from below. Thus, while the hawk is flying, it sees both its projected flight path and the ground below, where it might detect a mouse scurrying in the grass.

496 531 559 nm

Rod cells

Cone cells

Hawk Fovea ConeDiplomonads

496 531 559 nm

400 450 500 550

Wavelength (nm)

400 450 500 550

Wavelength (nm)

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Responses

  • Almaz Asfaha
    When a rod is stimulated by the plasma membrane becomes?
    8 years ago
  • jordan
    How rod cells in the dark and light with ion channel in membrane?
    8 years ago
  • Kimberley
    How do rod cells work in dark resting potential?
    8 years ago
  • natascia
    How do rod cells work in dark synaptic terminal?
    8 years ago
  • taija koskinen
    How rod cell work with light in eye?
    8 years ago
  • Ursula
    What diminish the amount of light that photoreceptors receive?
    8 years ago
  • may
    When a rod cell is stimulated with light its membrane potential?
    7 years ago
  • Mark
    What is a membrane on optical nerve?
    5 years ago
  • sarah
    What is the function of a optic nerve in a squid?
    5 years ago
  • ELIZABETH
    When a photoreceptor absorb light what happens to the Na channels in its outer segments?
    1 year ago

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