Human Physiology, 14th edition (2016)

298 Chapter 10

and these new discs migrate toward the tips to replace the lost

material.

The retinal pigment epithelium is a simple, one-cell-layer-

thick membrane. Microvilli project from the apical surface of

the pigment epithelial cells toward the photoreceptors, aiding

interactions. The basal surface of the retinal pigment epithe-

lium contacts Bruch’s membrane, the connective tissue base-

ment membrane separating the pigment epithelium from the

blood vessels of the choroid. Research reveals many functions

of the retinal pigment epithelium important for vision, includ-

ing the following:

1. Phagocytosis of the shed outer segments of the photoreceptors

2. Absorption of scattered light in the retina by melanin

pigment

3. Delivery of nutrients from the blood to the photoreceptors

4. Suppression of an immune attack of the retina (thereby

helping to make the retina an immunologically privileged

site; chapter 15, section 15.3)

5. Conversion of visual pigment from the photoreceptors

into its active form, which is recycled back to the pho-

toreceptors in a process called the visual cycle of retinal

(discussed shortly)

dissociation in response to light, and it is this photochemi-

cal reaction that eventually results in the production of

action potentials in the optic nerve.

LEARNING OUTCOMES

After studying this section, you should be able to:

14. Describe the structure of the retina, and how light
    affects rhodopsin.


15. Explain how light affects synaptic activity in the
    retina, and describe the neural pathways of vision.


16. Compare the function of rods and cones, and
    describe the significance of the fovea centralis.


17. Describe the neural pathways required for vision.

The retina consists of a single-cell-thick pigmented epithe-
lium, photoreceptor neurons called rods and cones, and layers
of other neurons. The neural layers of the retina are actually a
forward extension of the brain. In this sense, the optic nerve
can be considered a tract, and indeed the myelin sheaths of
its fibers are derived from oligodendrocytes (like other CNS
axons) rather than from Schwann cells.
Because the retina is an extension of the brain, the neural
layers face outward, toward the incoming light. Light, there-
fore, must pass through several neural layers before striking
the photoreceptors ( fig.  10.36 ). The photoreceptors then syn-
apse with other neurons, so that synaptic activity flows out-
ward in the retina.
The outer layers of neurons that contribute axons to the
optic nerve are called ganglion cells. These neurons receive
synaptic input from bipolar cells, which in turn receive input
from rods and cones. In addition to the flow of information
from photoreceptors to bipolar cells to ganglion cells, neurons
called horizontal cells synapse with several photoreceptors
(and possibly also with bipolar cells), and neurons called ama-
crine cells synapse with several ganglion cells.
Each rod and cone consists of an inner and an outer seg-
ment ( fig.  10.37 ). The inner segment contains most of the
cell’s organelles; the outer segment contains hundreds of
flattened membranous sacs, or discs ( fig.  10.38 ), where the
photopigment molecules required for vision are located. The
photoreceptor cells continuously add new discs at the base of
the outer segment as the tip regions are removed by the cells
of the retinal pigment epithelium (see fig. 10.36 ) through a
process of phagocytosis. Each retinal pigment epithelial cell
is in contact with 50 to 100 photoreceptor outer segments,
and daily removes the distal 10% of these outer segments by
phagocytosis. This amounts to the phagocytosis of hundreds
of thousands of discs over the course of a lifetime by each
retinal pigment cell, making these cells the most phagocyti-
cally active cells in the body. The photo receptors continu-
ously produce new discs at the base of their outer segments,

Fibers of the

optic nerve

Ganglion cells

Amacrine cells

Direction

of neural

activity

Bipolar cells

Horizontal

cells

Photoreceptor

cells

Cone

Retina

Direction

of light

Rod

Pigment epithelium

Choroid layer

Sclera

Figure 10.36 Layers of the retina. Because the retina

is inverted, light must pass through various layers of nerve cells

before reaching the photoreceptors (rods and cones).