Keywords
accommodation, blood retinal barrier, pupillary light reflex, phototransduction, visual fields, Horner syndrome, ptosis, strabismus
Chapter Outline
The Eye Has Three Concentric Tissue Layers and a Lens, 108
Intraocular Pressure Maintains the Shape of the Eye, 108
The Cornea and Lens Focus Images on the Retina, 109
The Iris Affects the Brightness and Quality of the Image Focused on the Retina, 109
A System of Barriers Partially Separates the Retina From the Rest of the Body, 109
The Retina Contains Five Major Neuronal Cell Types, 109
The Retina Is Regionally Specialized, 110
Retinal Neurons Translate Patterns of Light Into Patterns of Contrast, 110
Photopigments Are G Protein–Coupled Receptors That Cause Hyperpolarizing Receptor Potentials, 111
Ganglion Cells Have Center-Surround Receptive Fields, 111
Half of the Visual Field of Each Eye Is Mapped Systematically in the Contralateral Cerebral Hemisphere, 111
Damage at Different Points in the Visual Pathway Results in Predictable Deficits, 112
Some Fibers of the Optic Tract Terminate in the Superior Colliculus, Accessory Optic Nuclei, and Hypothalamus, 112
Primary Visual Cortex Sorts Visual Information and Distributes It to Other Cortical Areas, 113
Visual Cortex Has a Columnar Organization, 113
Visual Information Is Distributed in Dorsal and Ventral Streams, 113
Early Experience Has Permanent Effects on the Visual System, 113
Reflex Circuits Adjust the Size of the Pupil and the Focal Length of the Lens, 114
Illumination of Either Retina Causes Both Pupils to Constrict, 114
Both Eyes Accommodate for Near Vision, 115
The visual system is the most studied sensory system, partly because we are such a visually oriented species and partly because of its relative simplicity. In addition, the visual pathway is highly organized in a topographical sense, so even though it stretches from the front of your face to the back of your head, damage anyplace causes deficits that are relatively easy to understand.
The Eye Has Three Concentric Tissue Layers and a Lens
Vertebrate eyes perform functions analogous to those performed by cameras, but do so using three roughly spherical, concentric tissue layers either derived from or comparable to the dura mater, the pia-arachnoid, and the CNS ( Fig. 17.1 ). The thick, collagenous outer layer forms the sclera —the white of the eye—and continues anteriorly as the cornea and posteriorly as the dural optic nerve sheath . The middle layer is loose, vascular connective tissue that forms the pigmented choroid that lines the sclera; it continues anteriorly as the vascular core of the ciliary body , the ciliary muscle , and most of the iris . The innermost layer, itself a double layer because of the way the eye develops, forms the neural retina (closer to the interior of the eye) and the retinal pigment epithelium (adjacent to the choroid); it continues anteriorly as the double-layered epithelial covering of the ciliary body and the posterior surface of the iris. Suspended inside the eye, and not really part of any of these tissue layers, is the lens .
Collectively, structures derived from these three layers, together with the lens, take care of the functions dealt with by cameras: keeping a photosensitive surface in a stable position, focusing images of objects at different distances onto this surface, regulating the amount of light that reaches the photosensitive surface, and absorbing stray light.
Intraocular Pressure Maintains the Shape of the Eye
The shape of the eye is maintained by having it blown up like a soccer ball. The sclera and cornea provide the tough wall, and the inflation pressure is generated by a fluid secretion-reabsorption system much like the cerebrospinal fluid (CSF) system. The ciliary epithelium secretes a CSF-like aqueous humor into the posterior chamber (the space between the iris and the lens). Just as CSF circulates through the ventricles and subarachnoid space and then filters through arachnoid villi, aqueous humor moves through the pupil , into the anterior chamber (between the iris and cornea), gets filtered into a scleral venous sinus (the canal of Schlemm ) near the corneoscleral junction, and from there reaches the venous system. The resistance to flow at the filtration site results in a pressure buildup in the aqueous humor. Because the space behind the lens is filled with gelatinous, incompressible vitreous humor , the pressure in the aqueous humor is transmitted throughout the interior of the eye and the shape of the eye is maintained. The overproduction of fluid by the ciliary epithelium and/or the decrease in outflow can result in increased pressure on the optic nerve, resulting in the loss of vision over time (glaucoma). Medications to treat glaucoma include drugs that inhibit fluid production in the ciliary epithelium (i.e., timolol, apraclonidine) or drugs that will increase the outflow (i.e., latanoprost, physostigmine, pilocarpine).
The Cornea and Lens Focus Images on the Retina
There’s a big change in refractive index at the interface between air and the front of the cornea, so this is where most of the focusing happens. The lens contributes less because there’s much less change in refractive index going from aqueous humor to lens or from lens to vitreous humor. The major importance of the lens is in adjusting the focus of the eye during accommodation for near vision (see Fig. 17.9 later in this chapter). Contraction of the ciliary muscle relaxes some of the tension on the capsule suspending the lens, allowing the lens to get fatter and the eye to focus on near objects.
The Iris Affects the Brightness and Quality of the Image Focused on the Retina
The pigmented posterior epithelial layers of the iris prevent light from getting into the eye except through the pupil, so regulating the size of the pupil regulates the amount of light reaching the retina (although neural changes in the retina are much more important for regulating the sensitivity of the eye). The pupillary sphincter , innervated by the oculomotor nerve via the ciliary ganglion, makes the pupil smaller by releasing acetylcholine onto muscarinic receptors (see Figs. 17.7 and 17.8 later in this chapter). The pupillary dilator , innervated by upper thoracic sympathetics via the superior cervical ganglion, makes the pupil larger by releasing norepinephrine onto adrenergic receptors. During an eye exam to look at the retina, medications such as cholinergic antagonists (i.e., atropine), along with some adrenergic agonists (i.e., phenylephrine), can dilate the pupil ( mydriasis ). Medications that are cholinergic agonists (i.e. carbachol) or adrenergic antagonists (i.e., timolol) will result in small pupils ( miosis ).
A System of Barriers Partially Separates the Retina From the Rest of the Body
Another indication that the neural retina is really an outgrowth of the CNS is the way its neurons are protected by a similar three-part barrier system. The endothelial cells of retinal capillaries are zipped up by tight junctions, forming a blood-retinal barrier system in the literal sense. The ciliary epithelium, just like the choroid epithelium, prevents diffusion (i.e., is a barrier) from the ciliary body into aqueous humor. Finally, the retinal pigment epithelium, in a way analogous to the barrier function of the arachnoid, prevents diffusion from the choroid into the retina.
The Retina Contains Five Major Neuronal Cell Types
Retinal neurons and synapses are arranged in layers.
Standard descriptions of the retina as a 10-layered structure also include a row of junctions between adjacent photoreceptors ( outer limiting membrane ), the layer of ganglion cell axons ( nerve fiber layer ), and the basal lamina on the vitreal surface of the retina ( inner limiting membrane ). Oddly enough, the layers are arranged so that the last part of vertebrate neural retinas reached by light is the photosensitive parts of the rod and cone cells, embedded in processes of pigment epithelial cells.
The Retina Is Regionally Specialized
Rods function in dim light.
Populations of cones signal spatial detail and color.
The center of the visual field corresponds to the fovea , a small retinal region in the middle of a pigmented zone called the macula . The fovea is filled with thin, densely packed cones and no rods. All the other neuronal types are pushed toward the periphery, so the center of the fovea is a small pit. Outside the fovea, the number of cones diminishes quickly. The packing density of rods, in contrast, first increases rapidly and then declines slowly. We have three different types of cones in terms of the wavelength to which each is most sensitive, so the total cone population can be used for color vision. Rods, on the other hand, come in only one variety but function at lower light levels than do cones. The fovea, with its densely packed cones, is therefore specialized for high spatial acuity and color vision , but only at moderate or high levels of illumination. The region around the fovea, with many rods and few cones, has reasonably good spatial acuity, works at low light levels, but is not very useful for color vision. Finally, the peripheral retina, with few rods and even fewer cones, is mostly good for telling us that something is moving around out there.
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