Focusing On The Retina: A Lesson About How Our
Eyes Work
The retina is a lining inside the eye
covering the entire back half of the eyeball. With respect to
vision, this is where the real action takes place. Millions of
retinal cells, nature's microchips, constantly flash messages
to your brain. These countless bits of information are carried
on nerve fibers, somewhat like electrical impulses that travel
on fiber-optic strands.
Your brain decodes these messages, much as your telephone
and television set decode electrical impulses. For the
transmission of information to the brain, the cells of the
retina are divided into two basic cell types: the rods and the
cones. The rods function best in dim light, the cones under
lighted conditions.
There are about 7 million cones and 125 million rods. The
cones are snappy little numbers that spring to attention the
moment light hits them. They handle fine details and color. The
rods, on the other hand, are laggards, taking about ten minutes
to come to attention when you enter a dim or darkened room.
Their job, in addition to helping you see in the dark, is to
distinguish black and white.
Your sight is further refined by the connection of each rod
and cone to a type of cell called a bipolar cell. The job of
the bipolar cells is to relay messages to the over 1.9 million
ganglion cells, which carry the messages to the brain. The
bipolar cells have a lot of responsibilities. They are divided
into two types, the magno cells, numbering about 10% of the
total, and the parvo cells, which are much more numerous at 90%
of the total.
The magno cells make it possible to distinguish shapes in
indistinct light and to see in the dark. Thanks to these cells,
you can find a quarter on a wood floor in dim light or locate
that black suit in the back of your closet. Loss of magno cells
may impair your ability to read, causing difficulty in
separating the details of letters, spaces, and the breaks
between words. The parvo cells fill in the details, like a
painter adding texture, line, and color to the sketch.
Since there are fewer ganglion cells than there are rods and
cones, several hundred rods and cones are connected to each
ganglion cell, forming a field around the cell. If you were to
lose ganglion cells, it would affect your ability to see well
in dim light, cause difficulty in contrasting similar shades of
color, and make detail fuzzy. Cataracts can cause similar
symptoms.
The macula, in the center of the retina, contains the
richest supply of ganglion cells. It is the hub of sight. If
you lose cells in this region, it robs you of the ability to
see things in the center of the visual field and compromises
your ability to fixate (to focus the eyes on a specific point
or object). You need to fixate in order to read, for example.
Otherwise, the letters seem to jump around on the page.
Macular failure is a condition most often associated with
age-related macular degeneration (AMD), a disease that
primarily affects the elderly. With glaucoma, damage to the
macula generally does not occur until the final stages of the
disease. Glaucoma patients experience visual loss in their
peripheral areas first.
Unfortunately, there are fewer cells in this area, so if you
lose any of them, your peripheral vision decreases. While this
type of vision loss is not as disabling as the loss of central
vision experienced by people with AMD, it is, nevertheless,
worrisome, for you find your side vision, and your top and
bottom vision, falling away.
In the center of the macula is a tiny body of densely packed
cone cells, each with its own connecting ganglion cell. The
fact that each cone cell in this area gets its own ganglion
cell is an indication of how important this area is for seeing.
This body, the fovea, is your spyglass; it makes you able to
spy a squirrel at the side of the tree, see a statue decorating
the corner of a high building, or look miles into the distance
on a clear day.
Birds of prey, such as hawks and vultures, possess not one,
but two, foveas that act as magnifiers, enabling them to locate
objects that even the sharpest of human eyes cannot detect.
Such birds can detect the presence of a creature that might
become lunch from 9,000 to 13,000 feet in the air.
Perhaps the most remarkable feature of our sight apparatus
is that the eye produces actual images on the retina. A tiny
inverted image appears continuously during the act of seeing.
This image is slowed down or sped up depending upon the
strength of the light source, the eye's ability to fixate, and
the general health of the rods, cones, and nerve cells. For
example, in a dim light, you may find it takes a bit longer to
discern the outlines or details of an object. A similar
situation occurs if you have cataracts or have trouble fixating
because of a loss of cone cells in the eye.
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