Accommodation

Structures of accommodation

Ciliary Muscle - The ciliary muscle is a smooth muscle that is shaped like a ring and it’s located in the middle of the eyes. It holds the lens with the suspensory ligaments and also adjusts the optical power or shape of the lens during accommodation.

Lens - This is a transparent structure in the eye, it is biconvex in shape (both surfaces are shaped like the exterior of a circle). It is bordered anteriorly (in front) by a ring it forms with the posterior side of the iris. The lens is held by the suspensory ligament and has a diameter of 10 mm and a height of 4 mm in an adult. These measurements vary due to change in the lens structure during accommodation and aging.

Pupil - The pupil is located in the middle of the eyes, it is black in color and constricts to prevent light rays that have diverged from touching the retina and causing blurred vision.

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Theories of accommodation

Different theories which are still in contention were proposed to describe the mechanism of accommodation. Some of those theories are:

Helmholtz theory of accommodation (1855) – This theory is also called the capsular theory of accommodation. Helmholtz theorized that when the eyes are viewing a distant object, the ciliary muscle relaxes and the zonular fibers between the ciliary body and the equator of the lens stay flattened but when the object of focus is close, the ciliary muscles contract and the zonular fibers loosen. In the Helmholtz theory, the lens equator slides away from the sclera during accommodation and closer to the sclera when accommodation ends. In this theory, the zonular fibers are relaxed during accommodation and the zonular fibers are under tension when there is no accommodation reflex.

Schacher theory of accommodation (2006) - This theory states that when the lens is in focus, there is increased tension on the lens through the equatorial zonular fibers and when there is contraction of the ciliary muscle, the zonular fibers located equatorially increase their tensile strength. This results in the steepness of the central surface of the lens, an increase in thickness of the lens and a flattening of the lens edges. As the tension on the equatorial zonular fibers increase during accommodation, the anterior and posterior zonular fibers relax. The anterior and posterior zonular fibers serve as passive support structures for the lens, but the equatorial zonular fibers determines the refractive power of the lens.

Catenary theory of accommodation (1970) – This theory is also called the Coleman theory of accommodation. It states that the lens and the zonula fibers form a diaphragm, which is held in a catenary (a curve formed by a wire, rope, or chain hanging freely from two points that are not in the same vertical line) shape due to the difference in pressure between the aqueous and vitreous bodies of the lens. A change in diameter of the ciliary body results in a change of the catenary shape. It means there is a continuous pressure difference on the lens. The strength of this pressure difference is approximately 2.3 cm of water column, with major changes occurring during the initial seconds of the accommodation phase. The anterior capsule and the zonular fibers form hammock shaped surface that is duplicable but depends on the diameter of the ciliary body. The ciliary body however makes a shape like the pillars of a suspension bridge, but does not need to support the force around the equator to flatten the lens.

Accommodation pathway

When the eyes focuses on closer objects, the diotropic power of the eye increases by constriction of the pupil and by increasing the lens curvature. At rest the lens is flattened and the tensile strength is high because of the zonular ligaments. During accommodation, the ciliary muscle contracts and moves the ciliary body anteriorly and deep towards the optic axis. All the muscles work simultaneously and tension on the zonular ligaments is relaxed. When the lens releases tension it increases its biconvexity and this enables focusing on closer objects easier. The anterior lens curvature radius changes most during accommodation.

Visual messages from the retina pass to the visual cortex located in the occipital lobe of the brain which interprets the objects in the visual field. The information is sent back through efferent fibers to the pretectal area and then to the Edinger-Westphal nucleus, which contains preganglionic parasympathetic neurons whose axons travel in the oculomotor nerve. Efferent impulses pass in the oculomotor nerve to the orbit where they form a synapse in the ciliary ganglion. Postganglionic fibres (short ciliary nerves) innervate the ciliary muscle causing it to contract. There is also a sparse sympathetic innervation of ciliary muscle, which has a very limited capacity to relax the muscle. Accommodation also leads to constriction of the pupil due to contraction of the sphincter pupillae and convergent eye movements caused by contraction of the medial, superior, and inferior recti which are all innervated by the oculomotor nerve. When focusing on close objects, the optical power of the eye's lens permits the eye to create clear focused images. Objects appear blurred in the plane behind the retina, but due to an increase in the optical power of the eyes, this object image becomes clear. The optical power increases when the lens changes shape. In changing the shape of the lens, the ciliary muscle contracts to decrease the size (or diameter) of the lens, the suspensory ligaments relax and tension is released around the radius of the lens, allowing the lens to form a more spherical shape with a higher optical power.

Similarly, when focusing on distant objects, the lens stays flattened because of the grip from the suspensory ligament. The suspensory ligament draws the edges of the lens capsule (which is elastic) near to the ciliary body and this internal pressure within the lens keeps it flattened in shape. To do this, the ciliary muscle relaxes in conjunction with the increase in suspensory ligament tension, and thus, together they increase the lens diameter and size.