386 Neuroanatomy: Draw It to Know It
The Eye (Cont.)
Now, we will address the specifi c layers of the retina, dis-
cuss the passage of light through the retina, and discuss
the transformation of light into neural signal, called
phototransduction. By convention, the retina contains
ten distinct layers, which are reviewed in detail in
Drawing 22-4 ; here, we will simply group the retinal
layers into four diff erent functional layers and skip the
retinal membranes. First, let’s orient ourselves: indicate
that internal to the retina lies the vitreous chamber, and
external to it lies the choroid. Next, label the innermost
retinal layer as the nerve fi ber layer; then, label the
surrounding layers as the synaptic and cell body layers;
then, label the surrounding layer as the photoreceptor
cell segment layer (the rods and cones); and fi nally, label
the outermost layer as the retinal pigmented epithelium.
Indicate that light passes through the retina and is
captured by the photoreceptor cell segments. Show
that the phototransduction cascade occurs, here, which
transforms light into neural signal, and indicate that
the signal is passed back through the retina and passed
out of the eye through the optic nerve. Th e nerve fi ber
layer is unmyelinated to avoid blocking the passage
of light: the nerve fi bers become myelinated only aft er
they exit the eye as the optic nerve. Note that the pig-
mented epithelium captures light not picked up by the
photoreceptors and that the photoreceptor cell seg-
ments are metabolically dependent upon the pigmented
epithelium for photoreceptor regeneration and waste
disposal.
Finally, show that the fovea lies in the center of the
retina. Draw the center of the fovea as a pin-sized depres-
sion; in this central pit, the ganglion and bipolar cells are
pushed aside so as not to impede the path of light rays to
the photoreceptor layer. Th is unimpeded path, in com-
bination with the pure cone composition of the central
foveal photoreceptor layer, makes the center of the fovea
the area of highest visual acuity. Note that in contrast,
the optic nerve head is devoid of photoreceptors and we
do not perceive the visual region that corresponds to the
optic nerve: it forms the blind spot. As a corollary, when
disc edema occurs (ie, when the optic nerve head swells),
the blind spot enlarges. Enlarged blind spots are com-
monly observed in the clinical syndrome of idiopathic
intracranial hypertension (aka pseudotumor cerebri).