Accommodation

The refractive power of the eye described in the previous section is not a constant value. The eye's refractive power must alter to allow visualization of

Table 16.1 Important refractive indices n of the various tissues of the eye (from Krause, K. Methoden der Refraktionsbestimmung. Biermann, Münster, Germany, 1985)

Eye tissue

Refractive index n

Cornea

1.376

Aqueous humor

1.336

Lens at the poles

1.385

Lens at the core

1.406

Vitreous body

1.336

426 16 Optics and Refractive Errors — Refraction of light rays traveling through converging and diverging lenses. —

Fig. 16.3 a The converging lens (biconvex) concentrates incident light rays at a focal point behind the lens. b A diverging lens (biconcave) ensures that the light rays do not meet at all. The light rays appear to originate at a virtual focal point in front of the lens.

both near and distant objects with sharp contours. This accommodation is made possible by the elasticity of the lens.

Accommodation mechanisms: Accommodation involves the lens, zonule fibers, and ciliary muscle.

❖ Lens: The soluble proteins of the lens are surrounded by a thin elastic capsule. The curvature of the posterior capsule of the lens is greater than its anterior curvature, with a posterior radius of 6.0 mm as opposed to an anterior radius of 10.0 mm. The intrinsic elasticity of the lens capsule tends to make the lens assume a spherical shape. However, in the unaccommodated state this is prevented by the pull of the zonule fibers. The elasticity of the inner tissue of the lens progressively decreases with age due to deposits of insoluble proteins.

❖ Zonule fibers: The radiating zonule fibers insert into the equator of the lens and connect it to the ciliary body. They hold the lens securely in position and transmit the pull of the ciliary muscle to the lens.

❖ Ciliary muscle: Contraction of the ring-shaped ciliary muscle decreases the tension in the zonule fibers. The lens can then approach the spherical shape (with a radius of curvature of 5.3 mm) that its physical configuration and chemical composition would otherwise dictate. This change in the curvature of the lens is especially pronounced in its anterior surface. The deformation increases the refractive power; the focus of the eye shifts to the near field (Fig. 16.4), and close objects take on sharp contours. As the ciliary muscle relaxes, the tension on the lens increases and the lens flattens. The resulting decrease in refractive power shifts the focus of the eye into the distance (Fig. 16.4), and distant objects take on sharp contours.

The ciliary muscle is innervated by the short ciliary nerves, postganglionic parasympathetic fibers of the oculomotor nerve. Parasympatholytics such as atropine, scopolamine, and cyclopentolate inhibit the function of the ciliary muscle and therefore prevent accommodation. Referred to as cycloplegics, a

426 16 Optics and Refractive Errors — Refraction of light rays traveling through converging and diverging lenses. —

Fig. 16.3 a The converging lens (biconvex) concentrates incident light rays at a focal point behind the lens. b A diverging lens (biconcave) ensures that the light rays do not meet at all. The light rays appear to originate at a virtual focal point in front of the lens.

16.1 Basic Knowledge 427 Morphologic changes in accommodation. -

Ciliary muscle Accommodation

16.1 Basic Knowledge 427 Morphologic changes in accommodation. -

Ciliary muscle Accommodation

Fig. 16.4 Upper half of figure: In accommodation, the lens becomes increasingly globular. The curvature of the anterior surface in particular increases. The ciliary muscle is shifted slightly anteriorly, and the anterior chamber becomes shallower. Objects in the near field (continuous line) are represented on the retina with sharp contours.

Lower half of figure: With the ciliary body relaxed, parallel incident light rays (dotted line) are focused on the retina. Distant objects are represented on the retina with sharp contours.

Fig. 16.4 Upper half of figure: In accommodation, the lens becomes increasingly globular. The curvature of the anterior surface in particular increases. The ciliary muscle is shifted slightly anteriorly, and the anterior chamber becomes shallower. Objects in the near field (continuous line) are represented on the retina with sharp contours.

Lower half of figure: With the ciliary body relaxed, parallel incident light rays (dotted line) are focused on the retina. Distant objects are represented on the retina with sharp contours.

these medications also cause mydriasis by inhibiting the sphincter pupillae. Parasympathomimetics such as pilocarpine cause the ciliary muscle and sphincter pupillae to contract, producing miosis.

H When the ciliary muscle is at rest, the zonule fibers are under tension and the eye focuses on distant objects.

Accommodation is regulated by a control loop. The control variable is the sharpness of the retinal image. The system presumably uses the color dispersion of the retinal image to determine the direction in which accommodation should be corrected.

Range of accommodation: This specifies the maximum increase in refractive power that is possible by accommodation in diopters (Fig. 16.5). In mathematical terms, the range of accommodation is obtained by subtracting near-point refractive power from far-point refractive power. The near point is shortest distance that allows focused vision; the far point describes the farthest point that is still discernible in focus. The near and far points define the range of accommodation; its specific location in space is a function of the refractive power of the eye.

Example: In one patient, the near point lies at 0.1 m and the far point at 1 m. This patient's range of accommodation is then 10 diopters -1 diopter = 9 diopters.

In an emmetropic eye, the far point is at optical infinity. However, accommodation can also bring near-field objects into focus (Fig. 16.6b). The elasticity of the lens decreases with increasing age, and the range of accommodation decreases accordingly (Fig. 16.5). Presbyopia (physiologic loss of accommodation in advancing age) begins when the range of accommodation falls below 3 diopters. The gradual loss of accommodation causes the near point to recede; that patient's arms become "too short for reading". Depending on age and limitation of accommodation, presbyopia can be compensated for with converging lenses of 0.5-3 diopters (see Fig. 16.6c and d).

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