The Physiology of Visual Phototransduction in the Human Eye
Imagine a constant field of complete darkness in place of your normal vision. The swirling colors
and rapid movements of objects that used to paint the world around you fail to appear, and your
visual perception of the world could be defined by one word: black. The sense of sight provides the
majority of external sensory stimuli for human beings, and therefore comprises the main way that
each of us interprets our surroundings. Without it, we must depend on our less complex senses to
analyze our environment, voiding us of both the aesthetics of vision and a complete understanding
of the world around us.
Visual phototransduction is the physiological, or bodily, process by which the human eye adapts to
changes in levels of light in the surrounding environment in order to see. It involves a cascade of
rapid events leading to sight that begins with the entrance of light into the eye, and ends with the
realization of human vision.
Biological Components of Visual Phototransduction
Vision requires several key molecules and biological factors to take place. Some of these necessary
components include simple charged particles, known as ions, while others consist of complex
enzymes (proteins) that pump said ions through cell membranes (barriers between cellular
compartments). These components ensure the continual production of human sight at each of their
respective levels of visual phototransduction. They depend upon one another to complete the entire
cascade that produces vision quicker than the blink of an eye.
Enzymes and Complex Molecules Involved in Phototransduction
1) The photopigment rhodopsin functions as an unstable pigment that undergoes a chemical
change when it absorbs light. It can be found in the cells of the retina (the back of the eye).
Two subcomponents make up rhodopsin: retinal and opsin, which can be separated to produce
changes in the eye.
2) The g-protein transducin acts as a molecular “switch” that transmits signals from outside of
the cell to the inside. Another protein, phosphodiesterase, cleaves off phosphate groups from
molecules “deactivating” them to induce change.
3) Sodium ion channels, “ion pumps” that move sodium ions from areas of high concentration
to low concentration, move only sodium ions. The pumps perform this action in order to alter
the electric energy inside cells by allowing more positive charge, in the form of sodium ions, to
enter the cells.
4) The neurotransmitter glutamate responds to changes made by the previous molecules by
either increasing or decreasing its concentration. Glutamate does this in order to activate the
action potentials, quick changes in electrical energy that produce a bodily response, of retinal
cells.
Ions Involved in Phototransduction
Sodium ions, denoted as Na+, drive this process. All of the changes that take place within the
proteins and enzymes of the human eye ultimately affect the levels of sodium ions present. The
changing concentration of this ion within visual rod and cone cells, responsible for black/white
and color vision respectively, determines the magnitude of action potentials produced by retinal
cells. These action potentials represent the keys to the ignition of vision.
Cascade of Visual Phototransduction
Defined as a cascade, visual phototransduction consists of a stepwise procedure in which each step
produces a result or change that starts the next step of the process. Figure 10-40 presented below
gives a visual representation of phototransduction in retinal cells. Refer back to it while reading the
description of the cascade of phototransduction that begins on the next page. The figure provides
extra details that do not need to be understood in order to gain an understanding of the process.
Therefore, you should focus only on the components of the figure that have been directly mentioned
in this description.
Figure 10-40: Phototransduction in Retinal Cells
Strauss, James A. "Physiology of Vision." BIOL 472 - The Pennsylvania State University. N.p.: n.p., n.d. 40-42. Print.
Resting Status in Darkness (Portion “a” of Figure 10-40)
In an unaltered state without any light, the retinal cells of the eye have open Na+ channels
allowing free passage of these ions through cell membranes. This results in a cell interior that is
more positively charged since more Na+ ions are able to enter. This positive electric charge
additionally permits the release of glutamate onto retinal cells, where they inhibit action
potentials from being produced.
Introduction of Light (Portion “b” of Figure 10-40)
As light enters the eye and is projected onto the retinal cells, it causes rhodopsin to undergo a
chemical change resulting in the beginning of the cascade of phototransduction through the
separation of rhodopsin’s two components: retinal and opsin. The now free opsin goes on to
activate transducin, which in turn activates phosphodiesterase. The phosphodiesterase protein
then deactivates the Na+ channels that allow the positive ions to enter the retinal cells.
Since the influx of Na+ ceases, the interior of these cells becomes negatively charged. This
buildup of negative charge urges the retinal cell to create a response. Therefore, the cell
decreases the levels of glutamate within itself, since the glutamate inhibits the retinal cell’s
ability to produce an action potential. With the levels of glutamate lowered, the retinal cells can
now create action potentials.
Activation of Vision
These action potentials travel from the eye to the brain through the optic nerve, and upon
arrival at the brain, these signals enter the occipital lobe of the rear of the brain. This part of the
brain takes the sensory information from the action potentials and makes sense of them,
forming the images each human sees with their eyes. These images produced from the occipital
lobe then travel to the visual cortex in the brain. This portion of the brain completes the final
step of vision, and makes sense of what the brain is showing us through our eyes, connecting
what we see to what we know.
Phototransduction Cascade Results in Operational Vision
The ability to see comes from a complex cascade full of mechanisms located in an area of the body
smaller than a quarter. The human body has adapted itself to create these processes in order to gift
each of our eyes with vision. An understanding of this seemingly simple, yet intricate, process can
alert anyone to how hard the body works to make sure you keep your eyes on the prize.