Vision mechanics

Wooten, B. R. and G. Wald (1973). Color-vision mechanisms in the peripheral retinas of normal and dichromatic observers. Journal of General Physiology 61 125-145. 

As well as their many important technological applications, conjugated polymers are also active components in many biological optophysical processes, for example, as light collectors in photosynthesis, and in the vision mechanism via photoisomerization. Charge transport in organic molecules is also an important component of cellular function. Thus, many of the concepts developed in this book are applicable to these biological systems. 

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

W. S. Stiles, Mechanisms of Colour Vision, Academic Press, Inc., New York, 1978. 

The problem could be maintenance, operation, or design, or a combination of any or all these factors. In all honesty, you should never. see this set of evidence marks because it indicates a lack of control. Now because the mechanic cannot control operational problems or design problems, the first phase to correct this situation is to control the mechanical maintenance factors, like alignment, proper bolting and torque. sequences, be sure shafts are straight and round, and dynamically balance all rotary components. Reinstall the pump and wait for the next failure. Once the maintenance factors are under control, there should appear a clear vision and path to resolve any operational and/or design weaknesses. 

Baskes (1999) has discussed the status role of this kind of modelling and simulation, citing many very recent studies. He concludes that modelling and simulation of materials at the atomistic, microstructural and continuum levels continue to show progress, but prediction of mechanical properties of engineering materials is still a vision of the future . Simulation cannot (yet) do everything, in spite of the optimistic claims of some of its proponents. 

Cardiac glycosides have a small ratio of toxic to therapeutic concentration. Possible adverse effects are nausea, vomiting, abdominal pain, diarrhoea, fatigue, headache, drowsiness, colour vision disturbances, sinus bradycardia, premature ventricular complexes, AV-block, bigeminy, atrial tachycardia with AV-Block, ventricular fibrillation. There are several mechanisms relevant for their toxic action (Table 2). 

Because nonmetals do not form monatomic cations, the nature of bonds between atoms of nonmetals puzzled scientists until 1916, when Lewis published his explanation. With brilliant insight, and before anyone knew about quantum mechanics or orbitals, Lewis proposed that a covalent bond is a pair of electrons shared between two atoms (3). The rest of this chapter and the next develop Lewis s vision of the covalent bond. In this chapter, we consider the types, numbers, and properties of bonds that can be formed by sharing pairs of electrons. In Chapter 3, we revisit Lewis s concept and see how to understand it in terms of orbitals. 

Nortriptyline. Nortriptyhne, a tricychc antidepressant, has been shown in double-blind, placebo-controlled randomized trials to be superior to placebo for smoking cessation (Prochazka et al. 1998). Nortriptyline appears to have efficacy comparable to that of bupropion for smoking cessation (Hall et al. 2002). The efficacy of this agent may be improved with more intensive behavioral therapies (Hall et al. 1998). Nortriptyline s mechanism of action is thought to relate to its noradrenergic and serotonergic reuptake blockade, because these two neurotransmitters have been implicated in the neurobiology of nicotine dependence. Side effects of nortiptyline are typical of tricyclic antidepressants and include dry mouth, blurred vision, constipation, and orthostatic hypotension. Nortriptyline appears to have some utility for smokers with a past history of major depression, and it can be recommended as a second-... 

The Physiome Project should be viewed as both a vision and a route. It has been portrayed as consisting of two parts (Bassingthwaighte et al. 1998) (i) the databasing of biological information (the Mechanic s touch ), and (ii) the development of descriptive and, ultimately, analytical models of biological function (the Orbiter s view ). These are by no means sequential stages of the development. 

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

Cannabinoids have antiemetic activity when used alone or in combination with other antiemetics.5 Dronabinol and nabilone are commercially available oral formulations used for preventing and treating refractory CINV.5,10 Dronabinol is also used to treat anorexia and weight loss associated with human immunodeficiency virus (HIV) infection. Cannabinoids are thought to exert their antiemetic effect centrally, although the exact mechanism of action is unknown.1,10 Sedation, euphoria, hypotension, ataxia, dizziness, and vision difficulties can occur with cannabinoids. 

Thus far in this book we have discussed one- or two-component photochemical systems which because of their relative simplicity lend themselves quite well to laboratory study. Consequently the mechanisms of many of the photoreactions we have discussed have been elucidated in exquisite detail. As we turn our attention in this chapter to some photochemical aspects of living systems, we shall find much more complex situations in which mechanistic details are just now beginning to be obtained. In some systems, such as those which exhibit phototaxis or phototropism, so little is known that our treatment must as a consequence be limited to only a brief discussion of these phenomena. The topics we will consider here are photosynthesis, vision, phototaxis and phototropism, and damage and subsequent repair of damage by light. Due to space limitations, a discussion of the very fascinating area of bioluminescence must be omitted. 

Brindley, G. S. et al. (1966). The flicker fusion frequency of the blue-sensitive mechanism of colour vision. Journal of Physiology (London) 183 497-500. 

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