Figure 35.13 A Rod Cell Responds to Light
Rod cells are very sensitive to light and can even detect a single photon of light. Conscious perception of light in dark-adapted humans requires only 5 to 10 photons. The response of a rod cell to light was first measured with electrodes that penetrated the cells and recorded their changes in membrane potential when stimulated with pulses of dim light. The results were very surprising. Since other sensory cells depolarize and generate action potentials when stimulated, the same was expected of the rod cells. However, when the rod cell was exposed to light, the inner segment membrane hyperpolarized, or became more negative. Further, the brighter the light, the greater the degree of hyperpolarization. A very dim light resulted in a slight hyperpolarization, while a bright light resulted in a strong hyperpolarization. When the rod cell was kept in the dark, the membrane depolarized, or became less negative. Another approach was used to investigate the mechanisms underlying this strange responsiveness of rod cells to light. A suction electrode is a glass pipette with a polished tip. It can be placed up against a membrane and sealed to that membrane by a slight suction. Since rod cells are long and thin, it is possible to suck most of a single rod cell into the pipette without rupturing its membrane. When light was shone through the pipette, the electrode could record the resulting electrical current that flowed across the rod cell membrane. It was found that the light stimulus caused a decrease of a positive current carried by Na+ ions flowing into the cell. Subsequent work (see Figure 35.14 of the textbook) showed that the excited photopigment rhodopsin activates a G protein called transducin. Transducin activates a phosphodiesterase, which converts cyclic GMP to GMP. In the dark, cyclic GMP maintains open Na+ channels in the rod cell membrane, and as the cyclic GMP is destroyed by the phosphodiesterase, the Na+ channels close and the cell hyperpolarizes because of the outward flow of K+ ions.
Original Papers
Baylor, D. A., T. D. Lamb, and K. W. Yau. 1979. The membrane current of single rod outer segments. The Journal of Physiology 288: 589–611.
http://jp.physoc.org/cgi/reprint/288/1/589
Baylor, D. A., T. D. Lamb, and K. W. Yau. 1979. Responses of retinal rods to single photons. The Journal of Physiology 288: 613–634.
http://jp.physoc.org/cgi/reprint/288/1/613
Schnapf, J. L., and D. A. Baylor. 1987. How photoreceptor cells respond to light. Scientific American 256: 40–47.
http://www.ncbi.nlm.nih.gov/pubmed/3031814
Links
Kimball’s Biology Pages: The Human Eye
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html
NCBI: Neuroscience, 2nd ed.: Phototransduction
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A747
University of Utah: John Moran Eye Center: Webvision: Photoreceptors
http://webvision.med.utah.edu/photo1.html
Duke University: Arshavsky Lab Research: Arshavsky’s Research Projects
http://www.duke.edu/~arsha001/research.html