Almost at the very instant that light hits a cell in the retina, or a sound wave nudges the tip of a receptor cell in the ear, the receptor cell converts this stimulus into an electrical signal—the language of the brain.

This conversion, or transduction, is swift and precise. But it is also surprisingly intricate—so intricate that the process is not yet fully understood for most of the senses.

In the past decade, however, it has been worked out quite thoroughly for vision. It begins when a photon of light meets one of the photoreceptor cells of the retina (either a rod or a cone cell). A photon that strikes a rod cell is immediately absorbed by one of the 100 million molecules of a receptor protein— rhodopsin—that are embedded in the membranes of a stack of disks in the top part, or "outer segment," of each cell.

These rhodopsin molecules have a snakelike shape, crisscrossing the membrane seven times, and contain retinal (a form of vitamin A), which actually absorbs the light. In the dark, the retinal fits snugly into a binding pocket in rhodopsin. But on exposure to light, it straightens out. This alters the three-dimensional structure of the entire rhodopsin molecule, activating it and triggering a biochemical cascade.

The activated rhodopsin then stimulates transducin, a protein that belongs to the large family of so-called G proteins. This in turn activates an enzyme that breaks down cyclic GMP, a "second messenger," dramatically lowering its level. Cyclic GMP carries signals from the disks, where light is absorbed, to the cell's surface membrane, which contains a large number of channels. These channels control the flow of ions (charged atoms) into the cell. As ions move into the cell, they alter its electrical potential.

"In the dark, the channels are constantly open because of a high level of cyclic GMP. This allows sodium and calcium ions, which carry positive charges, to flow into the cell," explains King-Wai Yau, an HHMI investigator at the Johns Hopkins University School of Medicine who played an important role in deciphering the transduction process.

"But in the light, the channels close. Then the electrical potential inside the cell becomes more negative. This reduces the amount of neurotransmitter that is released from the base of the cell to other cells"—and thus alerts neurons in the next layer of retinal cells that a photon of light has arrived.

This complex cascade of transduction events is repeated in a remarkably similar way in olfactory receptor cells, which respond to odors, says Yau. But the receptor cells that respond to sound use a very different system: their channels open and close as a direct response to a mechanical force—either tension or relaxation.

Whatever the means, the end result of transduction is the same: the cell generates an electrical signal that flashes through a dense thicket of nerve cell connections in the brain, bringing news from the outside world in a Morse-code-like language the brain can understand.

— Maya Pines

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Illustration: Eade Creative Services Inc./George Eade illustrator (rod and cone cells adapted from Scientific American Vol. 256, No. 2, page 42, 1987; transduction adapted from figure 28-3 page 404, Principles of Neuroscience by Eric Kandel, James H. Schwartz and Thomas M. Jessell ©1991 Elsevier Science Publishing Company, Inc.)