Retina is a tissue present inside your eyeballs that lines the backside of the eyeballs. It converts the incoming light into nerve signals, and that is how you can see objects clearly under different conditions such as under the sunlight or starlight. It helps in differentiating between the light wavelengths so that you can see colours. Not only that, Retina also gives you the precision to see and detect minute objects including a strand of human hair or speck of dust a few metres away.
Retina is a part of the brain; to be precise, it is a part of the Central Nervous System (CNS). During the embryonic developmental stage in vertebrates, the Retina and the Optic Nerve develop as an outgrowth of the developing brain.
Here, we have explored interesting facts about the design, working and all related facts about the Retina:
- Similar to the other structures in the Central Nervous System, the shape of the retina is like a plate. It is about a quarter millimetres in thickness. Similar to the layers of cake, the Retina also consists of three layers of nerve-cell bodies. These bodies are separated by two layers that contain the synapses made by the axons and the dendrites of these cells.
- Retina consists of sensory neurones and the intricate neural circuits. The sensory neurones respond to light while the intricate neural circuits perform the primary stages of image processing. Finally, an electrical message travels through the optic nerve to your brain and further processes your visual perception.
- The photoreceptors or the sensory cells lie at the back of the retina. The light rays need to pass through the entire retina before it reaches the pigment molecules and excites them. This is so because the membranes of the photoreceptors containing the pigments must be in touch with the Epithelial Layer of the Eye. This layer provides a steady stream of Retinal or Vitamin A, one of the vital molecules.
- After these Retinal molecules are exposed to light, they undergo a conformational change and are recycled back into the pigment epithelium. This tissue is located behind the retina. It is very dark as its cells contain melanin granules. These granules absorb stray photons thus preventing them from creating a reflection on the photoreceptors, which might make the images appear blurred. These melanin granules also safeguard the cells from overexposure to light radiation.
The Design of Retina
- The Retina of all vertebrate contains two types of photoreceptors. These are the rods and the cones. Rods are used for low-light vision and Cones for daylight and any bright-coloured vision. The Retinas of frogs, fishes, turtle and bird contain at least 3-5 different types of cones. And that is why they have a very good colour vision. On the other hand, the Retinas of most of the mammals predominantly consist of rods.
- The Retinas of most of the mammals contain two types of cones: the green-sensitive ones and the others are blue-sensitive cones. However, primates have one extra type of cones in their eyes. These cones are red-sensitive cones. With the cone vision of your eye, you can see images starting from the grey shades of dawn to the eye-dazzling brightness of noon when the sun is the brightest.
- The cone photoreceptors can adapt to the surrounding brightness, and the circuitry in the retina further regulates the eye’s response to the brightness. Similar to the cone photoreceptors, the rod photoreceptors and the neural circuitry (to which they are connected) can adapt to the low intensity of light.
Anatomy and Physiology of the Retina
- The brightest and the most brilliant colour vision are experienced by the eye when the light directly falls on the small dimple on the Retina, also known as the Macula or the Fovea Centralis. This region has a high presence of cones which are smaller and tightly packed compared to any other region on the Retina.
- Your eyes receive data from a field that ranges up to 200 degrees. The visual acuity over most of that range is very poor. In order to form high-resolution images, the light needs to fall on the Fovea. This limits your acute vision angle to about 15 degrees. In low light, the Fovea constitutes a second blind spot as because it is only the cones that have low light sensitivity. So, during night. If you want to get the maximum visual acuity, shift the vision slightly to one side by at least 4 to 12 degrees. In this way, the light will fall on some rods.
- The diameter of the Anatomic Macula or Area Centralis is about 4.5–6 mm. These are centred on the fovea and are located between the superior and inferior temporal arcades. The macula or the fovea centralis is located about 3 mm temporal to the optic disc. It has a diameter of about 1.5 mm.
- The Fovea is the central part of the macula. It is formed by a central depression of about 0.35 mm wide. Clinically, one can recognise the area by the foveal reflex. If it is lost, then it indicates an early macular disease.
- The peripheral retina constitutes the rest of the retina which is outside the temporal retinal arteries. If considered anatomically, the peripheral retina contains only a layer of ganglion cells.
- Each adult human retina consists of 3.5 million Retinal Pigment Epithelium (RPE) cells.
- The human retina constitutes about 4 to 5 million cones and about 77 to 107 million rods.
- Each photoreceptor cell in your eye contains an outer segment (photopigment), the inner segment (mitochondria, endoplasmatic reticulum), a nucleus, an inner fibre, and the synaptic terminal.
- The ganglion cell layer and the photoreceptor layer are connected by the Interneurons in the inner nuclear retinal layer. These interneurons contain the bipolar, horizontal, amacrine, and the inner plexiform cells. These cells constitute the complex neuroretinal circuits that help in processing the photoreceptor signal and then transmit that information to the ganglion cells. In other words, the photoreceptor cell is directly connected to the ganglion cell through bipolar cells. The rods and cones provide input to these bipolar cells.
- The visual information is transmitted by the ganglion cells from the retina to your brain. The ganglion perikarya are the largest retinal neurones. These are located in the ganglion cell layer. Their axons run in on the inner surface of the retina in a separate layer and are collected at the optic disk. From there, they exit the eye as the optic nerve.
A healthy Retina is essential for good vision. Retinal disorders affect the vital tissues of the eye, thus affecting your vision. It can even cause blindness. Common Retinal disorders include:
- Macular Degeneration: Age-related macular degeneration (AMD) is cased when the Macula of your Retina is damaged. People suffering from AMD lose their central vision. You won’t be able to see fine details. Your peripheral or side vision remains normal. AMD is of two types:
This type of AMD is very common. About 80% of the people who suffer from AMD suffer from the dry form. When parts of your macula get thinner with age, tiny clumps of protein called drusen grow on them. With this, you slowly lose your central vision. There is no way to treat dry AMD.
This is less common; however is a serious one. When there is an abnormal growth of blood vessels under the retina, the condition is called Wet AMD. Blood or other fluid often leaks from these vessels that cause scarring of the macula. Compared to dry AMD, you tend to lose vision faster with wet AMD.
People often do not realise they are suffering from AMD until their vision gets extremely blurred.
- Diabetic Retinopathy
People with diabetes suffer from this eye disease. High blood sugar levels often damage the blood vessels in the retina. This causes the blood vessels to swell and leak. They even close, thereby completely stopping blood flow. Many times, abnormal new blood vessels grow on the retina. These changes too can cause vision loss.
- Retinal Vein Occlusion
The veins that carry blood away from the retina get blocked causing Retinal Vein Occlusion. Retinal vein occlusion is often caused when the arteries harden, thus, causing blood clots.
Blockage of smaller veins in the retina often occurs in places where the retinal arteries have thickened. This puts excess pressure on the retinal vein. Several factors can cause Retinal Vein Occlusion. This includes:
- High blood pressure (hypertension)
- Other eye conditions, such as macular edema, glaucoma, and vitreous haemorrhage
The risk of Retinal Vein Occlusion increases with age, and thus, older adults are vulnerable to retinal vein occlusion.
- Retinal Tear or Detachment
Often it happens that the vitreous moves away from the retina without causing any serious problem. However, sometimes it pulls so hard that it tears the retina. Fluid tends to pass through this retinal tear, thus lifting it off the back of the eye similar to the way you peel wallpaper off the wall. This is known as retinal detachment. As the retina gets detached from the eye, your vision becomes blurry. This is a very serious problem that results in blindness unless you undergo a detached retina surgery.
- Retinitis Pigmentosa
This is a group of eye problems that affect your retina. Retinitis Pigmentosa affects the way your retina responds to light, thus making it difficult for you to see clearly. People suffering from Retinitis Pigmentosa may slowly lose their vision over time. However, people do not become completely blind.
- Macular Hole
People suffering from this condition often notice a sudden decrease in vision in one eye. Macular hole often occurs due to Vitreous shrinkage and/or separation. This condition is caused by a number of other conditions such as:
This condition is commonly known as cellophane maculopathy or macular puckers. In this condition semitranslucent, avascular (having few or no blood vessels), fibrocellular membranes develop on the inner surface of the retina. They cause minimal symptoms and can be diagnosed only after close observation. However, in a number of other cases, they also cause painless loss of vision and metamorphopsia or distortion of vision. Generally, ERMs give symptoms when it affects the macula or the central portion of the retina. The macula is the part that helps us to identify fine detail, helps in reading and recognising faces. So, distortion of vision is more evident when ERM affect eyes.
Most of the patients suffering from ERMs do not experience any symptoms. After close examination of the retina or during a dilated retinal exam the eye specialist detects the condition. Retinal imaging such as with ocular coherence tomography (OCT) is also used for identifying ERM. In these cases, patients usually have a normal or a near-normal vision. However, this condition can get worse with time causing great visual distortion. Defect on the surface layer of the retina causing the glial cells to migrate there and start growing on the retinal surface can give rise to this condition. This extra growth appears like cellophane and may contract with time, thus causing traction and macular puckers and poor vision quality.
This is why it is crucial visits ophthalmologists or retina specialists regularly. They can examine your eyes closely and determine the early symptoms of any retinal problem.
Wondering where to get the best retina specialist in Kolkata? We are here to help you. Book an appointment with us today at http://www.dishaeye.org/appointment.