The human eye is an organ that reacts with light and allows light perception, color vision, and depth perception.

The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina (shown in Figure 1) on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina (Figure 1b). The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The iris, which is conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.

The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.

There are two types of photoreceptors in the retina: rods and cones, named for their general appearance as illustrated in Figure 2. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.

The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.

 

 

Practice Questions

 

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Key Points

• The cornea and the lens bend light to focus the image on the retina; the iris and pupil regulate the amount of light entering the eye.

• The aqueous humour maintains the convex shape of the cornea; the vitreous humour supports the lens and maintains the shape of the entire eye.

• Presbyopia occurs because the image focuses behind the retina; it is similar to hyperopia (farsightedness), which is caused by an eyeball that is too short.

• Myopia (nearsightedness) occurs when an eyeball is elongated; images in the distance appear blurry, but images nearby are clear.

• Rods are used for peripheral and nighttime vision; cones are used for daytime and color vision.

• The fovea is responsible for acute vision because it has a high density of cones.

Key Terms

Cornea: The transparent layer forming the front of the eye.

Lens: a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retinaA

Iris: the opaque contractile diaphragm perforated by the pupil and forming the colored portion of the eyeT

Presbyopia: Farsightedness caused by loss of elasticity of the lens of the eye, occurring typically in middle and old age.

Hyperopia: A condition in which visual images come to a focus behind the retina of the eye and vision is better for distant than for near objects.

Myopia: A vision condition in which people can see close objects clearly, but objects farther away appear blurred.

Retina: the thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain.

Rod: A rod-shaped cell located in the outer retina of the eye that is extremely sensitive to light.

Cone: A cell located near the center of the retina that is weakly photosensitive and is responsible for color vision in relatively bright light.