Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 6

Macula

Fovea

Optic disc

Figure 1.6. The fundus as viewed through the pupil; retinal arteries are shown in red and
retinal veins in blue.

visual system. After the initial processing of a given image, the retina sends
coded signals to the brain via the optic nerve.
The outermost layer of the retina is the retinal pigment epithelium (RPE), a
monocellular layer firmly attached to the underlying choroid (Figure 1.7). The
remainder of the retina is composed of several layers that collectively form the
neurosensory retina. Although the RPE is often considered a part of the retina,
the term “retina,” in practice and in literature, implies neurosensory retina; in
this chapter, retina also implies the neurosensory retina unless stated otherwise.
The retina is generally firmly attached to the RPE but, in a number of patho-
logical conditions, detachment can occur (Figure 1.8). Trauma, age, and disease
processes can all lead to separation, often with ensuing photoreceptor cell death.
The integrity of RPE cells is vital for the viability of the photoreceptors. In
addition to absorbing stray light, RPE cells also supply nutrients and oxygen
to photoreceptors, remove and digest discs shed by photoreceptors, and have a
regulatory role in ion and metabolite transport. Photoreceptors detect light and
are divided into two main classes, based on wavelength sensitivity and their
location in the retina: the cones are highly concentrated at the central part of the
retina (macula) and are responsible for both fine and color vision, whereas rods
are more prevalent in the periphery and mainly function in peripheral vision,
movement detection, and night vision. Color vision is due to the presence of three
types of cones: blue, green, and red. Each type is sensitive to a different spectrum
of light, but there is a considerable overlapping amongst them. The nuclei of
photoreceptors are located within the outer nuclear layer (ONL). Another major
group of cells are bipolar cells, located within the inner nuclear layer (INL).
Bipolar cells are involved in the processing and transferring of visual signals
from the photoreceptors to the ganglion cells. Other cells within the INL include
amacrine and horizontal cells, which modify the neural response as it passes
through the retina. The bipolar cells connect to ganglion cells; the axons of the
ganglion cells form the nerve fiber layer. These axons converge at the optic
disc to form the optic nerve; the axons travel via the optic nerve and make