Sensory Transduction in Vision, Olfaction, and Gustation: - Light Hyperpolarizes Rod and Cone Cells of the Vertebrate Eye
In the vertebrate eye, light entering through the pupil is focused on a highly organized collection of light sensitive neurons (Fig. 12–31). The light-sensing cells are of two types: rods (about 109 per retina), which sense low levels of light but cannot discriminate colors, and cones (about 3X106 per retina), which are less sensitive to light but can discriminate colors. Both cell types are long, narrow, specialized sensory neurons with two distinct cellular compartments: the outer segment contains dozens of membranous disks loaded with the membrane protein rhodopsin, and the inner segment contains the nucleus and many mitochondria, which produce the ATP essential to phototransduction.
FIGURE 12–31 Light reception in the vertebrate eye. The lens of the eye focuses light on the retina, which is composed of layers of neurons. The primary photosensory neurons are rod cells (yellow), which are responsible for high-resolution and night vision, and cone cells of three subtypes (pink), which initiate color vision. The rods and cones form synapses with several ranks of interconnecting neurons that con vey and integrate the electrical signals. The signals eventually pass from ganglion neurons through the optic nerve to the brain.
Like other neurons, rods and cones have a trans membrane electrical potential (Vm), produced by the electrogenic pumping of the Na+ K+ ATPase in the plasma membrane of the inner segment (Fig. 12–32). Also, con tributing to the membrane potential is an ion channel in the outer segment that permits passage of either Na or Ca2+ and is gated (opened) by cGMP. In the dark, rod cells contain enough cGMP to keep this channel open. The membrane potential is therefore determined by the net difference between the Na+ and K+ pumped by the inner segment (which polarizes the membrane) and the influx of Na+ through the ion channels of the outer seg ment (which tends to depolarize the membrane). The essence of signaling in the retinal rod or cone cell is a light-induced decrease in the concentration of cGMP, which causes the cGMP-gated ion channel to close. The plasma membrane then becomes hyperpolarized by the Na+ K+ ATPase. Rod and cone cells synapse with interconnecting neurons (Fig. 12–31) that carry information about the electrical activity to the ganglion neurons near the inner surface of the retina. The ganglion neurons integrate the output from many rod or cone cells and send the resulting signal through the optic nerve to the visual cortex of the brain.
FIGURE 12–32 Light-induced hyperpolarization of rod cells. The rod cell consists of an outer segment that is filled with stacks of membra nous disks (not shown) containing the photoreceptor rhodopsin and an inner segment that contains the nucleus and other organelles. Cones have a similar structure. ATP in the inner segment powers the Na+ K+ ATPase, which creates a transmembrane electrical potential by pumping 3 Na+ out for every 2 K+ pumped in. The membrane potential is reduced by the flow of Na+ and Ca2+ into the cell through cGMP gated cation channels in the plasma membrane of the outer segment. When rhodopsin absorbs light, it triggers degradation of cGMP (green dots) in the outer segment, causing closure of the cation channel. Without cation influx through this channel, the cell becomes hyper polarized. This electrical signal is passed to the brain through the ranks of neurons shown in Figure 12–31.
