Sensitivity to light—photosensitivity—confers on the simplest animals the ability to orient to the sun and sky and gives more complex animals rapid and extremely detailed information about objects in their environment. It is not surprising that both simple and complex animals can sense and respond to light. What is remarkable is that across the entire range of animal species, evolution has conserved the same basis for photosensitivity: a family of pigments called rhodopsins. In this animation we will describe how rhodopsin molecules respond when stimulated by light energy and how that response is transduced into neural signals.
Each rod cell has an outer segment packed with membranous discs. The disc membranes are stacked inside the rod cell outer membrane. In addition to rhodopsin molecules, the disc membranes carry G proteins (also called transducin) and molecules of the enzyme phosphodiesterase. The rod cell outer membrane contains a type of sodium channel that is held open by the binding of cyclic GMP (cGMP) molecules. The open sodium ion channels allow sodium ions to flow spontaneously from a higher sodium ion concentration outside the cell to a lower concentration inside the cell, thereby depolarizing the cell.
Light sensitivity begins with the absorption of light by rhodopsin. Rhodopsin consists of a protein part, called opsin, and a light-absorbing part, called 11-cis-retinal, which is cradled in the center of the opsin and bound covalently to it. When 11-cis-retinal absorbs a photon of light energy, the molecule rotates around an internal bond—straightening out—and becomes all-trans-retinal. The shift in retinal forces a conformation change in the opsin that signals the detection of light.
The photoexcited rhodopsin activates transducin by stimulating it to release GDP and pick up GTP. Activated transducin in turn activates phosphodiesterase. Activated phosphodiesterase then converts cGMP to GMP. The reduction in cGMP inside the cell allows the sodium channels to close. Pumps in the cell membrane continuously work to drive sodium ions out of the cell, so the cell can now become more negative. The membrane hyperpolarizes.
Signal transduction cascades have enormous amplification abilities. In this example, each molecule of photoexcited rhodopsin can activate several hundred transducin molecules, thus activating a large number of phosphodiesterase molecules. The catalytic capacity of phosphodiesterase is great: one molecule can hydrolyze more than 4,000 molecules of cGMP per second. Thus, a single photon of light can cause a huge number of sodium channels to close.
To test how a rod cell responds to light, we can penetrate a single rod cell with an electrode and record its membrane potential in the dark and in the light. In the dark, the membrane potential is around –35 mV. This value is considered depolarized—that is, relatively close to zero. In the dark, when the cell is depolarized, it continually trickles out neurotransmitter from its synaptic terminals.
A brief light stimulus triggers the membrane to get more negative—that is, to hyperpolarize. As the membrane hyperpolarizes, the cell releases less neurotransmitter onto the next neurons in the visual pathway. A brighter light stimulus results in an even stronger hyperpolarization, and an even greater reduction in neurotransmitter release. Note that this is the opposite of what other types of sensory receptors do; they typically depolarize and release neurotransmitters in response to a stimulus.
To be a system sensitive to light changes, the signal transduction cascade needs to turn off soon after the signal has stopped. With vertebrate eyes, the retinal and the opsin separate from each other soon after light absorption, causing the molecule to lose its photosensitivity. A series of enzymatic reactions is then required to return the all-trans-retinal to the 11-cis isomer, which then recombines with opsin so that it once again becomes the photosensitive pigment rhodopsin.
Additionally, the G protein has an automatic shut-off mechanism. After a short while, it cleaves its GTP to form GDP, and thereby becomes inactive again. Without the activated G protein, the phosphodiesterase stops its activity, while another enzyme in the cell replenishes the supplies of cGMP. The sodium channels open again, bringing the cell to its resting state.
In this animation we examined the function of rod cells in the human eye. The other class of photoreceptor—the cone cell—functions in a similar way. Rod cells are responsible for highly sensitive black-and-white vision and cone cells are responsible for the less sensitive color vision.
The human retina has three kinds of cone cells, each containing slightly different opsin molecules. The three cone opsins and the single rod opsin all differ in the wavelengths of light they absorb best. Although the same 11-cis-retinal group is the light absorber in all, its molecular interactions with opsin determine the spectral sensitivity of the rhodopsin molecule. Because different wavelengths of light are differentially absorbed by the different visual pigments, the brain can interpret the relative inputs from the different classes of cones as a full range of color. Color blindness results from the absence or dysfunction of one or more classes of cone cells. The most common form is red–green color blindness, which occurs in about 10 percent of men of European descent.