ASSIGNMENT 2: ACCOMMODATION & THE LENS

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CRYSTALLINE LENS STRUCTURAL OVER-VIEW

The Crystalline lens (or just lens for short) is a transparent structure that helps focus light on the retina. It is held in place by zonular ligaments. The zonules attach to the ciliary body. The lens is made up of the following parts

1. The Capsule

2. The Cortex (anterior and posterior)

3. The Nucleus

The Capsule is the epithelium covering of the lens. It helps keep the contents of the lens in place and provide a barrier for exchange of nutrients

The Cortex is the place occupied by lens cells. They extend anteriorly and posteriorly from the equator

The Nucleus is the central portion of the lens. This is made up of compressed mass of lens cells. The lens grows through out life. Thus the nucleus increases in size as an individual ages (more lens cells are compacted and become a part of the nucleus). This adds to the stiffness of the lens as the nucleus is a rigid structure, while the rest of the lens (cortex and capsule) is flexible.

The zonules attach the lens to the ciliary body. The support the lens and also help to change it's shape during the process of accommodation.

CILIARY BODY

The ciliary body is a triangular structure. It forms a part of the uveal tract (Iris, Ciliary body and the Choroid). The mass of the ciliary body is composed of muscles that control accommodation.

The ciliary body run like a ring (360 degrees) inside the eye. The zonular ligaments, attach the lens to the ciliary body.

It is innervated by parasympathetic fibers from the ciliary ganglion. The contraction of the ciliary muscles results in the narrowing of the ring, relaxation of the zonules. This makes the lens more globular during the process of accommodation.

The ciliary body serves another very important function. It is responsible for the production of the aqueous humor.

ACCOMMODATION & NEAR VISION

The process by which your eyes adjust for near vision is called "accommodation" i.e. your eyes accommodate for focusing near targets. Three things make up the accommodation reflex which enables your eyes to see clearly for near:

1. Your eyes converge

2. The pupil's size decrease

3. The lens become more globular.

The first two processes help to improve focus at a near distance. The third factor is responsible for actually focusing light. Why is this necessary?

Light rays must be coming from infinity to be focused on the retina with the eye at rest (i.e. not accommodating; light coming from infinity is in the form of parallel rays, which do not require an effort to focus).

For all distances less than infinity (in eye optics 6 meters and beyond is considered "infinity". ), the eye must "act" to focus light.

This act is the accommodation reflex.

The process helps focus diverging light rays focus on the retina. As more power is required to focus diverging rays the total power of the eye must be increased. This is done by making the lens more globular. The following figure shows the change that takes place during accommodation.

The lens shape is change by contraction of the ciliary muscles, which reduce the diameter of the ciliary body ring (remember the ciliary body is a circular structure that extends 360 in the eye), relaxing the zonules that support the lens and making the lens more round (globular).

This video shows how the ciliary body ring is altered during accommodation

PRESBYOPIA

As you age, the ability to accommodate is gradually lost. This is part due to weakness in the ciliary body muscles, zonular weakness and stiffness of the lens. As the ability to accommodate is lost, near vision becomes blurred, thus the need for reading glasses.

Here is video that basically shows what actually happens in presbyopia

Your task is look for evidence that suggests that presbyopia is caused by change in lens stiffness with age.