I think it would be unfair to call any one of our sense organs the most important as they all are very important in their own ways. Our Ears help us in hearing, our Tongue helps us taste food, our Skin (The largest organ in the body) prevents our inner organs and muscles from all sorts of things and our Nose helps us smell. But today, I am particularly interested in the sense organ, which is the Eye. Our Eyes help us in seeing things, the brain can understand information because of the eyes. In fact, the eyes play such an important role that 70% of the body’s sensory receptors are in the eyes. In case you’re wondering, sensory receptors sense information and send it to the Brain. Gotta tell you, I’m excited for this one, so let’s start by talking about the Anatomy of the Eye!
This layer also contains 2 types of neurons, which are Bipolar Neurons and Ganglion Neurons. The Cones and Rods are present right at the start of the retina, after that the bipolar neurons act as a bridge between the Ganglion Neurons and the Cones and Rods.
Anatomy of the Eye
Let’s first talk about the exterior part of the eye.
• Our eyebrows help keep sweat away from the eyes.
• The Eyelashes help trap dust particles so they don’t get into the eye and they also help us blink.
Our eyes also contain a lot of lacrimal (tear) glands, which are exocrine glands, which you should know if you read my last blog. And I think it would be absurd to mention this, but these glands secrete tears. Overall our eye is filled with lots of fluids, pretty hollow actually. Covered in a layer of protective fat around it.
It is strapped around by 6 muscles, which are the:
1. Lateral Rectus
2. Medial Rectus
3. Superior Rectus
4. Inferior Rectus
5. Superior Oblique
6. Inferior Oblique
These muscles help the eye move. Moving on to the main anatomy. Our Eyes are made up of 3 layers, namely- Fibrous, Vascular and Inner.
The Fibrous layer includes:
• Sclera- The white part we see in our eyes.
• Cornea- This is the transparent part of the eye, it’s like a window for light to pass through from and you can’t see it. It is like your eye’s windshield.
• Conjuctiva- It is a very thin membrane, covering your cornea.
Now let’s talk about the next layer which is the Vascular Layer. As the name suggests, this layer has a rich supply of blood vessels. This layer includes the following:
• Choroid- It surrounds the middle and rear part of the eye. Has a rich supply of blood vessels, so it helps bring blood to the eyes.
• Iris- The coloured part of the eye, just after the cornea is the Iris. It expands and contracts to help the pupil.
• Pupil- It is the black dot in between the Iris, which contracts when there is too much light falling on it, and expands when there is little light. After passing through the cornea, light passes through the pupil.
• Ciliary Muscle- It is attached to the eyeball, and helps the eye adjust its shape for accommodation, which I will tell you more about afterwards.
• Eye Lens- This is a convex lens, behind the Iris and Pupil. Light hits the eye lens after it passes through the pupil, which helps converge light (join the different light waves into one) by adjusting its shape.
The space between the Cornea and the Eye Lens is filled by a liquid known as Aqueous Humour.
The Inner Layer of the Eyes consists of only one thing, which is the Retina. The hero of the Eye. The space between the Eyer Lens and the Retina is filled with a liquid known as Vitreous Humour. I think we need to dedicate a whole paragraph to the Retina.
Retina: The Hero of the Visual System
The Retina is what helps us see, it literally forms a raw image of what we actually are gonna see before sending it to the brain. It consists of 2 layers-
1. The Pigmented Layer
2. The Neural Layer
The Pigmented Layer contains Cuboidal Epithelium (a type of epithelial tissue), and it contains the Melanin pigment which helps absorb light so it doesn’t scatter around everywhere. The Neural Layer is the real hero.
The Neural Layer consists of photoreceptors, which help detect light and convert into information (action potentials). These photoreceptors have 2 types-
1. Cones- They help us see bright colours and define details. They provide detailed ad fine imagery.
2. Rods- These photoreceptors on the other hand, can only recognize black and white and are more light-sensitive. They only define basic aspects such as the shape and size of the object. They react more in the dark.
The Ganglionic Neurons go on to form a chain of neurons and eventually form the 2nd Cranial Nerve, which if you read my blogs, know is the Optic Nerve.
As much as 100 rods maybe linked to the synapse of only one Bipolar Neuron, but only one cone can be linked to the synapse of one Bipolar neuron. Because so many rods are sending signals, the brain gets confused which one and as there is only one cone sending a signal at a time, cones help produce more detailed imagery.
Accomodation Of The Eye
Accomodation is the process by which the Eye Lens bend or flatten to converge the light waves. When the light waves hit the Eye Lens, it decides to either bend or flatten depending on the distance from where the light is coming. If you are looking at your phone, 20 cm away, the Eye Lens bends to converge the Light Waves. If you are looking somewhere far away, the Eye Lens flattens to converge the Light Waves. The Eye Lens does this all with the help of the Ciliary Muscle.
How this all works?
Light reaches our cornea, which lets it pass through to go inside the pupil and through the pupil, it hits the Eye Lens which makes all the light waves converge into one and makes it easier for our retina to understand. The Eye Lens do this by adjusting their shape with the help of the Ciliary Muscle, a process known as Accomodation. The Melanin present in the Pigmented Layer of the Retina, absorbs some light so it doesn’t scatter everywhere. Then, the light hits the photoreceptors (rods and cones) in the neural layer of the retina. These photoreceptors process the light into information and send to the bipolar neurons which convert the information into action potentials and send it to the Ganglionic Neurons. These neurons form a long chain, which eventually forms an Optic Nerve. This nerve takes the information to the thalamus and then to the visual cortex of the brain, where the image is inverted so the brain converts it into meaningful information, sets it upright, sends a signal and boom! You can see! All of this happens in a matter of milliseconds.
Eye Disorders
Disorders associated with the eye usually aren’t that serious and need the attention of an Ophthalmologist.
• Myopia- Myopia means short-sightedness in which a person cannot see far away objects. It happens when people look at a screen for too much time, so their eye lens which is bent all this time, remains bent. They basically strain out their ciliary muscles, because the ciliary muscles too need some rest. That’s why it is advised to look away from a screen after every 20 minutes. Myopia means you get a power in negative terms, because the lens which is used to correct myopia has a negative power. The lens used to correct myopia is a concave lens.
• Hypermetropia- Hypermetropia means far-sightedness in which a person cannot see nearby objects. It has a number of causes, such as Presbyopia. In presbyopia, the eye’s ability to bend decreases over time, it an age-related factor and starts usually from 40 years of age. Congenital hypermetropia means that it is present from birth because of some genetic mutation or it can be inherited. It is corrected by using a convex lens of positive power.
• Cataract- A cataract is a cloudy lens in the eye. It forms when proteins in the eye break down and clump together, causing a cloud-like formation. It can be congential (present from birth). Diabetes, or family history can also contribute to the formation of a cataract. It is corrected through surgery. Cataract surgery is usually not done on people below the age of 20 years, because at that time the eyes are still developing and there is a higher risk of complications.
Cataract Surgery
There are a number of ways to doing a cataract surgery:
• Phacoemulsification- It is the most common technique to doing a cataract surgery. In this procedure, the surgeon makes a tiny incision in the cornea, then uses ultrasound waves to break the cloudy lens into small pieces, which are then suctioned out. After that, a new lens is placed in the eye, known as an Intraocular Lens (IOL).
• Extracapsular Cataract Extraction (ECCE)- This type is used for more larger or advanced cataracts. The incision is larger than in the previous technique and requires stitches afterwards. The surgeon removes the cloudy lens in one piece and places an IOL.
• Femtosecond Laser-Assisted Cataract Surgery- It can be a part of an ECCE or Phacoemulsification. It is a newer technique in which lasers are used to make incisions and break the cataract, offering more precision.
• Manual Small Incision Cataract Surgery (MSICS)- It uses a smaller incision and removal of the cataract in one piece. It’s used in some regions due to its simplicity and lower costs.
Precautions to Avoid Eye Diseases
Below her are listed some precautions to avoid eye diseases:
• Limit your screen time to about 2 hours per day.
• Do not touch your eyes with dirty hands.
• Sleep well.
• Maintain a healthy diet and eat foods rich in Vitamins and Minerals good for eye health.
• Follow the 20-20 rule. In this rule, you should look away from the laptop or mobile screen every 20 minutes and focus on something 20 m away for at least 20 seconds.
• Get regular eye exams even if you don’t have any symptoms.
Fun Fact of the Day!
Cornea is the only body part which can be transplanted without the fear of the new body rejecting it. It is because it is avascular. It means that it has no blood supply so it escapes the grip of the immune system, identifying it as a threat.
By,
Deeparsh Bhanot
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