The Visual System

Boyes WK, Tandon P, Barone S, et al. 1994. Effects of organophosphates on the visual system of rats. J Appl Toxicol 14 135-143. 

Sandberg M, Jacobson I (1981) Beta-alanine, a possible neurotransmitter in the visual system J Neurochem 37(5) 1353-1356... 

Finocchio DV, Luschei ES, Mottet NK, Body RL. 1980. Effects of methylmercury on the visual system of the rhesus macaque (Macaca mulatto). I. Pharmacokinetics of chronic methylmercury related to changes in vision and behavior. In Merigan WH, Weiss B, editors. Neurotoxicity of the visual system. New York Raven Press, p. 113-122. 

The retina comprises two principal components, the non-neural retinal pigment epithelium and the neural retina. The retinal pigment epithelium is an essential component of the visual system both structurally and functionally. It is important for the turnover and phagocytosis of photoreceptor outer segments, the metabolism of retinoids, the exchange of nutrients between the photoreceptors, and the choroidal blood vessels and the maintenance of an efficient outer blood-retinal barrier. 

Cooper GP, Fox DA, Howell WE, et al. 1980. Visual evoked responses in rats exposed to heavy metals. In MeriganWH, Weiss B, eds. Neurotoxicity of the visual system. New York, NY Raven Press, 203-218. 

More specific to the events after electrical signals have been generated in the retina are observations that critical flicker fusion thresholds, a classical measure of central processing speed relevant to the dynamic functioning of the visual system, are directly proportional (p < 0.001) to MPOD, as first reported by Hammond and Wooten (2005) and confirmed by Renzi et al. (2008a) in a larger population. 

Foster In view of the concentration of chromophore in the visual system, a 500-fold reduction of chromophore in the eye may have very httle effect on the inner retinal photoreceptors. It is striking that Drosophila carotenoid-depletion experiments only reduce the visual ERG but did not abolish it. 

Foster I don t know the mechanism whereby chromophore is retained. The bottom line is that we know chromophore is only depleted in flies and mice, and that most of the retinoid will be part of the visual system anyway. There must be some other way of retaining vitamin A. In fact in mammals, there are a whole range of potential vitamin A binding proteins, hke IRBP in the pigmented epithelium, that could serve to mop-up and act as a chromophore sink. 

Hallucinogens such as lysergic acid diethylamide (LSD) modulate serotonergic neurons via serotonergic autoreceptors and, by targeting receptors in the visual system, may cause prominent visual hallucinations (Aghajanian and Marek, 1999). 

S ADP + N-phospho-L-Arg <7, 28> (<7> the enzyme is an important component of the energy releasing mechanism in the visual system that has high and fluctuating energy demands [28] <28> the enzyme is a modulator of energetic reserves under starvation stress conditions, activity is post-transcriptionally regulated [37]) (Reversibility <7, 28> [28, 36]) [28, 37]... 

The cure for dream psychosis is awakening. As soon as REM terminates, the serotonin system escapes from inhibition and modulates the visual system in favor of the undistorted processing of external data. As the LSD effects wore off, Hofmann slept deeply, thereby restoring the efficacy of his own serotonin neuromodulatory system. Too bad we couldn t have recorded that first recovery night to detect Hofmann s REM, measure it, decide if it was more or less intense, and even perform an awakening or two to sample his post-LSD dreams. 

The emotions mescaline evokes are not always pleasant. Although one may feel euphoria and a sense of cosmic transcendence, one may also see grotesque faces and have a gripping sense of terror. By this time, the brain effects have spread far beyond the visual system. More specifically, they have by now clearly engaged those limbic structures that constituted Kliiver s second scientific focus of interest. 

Vertebrate Olfaction and Gustation Use Mechanisms Similar to the Visual System... 

The occurrence of a variety of neuropeptides in the brain has been discussed in Section A. The first of these to be discovered811 was the 11-residue substance P (Table 30-4), which was isolated in 1931. Like other neuropeptides it may function either as a transmitter or neuromodulator or perhaps both. Substance P, as well as many other neuropeptides, has been localized to specific neurons. Along with somatostatin, CCK, and enkephalins, it is found in high concentrations in the basal ganglia. Enkephalin and substance P are also found in specific neural elements in the visual system of lobsters.812 In some cases a neuron contains both synaptic vesicles containing a major neurotransmitter and also vesicles containing a peptide or other cotrans-mittor. The peptide pituitary hormones ACTH, MSH, and vasopressin as well as the hypothalamic neurohormones may have effects on learning and behavior.813... 

Evarts and his collaborators (1955) investigated the influence of LSD-25 on transmission in the visual system of cats. 

These investigators not only were interested in the effect of this drug on the visual system as a whole, but, by an ingen-... 

Human color perception correlates with integrated reflectance (McCann et al. 1976). Other experiments have shown that the human visual system does not actually estimate the reflectance of objects (Helson 1938). What is known about the visual system is that color processing is done in an area denoted as V4 (visual area no. 4). In V4, cells have been found that respond to different colors irrespective of the type of illuminant (Zeki... 

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