Low-light vision

In the eye, specialized photoreceptor cells of the retina, called rod cells are primarily responsible for low-light vision, with relatively little color detection. Rod cell outer segments contain lamellar protein disks rich in the protein opsin (Figure 19.26). Oxidation and isomerization of all-im/75 -retinol yields an intermediate, W-cis retinal, which is important in photoreception. The chemistry of photoreception is shown in Figure 19.27 and summarized as follows. 

High-purity lead oxide is used to make precision glasses needed for lasers, low-dose X-ray machines, fiber optic probes, medical camera systems, and low-light military equipment such as night vision scopes and goggles. 

Deficienqr of vitamin A results in night blindness (rod cells are responsible for vision in low light), metaplasia of the corneal epithelium, dry eyes, bronchitis, pneumonia, and follicular hyperkeratosis. 

In the absence of sufficient vitamin A, one consequence is a vision defect termed night blindness or, more technically, xerophthalmia. The rod cells in the retina are largely responsible for vision in low light. If they have too little retinal, they cannot do their job and night blindness, the inability to see in low light, results. If the condition persists, it is characterized by extreme dryness of the conjunctiva and can result in permanent blindness. 

A deficiency of vitamin A leads to vision defects, including a visual impairment at low light levels, termed night blindness. For the processes of vision, retinol needs to be converted first by oxidation into the aldehyde retinal, and then by enzymic isomerization to cw-retinal. c -Retinal is then bound to the protein opsin in the retina via an imine linkage (see Section 7.7.1) to give the red visual pigment rhodopsin. 

Under conditions of low light intensity, the all-trans-retinaldehyde released from rhodopsin is reduced to aU-fra s-retinol, which is then transported to the retinal pigment epithelium bound to the interphotoreceptor RBP. This protein also binds fatty acids, including palmitate and docosa-hexaenoic acid (C22 6 co3), which is known to be essential for vision and which comprises some 50% of the phospholipid of photoreceptor cells. 

The retina has been considered" " to be a doubic-.sense organ in which the rods arc concerned with colorless vision at low light intensities and the cones are concerned with color vision at high light inicn.silics. A dark-adapted, excised retina is rose red when it is exposed to light. it.s color... 

Both chemical and visual information is available when a prey species assesses local predation risk. However, contradictory models exist regarding the relative importance of visual versus chemical cues during risk assessment. Laboratory experiments conducted by Hartman and Abrahams (2000) demonstrated that fathead minnows reacted to alarm cues under low light conditions with significantly more dashes than they did in clear water. They concluded that vision was the primary source of information used in predator assessment (visual compensation model), as the minnows would respond to alarm cues with greater intensity when visibility was reduced. 

Increasing the size and contrast of the letters on a remote control will not only aid users with low vision but also will aid the general population in low light. Choose color combinations that can be seen by those with color blindness. Also, larger switches arranged in an expected, logical order will make for a better product. Those with limited mobility, including the millions with arthritis, will be able to operate such a remote with ease. 

Night vision involves the differences between the two kinds of receptor cells. Night vision refers to the function of rod cells in low light. Smdies indicate that rod cell sensitivity in low light diminishes somewhat with age. Compared to someone who is 20 years old, a 45-year-old person needs four times the light to achieve the same level of perception in low light. By age 60, the light levels needed double compared to age 45. 

In the retinal cells of the eye, vitamin A (all-trans-retinol) is converted into the 11-ds-isomer, which is then oxidised to 11-cts-retinaldehde. In the dark the latter then combines with the protein opsin to form rhodopsin (visual purple), which is the photoreceptor for vision at low light intensities. When light falls on the retina, the czs-retinaldehyde molecule is converted back into the aW-trans form and is released from the opsin. This conversion results in the transmission of an impulse up the optic nerve. The all-frans-retinaldehyde is converted to all-trans-retinol, which re-enters the cycle, thus continually renewing the light sensitivity of the retina (Rg. 5.2). 

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