What is the Significance of the Human Eye? 

The eye is the organ that helps in seeing and perceiving the world around. The eye, along with its entire system of muscles and nerves, helps in providing a mental representation of the environment. Thus, the eye contributes in the acuity to navigate properly through physical space, interact with individuals and the objects around.  

Anatomy of the Eye 

Humans have a pair of eyes. Each eye sits in a bony socket found in the skull called the orbit. The eyeball is a small sphere, spanning a diameter of about 24 millimeters. It is suspended in the orbits by a set of six muscles known as extraocular muscles, which control the eyeball movements. These muscles coordinate with each other to aid in the symmetrical movement (the ability to move in the same direction at the same time) of the eyes. There is also a thick layer of fatty tissue surrounding the eyeball that cushions and protects the eyeball during movements.  

The eye, contrary to belief, is not a perfectly spherical organ. Instead, it is made up of three layers that cover each eye: 

1. The fibrous tunic is the outermost layer that provides proper structure to the eye and supports the inner parts. 
2. The vascular tunic is the layer of the eye that lies between the fibrous tunic and the neural tunic. 
3. The neural tunic or retina is the innermost layer of the eye. 

The fibrous tunic consists of the cornea and sclera of the eye. The sclera is the white part and makes up about five-sixths of the eye’s surface. It is made of thick muscles and has a rich supply of nerves and blood vessels and provides attachment for the external muscles. The cornea is the clear frontal area of the eye that allows light into it. These two parts are connected by a ring-like structure called the limbus. The vascular tunic is composed of three structures: iris, choroid, and ciliary body. The choroid is a layer of blood vessels at the back of the eye, located behind the sclera. At the rear of the choroid is the ciliary body, which is a muscular structure attached to the lens by zonule fibers. The iris is the visible colored part of the eye and overlays the ciliary body. It is a smooth muscle that opens or closes the pupil, which is a small opening at the center of the eye that allows light to enter.  

The innermost layer, the neural tunic or the retina, is a neural layer at the back of the eye that senses light and performs phototransduction. The retina contains a patch of special light-sensitive cells called the macula lutea. At the center of the patch is a small pit called the fovea centralis which helps in providing sharp images to the eye for processing. The lens is attached to the ciliary body by a ligament known as Zonule of Zinn, a suspensor ligament made up of hundreds of fine transparent fibers. The key function of the ligament is to transmit muscular forces to change the shape of the lens for focusing. The eye consists of two cavities namely the posterior chamber and the anterior chamber. The anterior chamber is located between the cornea and lens and is filled with a clear, transparent liquid called the aqueous humor. The posterior chamber is the entire space behind the lens, up until the retina, and is filled with a jelly-like substance called the vitreous humor. The retina is composed of several layers of specialized cells for the initial processing of light. The fovea at the center of the retina lacks these cells and contains just the photoreceptors, rods, and cones. The axons of the neurons of the retina combine to form the optic disc. The optic disc leaves the eye as the optic nerve, a bundle of millions of nerve fibers carrying visual messages from the eye to the brain. 

Vision 

Vision occurs due to the functioning of the eye. To transmit accurate visual signals to the brain, the light has to be focused on the retinal cells with apposite intensity. Light from a source passes through the cornea and enters the pupil at the center of the cornea. The lens, along with the cornea, help focus the light rays on the retina. The cells in the retina then absorb and process the light waves into electrochemical impulses and send them to the brain. The size of the pupil is adjusted by an assortment of contractions by the iris muscles to control the amount of light entering the eye. When there is a bright light, the pupil constricts in size, thus reducing the amount of light that enters the eye. The pupil then dilates when there is very dull lighting and thereby helps to focus better in the dark. Several factors contribute to the processing of light waves by the eye, creating a sense of vision. The following context provides a detailed account of these factors. 

Refraction 

An important phenomenon known as refraction happens when light rays pass through the cornea and the lens and help in the final quality of the processed image. Refraction is the bending of light rays due to the difference in the densities of two substances from which the light passes. If the difference is higher, refraction occurs at a greater level. The degree of refraction also depends on the angle at which light rays strike the lens. The greater the angle, the greater would be the refraction. The cornea and lens are the eyes’ most important refractive surfaces, especially the cornea due to the difference in density between the outside air and the fluids present behind the cornea.  

The light source from an object at least 6 meters away from the eye is considered a parallel ray by the time it reaches the eye. The same is not the case with objects that are nearer than 6 meters. For the eye to have a better refractive ability, there should be a greater distance between the lens and the retina to focus all the divergent rays from a nearby source than a faraway one. However, the distance between the lens and the retina always remains constant. So, the strength of the lens needs to be adjusted through a process called accommodation. 

Accommodation 

Accommodation is the process through which the strength of the lens is adjusted by changing its shape by the action of the ciliary body. When the ciliary body is relaxed, the ligaments attached to the lens pull it flat, thereby making it less curved and weakly refractive. Contraction of the ciliary body makes the lens more curved, increasing the strength and refractive nature of the lens. Hence, it can be inferred that when it comes to objects that are far away, the ciliary muscles are relaxed and the lens is flat and if the object is nearby, the ciliary muscles contract and the lens assumes a more curved form. 

Phototransduction 

Phototransduction is the alteration of light stimuli into neural signals by the retinal cells. The eye’s major function, vision, happens through the process of phototransduction. The photoreceptor cells are the ones that perform this task. These cells consist of three parts: the outer segment, the inner segment, and the synaptic terminal. The synaptic terminal is closest to the interior of the eye and transmits signals generated in the photoreceptor to the bipolar cells upon light stimulation. The outer segment contains billions of light-sensitive molecules called photopigments, which undergo chemical altercations when activated by light. The photopigments are made up of two components, opsin, and chromophore. The chromophore is the part that absorbs light. There are four photopigments wholly, one in rods called rhodopsin and one in each of the three types of cones. Each of the pigments in the cones absorbs light of a different wavelength. 

Photoreceptor activity 

Photoreceptor activity involves a series of steps and happens during two circumstances, during bright light or dark. The photoreceptors contain sodium channels that respond to a nucleotide called cyclic guanosine monophosphate (cGMP). When cGMP is bound to these channels, they remain open. When light is absent, the concentration of cGMP remains high, which results in the opening of the sodium channels. Consequently, the sodium ions leak into the photoreceptors, changing the voltage of these cells. The cell’s charge becomes positive that triggers the opening of the voltage-gated calcium ion channels. The entry of calcium ions into the presynaptic neuron stimulates the release of neurotransmitters from the synaptic terminal during a dim light. 

When there is exposure to light, the concentration of cGMP reduces through a series of biochemical reactions prompted by the activation of photopigments. The fall in cGMP levels causes the sodium channels to close, thus inhibiting the sodium leak. As a result,the voltage of the photoreceptors leans more into the negative spectrum followed by the closing of the voltage-gated calcium channels that eventually reduce the neurotransmitter release. 

The retina, therefore, signals the brain regarding light stimulation and related information through an inhibitory response that occurs through a series of physiological reactions at the cellular level. Spikes of electrical discharges traveling along the cell surface known as action potential originate only in the ganglion cells. The ganglion cells are the first neurons in the chain of transfers that carry the visual stimuli to the brain.  

How Similar are the Eye and a Camera? 

The functioning of the eye is very similar to that of a camera, be it the conventional or the digital one. In the case of the eye, light enters through the cornea, pupil, and lens, and gets focused into the retina. The light signals are converted into neural signals and sent to the brain through the optic nerves for interpretation. This process is analogous to the working of a camera while capturing an image. When light hits the surface of the camera’s lens, the aperture controls the amount of light that enters the camera. The light then falls onto a light-sensitive surface. Before digital cameras, this light-sensitive surface was the camera’s film but now this surface is an imaging sensor chip. The retina, films, and imaging sensor chip all receive an inverted version of the image. This is because the lens in both an eye and a camera are convex or curved outwards. When light hits a convex object, it refracts and flips the image upside-down. The brain then processes the image and helps flip it back again. Digital cameras are programmed to make the necessary corrections internally whereas the non-digital cameras are inserted with a prism or mirror that flips the image accurately. 

Conclusion 

The eye is one of the most essential sense organs as it helps humans perceive and interact with the environment around them. The sense of vision is crucial for the survival and social well-being of humans. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for  

• Bachelors in Zoology  
• Bachelors in General Physiology 
• Masters in Human Physiology  
• Masters in Anatomy and Physiology