The Retina

The retina is a layer of nervous tissue that covers the inside
of the back two-thirds of the eyeball, in which stimulation
by light occurs, initiating the sensation of vision. The retina
is actually an extension of the brain, formed
embryonically from neural tissue and connected to the
brain proper by the optic nerve. The retina functions specifically
to receive light and to convert it into chemical
energy. The chemical energy activates nerves that conduct
the electrical messages out of the eye into the higher
regions of the brain.
Ten layers of cells in the retina can be seen microscopically.
In general, there are four main layers: (1) Next to the
choroid is the pigment epithelium. (2) Above the epithelium
is the layer of rods and cones, the light-sensitive cells.
The changes induced in the rods and cones by light are
transmitted to (3) a layer of neurons (nerve cells) called
the bipolar cells. These bipolar cells connect with (4) the
innermost layer of neurons, the ganglion cells; and
the transmitted messages are carried out of the eye along
their projections, or axons, which constitute the optic
nerve fibres. Thus, the optic nerve is really a central tract,
rather than a nerve, connecting two regions of the nervous
system, namely, the layer of bipolar cells, and the cells of
the lateral geniculate body, the latter being a visual relay
station in the diencephalon (the rear portion of the
forebrain).The arrangement of the retinal cells in an orderly manner
gives rise to the outer nuclear layer, containing the
nuclei of the rods and cones; the inner nuclear layer, containing
the nuclei and perikarya (main cell bodies outside
the nucleus) of the bipolar cells, and the ganglion cell layer,
containing the corresponding structures of the ganglion
cells. The plexiform layers are regions in which the neurons
make their interconnections. Thus, the outer
plexiform layer contains the rod and cone projections terminating
as the rod spherule and cone pedicle; these make
connections with the dendritic processes of the bipolar
cells, so that changes produced by light in the rods and
cones are transmitted by way of these connections to the
bipolar cells. (The dendritic process of a nerve cell is
the projection that receives nerve impulses to the cell; the
axon is the projection that carries impulses from the cell.)
In the inner plexiform layer are the axons of the bipolar
cells and the dendritic processes of the ganglion and amacrine
cells (the latter are neurons lacking long axons,
instead possessing shorter axon-like projections that relay
signals to ganglion cells). The association is such as to
allow messages in the bipolar cells to be transmitted
to the ganglion cells, the messages then passing out along
the axons of the ganglion cells as optic nerve messages