The Visual System

Brain Integration of Visual Information: V1

(posterior occipital cortex): Processes all aspects of vision

What is the theory of trichromaticity

-Humans and other primates have three populations of cone photoreceptors, which are sensitive to different wavelengths of light. The three cone classes have slightly different photopigments of the rhodopsin family, each with a somewhat dif- ferent opsin sequence that alters its sensitivity to light wavelengths. -The three types of cones are usually called red cones, green cones, and blue cones, loosely approximating the color of light to which they are most sensitive

fovea

-Some light scattering does occur, however, and many retinas have a central high-acuity region in which the intervening cell layers and blood vessels are displaced to the side. -a depression 1.5 mm in diameter (5o of visual angle) -The central 1o of the fovea contains tightly packed cones to the exclusion of other neurons. Rod photoreceptors are absent in this central part of the human fovea but greatly outnumber cones elsewhere in the retina. Primates and some birds have well- developed foveas

Summary Photoreception

-The vertebrate eye is a camera eye that focuses light onto retinal rod and cone photoreceptors. Rods and cones are unusual in that light produces a hyperpolarizing receptor potential. -The photopigment rhodopsin is a GPCR molecule conjugated to retinal. It is contained in membranes of outer segments of vertebrate rods and cones.When rhodopsin absorbs light, it acts via a G protein to decrease the concentration of cGMP in the cytoplasm, leading to closing of cGMP-gated Na+ channels that keep the photoreceptor depolarized in the dark. Light-induced closure of these channels hyperpolarizes the photoreceptors. -In arthropods such as Drosophila the photopigment rhodopsin is similar to that of vertebrates and activates a similar G protein, but it is linked to a different intracellular effector and leads to the production of DAG and IP3, opening ion channels and producing a depolarizing receptor potential. -Rhodopsin is deactivated and ultimately regenerated to 11-cis rhodopsin after activation. In vertebrates, most regeneration is a slow enzymatic process, part of which occurs outside the photoreceptors in the adjacent pigment epithelium.

two major kinds of image forming eyes

-camera eye -compound eye

The response to light is thus a ____________ in the dark current which causes the membrane potential to become more ____________, producing a ______________ receptor potential that is graded according to the intensity of the light flash

-decrease -negative -hyperpolarizing

rods

-more sensitive -used in dim light

dark current

-the inward sodium current that occurs in photoreceptors in the dark -must be constantly counteracted by the expenditure of ATP to pump Na+ out of the cell with a very active Na+-K+-ATPase located in the inner segment. This large expenditure of ATP makes the metabolic rate of a rod in darkness perhaps the highest of any cell in the body. -keeps the rod relatively depolarized. The light- induced decrease in cytoplasmic concentration of cGMP shifts the equilibrium of cGMP binding to the channel protein so that cGMP dissociates from the channel and the channel closes.

cones

-used in brighter light, for color vision, and for high- acuity vision in humans and other animals having a fovea

lateral pathways

-which extend along the retinal sheet, via horizontal cells and amacrine cells -Horizontal and amacrine -antagonistic surround

straight-through pathways

-which project radially through the retina at right angles to its surface (photoreceptors → bipolar cells → ganglion cells -The straight-through pathways give rise to the properties of the center of a contrast-sensitive ganglion cell's receptive field -Photoreceptors - bipolar cells - RGCs the center of a contrast-sensitive ganglions receptive field

The transduction of light into an electrical signal in rod and cone photoreceptors has four stages:

1) light activates rhodopsin; 2) activated rhodopsin stimulates a G protein to activate a phosphodiesterase enzyme; 3) the enzyme decreases the concentra- tion of cyclic GMP in the photoreceptor cytoplasm; 4) the decrease in cyclic GMP closes cyclic nucleotide-gated ion channels similar to the channels in olfactory receptor cells.

The form of retinal used in vision in most animals, including all mammals is ___________

11-cis retinal

Chromophore

A photopigment consists of a protein containing an associated nonpeptide organic molecule called a ________ (from the Greek meaning "color bearing"). The absorption of a photon of light by a _____________ produces a chemical reaction, and this triggers a transduction cascade.

PAX6

A single gene, PAX6, can initiate eye development in diverse animals, and PAX6 interacts similarly with other regulatory genes in the eye development of many phylogenetic groups of animals. It is likely that basic genetic plans for eye development and function evolved very early, but specific eye lineages (such as the camera eyes of cephalopods evolved separately, using parallel expressions of these developmental and functional modules.

dark adaptation.

If rods are stimulated by very bright light, the photoreceptors recover very slowly. This is because the bright light has converted 11-cis retinal to all-trans retinal in a large fraction of the pigment of the eye, a process called bleaching. After a bleach greater than about 1% of the photopigment, recovery of rod vision is prolonged; this slow adjustment to darkness is called

Eyes

Most animal phyla contain examples of eyes (light-responsive sense organs) that differ in type and complexity. Eyes, as noted earlier, are sense organs that contain photoreceptors mediating light responses; eyes may also form images of the visual world that provide information to allow an animal to localize and identify visual stimuli.

Visual Sensory Processing

Photoreceptors respond to light, but the vertebrate visual system responds to pattern: contrasts, or changes in light level and color over space and time. This conversion—from sensitivity to light into sensitivity to contrast—occurs partly in the retina and partly in higher visual-processing areas of the brain.

Rhodopsin activates a G protein signal-transduction cascade, like other members of the GPCR superfamily

The details of this cascade can differ in different kinds of animals and sometimes even in different photoreceptors of the same animal

Color vision is accomplished by populations of photoreceptors that contain different photopigments

The ability to distinguish color depends on the differential sen- sitivities of photopigments to different wavelengths of light. Al- though many animals are color-blind, many other animals with well-developed diurnal visual systems have evolved color vision. Examples include several orders of insects, crustaceans such as mantis shrimp, teleost fish, frogs, turtles, lizards, birds, and primates.

rhabdomere

The eight or more retinular cells of an ommatidium are arranged in a circle, like sections of an orange but more elongate. The transduction cascade of a retinu- lar cell is localized to the membranes of its microvilli, which are arranged along one edge of the retinular cell in an array called a rhabdomere. The microvillar membranes of the rhabdomere contain not only the rhodopsin photopigment, but also the G proteins and associated proteins, including the channels that produce the electrical response to light.

Camera eye

a lens forms an inverted image on an array of photoreceptors at the back of the eye

In contrast to the compound eye of insects

all vertebrates have camera eyes. In this type of eye, the cornea and lens focus an in- verted image of the visual field on the retina

off-center cell

also has a receptive field with a concentric, antagonistic center and surround. The off-center cell, however, is inhibited by light in its center and excited by light in its surround. Its receptive field is thus the opposite of the field of the on-center cell.

cgMP phosphodiesterase (PDe)

an enzyme in the disc membrane that hydrolyzes cGMP to 5′-GMP. Activation of PDE involves dissociation of its catalytic subunit from a regulatory subunit that inhibits its activity. The activated PDE decreases the cytoplasmic concentration of cGMP, and the cation channels close. Thus the second messenger-mediated response to light in a rod outer segment is decreased Na+ influx, producing hyperpolarization.

retinal ganglion cells

are the output of the retina; their axons form the optic nerve that extends to the brain -in contrast to photoreceptors, each respond to stimulation over a relatively large visual area—the ganglion cell's receptive field

many other vertebrates have a less elaborate and broader area of relatively high acuity called the ____________

area centralis

For terrestrial vertebrates the greatest amount of refraction occurs

at the interface between the air and the cornea, which differ greatly in refractive index and thus produce greater bending. Refraction by the lens is secondary because the lens and the aqueous humor have more similar refrac- tive indices. Lens refraction serves primarily to focus the image by changing the shape of the lens

lateral geniculate nucleus (lgN)

axons of retinal ganglion cells that form the optic nerve synapse in a region of the thalamus termed the

The retina contains the rod and cone photoreceptors and four kinds of integrating neurons (see Figure 14.25B):

bipolar cells, horizontal cells, amacrine cells, and ganglion cells

Brain Integration of Visual Information: V4

color information

Which photoreceptor predominate in retinas of strongly diurnal animals?

cones

inner segment of photoreceptors

containing the nucleus, mitochondria, other cell organelles, and the synaptic terminal

outer segment of photoreceptors

contains photosensitive membranes -contain many flattened lamellae of membranes derived from the cell membrane.

there is an intracellular messenger that conveys a change from the discs to the outer surface membrane. This intracellular messenger is

cyclic gMP (cyclic guanosine monophosphate, or cgMP

Drosophila pho- toreceptors, like most arthropod photoreceptors,

do not generate action potentials.

Horizontal cells

extend tangentially, connecting different regions of the outer plexiform layer.

Brain Integration of Visual Information: V3/V3A

form and dynamic form of an object

Brain Integration of Visual Information: TE/IT

identification of objects

rhabdomeric photoreceptor cells

in contrast, have collections of microvilli that increase the membrane surface area. Vertebrate photoreceptor cells are ciliary, and arthropod photoreceptor cells are rhabdomeric, but many other phyla have examples of both photoreceptor types, sometimes in the same animal.

on-center cell

increases its rate of impulse discharge when the center of its receptive field is illuminated by a spot of light (see Figure 14.29A). The same spot of light, however, suppresses activity when it is presented in the larger surrounding part of the receptive field. The on-center cell is maximally stimu- lated when the entire center of its receptive field, but none of its surround, is illuminated. The cell's activity is maximally inhibited or suppressed when the surround, but not the center, is illuminated by an annulus (ring) of light. Such suppression is followed by an increased discharge when the light is turned off (an "off response"). Diffuse light stimulation, covering the entire receptive field of the ganglion cell, has little effect on the cell's activity because the excitatory effect of light at the center and the inhibitory effect of light in the surround are antagonistic, canceling each other out.

In the eye of aquatic vertebrates, the cornea does little refraction, because

its refractive index is similarto that of water. Instead, a thick, nearly spherical lens accomplishes most of the refraction; the refractive index of the lens is greater at its center than at its edges, thereby minimizing distortion. A spheri- cal fish lens cannot change shape, but instead focuses by moving anteriorly and posteriorly, like a camera lens.

Compound eye

many facets called ommatidia (singular ommatidium), each with its own lens, together produce what is called a mosaic image—each ommatidium conveys information about one part of the visual world, and the animal's nervous system constructs the image as a mosaic of "tiles" of individual ommatidial responses.

Amacrine cells

mediate similar tangential interconnections in the inner plexiform layer. Tangential connections (IPL) Movement and direction

ciliary photoreceptor cells

modified cilia contain the rhodopsin molecules

Brain Integration of Visual Information: V5/MT

movement

receptive field

of a neuron such as the ganglion cell is defined as the area of the retina (or the area of the visual field) within which the membrane potential or impulse activity of that neuron can be influenced by light. For a rod or cone, the receptive field corresponds for the most part to the retinal area occupied by the receptor itself. For visual interneurons such as retinal ganglion cells, in contrast, the receptive field is typically much larger, embracing an area containing many photoreceptors, and can also include different regions giving responses of different polarity. -The area of the retina within which the membrane potential or impulse activity of the neuron (retinal ganglion cell) can be influenced by light --->On-center cell --->Off-center cell

Both rods and cones have an ________ segment and an ________ segment

outer and inner

The aspects of visual stimuli that are most important for the behavior of animals (including humans) are ____________ of light and darkness and color, rather than the overall light level

patterns

The absorption of light by 11-cis retinal produces a _______________, which twists the aldehyde tail of the chromophore around one of its double bonds and produces all-trans retinal

photochemical reaction

All organisms detect light by using a pigment, termed a ______________, that absorbs the light. Although bacteria have several kinds of photopigments, all animals employ one dominant photopigment, rhodopsin, for photoreception

photopigment -Because the photopigment is associated with cell membranes, all photoreceptor cells (light-responsive cells) have greatly increased membrane surface areas that increase their light sensitivity. Photoreceptor cells are subdivided into ciliary and rhabdomeric.

Neurons of the LGN project to the ________________ at the posterior end of the cerebrum. Unlike the optic projections of lower vertebrates, the projections of most mammals are only partially crossed at the optic chiasm.

primary visual cortex

Brain Integration of Visual Information: V2

process all aspects of vision, but more complex shapes than V1

bipolar cells

receive input from photoreceptors at the outer plexiform layer near the photoreceptors, and they synapse on amacrine and ganglion cells in the inner plexiform layer. Input from photoreceptors Synapse on amacrine and RGCs

Two kinds of color-opponent processes are present in the mam- malian retina

red-green opponency and blue-yellow opponency. These opponent processes explain why we can perceive a color as bluish green or as reddish yellow (orange), but we do not ordinarily perceive a color as reddish green or as bluish yellow

The chromophore of animal photoreceptors is

retinal -the aldehyde of vitamin A which is bound to the integral membrane protein opsin, to produce the light-sensitive pigment rhodopsin.

All insects and many other arthropods have compound eyes, which consist of many clustered cell groups called om- matidia, each ommatidium with its own lens and photoreceptors called __________________

retinular cells

all eyes employ the same kind of photopigment: ___________________

rhodopsin

Nocturnal animals tend to have retinas in which most or all photoreceptors are

rods

two kinds of photoreceptors in the retina,

rods and cones

Phototransduction in insects

similar at the outset to that in vertebrates but involves a different second messenger. Absorption of a photon causes a change in the conformation of rhodopsin, leading to activation of a G protein by exchange of GTP for GDP on the G-protein α subunit. The activated G protein then activates a phospholipase C, which produces the second messengers IP3 and diacylglycerol (DAG). DAG (or a lipid metabolite of it) is thought to directly open two cation channels, both of which are TRP channels. Opening of the cation-permeable channels produces a depolarization, which spreads along a short axon to the photoreceptor synaptic terminal and triggers the release of synaptic transmitter.

We can differentiate two major kinds of neurons in the visual cortex:

simple cells and complex cells

two sorts of retinal pathways

straight-through pathways lateral pathways

The retina of the vertebrate eye is a developmental outgrowth of

the brain

In mammals the major visual projection is __________________

the geniculostriate system

Lamellae in rods

the lamellae become separated from the outer membrane and form internalized flattened discs (see Figure 14.26). Several hundred to 1000 discs, stacked like pancakes, fill the rod outer segment. The membranes of rod discs and cone invaginated lamellae contain the photopigment rhodopsin. Because the disc membranes and the cell membrane are discontinuous in rods, the light-induced changes in rhodopsin at the disc membrane must somehow affect the ion permeability of the outer membrane. An electrical change in the disc membrane will not directly affect thesurface membrane because these two membranes are not in continuity.

Retina

the photoreceptor-containing layer at the back of the eye. Light is refracted (light rays are bent) at surfaces where materials differ in density (quantified as the refractive index) -It (in vertebrates) contains rod and cone photoreceptor cells, as well as a network of neurons—horizontal cells, bipolar cells, amacrine cells, and ganglion cells—which perform the first stages of visual integration

Lamellae in cones

these lamellae retain continuity with the outer cell membrane, so the lumen of each lamella is continuous with extracellular space.

Light-stimulated rhodopsin (metarhodopsin II) activates the G protein _______________ which is closely related to the G proteins that mediate metabotropic synaptic actions and chemoreception

transducin

The photoreceptor protein rhodopsin

was the first G protein-coupled receptor to be sequenced, cloned, and examined with X-ray crystallography, and the details of the G-protein cascade in vertebrate photoreceptors are more fully understood than any other G-protein cascade in nature.

The retina is said to be inverted

with the photoreceptors in the outermost layer, farthest away from incoming light. This inverted structure is a consequence of the way in which the retina develops in the embryo, as the more distal layer of a two-layered optic cup.

Photoreceptors

—sensory receptor cells that are sensitive to light—are easier to identify, isolate, grind up, clone, and study with physiological techniques than other receptor cells

Photoreception

—the response of a sensory cell to light—arose early in evolution