388 Neuroanatomy: Draw It to Know It
The Retina (Advanced )
Here, let’s address the organization of the ten distinct
layers of the retina in further detail. Label the top of the
page as internal to the retina (the vitreous chamber) and
the bottom of the page as external to it (the choroid).
From inner to outer, the retinal layers are the inner limit-
ing membrane; nerve fi ber layer; ganglion cell layer;
inner plexiform layer; inner nuclear layer; outer plexi-
form layer; outer nuclear layer; outer limiting mem-
brane; photoreceptor cell segment layer; and retinal
pigmented epithelium.
Once again, indicate that light passes through the
retina and is captured by the photoreceptor cell seg-
ments. Show that the phototransduction cascade occurs
here, which transforms light into neural signal, and indi-
cate that the signal is passed back through the retina and
passed out of the eye through the optic nerve. Note that
the photoreceptor cell segments are metabolically depen-
dent upon the pigmented epithelium.
Indicate that the inner limiting membrane is a thin,
basal lamina that separates the nerve fi ber layer from the
vitreous chamber and that the outer limiting membrane
is a row of intercellular junctions that separates the pho-
toreceptor cell segments from the outer nuclear layer.
Müller glial cells extend across the retina: their proximal
endings oppose the inner limiting membrane and their
distal processes help form the outer limiting membrane.
Next, show that the ganglion cell layer contains ganglion
cell bodies, the axons of which form the nerve fi ber layer.
Th en, indicate that the plexiform layers are synaptic zones:
the inner plexiform layer is relatively thick whereas the
outer plexiform layer is much thinner. Now, show that the
nuclear layers contain cell bodies: the inner nuclear layer
comprises retinal interneuronal cell bodies and the outer
nuclear layer comprises photoreceptor cell bodies.
Lastly, let’s further defi ne the fi ve neuronal cell types
that exist within the retina. Th e outermost cells are the
rods and cones, which are the photoreceptor cells, and
the innermost cells are the ganglion cells, whose unmy-
elinated axons form the nerve fi ber layer. Sandwiched in
between the photoreceptor cells and ganglion cells are
the bipolar cells, which pass forward visual information
from the photoreceptor cells to the ganglion cells, and
also the horizontal and amacrine cells, which enhance
visual contrast. It is well recognized that the visual system
relies more on visual contrast than the overall level of
illumination for visual perception: Th e visual system
attends to the borders between light and dark areas or
color diff erences more so than light intensity. As long as
we can read the page of a book comfortably, we perceive
the words on it just the same in varying levels of illumi-
nation; it is the contrast of the ink from the page that
makes the largest impression in our mind.