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The Retina

You will now learn about the retinal neurons and the laminar structure of the retina, and the ways in which the light-sensitive receptors of the eye convert the image projected onto the retina into neural responses. The light sensitive retina forms the innermost layer of the eye (Figure 14.16).

Figure 14.16 The eye, the three coats of the eye and the layers of the retina. The retina is the innermost coat of the eye and consists of the retinal pigment epithelium and neural retina.

The retina covers the choroid and extends anteriorly to just behind the ciliary body. The retina consists of neurons and supporting cells.

Components of the Retina

The retina is derived from the neural tube and is, therefore, part of central nervous system. It consists of two parts, the retinal pigment epithelium, which separates the middle, choroid coat of the eyeball from the other innermost component and the neural retina (Figure 14.16) – the dark pigments within the retinal pigment epithelium and choroid coat function to absorb light passing through the receptor layer, thus reducing light scatter and image distortion within the eye. The neural retina contains five types of neurons (Figure 14.17): the visual receptor cells (the rods and cones), the horizontal cells, the bipolar cells, the amacrine cells, and the retinal ganglion cells.

Retinal Layers

The retina is a laminated structure consisting of alternating layers of cell bodies and cell processes (Figure 14.18).

Figure 14.17 The components of the neural retina. The neural retina consists of at least five different types of neurons: the photoreceptors (rods and cones), horizontal cell, bipolar cell, amacrine cell and ganglion cell.

Figure 14.18 The neural retina is formed by alternating layers of neuron cell bodies that appear dark and neuron processes that appear light in Nissl stained tissue. The receptor cells synapse with bipolar and horizontal cells in the outer plexiform layer. The bipolar cells, in turn, synapse with amacrine and ganglion cells in the inner plexiform layer The axons of the retinal ganglion cells exit the eye to form the optic nerve.

The innermost layers are located nearest the vitreous chamber, whereas the outermost layers are located adjacent to the retinal pigment epithelium and choroid. The most important layers, progressing from the outer to inner layers, are:

• the retinal pigment epithelium, which provides critical metabolic and supportive functions to the photoreceptors;
  
• the receptor layer, which contains the light sensitive outer segments of the photoreceptors;
  
• the outer nuclear layer, which contains the photoreceptor cell bodies;
  
• the outer plexiform layer, where the photoreceptor, horizontal and bipolar cells synapse;
  
• the inner nuclear layer, which contains the horizontal, bipolar and amacrine cell bodies;
  
• the inner plexiform layer, where the bipolar, amacrine and retinal ganglion cells synapse;
  
• the retinal ganglion cell layer, which contains the retinal ganglion cell bodies; and
  
• the optic nerve layer, which contains the ganglion cell axons traveling to the optic disc.
  

Notice that light passing through the cornea, lens and vitreous must pass through most of the retinal layers before reaching the light-sensitive portion of the photoreceptor; the outer segment in the receptor layer. Notice also that in the region of the fovea where the image of the central visual field center is focused, the retina consists of fewer layers (Figure 14.19): thereby minimizing the obstacles to forming a clear image on the fovea. The area around the fovea, the surrounding macula, is thicker because it contains the cell bodies and processes of retinal neurons receiving information from the receptors in the fovea.

The optic disc is formed by the retinal ganglion cell axons that are exiting the retina. It is located nasal to the fovea (Figure 14.19). This region of the retina is devoid of receptor cells and composed predominantly by the optic nerve layer. Consequently, it is the structural basis for the 'blind spot" in the visual field.

Figure 14.19 The fovea of the retina and the layers of the retina in the surrounding macula. The fovea and macula are colored as they appear when stained for Nissl substance, which is most abundant in the neuron cell body.

The Photoreceptors

The human has two types of photoreceptors: the rods and cones (Figure 14.20). They are distinguished structurally by the shapes of their outer segments. The photopigments of the rods and cones also differ. The rod outer segment disks contain the photopigment rhodopsin, which absorbs a wide bandwidth of light. The cones differ in the color of light their photopigments absorbs: one type of photopigment absorbs red light, another green light, and a third blue light. As each cone receptor contains only one of the three types of cone photopigment, there are three types of cones; red, green or blue. Each cone responds best to a specific color of light, whereas the rods respond best to white light². The rod and cone photopigments also differ in illumination sensitivity; rhodopsin breaks down at lower light levels than that required to breakdown cone photopigments. Consequently, the rods are more sensitive - at least at low levels of illumination.