What Retina is meant by?
The retina is a layer of nerve tissue that covers the interior of the eyeball's back two-thirds, where light stimulation occurs, causing the illusion of vision. Actually, the Retina is an extension of the brain, which is formed embryonically from the neural tissue and is connected to the brain properly by the optic nerve. Retinal detachment is an emergency situation that the eye’s part (retina) pulls away from supportive tissue.
About the Retina Tissue
The Retina is given as a complex transparent tissue that consists of many layers, only one of which has light-sensitive photoreceptor cells. Light can pass through overlying layers to enter photoreceptor cells, which are divided into two groups: rods which cones, and are functionally and structurally distinguished by their response to different types of light. Rods predominate in the nocturnal animals, and they are most sensitive to reduced light intensities. At the same time, humans provide night vision and aid in visual orientation of the retinal display.
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Cones are very prominent in humans and in some of the animals that are active during the day and provide a detailed vision (it means, as for reading) and perception of color. The more cones per unit area of the Retina, the better the detail that can be distinguished by that area. These rods are fairly well-distributed over the total retina part, but cones tend to concentrate at two sites, as given below:
Surrounding macula lutea, which is the circular patch of yellow-pigmented tissue with the size of up to 5 to 6 mm (0.2 to 0.24 inch) in diameter.
Fovea centralis, which is a pit at the rear of the Retina that contains no rods and holds the densest concentration of cones in the eye.
When light enters the eye, it passes through the lens and cornea and is refracted by focusing an image onto the Retina. Light-sensitive molecules present in the rods and cones react to a particular wavelength of light and trigger nerve impulses. Complex interconnections (which are called synapses) within and between the retinal cell layers assemble these impulses into the coherent pattern that, in turn, is carried out through the optic nerve to the brain's visual centers, at which they are further organized and interpreted.
Structure
Inverted Versus Non-Inverted Retina
The inverted Retina of vertebrates is characterized as having light-sensing cells in the back of the retina, requiring light to pass through layers of capillaries and neurons before reaching the cones and rods. The ganglion cells, whose axons form optic nerves, are at the Retina's front; thus, the optic nerve should cross via Retina en route to the brain. There are no photoreceptors in this region that give rise to the blind spot. Also, in contrast, in the cephalopod retina, the photoreceptors are in the front part, with processing capillaries and neurons behind them. Due to this, cephalopods do not have a blind spot.
Although the overlying neural tissue is partially transparent, and the accompanying glial cells have been represented to act as fiber-optic channels to transport the photons directly to the photoreceptors, light scattering does take place. A few vertebrates, including humans, hold an area of the central Retina adapted for a high-acuity vision. This region, known as the fovea centralis, is avascular (meaning it lacks blood vessels) and has very little neural tissue in front of the photoreceptors, reducing light scattering.
Development
Retinal development starts with the establishment of the eye fields, which are mediated by the SIX3 and SHH proteins, with subsequent development of the optic vesicles, which are regulated by the LHX2 and PAX6 proteins. The Pax6 role in eye development was elegantly demonstrated by Walter Gehring with his colleagues, who represented that ectopic expression of Pax6 can lead to the eye formation in Drosophila antennae, legs, and wings. The optic vesicle will give rise to 3 structures as given below:
The retinal pigmented epithelium,
The neural Retina, and
The optic stalk.
The neural Retina has the retinal progenitor cells (RPCs), which give rise to seven Retina's cell types. The differentiation starts with the retinal ganglion cells and concludes with the production of Muller glia. Although every cell type varies from the RPCs in sequential order, there is a considerable overlap in timing, where the individual cell types vary. The cues, which determine an RPC daughter cell fate are coded by the multiple transcription factor families with the homeodomain and bHLH factors.
Blood Supply
The Retina can be divided into layers, each with its own set of cellular compartments or cell types, each with its own metabolism and nutritional requirements. To satisfy such requirements, the ophthalmic artery will bifurcate and supply the Retina through two distinct vascular networks, which are given below:
The choroidal network that supplies the outer Retina and choroid,
The retinal network supplies the inner layer of the Retina.
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Circulatory Mechanisms
At first glance, one can think that the vertebrate Retina is "badly designed" or "wired wrongly," but it's a fact that the Retina could not function if it were not inverted. The photoreceptor layer should be embedded in Retinal Pigment Epithelium (RPE) that performs at least 7 vital functions, one of the most obvious being to supply the oxygen and the other required nutrients, that are needed for the photoreceptors to function.
FAQs on Retina
1. What is the retina and where is it located in the eye?
The retina is a very thin layer of tissue located at the back of the eye. Its main job is to receive light that the lens has focused, convert this light into electrical signals, and send these signals to the brain. You can think of it as the digital sensor in a camera.
2. How does the retina help us see different colours and see in the dark?
The retina has two types of special light-sensitive cells called photoreceptors. These are:
- Cones: These cells work in bright light and are responsible for our sharp, detailed, full-colour vision.
- Rods: These cells are much more sensitive to light and are responsible for our vision in dim light or at night, but they do not detect colour.
3. What is the main difference between rod and cone cells in the retina?
The main difference is what they help us see. Cones are for seeing colour and fine details in bright light, which is why you see vividly during the day. Rods are for seeing in low light and detecting motion, which is why it's hard to distinguish colours in a dark room but you can still see shapes.
4. What are some common examples of problems or diseases that can affect the retina?
Several conditions can harm the retina. A common example is Retinal Detachment, where the retina lifts or pulls away from its normal position. Another is Macular Degeneration, which affects the central part of the retina (the macula) and causes loss of central vision, making it hard to read or see faces clearly.
5. How do doctors examine the health of a person's retina?
Doctors, specifically ophthalmologists, use an instrument called an ophthalmoscope to look through the pupil and directly see the retina, optic nerve, and blood vessels at the back of the eye. This allows them to check for signs of damage or disease. This examination is often part of a routine eye check-up.
6. Why do we have a 'blind spot' in our vision?
The blind spot exists because of the way the retina is built. There is a small area on the retina where the optic nerve, which carries signals to the brain, connects and leaves the eye. In this specific spot, there are no photoreceptor cells (no rods or cones) to detect light. Since no light signals can be sent from this point, it creates a small gap, or blind spot, in our vision.
7. Why is it so important to protect the retina from damage?
It's crucial because the cells in the retina are part of the central nervous system, just like the brain. These highly specialised nerve cells have a very limited ability to heal or regenerate if they are damaged. Therefore, any significant injury or disease affecting the retina can lead to permanent vision loss, which is why eye protection and regular health check-ups are so important.
