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Section III: Systems Neuroscience: Sensory & Motor Systems

Visual System I: The Eye

OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Describe the optical functions of the cornea and lens, problems that occur in focus, and the correction of these problems.

  • Describe the process of phototransduction from photon capture to neurotransmitter release.

  • Diagram the neural circuit of the retina, and outline the function of each major cell class.

  • List the central projection targets of retinal ganglion cells and describe the functions of these brain areas.

IMAGING & CAPTURING LIGHT

Vision begins with light passing through the cornea and lens, the optical elements that refract, focus, and transmit light to the innermost layer of the eye, the retina, the region that transduces light information to electrical signals and transmits them to the brain. Because the cornea is at the interface between air (index of refraction close to 1.0) and the corneal tissue (index of refraction about 1.38), the cornea does most of the focusing. The lens, with a slightly higher index of refraction than the aqueous humor (variable, but about 1.4 in its center), fine tunes the focus. Ciliary muscles modulate refraction by the lens, controlling focus. This process is called accommodation. Because the cornea and lens are converging and positive lenses, they project an inverted image on the retina.

Anatomy of the Eye

The main structural elements of the eye are shown in Figure 11–1.

FIGURE 11–1

The anatomy of the eye. A. An axial cross-section of the eye. The outer surface of the eye is the sclera. Inside that is the choroid containing blood vessels that nourish the retina and pigment epithelium. Inside the choroid is the retina. The vitreous (from the Latin for glass) is a gel-like substance that fills the inside of the eye. The area of highest acuity in the eye is the macula lutea. Within the macula is the small area of ultra-high acuity found only in mammals and some birds, the fovea centralis. The optic disk is where the ganglion cell axons leave the optic nerve. There are no photoreceptors there, so it constitutes the “blind spot” in the retina. B. Close-up of the anterior portion of the eye. The cornea does approximately 70% of the focusing. Between the cornea and lens is the aqueous humor. Just in front of the lens is the pupil made by the iris. Pupil size is controlled by the ciliary muscle, which is driven by the ciliary nerve. C. How accommodation, the control of focus, works: The lens fine-tunes the focusing of light entering the eye. Light from a distance is refracted a small amount by the stretched, flat lens to strike the retina in focus. D. Light from a source nearby is refracted more by the relaxed, more convex lens to be focused on the retina. This is controlled by the third cranial ...

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