Receptor and Neural Function of the Retina pdf - The Nervous System

in the cones, it is one of three “color”

In the case of the rods, this is

The light-sensitive photochemical is found in the outer segment.

synaptic body.

nucleus,

rod or a cone: (1) the

To the right in Figure 50–3 are labeled the major functional segments of either a

rods, and the cones are slender and have a diameter of only 1.5 micrometers.

8 micrometers in diameter; in the central part of the retina, in the fovea, there are

the retina, the rods are 2 to 5 micrometers in diameter, whereas the cones are 5 to

longer than the cones, but this is not always the case. In the peripheral portions of

outer segment of the cone is conical in shape. In general, the rods are narrower and

ponents of a photoreceptor (either a rod or a cone). As shown in Figure 50–4, the

Figure 50–3 is a diagrammatic representation of the essential com-

of the cones. This allows light to pass unimpeded to the cones.

in the foveal region, the blood vessels, ganglion cells, inner nuclear layer of cells,

tradistinction to the much fatter cones located more peripherally in the retina. Also,

image. That is, the foveal cones have especially long and slender bodies, in con-

only 0.3 millimeter in diameter, is composed almost entirely of cones;

central fovea,

than 1 square millimeter; it is especially capable of acute and detailed vision. The

in the center of the retina, shown in Figure 50–2, occupying a total area a little more

fovea

The

are pulled aside to decrease this loss of acuity.

in the central foveal region of the retina, as discussed subsequently, the inside layers

acuity is decreased by this passage through such nonhomogeneous tissue. However,

of the retina. This distance is a thickness of several hundred micrometers; visual

(see Figure 50–1); that is, it passes first

ous humor, it

fibers, and (9) inner limiting membrane.

nuclear layer, (6) inner plexiform layer, (7) ganglionic layer, (8) layer of optic nerve

taining the cell bodies of the rods and cones, (4) outer plexiform layer, (5) inner

(2) layer of rods and cones projecting to the pigment, (3) outer nuclear layer con-

are arranged in layers from the outside to the inside as follows: (1) pigmented layer,

Figure 50–1 shows the functional components of the retina which

signals.

optic nerve fibers and the cerebral cortex. The

layers of neurons in the retina itself and, finally, into

the dark. When either rods or cones are excited,

rods,

for color vision, and (2) the

cones,

The retina is the light-sensitive portion of the eye

The Eye: II. Receptor and

626

Neural Function of the Retina

that contains (1) the

which are responsible

which are mainly

responsible for black and white vision and vision in

signals are transmitted first through successive

purpose of this chapter is to explain the mechanisms by which the rods and
cones detect light and color and convert the visual image into optic nerve

Anatomy and Function of the Structural
Elements of the Retina

Layers of the Retina.

After light passes through the lens system of the eye and then through the vitre-

enters the retina from the inside

through the ganglion cells and then through the plexiform and nuclear layers before
it finally reaches the layer of rods and cones located all the way on the outer edge

Foveal Region of the Retina and Its Importance in Acute Vision.

is a minute area

these cones have a special structure that aids their detection of detail in the visual

and plexiform layers are all displaced to one side rather than resting directly on top

Rods and Cones.

outer segment, (2) the inner segment, (3) the

and (4)

the

rhodopsin;

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

627

Pigmented layer

Cone

Vertical

pathway

Lateral pathway

Rod

Outer nuclear layer

Horizontal cell

Amacrine cell

Bipolar
cell

Amacrine

cell

Distal

Proximal

Inner nuclear layer

Inner plexiform
layer

Ganglion cell

To optic nerve

Ganglion cell layer

Stratum opticum

Inner limiting
membrane

Outer plexiform
layer

DIRECTION OF LIGHT

Layers of retina.

Figure 50–1

that connects with subsequent neuronal cells, the

The

ing energy for function of the photoreceptors.

larly important are the mitochondria; as explained later,

usual cytoplasm with cytoplasmic organelles. Particu-

The

proteins. The concentrations of these photosensitive

gated proteins. They are incorporated into the mem-

cone.

brane. There are as many as 1000 discs in each rod or

discs.

Figures 50–3 and 50–4 the large numbers of

except for differences in spectral sensitivity.

color pigments,

photochemicals, usually called simply

1986; courtesy H. Mizoguchi.)

interference with light transmission. (From Fawcett DW: Bloom and Fawcett: A Textbook of Histology, 11th ed. Philadelphia: WB S

o decrease

Photomicrograph of the macula and of the fovea in its center. Note that the inner layers of the retina are pulled to the side t

Figure 50–2

aunders,

that function almost exactly the same as rhodopsin

Note in the outer segments of the rods and cones in

Each

of the discs is actually an infolded shelf of cell mem-

Both rhodopsin and the color pigments are conju-

branes of the discs in the form of transmembrane

pigments in the discs are so great that the pigments
themselves constitute about 40 per cent of the entire
mass of the outer segment.

inner segment of the rod or cone contains the

these mitochondria play the important role of provid-

synaptic body is the portion of the rod or cone

color pigments,

cones,

nerve fibers leading from the eye. The light-sensitive

pose on exposure to light and, in the process, excite the

Photochemistry of Vision

will be unable to function even after surgical repair.

replaced soon, however, the retina will be destroyed and

normal relation with the pigment epithelium. If it is not

to the neural retina through the retinal artery, the

toward the interior of the globe.

in the vitreous humor, which pull areas of the retina

retina and the pigment epithelium. Detachment is occa-

. In some instances,

detaches from the pigment epithelium

The neural retina occasionally

blood vessels for their nutrition, especially for their

and cones, depend mainly on diffusion from the choroid

of the retina, especially the outer segments of the rods

lying between the retina and the sclera. The outer layers

choroid,

However, the outermost layer of the retina is adher-

structures of the eye.

Thus, the inner layers of the retina

inside retinal surface.

artery, which enters the eyeball through the center of

The nutrient blood supply for the internal

Blood Supply of the Retina—The Central Retinal Artery and the

rods and cones.

the pigment. We show later that vitamin A is an impor-

the rods and cones, which themselves are embedded in

This vitamin A is exchanged back and forth

The pigment layer also stores large quantities of

normal 20/20 values.

of albinos, even with the best optical correction, is

and excites many receptors. Therefore, the visual acuity

the retina and by the underlying sclera, so that a

their bodies. When an albino enters a bright room, light

albinos,

The importance of melanin in the pigment layer is

precise images.

camera. Without it, light rays would be reflected in all

clear vision. This pigment performs the same function

the globe of the eyeball; this is extremely important for

The black pigment

, that represent the next

horizontal

The Nervous System: B. The Special Senses

628

Unit X

and bipolar cells

stages in the vision chain.

Pigment Layer of the Retina.

melanin in

the pigment layer prevents light reflection throughout

in the eye as the black coloring inside the bellows of a

directions within the eyeball and would cause diffuse

lighting of the retina rather than the normal contrast
between dark and light spots required for formation of

well illustrated by its absence in

people who are

hereditarily lacking in melanin pigment in all parts of

that impinges on the retina is reflected in all directions
inside the eyeball by the unpigmented surfaces of

single discrete spot of light that would normally
excite only a few rods or cones is reflected everywhere

seldom better than 20/100 to 20/200 rather than the

vitamin A.
through the cell membranes of the outer segments of

tant precursor of the photosensitive chemicals of the

Choroid.

layers of the retina is derived from the central retinal

the optic nerve and then divides to supply the entire

have their own blood supply independent of the other

ent to the

which is also a highly vascular tissue

oxygen.

Retinal Detachment.

the cause of such detachment is injury to the eyeball
that allows fluid or blood to collect between the neural

sionally caused by contracture of fine collagenous fibrils

Partly because of diffusion across the detachment gap

and partly because of the independent blood supply

detached retina can resist degeneration for days and can
become functional again if it is surgically replaced in its

Both rods and cones contain chemicals that decom-

chemical in the rods is called rhodopsin; the light-
sensitive chemicals in the

called cone pigments

have compositions only slightly dif-

ferent from that of rhodopsin.

Outer segment

Membrane shelves

lined with rhodopsin

or color pigment

Mitochondria

Inner segment

Nucleus

Outer limiting
membrane

Synaptic body

Schematic drawing of the functional parts of the rods and cones.

Figure 50–3

(Courtesy Dr. Richard Young.)

Membranous structures of the outer segments of a rod

Figure 50–4

(left) and

a cone (right).

When the rod is exposed to light, the resulting

Once night blindness develops, it can sometimes be

stored in the liver and can be made available to the eyes.

because large quantities of vitamin A are normally

remain on a vitamin A–deficient diet for months,

For night blindness to occur, a person usually must

adequate vision in vitamin A–deficient persons.

This condition is called

this is that without vitamin A, the amounts of retinal and

with severe vitamin A deficiency. The simple reason for

light intensities.

sion between retinal and vitamin A is especially impor-

in the retina. We shall see later that this interconver-

A, thus reducing the amount of light-sensitive pigment

retinal in the retina, it is converted back into vitamin

retinal when needed. Conversely, when there is excess

vitamin A is normally always available to form new

rods and in the pigment layer of the retina. Therefore,

Vitamin A is present both in the cytoplasm of the

retinal, which combines with scotopsin to form new

Finally, the 11-

retinol under the influence of the enzyme isomerase.

Then the all-

retinol, which is one form of vitamin A.

into all-

retinal. This is by conversion of the all-

Figure 50–5 that there is a second chemical route by

light energy.

to re-form rhodopsin, which then remains stable until

formed, it automatically recombines with the scotopsin

retinal isomerase.

retinal. This process

of rhodopsin, as shown in Figure 50–5, is to reconvert

The first stage in re-formation

action potential, as we discuss later.

that excites electrical changes in the rods,

It is the metarhodopsin II, also called

seconds), into the completely split products

and finally, much more slowly (in

This then decays in microseconds

retinal and scotopsin. Bathorhodopsin

away from the scotopsin. The immediate product is

a straight molecule rather than an angulated molecule.

in the retinal portion of the rhodopsin, which leads to

50–5. The cause of this is photoactivation of electrons

fraction of a second, as shown at the top of Figure

When light energy is absorbed by rhodopsin, the

retinal. This

(also called “retinene”). Furthermore, the retinal is a

This substance is a combination of the

visual purple.

The outer

Rhodopsin and Its Decomposition by Light Energy.

Excitation of the Rods

Rhodopsin-Retinal Visual Cycle, and

applied to the cone pigments.

chemistry of rhodopsin, but the same principles can be

In this section, we discuss principally the photo-

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

629

segment of the rod that projects into the pigment layer
of the retina has a concentration of about 40 per cent
of the light-sensitive pigment called rhodopsin, or

protein scotopsin and the carotenoid pigment retinal

particular type called 11-cis

cis form of

retinal is important because only this form can bind
with scotopsin to synthesize rhodopsin.

rhodopsin begins to decompose within a very small

instantaneous change of the cis form of retinal into an
all-trans form that still has the same chemical structure
as the cis form but has a different physical structure—

Because the three-dimensional orientation of the
reactive sites of the all-trans retinal no longer fits
with the orientation of the reactive sites on the protein
scotopsin,

the all-trans

retinal begins to pull

bathorhodopsin, which is a partially split combination
of the all-trans

is extremely unstable and decays in nanoseconds to
lumirhodopsin.
to metarhodopsin I, then in about a millisecond to
metarhodopsin II,

scotopsin

and all-trans retinal.

activated

rhodopsin,
and the rods then transmit the visual image into the
central nervous system in the form of optic nerve

Re-formation of Rhodopsin.

the all-trans retinal into 11-cis
requires metabolic energy and is catalyzed by the
enzyme

Once the 11-cis retinal is

its decomposition is again triggered by absorption of

Role of Vitamin A for Formation of Rhodopsin.

Note in

which all-trans retinal can be converted into 11-cis

trans retinal first

trans

trans retinol is converted into 11-cis

cis retinol is converted into 11-cis

rhodopsin.

tant in long-term adaptation of the retina to different

Night Blindness.

Night blindness occurs in any person

rhodopsin that can be formed are severely depressed.

night blindness because the

amount of light available at night is too little to permit

reversed in less than 1 hour by intravenous injection of
vitamin A.

Excitation of the Rod When Rhodopsin
Is Activated by Light

The Rod Receptor Potential Is Hyperpolarizing, Not Depolariz-
ing.

Rhodopsin

Scotopsin

11-cis retinal

11-cis retinol

all-trans retinal

all-trans retinol

(Vitamin A)

Bathorhodopsin

(nsec)

Lumirhodopsin

(

sec)

Metarhodopsin I

(msec)

Metarhodopsin II

(sec)

Light energy

Isomerase

(p sec)

(minutes)

formation of rhodopsin by the chemical processes.

of rhodopsin during exposure to light and subsequent slow re-

Rhodopsin-retinal visual cycle in the rod, showing decomposition

Figure 50–5

phosphodiesterase.

3. The

2. The

shown in Figure 50–5.

form of rhodopsin, as already discussed and

retinal portion of the rhodopsin; this leads to the

1. The

excitation? The answer is that the photoreceptors

of light will cause half saturation of the rod. How

potential in a rod of about 1 millivolt. Only 30 photons

unit of light energy, can cause a measurable receptor

a single photon of light, the smallest possible quantal

Under optimal conditions,

Mechanism by Which Rhodopsin Decomposition

would be possible otherwise.

of the light intensity. This is exceedingly important,

as in the rods. A visual image impinged on the rods

second. In cones, the change occurs four times as fast

that is, the

When a sudden pulse of light strikes the retina, the

Relation of the Receptor Potential to Light Intensity.

Duration of the Receptor Potential, and Logarithmic

potassium ions across the membrane.

millivolts, which is near the equilibrium potential for

sity, the membrane potential approaches –70 to –80

tronegativity becomes—that is, the greater is the

light energy striking the rod, the greater the elec-

inside the membrane, and the greater the amount of

than leak back in. Because they are positive ions, their

segment. Thus, more sodium ions now leave the rod

the rod, even though sodium ions continue to be

to decompose, and this

of the rod is exposed to light, the rhodopsin begins

Then, when the rhodopsin in the outer segment

receptors.

of the rod, measuring about –40 millivolts rather than

normal dark conditions, when the rod is not excited,

negativity on the inside of the entire cell. Thus,

is very leaky to sodium ions. Therefore, positively

state,

different; here, the rod membrane, in the

where the photoreceptor discs are located, is entirely

the entire cell. However, the outer segment of the rod,

pumps sodium from inside the rod to the outside,

segments of the rod. The inner segment continually

Figure 50–6 shows movement of sodium ions in a

brane in the following way.

This causes hyperpolarization of the entire rod mem-

decomposes, it decreases the rod membrane conduc-

polarization? The answer is that

sensory receptors.

of “depolarization”) that occurs in almost all other

the rod membrane. This is exactly

intrarod membrane potential, which is a state of

tials in almost all other sensory receptors. That is, exci-

The Nervous System: B. The Special Senses

630

Unit X

receptor potential is different from the receptor poten-

tation of the rod causes increased negativity of the

hyper-

polarization, meaning that there is more negativity
than normal inside
opposite to the decreased negativity (the process

But how does activation of rhodopsin cause hyper-

when rhodopsin

tance for sodium ions in the outer segment of the rod.

complete electrical circuit through the inner and outer

thereby creating a negative potential on the inside of

dark

charged sodium ions continually leak back to the
inside of the rod and thereby neutralize much of the

under

there is reduced electronegativity inside the membrane

the usual –70 to –80 millivolts found in most sensory

decreases the outer segment

membrane conductance of sodium to the interior of

pumped outward through the membrane of the inner

loss from inside the rod creates increased negativity

degree of hyperpolarization. At maximum light inten-

transient hyperpolarization that occurs in the rods—

receptor potential that occurs—reaches a

peak in about 0.3 second and lasts for more than a

of the retina for only one millionth of a second can
sometimes cause the sensation of seeing the image for
longer than a second.

Another characteristic of the receptor potential is

that it is approximately proportional to the logarithm

because it allows the eye to discriminate light intensi-
ties through a range many thousand times as great as

Decreases

Membrane Sodium Conductance—The

Excitation “Cascade.”

can such a small amount of light cause such great

have an extremely sensitive chemical cascade that
amplifies the stimulatory effects about a millionfold,
as follows:

photon activates an electron in the 11-cis

formation of metarhodopsin II, which is the active

activated rhodopsin functions as an enzyme to

activate many molecules of transducin, a protein
present in an inactive form in the membranes of
the discs and cell membrane of the rod.

activated transducin activates many more

molecules of

- -

Leakage current
decreased by
decomposing
rhodopsin

Sodium

current

Sodium
pump

of the rod.

decreases

Theoretical basis for generation of a “hyperpolarization receptor

Figure 50–6

potential” caused by rhodopsin decomposition, which
the flow of positively charged sodium ions into the outer segment

40 minutes, about 25,000-fold.

tivity has increased about 6000-fold, and at the end of

required intensity. At the end of 20 minutes, the sensi-

sensitivity has already increased 10-fold—that is, the

first entering the darkness, but within 1 minute, the

having been exposed to bright light for several hours.

Figure 50–8 shows the course of dark adaptation

rods and cones to combine with the retinal. This is

to give still more light-sensitive pigments, the final

Furthermore, vitamin A is converted back into retinal

are converted back into the light-sensitive pigments.

long time, the retinal and opsins in the rods and cones

Conversely, if a person remains in darkness for a

reduced. This is called

in the rods and cones are considerably reduced, and

into vitamin A. Because of these two effects, the con-

retinal and opsins. Furthermore, much of the retinal of

light for hours, large portions of the photochemicals in

Retinal Sensitivity—Light and

Automatic Regulation of

nanometers.

for the rhodopsin of the rods, with a peak at 505

in Figure 50–7. Also shown is the absorption curve

the retina differentiates the colors. The approximate

ity for each type of cone, which begins to explain how

These are also the wavelengths for peak light sensitiv-

lengths of 445, 535, and 570 nanometers, respectively.

The

blue-sensitive pigment, green-

are called, respectively,

colors: blue, green, or red. These color pigments

color pigments is present in each of the different cones,

In the discussion of color vision later in the chapter,

nations of retinal and photopsins.

sensitive pigments of the cones, therefore, are combi-

the same in the cones as in the rods. The color-

The

are slightly different from the scotopsin of the rods.

tions, or the opsins—called

in the rods. The only difference is that the protein por-

Photochemistry of Color Vision by the Cones

than the rods, but even this allows color vision at

The cones are about 30 to 300 times less sensitive

dark conditions.

This explains the extreme sensitivity of the rods under

of light to cause movement of millions of sodium ions.

Thus, the rods have developed an important chemi-

sodium channels.

(the metarhodopsin II), and the entire cascade

the rod, inactivates the activated rhodopsin

rhodopsin kinase,

5. Within about a second, another enzyme,

ion flow is what excites the rod, as already

channel opens again. This diminution of sodium

rhodopsin. Because the sodium flux through

channels to close. Several hundred channels

phosphodiesterase hydrolyzes the cGMP, this

“splints” it in the open state. But in light, when

of the rod’s outer membrane in a way that

destroying it. Before being destroyed, the cGMP

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

631

4. Activated phosphodiesterase is another enzyme;

it immediately hydrolyzes many molecules of
cyclic guanosine monophosphate (cGMP), thus

had been bound with the sodium channel protein

removes the splinting and allows the sodium

close for each originally activated molecule of

each of these channels has been extremely rapid,
flow of more than a million sodium ions is
blocked by the channel closure before the

discussed.

which is always present in

reverses back to the normal state with open

cal cascade that amplifies the effect of a single photon

any intensity of light greater than extremely dim
twilight.

It was pointed out at the outset of this discussion that
the photochemicals in the cones have almost exactly
the same chemical composition as that of rhodopsin

photopsins in the cones—

retinal portion of all the visual pigments is exactly

it will become evident that only one of three types of

thus making the cones selectively sensitive to different

sensitive pigment, and red-sensitive pigment.
absorption characteristics of the pigments in the three
types of cones show peak absorbencies at light wave-

absorption curves for these three pigments are shown

Dark Adaptation

Light and Dark Adaptation.

If a person has been in bright

both the rods and the cones will have been reduced to

both the rods and the cones will have been converted

centrations of the photosensitive chemicals remaining

the sensitivity of the eye to light is correspondingly

light adaptation.

limit being determined by the amount of opsins in the

called dark adaptation.

when a person is exposed to total darkness after

Note that the sensitivity of the retina is very low on

retina can respond to light of one tenth the previously

600

700

Yellow

Violet

Blue

Green

400

Blue

cone

Green

cone

Red

cone

Rods

500

Red

Orange

Wavelength (nanometers)

Light absorption

(per cent of maximum)

100

and cones of the human retina: direct measurements reveal mech-

1964, and by Brown PK, Wald G: Visual pigments in single rods

Jr: Visual pigments of single primate cones. Science 143:1181,

from curves recorded by Marks WB, Dobelle WH, MacNichol EF

of the three color-receptive cones of the human retina. (Drawn

Light absorption by the pigment of the rods and by the pigments

Figure 50–7

anisms of human night and color vision. Science 144:45, 1964.
“ 1964 by the American Association for the Advancement of
Science.)

ring to Figure 50–9, one can see that an orange

Refer-

Interpretation of Color in the Nervous System.

ently in Figure 50–9. They can explain most of the phe-

curves are shown in Figure 50–7 and slightly differ-

the three types of pigment found in the cones. These

basis of color vision tests, the spectral sensitivities of

Spectral Sensitivities of the Three Types of Cones.

mixed in different combinations.

almost all gradations of colors when only red, green,

Tricolor Mechanism of Color

This section is a discussion of the mechanisms by

From the preceding sections, we have learned that dif-

Color Vision

times that of starlight, yet the eye can function both in

adaptation, the intensity of sunlight is about 10 billion

ter. As an example of the extremes of light and dark

After dark adaptation, the light spots begin to regis-

Conversely, when a person first enters darkness, the

receptors excessively.

its different parts. This is poor vision, and it remains

visual image is bleached, having little contrast among

exceedingly bright, and as a consequence, the entire

sunlight. Then, even the dark spots in the images seem

lighter areas but not to the darker areas. An example

image, it is essential that the sensitivity of the retina

as much as 500,000 to 1 million times, the sensitivity

adaptation, the eye can change its sensitivity to light

Value of Light and Dark Adaptation in Vision.

adaptation by the photochemicals.

neural adaptation occurs in a fraction of a second, in

the neural circuit. Although the degree of adaptation is

glion cells are all intense. However, most of these signals

bipolar cells, horizontal cells, amacrine cells, and gan-

intensity first increases, the signals transmitted by the

in the retina itself and in the brain. That is, when light

The other mechanism is

pupillary opening.

mately 30-fold within a fraction of a second, because of

in Chapter 49. This can cause adaptation of approxi-

change in pupillary size,

other mechanisms for light and dark adaptation. The

rhodopsin or color photochemicals, the eye has two

sensitivity, as discussed later in the chapter.

cell in the retina; these rods summate to increase their

sitivity increasing tremendously. In addition, still more

to adapt for many minutes and even hours, their sen-

a few minutes, while the slowly adapting rods continue

rapid adaptation, the cones cease adapting after only

change in darkness as the rods do. Therefore, despite

rapidly in cones as in rods. However, the cones do not

vision, including adaptation, occur about four times as

tation of the cones, because all the chemical events of

curve. The early portion of the curve is caused by adap-

Note, however, the inflection in the

adaptation curve.

The resulting curve of Figure 50–8 is called the

The Nervous System: B. The Special Senses

632

Unit X

dark

achieve anywhere near the same degree of sensitivity

sensitivity of the rods is caused by neuronal signal con-
vergence of 100 or more rods onto a single ganglion

Other Mechanisms of Light and Dark Adaptation.

In addition

to adaptation caused by changes in concentrations of

first of these is a

as discussed

changes in the amount of light allowed through the

neural adaptation, involving

the neurons in the successive stages of the visual chain

decrease rapidly at different stages of transmission in

only a fewfold rather than the many thousandfold that
occurs during adaptation of the photochemical system,

contrast to the many minutes to hours required for full

Between the

limits of maximal dark adaptation and maximal light

automatically adjusting to changes in illumination.

Because registration of images by the retina

requires detection of both dark and light spots in the

always be adjusted so that the receptors respond to the

of maladjustment of retinal adaptation occurs when a
person leaves a movie theater and enters the bright

poor until the retina has adapted sufficiently so that
the darker areas of the image no longer stimulate the

sensitivity of the retina is usually so slight that even
the light spots in the image cannot excite the retina.

bright sunlight after light adaptation and in starlight
after dark adaptation.

ferent cones are sensitive to different colors of light.

which the retina detects the different gradations of
color in the visual spectrum.

Detection

All theories of color vision are based on the well-
known observation that the human eye can detect

and blue monochromatic lights are appropriately

On the

the three types of cones in humans have proved to be
essentially the same as the light absorption curves for

nomena of color vision.

Minutes in dark

Retinal sensitivity

100

1000

10,000

100,000

Rod adaptation

Cone adaptation

rod adaptation.

Dark adaptation, demonstrating the relation of cone adaptation to

Figure 50–8

inner plexiform layer, where they synapse with

from the rods, cones, and horizontal cells to the

bipolar cells,

3. The

horizontal cells,

2. The

plexiform layer, where they synapse with bipolar

1. The photoreceptors themselves—the

foveal retina. The different neuronal cell types are as

neural connections, showing at the left the circuit in

Figure 50–11 presents the essentials of the retina’s

neural organization in the retina. To simplify this,

Figure 50–1 shows the tremendous complexity of

Neural Circuitry of the Retina

certain colors by color-blind people.

cones depicted in Figure 50–9, it can be readily under-

“2,” and the green-blind person reads “4.”

vision reads “42,” whereas the red-blind person reads

“21.” In the bottom chart, the person with normal color

“74,” whereas the red-green color-blind person reads

top chart, the person with normal color vision reads

a confusion of spots of several different colors. In the

shown in Figure 50–10. These charts are arranged with

Color Test Charts.

nomenon called blue weakness.

although sometimes they are underrepresented, which

Only rarely are blue cones missing,

Blue Weakness.

blindness is passed from mother to son, and the mother

inherited from the mother, never from the father, color

can lead to color blindness.

the male has only one X chromosome, a missing gene

always has a normal gene for each type of cone. Because

Yet color blindness almost never occurs in females

female X chromosome code for the respective cones.

occurs almost exclusively in males. That is, genes in the

detect the long wavelength red color.

cones. A color-blind person who lacks green cones is

blindness.

colors; the person is especially unable to distinguish red

cones. If either of these two cones is missing, the person

wavelengths of 525 and 675 nanometers, are normally

orange, and red colors, which are the colors between the

instance, one can see in Figure 50–9 that green, yellow,

unable to distinguish some colors from others. For

receptive cones is missing from the eye, the person is

When a single group of color-

types of cones about equally.

thermore, the perception of white can be achieved by

bination of all the wavelengths of the spectrum. Fur-

of light corresponding to white; instead, white is a com-

tion of seeing white. Yet there is no single wavelength

all the red, green, and blue cones gives one the sensa-

preted as yellow, and 31:67:36 as green.

system as blue. Likewise, ratios of 83:83:0 are inter-

value of 0, and the blue cones to a value of 97. This set

cones to a stimulus value of 0, the green cones to a

orange. Conversely, a monochromatic blue light with a

of cones in this instance are 99:42:0. The nervous

at all. Thus, the ratios of stimulation of the three types

to a stimulus value of about 42, but the blue cones not

at optimum wavelength); it stimulates the green cones

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

633

monochromatic light with a wavelength of 580
nanometers stimulates the red cones to a stimulus
value of about 99 (99 per cent of the peak stimulation

system interprets this set of ratios as the sensation of

wavelength of 450 nanometers stimulates the red

of ratios—0:0:97—is interpreted by the nervous

Perception of White Light.

About equal stimulation of

stimulating the retina with a proper combination of
only three chosen colors that stimulate the respective

Color Blindness

Red-Green Color Blindness.

distinguished from one another by the red and green

cannot use this mechanism for distinguishing these four

from green and is therefore said to have red-green color

A person with loss of red cones is called a protanope;

the overall visual spectrum is noticeably shortened at
the long wavelength end because of a lack of the red

called a deuteranope; this person has a perfectly normal
visual spectral width because red cones are available to

Red-green color blindness is a genetic disorder that

because at least one of the two X chromosomes almost

Because the X chromosome in the male is always

is said to be a color blindness carrier; this is true in about
8 per cent of all women.

is a genetically inherited state giving rise to the phe-

A rapid method for determining color

blindness is based on the use of spot charts such as those

If one studies these charts while at the same time

observing the spectral sensitivity curves of the different

stood how excessive emphasis can be placed on spots of

Neural Function of the Retina

the peripheral retina and at the right the circuit in the

follows:

rods and

cones—which transmit signals to the outer

cells and horizontal cells

which transmit signals

horizontally in the outer plexiform layer from the
rods and cones to bipolar cells

which transmit signals vertically

ganglion cells and amacrine cells

600

700

Yellow

Violet

Blue

Green

400

Blue

cone

Green

cone

Red

cone

500

Red

Orange

Wavelength (nanometers)

Light absorption

(per cent of maximum)

Blue

Green

Yellow

Orange

100

green, yellow, and orange.

sensitive cones by monochromatic lights of four colors: blue,

Demonstration of the degree of stimulation of the different color-

Figure 50–9

trast in the visual image.

Their role may be to help control the degree of con-

by the horizontal cells in the outer plexiform layer.

outer plexiform layer. These signals are inhibitory and

cell. This cell transmits signals in the retro-

prominent and not shown in the figure, is the

A sixth type of neuronal cell in the retina, not very

ganglion cells,

5. The

directions, either directly from bipolar cells to

amacrine cells,

4. The

The Nervous System: B. The Special Senses

634

Unit X

which transmit signals in two

ganglion cells or horizontally within the inner
plexiform layer from axons of the bipolar cells to
dendrites of the ganglion cells or to other
amacrine cells

which transmit output signals

from the retina through the optic nerve into the
brain

inter-

plexiform
grade direction from the inner plexiform layer to the

are believed to control lateral spread of visual signals

for Colour Blindness. Tokyo: Kanehara & Co., but tests for

normal person reads “42.” (Reproduced from Ishihara’s Tests

“2,” but the green-blind person (deuteranope) reads “4.” The

In this chart, the red-blind person (protanope) reads

reads “74,” but the red-green color-blind person reads “21.”

In this chart, the normal person

Two Ishihara charts.

Figure 50–10

Upper:

Lower:

color blindness cannot be conducted with this material. For
accurate testing, the original plates should be used.)

light focused on the retina. The visual pathway from

strated in Figure 50–12, which shows a minute spot of

proper visual contrast. This phenomenon is demon-

that is important in all other sensory systems—that is,

Therefore, this lateral connection

always inhibitory.

bipolar cells. The outputs of the horizontal cells

cones, as well as connecting with the dendrites of the

The horizontal cells, shown in Figure 50–11, connect

Lateral Inhibition to Enhance Visual Contrast—

illumination; the signal is not all or none, as would be

the rods and cones, the strength of the hyperpolariz-

of signal strength. Thus, for

The importance of electrotonic conduction is that it

tric current flow, not by action potentials.

bipolar cell or horizontal cell, once again the signal is

tic body, and no action potential is required. Then,

current flow in the cytoplasm all the way to the synap-

segment of a rod or a cone, almost the same degree of

is by electrotonic conduction. That is, when hyperpo-

the visual signals are generated, to the synaptic bodies

cones, conduction from their outer segments, where

the way to the output synapses. Even in the rods and

tric current, not action potentials, in the neuronal cyto-

explained as follows.

Otherwise, all the retinal neurons conduct their visual

importance of these action potentials is questionable.

also been recorded in amacrine cells, although the

the optic nerve. Occasionally, action potentials have

means of action potentials are the ganglion cells, and

The only

Transmission of Most Signals Occurs in the Retinal Neurons by

release inhibitory transmitters.

unclear, but at least some of the horizontal cells

bipolar, horizontal, and interplexiform cells are

as inhibitory transmitters. The transmitters of the

indolamine,

choline,

gamma-aminobutyric acid, glycine, dopamine, acetyl-

least eight types of transmitter substances, including

at their synapses with the bipolar cells.

eated. However, both the rods and the cones release

directly to ganglion cells and by way of amacrine cells.

and cones; the outputs of these bipolar cells pass both

retinal circuitry of Figure 50–11 connect with both rods

The other two bipolar cells shown in the peripheral

connectivity.

Also, horizontal and amacrine cells provide lateral

cells, (3) amacrine cells, and (4) ganglion cells.

in the direct visual pathway: (1) rods, (2) bipolar

cells. Thus, for pure rod vision, there are four neurons

amacrine cells, which relay the signals to the ganglion

The output from the bipolar cell passes only to

type of visual system present in many lower animals.

middle of these connects only to rods, representing the

cones are present. Three bipolar cells are shown; the

tions for the peripheral retina, where both rods and

To the left in Figure 50–11 are the neural connec-

inner plexiform layer.

inhibitory signals laterally in the outer plexiform layer,

ganglion cells. In addition, horizontal cells transmit

the direct pathway: (1) cones, (2) bipolar cells, and (3)

new, fast cone system. This shows three neurons in

foveal portion of the retina,

To the right in Figure 50–11 is the visual pathway

follows.

the circuitry for the two systems is slightly different, as

ducted to the brain two to five times as rapidly. Also,

visual signals for rod vision, and the signals are con-

of vision based on cone vision. The neurons and nerve

of our other sensory systems, the retina has both an

tions Differently from the Rod Pathway.

The Visual Pathway from the Cones to the Ganglion Cells Func-

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

635

As is true for many

old type of vision based on rod vision and a new type

fibers that conduct the visual signals for cone vision
are considerably larger than those that conduct the

from the

representing the

and amacrine cells transmit signals laterally in the

Neurotransmitters Released by Retinal Neurons.

Not all the

neurotransmitter chemical substances used for synap-
tic transmission in the retina have been entirely delin-

glutamate
Histological and pharmacological studies have shown
there to be many types of amacrine cells secreting at

and

all of which normally function

Electrotonic Conduction, Not by Action Potentials.

retinal neurons that always transmit visual signals by

they send their signals all the way to the brain through

signals by electrotonic conduction, which can be

Electrotonic conduction means direct flow of elec-

plasm and nerve axons from the point of excitation all

larization occurs in response to light in the outer

hyperpolarization is conducted by direct electric

when the transmitter from a rod or cone stimulates a

transmitted from the input to the output by direct elec-

allows graded conduction

ing output signal is directly related to the intensity of

the case for each action potential.

Function of the Horizontal Cells

laterally between the synaptic bodies of the rods and

are

provides the same phenomenon of lateral inhibition

helping to ensure transmission of visual patterns with

Rods

Rod nuclei

Bipolar

cells

Horizontal

cells

Amacrine

cells

Ganglion

cells

Cones

Pigment layer

area to the right.

Neural organization of the retina: peripheral area to the left, foveal

Figure 50–11

peripherally.

cones, as shown to the right in Figure 50–11. This

cones—about 35,000 of them—and no rods. Also, the

central fovea,

the center, in the

increase the acuity of vision in the central retina. In

become more slender. These effects progressively

on each optic fiber, and the rods and cones also

approaches the fovea, fewer rods and cones converge

peripheral retina and the central retina. As one

However, major differences exist between the

about 1.6 million. Thus, an average of 60 rods and 2

million cones; yet the number of ganglion cells is only

In a sense, then, many or most amacrine cells are

directional sensitive.

direction; therefore, these amacrine cells are said to be

illumination, irrespective of direction.

turned either on or off, signaling simply a change in

of visual signals, but again, the response dies quickly.

dies rapidly.

the onset of a continuing visual signal, but the response

amacrine cells to ganglion cells.

for rod vision—that is, from rod to bipolar cells to

been characterized, and all of them are different. One

by morphological or histochemical means. The func-

ates over a much greater distance.

between two adjacent photoreceptors. In contrast, the

in the visual image, even when the border lies exactly

bipolar cells lie immediately against each other, this

mechanism. Because depolarizing and hyperpolarizing

lateral inhibition, in addition to the horizontal cell

signals. We shall see later that both positive and nega-

bipolar responses, the importance of this phenomenon

polarity of the electrical response.

zontal cell is an inhibitory cell, this would reverse the

indirectly through a horizontal cell. Because the hori-

rods and cones, whereas the other receives its signal

ing by hyperpolarizing. The other possibility is that one

released by the rods and cones, and the other respond-

ence. One explanation is that the two bipolar cells are

There are two possible explanations for this differ-

hyperpolarize.

when the rods and cones are excited, and others

That is, some bipolar cells depolarize

and inhibitory signals in the visual pathway: (1) the

Two types of bipolar cells provide opposing excitatory

borders in the visual image.

allow high visual accuracy in transmitting contrast

inhibition in the surrounding areas. This is essential to

in the plexiform layers, transmission through the hor-

whereas an area to the side is inhibited. In other words,

The Nervous System: B. The Special Senses

636

Unit X

the centralmost area where the light strikes is excited,

instead of the excitatory signal spreading widely in the
retina because of spreading dendritic and axonal trees

izontal cells puts a stop to this by providing lateral

Some of the amacrine cells probably provide addi-

tional lateral inhibition and further enhancement of
visual contrast in the inner plexiform layer of the
retina as well.

Excitation of Some Bipolar Cells and
Inhibition of Others—The Depolarizing
and Hyperpolarizing Bipolar Cells

depolarizing bipolar cell and (2) the hyperpolarizing
bipolar cell.

of entirely different types—one responding by depo-
larizing in response to the glutamate neurotransmitter

of the bipolar cells receives direct excitation from the

Regardless of the mechanism for the two types of

is that it allows half the bipolar cells to transmit

positive signals and the other half to transmit negative

tive signals are used in transmitting visual information
to the brain.

Another important aspect of this reciprocal relation

between depolarizing and hyperpolarizing bipolar
cells is that it provides a second mechanism for

provides a mechanism for separating contrast borders

horizontal cell mechanism for lateral inhibition oper-

Amacrine Cells and Their Functions

About 30 types of amacrine cells have been identified

tions of about half a dozen types of amacrine cells have

type of amacrine cell is part of the direct pathway

Another type of amacrine cell responds strongly at

Other amacrine cells respond strongly at the offset

Still other amacrine cells respond when a light is

Still another type of amacrine cell responds to

movement of a spot across the retina in a specific

interneurons that help analyze visual signals before
they ever leave the retina.

Ganglion Cells and Optic Nerve Fibers

Each retina contains about 100 million rods and 3

cones converge on each ganglion cell and the optic
nerve fiber leading from the ganglion cell to the brain.

there are only slender

number of optic nerve fibers leading from this part of
the retina is almost exactly equal to the number of

explains the high degree of visual acuity in the central
retina in comparison with the much poorer acuity

Light beam

Neither excited
nor inhibited

Excited area

Inhibited area

Excitation and inhibition of a retinal area caused by a small beam

Figure 50–12

of light, demonstrating the principle of lateral inhibition.

change

This capability of the eyes to detect

have similar transient responses themselves.

partly generated by the amacrine cells, many of which

depolarizing and hyperpolarizing bipolar cells, and the

these responses to light are caused, respectively, by the

and “off-on” responses. The opposite directions of

effects occur. Thus, these records are called “on-off”

inhibition. Then, when the light is turned off, opposite

located lateral to the spot of light; this cell is markedly

second. The lower tracing is from a ganglion cell

on, but decreasing rapidity in the next fraction of a

Figure 50–13. The upper panel shows rapid impulses

in light intensity. This is

changes

As noted previously, many ganglion cells are

Transmission of Changes in Light Intensity—The On-Off

background ganglion cell firing.

The visual signals, in turn, are superimposed onto this

pulses at rates varying between 5 and 40 per second.

when unstimulated, they still transmit continuous im-

itive action potentials instead. Furthermore, even

within the retina is no longer appropriate; therefore,

tion employed in the rods, cones, and bipolar cells

distance involved, the electrotonic method of conduc-

of the optic nerve lead into the brain. Because of the

Excitation of the Ganglion Cells

location of the event, other than to give appropriate

field, but without specifying with great accuracy the

of a second. These ganglion cells presumably apprise

amacrine cells, to rapid changes in the visual image—

The Y ganglion cells respond, like many of the

from widespread retinal areas.

dritic fields, so that signals are picked up by these cells

only 5 per cent of the total. Also, they have broad den-

least numerous of all the ganglion cells, representing

signals to the brain at 50 m/sec or faster. They are the

35 micrometers in diameter, and they transmit their

The Y cells are the largest of all, up to

Function of the Y Cells to Transmit Instantaneous Changes in

cone, X cell transmission is probably responsible for

details of the visual image are transmitted. Also,

fore, it is mainly through the X cells that the fine

their signals represent discrete retinal locations. There-

do not spread widely in the retina. Because of this,

The X cells have small fields because their dendrites

14 m/sec.

medium diameter, between 10 and 15 micrometers,

representing 55 per cent of the total. They are of

most numerous of the ganglion cells are the X cells,

The

Transmission of the Visual Image and Color by the X Cells.

our crude rod vision under dark conditions.

vision, and they are probably important for much of

experiments, the W cells seem to be especially sensi-

areas.

inner plexiform layer, receiving signals from broad

have broad fields in the peripheral retina because the

by way of small bipolar cells and amacrine cells. They

receive most of their excitation from rods, transmitted

slow velocity of only 8 m/sec. These ganglion cells

small, having a diameter less than 10 micrometers, and

stituting about 40 per cent of all the ganglion cells, are

The W cells, con-

Transmission of Rod Vision by the W Cells.

nated W, X, and Y cells. Each of these serves a differ-

There are three distinct types of ganglion cells, desig-

Three Types of Retinal Ganglion Cells and

nerve fibers.

peripheral portions of the retina, so that signals from

times more sensitive to light than cones are, but it is

sitivity of the peripheral retina to weak light. This

The Eye: II. Receptor and Neural Function of the Retina

Chapter 50

637

Another difference between the peripheral and

central portions of the retina is the much greater sen-

results partly from the fact that rods are 30 to 300

further magnified by the fact that as many as 200 rods
converge on a single optic nerve fiber in the more

the rods summate to give even more intense stimula-
tion of the peripheral ganglion cells and their optic

Their Respective Fields

ent function.

they transmit signals in their optic nerve fibers at the

dendrites of the ganglion cells spread widely in the

On the basis of histology as well as physiologic

tive for detecting directional movement in the field of

and transmit signals in their optic nerve fibers at about

because every X cell receives input from at least one

all color vision.

the Visual Image.

either rapid movement or rapid change in light inten-
sity—sending bursts of signals for only small fractions

the central nervous system almost instantaneously
when a new visual event occurs anywhere in the visual

clues that make the eyes move toward the exciting
vision.

Spontaneous, Continuous Action Potentials in the Ganglion
Cells.

It is from the ganglion cells that the long fibers

ganglion cells transmit their signals by means of repet-

Response.

specifically excited by
demonstrated by the records of nerve impulses in

for a fraction of a second when a light is first turned

inhibited when the light is turned on because of lateral

transient nature of the responses is probably at least

in light

intensity is strongly developed in both the peripheral

Excitation

off

Lateral inhibition

Haven, Conn: Yale University Press, 1955.)

physiological Research into the Process of Reception. New

Perception: A Discussion of Aims, Means, and Results of Electro-

(Modified from Granit R: Receptors and Sensory

glion cell in this area is inhibited by the mechanism of

spot of light and (2) an area adjacent to the excited spot; the gan-

Responses of a ganglion cell to light in (1) an area excited by a

Figure 50–13

lateral

inhibition.

ited by the “opponent” color. Therefore, color analysis

begins to differentiate colors. Thus, each color-contrast

The importance of these color-contrast mechanisms

depolarizing bipolar cell, whereas the other color type

the following: One color type of cone excites the gan-

The mechanism of this opposing effect of colors is

yellow colors.

by yellow) on the other hand, giving a reciprocal

The same type of reciprocal effect occurs between

causing inhibition, or vice versa.

and green cones, with red causing excitation and green

type. For instance, this frequently occurs for the red

Conversely, some of the ganglion cells are excited by

“white” signal.

role in the detection of different colors. Instead, it is a

Therefore, the signal from the ganglion cell plays no

ganglion cell, the signal transmitted through the gan-

the red, blue, and green types—stimulate the same

cones or by only a few. When all three types of cones—

Transmission of Color Signals by the

In summary, the mechanism of lateral inhibition

tuate one another.

bipolar cell. Thus, where visual contrasts occur, the

the bipolar cell, and this allows extra excitation of the

unstimulated. Therefore, this cell does not inhibit

cell. The fact that one of the lateral photoreceptors is

two lateral receptors is in the dark. The bright spot of

Figure 50–14, assume that the central photoreceptor is

border occurs in the visual scene. Referring again to

Now, let us examine what happens when a contrast

cell. The two receptors on each side are connected

The central receptor excites a depolarizing bipolar

50–14, which shows at the top three photoreceptors.

ways. One circuit for this is demonstrated in Figure

horizontal cells are mainly inhibitory. Thus, the direct

tory, while the signals transmitted

is neither stimulated nor inhibited. The reason for this

is, when all the photoreceptors are stimulated equally

When flat light is applied to the entire retina—that

occurs.

mitted to the brain, let us explain how this process

borders in the scene. Because this seems to be the

in the Visual Scene—The Role of

Transmission of Signals Depicting Contrasts

detected. Conversely, the same gnat sitting quietly

retina and the central retina. For instance, a minute

The Nervous System: B. The Special Senses

638

Unit X

gnat flying across the field of vision is instantaneously

remains below the threshold of visual detection.

Lateral Inhibition

Many ganglion cells respond mainly to contrast

major means by which the pattern of a scene is trans-

by the incident light—the contrast type of ganglion cell

is that signals transmitted directly from the photore-
ceptors through depolarizing bipolar cells are excita-

laterally through

hyperpolarizing bipolar cells as well as through

excitatory signal through one pathway is likely to be
neutralized by inhibitory signals through lateral path-

to the same bipolar cell through inhibitory horizontal
cells that neutralize the direct excitatory signal if
all three receptors are stimulated simultaneously by
light.

stimulated by a bright spot of light while one of the

light excites the direct pathway through the bipolar

in the dark causes one of the horizontal cells to remain

signals through the direct and lateral pathways accen-

functions in the eye in the same way that it functions
in most other sensory systems—to provide contrast
detection and enhancement.

Ganglion Cells

A single ganglion cell may be stimulated by several

glion cell is the same for any color of the spectrum.

only one color type of cone but inhibited by a second

blue cones on the one hand and a combination
of red and green cones (both of which are excited

excitation-inhibition relation between the blue and

glion cell by the direct excitatory route through a

inhibits the ganglion cell by the indirect inhibitory
route through a hyperpolarizing bipolar cell.

is that they represent a means by which the retina itself

type of ganglion cell is excited by one color but inhib-

begins in the retina and is not entirely a function of
the brain.

Excitation

Inhibition

cells, but inhibition from the horizontal cells to the bipolar cell.

synapses between the rods and the bipolar cell and horizontal

and a ganglion cell (G) in the retina, showing excitation at the

Typical arrangement of rods, horizontal cells (H), a bipolar cell (B),

Figure 50–14

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