The crystalline lens and ciliary muscle

The crystalline lens and ciliary muscle. Like the cornea, the crystalline lens is a transparent structure. Unlike the cornea, it has the ability to change its shape in order to increase or decrease the amount of refracting power applied to light coming into the eye. Transparency is maintained by the regularity of elongated fiber cells within the lens. These cells originate at the equator of the lens and lay down across the surface of other fiber cells while growing toward the anterior portion of the lens and the posterior portion of the lens until they meet at the central sutures. During elongation they pick up crystallins, hence the name “crystalline lens.” It is these crystallins that give the lens a higher index of refraction than the aqueous and vitreous humors. The gradient index of refraction of the lens ranges from about 1.406 through the center to about 1.386 through the more peripheral portions of the lens (Hecht, 2002). This isdue to the fiber cells near the surface having a lower index of refraction than deeper cells, which results in a decrease in spherical aberrations and therefore a more refined quality of focus. The lens is surrounded by an elastic extracellular matrix known as the “capsule.” The capsule not only provides a smooth optical surface, but it provides an anchor for the suspension of the lens within the eye. A meshwork of nonelastic microfibrils or “zonules” anchor into the capsule near the equator of the lens and, much like a suspension system around a trampoline, connect into the ciliary muscle. When the ciliary muscle is relaxed, the tension on the zonules is highest and the lens is “pulled” to its flattest curvature. This generally results in focus for a distant object when the eye is emmetropic (e.g. does not have any refractive errors, such as myopia or hyperopia). When the ciliary muscle contracts, it moves slightly forward, but mostly inward towards the center line of the eye. This releases the tension on the zonules and allows the lens to take up its preferred shape, which is more rounded and thereby more powerful. This increases the focal power of the eye to focus on nearby objects.

Since the lens continues to lay down fiber cells throughout life, it becomes denser and less flexible resulting in

a loss of the ability to change focus for near objects with age. This process called presbyopia will be covered in a later chapter. A cataract is a condition in which the crystalline lens starts to develop opacities or lose its

transparency. Cataracts can be associated with environmental factors such as smoking, health conditions such as diabetes, or the use of certain medications such as corticosteroids (Delcourt et al., 2000; Rowe et al., 2000). The effect of cataracts on vision is generally a reduction in contrast sensitivity, an increase in glare and halos at night and some shift in color sensitivity due to the “yellowing” of the lens.

The Posterior Segment of the Eye The retina lines the interior of the posterior portion of the globe and is where images are formed. Initial processing of the image occurs at this highly specialized sensory tissue. Vitreous is the clear gel that fills the posterior segment and serves to provide for light transmission through the eye and to protect the retina.

The retina

The retina is a mostly transparent thin tissue designed to capture photons of light and initiate processing of the

image by the brain. The average thickness of the retina is 250 μm and it consists of 10 layers. From the surface of the retina to the back of the eye the layers are the inner limiting membrane, the nerve fiber layer

(axons of the ganglion cells), the ganglion cell layer, the inner plexiform layer (synapses between ganglion and

bipolar or amacrine cells), the inner nuclear layer (horizontal, bipolar amacrine and interplexiform cells, along

with the retina spanning glial cells), the outer plexiform layer (synapses between bipolar, horizontal and photoreceptor cells), the outer nuclear layer (photoreceptor cells), the outer limiting membrane, the receptor layer (outer and inner segments of the photoreceptor cells) to the retinal pigment epithelium (RPE). The RPE is the outmost layer of the retina and serves as the primary metabolic support for the outer segment of the receptor cells and also acts as the final light sink for incoming photons that reduces intraocular glare. Its light absorbing pigmentation is why the pupil appears black.

The fact that the receptor layer is deep within the retina means that photons of light actually must pass through most layers of the retina before reaching the receptors. The receptors absorb and convert photons to neural signals, which are than processed through the network of bipolar, horizontal, amacrine and ganglion cells. The output axons of the ganglion cells form the nerve fiber layer that collects at the optic nerve to exit the eye. It’s the intricate interconnections of the various neural cells in the retina that complete the first processing of the visual information being sent to the brain. There are two types of receptors in the receptor layer, rods and cones, essentially named for their shape. The outer segment of the receptor cells contain the light sensitive visual pigment molecules called “opsins” in stacked disks (rods) or invaginations (cones). There are approximately 5 million cones and 92 million rods in the normal adult retina. Cones provide the ability to discern color and the ability to see fine detail and are more concentrated in the central retina. Rods are mainly responsible for peripheral vision, vision under low light conditions and are more prevalent in the mid-peripheral and peripheral retina.

At the most posterior aspect of the retina, where most of the light that the eye receives is focused, is a region

called the macula lutea. The macula is an area approximately 5 to 6 mm in diameter which has a greater density of pigments (lutein and zeaxanthine). These pigments help to protect the retinal neural cells against oxidative stress. Within the macular area is the fovea centralis, the small region at the center of the retina where vision is most acute. In this small 1.5 mm (0.06 in) diameter area there are no rods, only cones and the overlying neural layers are effectively swept away so that there is a depression in the retina. The average thickness of the retina drops to around 185 ìm in this “foveal pit.” The area immediately outside the fovea is called the parafoveal region and is where there is a transition from cone-dominated to rod-dominated retina.

The retina receives its nourishment from two sources, the retinal vasculature serves the inner layers of the retina and the choroidal vasculature, which lies between the RPE and the sclera, serves the metabolically active RPE and outer layers of the retina. In order to maximize photon capture in the central retina, the retinal capillary system does not extend in to the fovea centralis, an area known as the foveal avascular zone. This area depends on the blood supply provided by the choriocapillaris.

One of the most common conditions that can affect the retina is age-related macular degeneration (ARMD), in which there is a loss of vision in the center of the visual field (Klein et al., 2004; Nicolas et al., 2003; van

Leeuwen et al., 2003). In ARMD, the ability of the retinal pigment epithelium to remove the waste produced by the photoreceptor cells after processing light coming into the eye is reduced. As a result, waste builds up in the form of “drusen.” These drusen further disrupt the metabolic process and eventually the retina starts to deteriorate. If blood vessels from the choriocapillaris break through (“wet ARMD”) the condition can become significantly worse. ARMD is generally hereditary and early signs are detectable through routine eye exams.

The vitreous humor
The vitreous body is a gel-like structure that fills the posterior portion of the globe. Vitreous humor is comprised of collagen fibrils in a network of hyaluronic acid and is a clear gel (Kaufman and Alm [eds.], 2003). The vitreous body is loosely attached to the retina around the optic nerve head and the macula and more firmly attached to the retina at the ora serrata just posterior to the ciliary body. The connections at the anterior portion of the vitreous body help to keep the anterior and posterior chamber fluids separated. The connections around the optic nerve and macula help to hold the vitreous body against the retina.
With aging, the vitreous starts to liquefy and shrink. When this happens, aqueous from the anterior chamber can get into the posterior chamber of the eye. Additionally, there can be increased tugging at the attachment points on the retina causing a release of cells that the individual sees as “floaters.” If there is significant traction at the attachment points, the retina can be pulled away from the inner globe and a retinal tear or detachment can result.