Eyes: Structure, Mechanism and Disorders

The eye is the most complex sensory organ. Eye vision is the act of seeing and is important to human survival because it allows us to view potentially dangerous objects in our surroundings. Most sensory receptors in the human body are located in the eyes. The diameter of an adult eyeball measures about 2.5 cm. From the total surface area of the eye, only the anterior one-sixth has been exposed the remainder is recessed and protected by the orbit, into which it fits.

A must read- Structure and function of the Human Eye – My Biology Dictionary

Parts of eyes

Eyeball consists of three layers: A fibrous tunic, a  vascular tunic, retina.

Fibrous tunic

A fibrous tissue is the outermost layer consisting of the sclera and cornea.

The cornea is the outermost part and is transparent and covers the coloured iris. The cornea helps to focus the light onto the retina. The outer surface consists of non-keratinized squamous epithelium. The middle coat of the cornea consists of collagen fibers and fibroblasts, and the inner surface is the simple squamous epithelium.

Sclera is the white of the eye and consists of dense connective tissue made up of fibroblasts and collagen fibers. The sclera covers all the parts of the eyeballs except the cornea. It provides the shape of the eyeball and also protects the inner part of the eyeball. Posteriorly, the external two-thirds of the sclera is continuous with the dural sheath of the optic nerve and the inner one-third becomes lamina cribrosa. At the junction of the sclera and cornea, there is an opening known as the scleral venous sinus and also known as ‘canal of schlemm’.  The space between the cornea and the lens filled with fluid is called Aqueous humor.                                                                  

Vascular tunic

The middle and the second layer is the vascular tunic or uvea. It consists of three layers choroid, iris, and ciliary body.

The anterior part of the uvea forms the bulk of the iris. Iritis and uveitis mean inflammation in the iris. The posterior part of the uvea is the choroid. The coloured portion of the eye is known as the iris and is shaped like a flattened doughnut.  It consists of melanocytes and circular and radial smooth muscle fibers. The amount of melanin in the iris determines the colour of the iris. When melanin is high eye appears black or brown. When it is less, then it appears blue and when moderate melanin is present then it appears green. A principal function of the iris is to regulate the amount of light entering the eyeball through the pupil, the hole in the centre of the iris.

The ciliary body consists of ciliary muscles and ciliary processes. The ciliary processes are protrusions or folds on the internal surface of the ciliary body. They contain blood capillaries that secrete aqueous humor. Extending from the ciliary process are zonular fibers (suspensory ligaments) that attach to the lens. The ciliary muscle is a circular band of smooth muscle. Contraction or relaxation of the ciliary muscle changes the tightness of the zonular fibers, which alters the shape of the lens, adapting it for near or far vision.

The lens is the highly elastic circular biconvex which is present behind the pupil.  Ciliary muscles control the thickness of the lens. It is the only structure of the eye that varies in thickness and focuses the light onto the retina. When the ciliary muscle contracts, it releases its pull on the lens, increasing its thickness.

Retina

The retina is the third and inner layer of the eyeball. It lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway. The retina consists of a neural layer and pigmented layer. The pigmented layer is a sheet of melanin-containing epithelial cells located between the neural part of the retina and the choroid. The melanin in the pigmented layer of the retina, like in the choroid, also helps to absorb stray light rays. The neural (sensory) layer of the retina is a multilayered outgrowth of the brain that processes visual data. Three distinct layers of retinal neurons- are the photoreceptor layer, the bipolar cell layer, and the ganglion cell layer. Two types of cells are present in the bipolar cell layer of the retina that is horizontal cells and amacrine cells.

Image source: https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work#:~:text=When%20light%20hits%20the%20retina,into%20the%20images%20you%20see.

Photoreceptor cell

Photoreceptors are specialized cells in which light rays are ultimately converted into nerve impulses. There are two types of photoreceptors: cones and rods. Each retina has about 6 million cones and 120 million rods. Rods allow us to see in dim light or in the moonlight. Because rods do not provide colour vision, in the dim light we can see only black, white, and all shades of grey in between. Brighter lights stimulate cones, which produce colour vision.  Visual acuity is high in cones as compared to rods. Three types of cones are present in the retina :

1. Blue cones, which are sensitive to blue light.
2. Green cones, which are sensitive to green light.
3. Red cones, which are sensitive to red light.

Mechanism of eye vision

Light rays hit on the retina through the cornea and the lens generates an impulse in rods and cones. The photosensitive compounds called photopigments in the human eyes are composed of opsin (a protein) and retinal (an aldehyde of vitamin A). After the light hits, there is the dissociation of the retinal from opsin resulting in changes in the structure of the opsin. Due to this membrane permeability changes. After that potential differences are generated in the photoreceptor cells. This produces a signal that generates action potentials in the bipolar cell as well as the ganglionic cell. These action potential impulses are further transmitted by the optic nerves to the visual cortex area of the brain. The neural impulses are detected and the image formed on the retina is recognized based on earlier memory and experience.

Image source- https://www.cns.nyu.edu/~david/courses/perception/lecturenotes/eye/eye.html

Symptoms associated with eye vision

• Severe pain in the eye.
• Change or sudden loss of vision.
• Floaters.
• Blurred vision of the eye.
• Watery and red eyes.
• The foreign body inside the eye may cause complexes.

Disorders related to eye vision

• Eyestrain -Tiredness of the eyes.
• Refractive Errors -Far-sightedness, short-sightedness, distorted vision.
• Dry eye syndrome – Tear glands are not able to make tears.
• Night Blindness [Nyctalopia] – Difficult in seeing in low light.
• Conjunctivitis [bacterial/Viral/Allergic] – Inflammation in the conjunctiva.
• Amblyopia– It is due to eye vision in one eye due to a vision development disorder.
• Eye floaters – Spots [ cobwebs, strings, rings] in the vision.
• Blepharitis – Inflammation of the eyelid causing crusty /dry eyelid, grittiness, and stickiness.
• Cataract:  A cataract is an opacity within the clear lens of the eye
• Glaucoma – When the intraocular pressure increases it may result in damage to the optic nerve.
• Photophobia-No clear image in presence of bright light.

Interesting facts about eye vision

• Eyes are thought to be first developed in animals around 550 million years ago.
• The eye can blink and lasts for 100-150 milliseconds. 
• The human eye can function without needing to rest.
• Brown is the most common colour of the eyeball.
• The first blue-eyed person is said to have lived 6,000-10,000 years ago.
• Babies are colour-blind at birth.
• Blind people can see their dreams as long as they weren’t born blind with non-keratinized squamous epithelium.
• The ostrich brain is smaller than the eye.
• Dogs can’t distinguish between green and red.
• Bees consist of five eyes.
• Humans can see only red, green, and blue.

                                                    YOUR EYES SHOW OFF THE STRENGTH OF YOUR SOUL

To summarize, eyes are complex sensory organs having a regulated mechanism of vision

Further reading- How the Eyes Work | National Eye Institute (nih.gov)