Vision signal transduction

See also G Proteins and Signal Transduction, G proteins in vision. Signal Transduction Agonists and Antagonists... 

Another regulatory Ca receptor is recoverin, which performs an important control function in the signal transduction cascade of the vision process, by inhibiting the activity of rhodopsin kinase (see Chapter 5.3.4). 

The detection of light, smells, and tastes (vision, olfaction, and gustation, respectively) in animals is accomplished by specialized sensory neurons that use signal-transduction mechanisms fundamentally similar to those that detect hormones, neurotransmitters, and growth... 

Demyelination. The role of myelin in the nervous system is to aid in signal transduction. Myelin acts like an electrical insulator by preventing loss of ion current, and intact myelin is critical for the fast saltatory nerve conduction discussed above. Neurotoxicants that target the synthesis or integrity of PNS myelin may cause muscle weakness, poor coordination, and paralysis. In the brain, white matter tracts that connect neurons within and between hemispheres may be destroyed, in a syndrome known as toxic leukoencephalopathy. A multifocal distribution of brain lesions is reflected in mental deterioration, vision loss, speech disturbances, ataxia (inability to coordinate movements), and paralysis. 

Figure 29-5. The visual cycle of vitamin A is central to vision. In the retina, light stimulates the conversion of 1 l-c/.s-retinal, part of rhodopsin (Rho), to all-iraws-retinal and activates rhodopsin (Rho ).This initiates the first step of the signal transduction cascade that results in the transmission of the visual signal to the brain.The visual cycle involves biochemical and metabolic events in both the photoreceptors (rods) and the retinal pigment epithelium.

Tlie second messengers that we have seen recur in many additional signal transduction pathways. For exampile, in a consideration of the sensory systems in Chapter 32, we will see how Ca -based signaling and cyclic nucleotide-based signaling play key roles in vision and olfaction. 

How do our sensory systems work How are the initial stimuli detected How are these initial biochemical events transformed into perceptions and experiences We have already encountered systems that sense and respond to chemical signals—namely, receptors that bind to growth factors and hormones. Our knowledge of these receptors and their associated signal-transduction pathways provides us with concepts and tools for unraveling some of the workings of sensory systems. For example, 7TM receptors (seven-transmembrane receptors. Section 14.1) play key roles in olfaction, taste, and vision. Ion channels that are sensitive to mechanical stress are essential for hearing and touch. 

Smell, taste, vision, hearing, and touch are based on signal-transduction pathways activated by signals from the environment. These sensory systems function similarly to the signal-transduction pathways for many hormones. "These intercellular signaling pathways appear to have been appropriated and modified to process environmental information. 

The visual transduction pathway is the best characterized G-protein-coupled signal transduction system. Study of the visual receptor, rhodopsin, over the past several decades has made it the archetype of the growing superfamily of heptahelical G-protein-coupled receptors (reviewed in Litman Mitchell, 1996a). The preeminent position of rhodopsin in this important superfamily will likely increase with the recent publication of the three-dimensional structure of rhodopsin (Palczewski et al., 2000). Many neurotransmitter receptors, as well as the olfactory and taste receptors, are members of this superfamily. Therefore, the effect of lipid membrane composition on various steps in visual signaling will be reviewed in some detail in this chapter. Given the similarity in mode of signaling, the observations made for the vision system should be of general applicability to other members of this receptor superfamily. 

The role of visual rhodopsins is to activate transducin, a heterotrimeric G protein, in the signal transduction cascade of vision [6,7,24]. Rhodopsin, a member of the... 

DHA is very abundant in excitable membranes in the retina and brain, particularly in PL of the rod outer segment of retina and of synaptic vesicles, and is important in vision. However, the mechanism by which DHA functions in retina is not well understood. Chen et al. (Y. Chen, 1993) suggest that DHA in retina might be involved in shuttling 11-c/j-retinal to photoreceptors, whereas Niu et al. (S. Niu, 2004) propose that DHA in PL increases the efficiency of G-protein-mediated signal transduction of rhodopsin. In humans, supplementation of infant formula with DHA accelerates the development of visual functions in pre-term infants. The novel protective lipid mediator docosanoids, namely, Protectin D1 (C. Serhan, 2002) and 17S-hydroperoxy-DHA (V. Marcheselli, 2003), have been suggested to mediate the beneficial effects of DHA. 

Smell, taste, vision, hearing, and touch are based on signal-transduction pathways activated by signals from the environment. 

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