Tuesday, December 18, 2012

Retinal Function

As we review the biological principle of “Form fits Function,” there is no better example than the remarkable sense of human light detection. Without the functional retina, the innermost layer of our eyeball, we would possess no vision. Most of the eight million extant species on our planet share with humans some degree of ability to detect light, ranging from simple detection of the mere presence of light in primitive organisms to the detailed color images perceived by birds of prey such as hawks. The visual acuity in such animals exceeds even that of humans.

The principle of “Form fits Function” amounts to a commentary on the ubiquitous design features of our universe. More specifically, it argues powerfully for the deliberate work of God whose activity in creating the multiplicity of living things was highlighted by the last product of creation--man in his image. In short, our lives operate successfully because this biological principle applies to virtually every phase of our physical experience. With respect to our sense of vision, we must not fail to understand extraordinary retinal tissue function.

The close range and distant matter surrounding us releases a constant flow of electromagnetic radiation. Visible light is but one type of radiation. That light ranges from dim to intense, along the familiar spectrum of colors. The retinal cells, termed rods and cones, detect light energy under dim conditions and color under bright conditions. There are four other types of retinal cells which possess other functions in the sight process. Rods and cones, however, are the key players in the drama.

All rods and cones contain a light-absorbing molecule called retinal which bonds to a protein called an opsin. In rods the visual pigment called rhodopsin is formed by this bond. When light, especially bright light, enters the eye we see clearly for a time. But the rhodopsin is changed somewhat and the rods become unresponsive. That explains why moving quickly from a bright environment to a dark room causes us to experience temporary difficulty seeing clearly while the rods regenerate.

Cone cells come in three classes. Three types of opsins, known as photopsins are each sensitive to a different color. One cone cell is sensitive to red light, another is sensitive to green light, and the third responds to blue light. Some cone cells are sensitive to an overlap of color wavelengths and together those wavelengths will produce intermediate colors when the brain organizes the image later in the visual process. In this way we may perceive every color on the visible light spectrum.

When we learned about rods and cones in elementary grades, our teachers may have wisely guarded her students against information overload. There was far more to the story. For example, what sort of signals do rods and cones send to the brain through the neurons? How does the light message arrive at the brain? After all, it is our brains which actually “see.” In the retina there are bipolar, horizontal, ganglion, and amacrine cells. They are the next players on stage, paving the way to discovery of “the rest of the story,” a phrase made famous by Paul Harvey, radio commentator for over fifty years.

The retina could not function as an entity apart from the energy of light flowing from the objects around us. The form of light and the function of the eye’s retina are complementary. This is one of hundreds of examples of the complementarity of design features present in our environment everywhere we look. Who is the author of these exquisite complementary design features? Our belief in the Designer is strengthened by recognizing the many form/function relationships evident everywhere in our created sphere.