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Review the anatomical structure of the eye, clicking on each part to practice identification.

Transduction of light

The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, rhodopsin    , has two main parts [link] ): an opsin, which is a membrane protein (in the form of a cluster of α-helices that span the membrane), and retinal—a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent ( cis ) form of the molecule to its linear ( trans ) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na + channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold ( [link] ).

 Molecular model A shows the structure of rhodopsin, a trans-membrane protein with seven helices spanning the membrane. A small organic molecule called retinal is tucked inside. B shows the molecular structure of retinal, which has a ring with a hydrocarbon chain attached. A ketone (double bonded oxygen) is at the end of the chain. In cis retinal the chain is kinked. In trans retinal the chain is straight.
(a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and retinal. When light strikes retinal, it changes shape from (b) a cis to a trans form. The signal is passed to a G-protein called transducin, triggering a series of downstream events.
Illustration A shows the signal transduction pathway for rhodopsin, which is located in internal membranes at the top of rod cells. When light strikes rhodopsin, a G protein called transducing is activated. Transducin has three subunits, alpha, beta and gamma. Upon activation, GDP on the alpha subunit is replaced with GTP. The subunit dissociates, and binds phosphodiesterase. Phosphodiesterase, in turn, converts cGMP to GMP, which closes sodium ion channels. As a result, sodium can no longer enter the cell, and the membrane becomes hyperpolarized. Illustration b shows that the tall, thin rod cell is stacked on top of a bipolar nerve cell. In the dark the membrane is depolarized, and glutamate is released from the rod cell to the axon terminal of the bipolar cell. In the light, no glutamate is released.
When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.

Trichromatic coding

There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive, as shown in [link] . Some cones are maximally responsive to short light waves of 420 nm, so they are called S cones (“S” for “short”); others respond maximally to waves of 530 nm (M cones, for “medium”); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or “long” cones). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations. Primates use a three-cone (trichromatic) system, resulting in full color vision.

The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about 2 million distinct colors.

 Graph plots normalized absorbance for rods and S, M and L cones against wavelength. For all four cell types, the trend is an approximately bell-shaped curve with a steeper decrease than increase. For S cones the peak absorbance is 420 nanometers. For rods the peak absorbance is 498 nanometers. For M cones the peak absorbance is 534 nanometers. For L cones the peak absorbance is 564 nanometers.
Human rod cells and the different types of cone cells each have an optimal wavelength. However, there is considerable overlap in the wavelengths of light detected.

Retinal processing

Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of processing of visual information occurs in the retina itself, before visual information is sent to the brain.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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