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Form submission: Article Submission Form - Zapatian Workers State - 02-16-2020

Which region or organization are you from?: Lazarus/LazCorp

Select an Article Type: News Report -

Article Title: Educational Bulletin: The Anatomy of the Eye

Add Author/s: By Marcus Vespasian (Zapatian Workers State)

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Add Article Text: Hello once again to the good people of Lazarus!

You may or may not know me as Zapatian Workers State, but I've been a proud shareholder of LazCorp since last summer/fall. Having written a few interview columns for the paper, I thought I would try something different this time. That is this time I'm publishing an educational/informational bulletin within the paper. Today's topic concerns the anatomy and physiology of the eye!

Let's begin, shall we?

The first thing to know about the structure of the eyeball as it rests in its orbit within the skull is that it can be characterized by a tripartite division between a superficial fibrous tunic, an deeper pigmented tunic, and farthest to the interior, the retina. To start out with, the fibrous tunic of the eye is largely generated by fibroblasts in embryonic development and can be further subdivided between the sclera, which is the opaque "white" of the eye, and the centrally located, transparent cornea, which refracts light as it passes through. Per the laws of optics, travel of light from the air (a lower-density material) into the cornea (a higher-density material) causes the angle of refraction for the light rays to be fewer degrees separated from the axis perpendicular to the cornea than the incident angle at which the light hit the cornea. This aids in preliminary focusing of light. The pigmented tunic of the eye consists of the choroid, which is a layer of vascular connective tissue containing dark pigments to absorb wavelengths of light that pass through the retina, the ciliary body, and the zonular fibers. The function of the ciliary body and the zonular fibers concerns the lens, a structure comprised principally of collagens which helps to further refract light passing through the cornea and focus it upon the retina. The ciliary body is a smooth muscle under control of the parasympathetic nervous system, whereas the zonular fibers are structures that connect the lens to the ciliary body. In its relaxed state, the ciliary body exerts tension force upon the zonular fibers, pulling the lens taut, whereas its contraction relieves tension upon the zonular fibers and allows the lens greater flexibility. This aim is of course to better enable the lens to focus incoming light upon the retina and improve visual acuity. When people report eye strain, what they actually have is usually ciliary body strain.

Before I begin discussion of the retina, I would discuss several other anatomical structures of the eye. The lens can be used to further subdivide the eye into an anterior segment (reaching from the lens to the cornea) and a larger posterior segment (reaching from the lens to the retina). Both of these delineated areas are filled with an aqueous, jellylike fluid that helps the eye keep its full volume through hydrostatic pressure and facilitates the refraction and focusing of light. The fluid that occupies the anterior segment is known as the aqueous humour, while the fluid occupying the posterior segment is called the vitreous humour. There is a minor difference in consistency, with the aqueous being somewhat thinner as a liquid than the vitreous. (Believe me, you would know if you dissected a cow eye.) In addition, the anterior segment of the eye can be sub-divided into an anterior chamber, reaching from the cornea to the iris, and a posterior chamber, reaching from the iris to the lens. Make sure not to get confused, as the posterior chamber of the anterior segment is still located anteriorly to the posterior segment. As for the iris, as most know, it contains a pupil and exists for the purpose of restricting passage of light through that pupil and blocking light rays that do not run in parallel with one another and would consequently impair visual resolution. The iris is a manner of smooth muscle controlled by the autonomic nervous system, consisting of an inner iris under parasympathetic control, which constricts the pupil with its contractions, and an outer iris under sympathetic control, which contracts to dilate the pupil. Naturally, it is desirable that the pupil should expand in situations which naturally activate the sympathetic nervous system, namely fight-or-flight scenarios, in order so that more light can reach the retina. Of course, the iris also contains pigmented molecules that gives rise to eye color, and it is the total lack of pigment in albino individuals that causes the color of the retina, posterior to the iris, to make the iris appear as red in these individuals.

Next, we may discuss the retina, which may very well be the most fascinating part of the eye, and is the eyeball's innermost layer. The retina is a sheet of photoreceptors (classified into rods and cones), bipolar cells, retinal ganglion cells (RGCs), and the axons from RGCs converging at the optic disc as they leave the eye into the optic nerve, which is also the second cranial nerve. The retina is a prime example of convergent signaling in a neural circuit. To begin true discussion of the retina, it is worth classifying the regions within the retina. Simply put, from deepest to most superficial, we have the ganglion cell layer, the inner plexiform layer, the inner nuclear layer, the outer plexiform layer, and the outer nuclear layer. Photoreceptors, which begin the circuit, are located in the outer nuclear layer and synapse with bipolar cells in the inner nuclear layer, which in turn synapse with RGCs in the ganglion cell layer. Horizontal cells in the outer plexiform layer and amacrine cells within the inner plexiform layer regulate the signal as it is transmitted from photoreceptors to bipolar cells to RGCs.

Now we must discuss the nature of photoreceptors and how they communicate information about vision. It is important to note that rods are adapted for a system of scotopic ("night") vision whereas cones use a system of photopic ("day") vision. This becomes relevant in that different pigment molecules are located within rods and cones. Cones use the pigment photopsin and come in three varieties: S-cones, which recognize short wavelengths of light as "blue," M-cones, which recognize medium/intermediate wavelengths as "green," and L-cones, which recognize long wavelengths are "red." The final output of color vision in the primary visual cortex of the brain is ultimately dependent upon the combined output of these 3 cone types. Then, rods use a pigment called rhodopsin, which exists as a complex between opsin and another protein named retinal. Light absorption by rhodopsin causes the conformational change in retinal from cis-retinal to all-trans-retinal, resulting in retinal dissociating from opsin and activating a signal transduction protein named transducin to activate the effect of phosphodiesterase enzymes. The function of phosphodiesterase is to convert cAMP (cyclic adenosine monophosphate) and cGMP (cyclic guanosine monophosphate) into regular AMP (adenosine monophosphate) and GMP (guanosine monophosphate). Within rods, cGMP acts as a second messenger in a signal transduction cascade to modulate the openings of Na+ channels and depolarize the rods to a point of tonic glutamate release. With phosphodiesterases active, cGMP levels are reduced, but in the dark, cGMP levels and Na+ influx increase. Unlike in most excitatory glutamatergic neurons, release of glutamate by rods into the synaptic cleft separating them from bipolar cells causes hyperpolarization in bipolar cells by binding to a certain class of GPCRs (G-protein-coupled receptors), ultimately terminating signaling. Consequently, in the dark, rhodopsin levels accumulate in rods, cGMP levels rise, tonic glutamate release increases, and bipolar cells experience hyperpolarization. In the day, rhodopsin is "bleached," cGMP levels fall, tonic glutamate release decreases, and bipolar cells no longer experience hyperpolarization and signal RGCs. This outcome in the bipolar cells is attributable to a phenomenon known as rebound firing, where the threshold for the openings of voltage-gated Ca2+ channels is relatively low, allowing any return to a state of resting membrane potential following a period of hyperpolarization to initiate momentary burst firing and transitory depolarization. (If you don't know what I'm talking about, don't worry, I'll get around to explaining neurons and action potentials in the future.) So now you see (or maybe not) why cones signal in the daylight while rods are mostly active at nighttime. In the retina, rods and cones are differentially distributed, with the highest proportion of cones and lowest proportion of rods being located in the fovea centralis, the area associated with the most vivid color vision. The fovea is located within the macula lutea, which is the area of the retina associated with "central" vision. By extrapolation, one might then correctly guess that proportions of rods increase as one goes out further towards the periphery of the retina; in fact, much of perceived color vision at the periphery is not "real" but a massive interpretive work by the brain using clues from areas of color which it is actively processing. This can be exploited in quite a few optical illusions. Lastly, there is the mucous membrane known as the conjunctiva, which covers over the sclera, and the lacrimal gland, which produces tears which are in turn drained into the nasolacrimal duct.

Alright, I think that covers pretty much everything relating directly to the anatomy of the eye. I realize there is a lot of highly specialized medical and scientific terminology some of our readers may not be familiar with, but I'm hoping that if this gets published it arouses people's natural curiosity to want to ask questions and know more!

Until next time, Marcus Vespasian signing out.