Specialized Features Of Rod And Cone Photoreceptors

The two types of retinal photoreceptor cells, rods and cones, are not evenly distributed across the retina (Fig. 3). Cones are concentrated in the foveal region, whereas rods are absent. Several of the cellular characteristics of cones make them particularly suited for high-acuity vision. Because of the tightly packed arrangement of foveal cones, the foveal region is capable of high-resolution (finegrained) responses. In addition, the outer segment, which contains most of the visual pigment of the cone, is relatively short and is most sensitive to direct axial rays of light. The photochemical cascade in cones is very fast, allowing the foveal region to have a very rapid response time.

Cones are adapted for color vision. Three types of cones have been described: one containing visual pigment that is most sensitive to blue light, one that is sensitive to green light, and one that is most sensitive to long wavelengths, including red and yellow light (referred to as the red-sensitive cone). As will be discussed later, the fovea is comprised primarily of red- and green-sensitive cones. The perception of color from a given light source is derived from a comparison of the relative amount of light activation that is generated in red- versus green- versus. blue-sensitive cones.

Cones are also adapted primarily for daytime (photo-pic) vision. They saturate only in intense light and are generally less sensitive to light than are rod photorecep-tors. During low-light (scotopic) conditions, such as those that exist during dawn or dusk, cones are not responsive, and we have no color or high-acuity vision at these times; visual images are achromatic, and we have difficulty distinguishing small shapes. Under these low-light conditions, vision is primarily mediated by rods. The same visual pigment, rhodopsin, is found in all rods. It maximally absorbs blue-green light, but because the perception of color is derived from a comparison of activation resulting from different photopigments, we do not perceive scotopic vision as having any specific color content.

Rods have much more photopigment than cones. Their outer segments, which contain the rhodopsin, are much longer than cones; thus, a single rod can respond to a much broader angle of incident light. Rods also respond more slowly than cones, which allows them to summate more light responses. These design features are responsible in part for the high sensitivity of individual rods which, in humans, has reached the theoretical limit of

A. Periphery

B. Fovea

C. Optic disk, optic nerve head

B. Fovea

C. Optic disk, optic nerve head

A. Periphery

FIGURE 2 Cellular organization of different regions of the retina. (A) The peripheral retina contains mostly rod photoreceptors, whereas the fovea contains no rods, only cones; other neurons in the retinal visual pathway are similar in the two regions. (B) In the fovea, all cellular elements except the outer segments are displaced radially out of the path of light. (C) The optic nerve head is comprised of ganglion cell axons and thus is devoid of other retinal neurons.

FIGURE 2 Cellular organization of different regions of the retina. (A) The peripheral retina contains mostly rod photoreceptors, whereas the fovea contains no rods, only cones; other neurons in the retinal visual pathway are similar in the two regions. (B) In the fovea, all cellular elements except the outer segments are displaced radially out of the path of light. (C) The optic nerve head is comprised of ganglion cell axons and thus is devoid of other retinal neurons.

detection. Under ideal conditions, the human retina can detect a single photon of light.

The outer segments of rods and cones are in close contact with a monolayer of pigmented epithelial cells (retinalpigment epithelium, or RPE), which serves important metabolic and supportive functions for photorecep-tors. The pigment granules within the RPE help absorb stray light and thus reduce glare. RPE cells produce a sticky extracellular matrix material that helps keep the outer segments straight and aligned. They participate in the breakdown of bleached photopigment and recycle the molecular byproducts to photoreceptors for resynthesis of pigments. They also ingest the outermost tips of the outer segments after they are shed by photoreceptors and thus aid in the continual renewal of outer segments.

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