Porth's Essentials of Pathophysiology, 4e

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Nervous System

U N I T 1 0

Axons of retinal ganglion cells

Direction of light

Optic disk

Synaptic body

Ganglion cell layer

Nucleus

Neural layer

Bipolar cell layer

Pigment epithelium

Inner segment Connecting structure

Sclera

Layer of rods and cones

Optic nerve

Outer segment

A

Pigment epithelium

B

C

FIGURE 38-6. Organization of the retina. (A) Organization of the retina showing the inner neural layer and the outer pigment epithelium. (B) The three layers of the neural retina: a posterior layer of photoreceptors (rods and cones), a middle layer of bipolar cells, and an inner layer of ganglionic cells. (C) Photoreceptor structure: a retinal rod, showing its component parts and distribution of organelles.

hemorrhage and the development of opacities; separa- tion of the pigment and sensory layers of the retina (i.e., retinal detachment); and macular degeneration. Because the retina has no pain fibers, most diseases of the retina do not cause pain, nor do they produce redness of the eye. The retina is composed of two layers: an outer pig- ment (melanin-containing) epithelium and an inner neural layer 16,22 (Fig. 38-6A). The outer surface of the pigmented layer, a single-cell–thick lining, abuts the choroid and extends anteriorly to cover the ciliary body and the posterior side of the iris. Its pigmented epithelial cells, like those of the choroid, absorb light and prevent it from scattering. The pigment layer also stores large quantities of vitamin A, which is an important precursor of the photosensitive visual pigments. The inner light-sensitive neural retina covers the inner aspect of the eyeball. 16,22 The neural retina is composed of three layers of neurons: a posterior layer of photore- ceptors (rods and cones), a middle layer of bipolar cells, and an inner layer of ganglion cells that communicate with the photoreceptors (see Fig. 38-6B). Light must pass through the transparent inner layers of the sensory retina before it reaches the photoreceptors. Impulses produced in response to light spread from the photore- ceptors to the bipolar neurons and other interneurons and then to the ganglionic cells, where action potentials are generated. The interneurons, which have cell bodies in the bipolar layer, play an important role in modu- lating retinal function. The optic disk, where the optic nerve exits the eye, is the weak part of the eye because it is not reinforced by the sclera. It also forms the blind

spot because it is not backed by photoreceptors, and light focused on it cannot be seen. People do not notice the blind spot because of a sophisticated visual func- tion called “filling in,” which the brain uses to deal with missing visual input. Two types of photoreceptors are present in the ret- ina: rods capable of black–white discrimination and cones capable of color discrimination. 16,22,24 Rod-based vision is particularly sensitive to detecting light, espe- cially moving light stimuli, at the expense of clear pat- tern discrimination. Rod vision is particularly adapted for night and low-level illumination. Cone receptors, which are selectively sensitive to different wavelengths of light, provide the basis for color vision. Three types of cones, or cone-color systems, respond to the blue, green, and red portions of the visible electromagnetic spectrum. Cones do not have the dark adaptation capability of rods. Consequently, the dark-adapted eye is a rod receptor eye with only black–gray–white expe- rience (scotopic or night vision). The light-adapted eye ( photopic vision ) adds the capacity for color discrimination. Both rods and cones contain chemicals that change configuration on exposure to light and, in the process, generate electric signals that lead to the action potentials generatedbytheganglioniccells.The light-sensitivechem- ical in rods is called rhodopsin , and the light-sensitive chemicals in cones are called cone or color pigments . Both types of photoreceptors are thin, elongated, mito- chondria-filled cells with a single, highly modified cil- ium (Fig. 38-6C). The cilium has a short base, or inner