Human Retina

Human Retina

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The eye has an important job in our body - it helps us see! But how does it work? Well, light enters our eye and is focused onto a small area at the back the retina. The retina has special cells called photoreceptors that respond light. There are two types of photoreceptors: rods and cones. These cells act like transducers, which means they convert light energy into electrical signals that our brain can understand. This is why we see things! So next time you look at something, remember that your amazing human retina is working hard to help you see it.

Where is the retina located in the human body?

Have you ever wondered how we see things? Well, it all starts with the retina - of tissue at the back of our eye. The retina is where the photoreceptors are located, and they help us see by responding to light. When light enters our eye and lands on the retina, the optic nerve sends electrical signals to our brain, which allows us to see. But did you know that there's a part of our eye where we can't see anything? It's called the blind spot and it's where the optic nerve connects to the retina. So next time you look at something, remember how amazing your retina is for helping you see, even with a small blind spot! 

Functions of other structures in the eye

This table outlines the structure of some other important features of the eye. Understanding the function of these will help you understand how light enters and is focused in the eye before it reaches the retina. It will also help you understand where the electrical signals produced by the photoreceptors go.

Features of the eye

Have you ever wondered how we convert light into something our brain can understand? Well, it all starts with the retina in our eye. When light enters our eye, it's focused onto the retina where light-sensitive pigments absorb it. These pigments undergo a chemical change called bleaching, which increases the permeability of the membrane to sodium ions. This causes depolarisation, which leads to a generator potential, and if it reaches a certain threshold, a nerve impulse called an action potential is sent! The impulse goes through a bipolar neurone before reaching the optic nerve and finally, the brain. A bipolar neurone is a special type of neurone with two extensions that act like an axon and a dendrite. So next time you see something, remember the amazing process happening in your retina!

Differences between rod and cone cells in the human retina

As mentioned above, the two types of photoreceptors we find in the eye are rod cells (rod-shaped) and cone cells (cone-shaped.) Rods and cones have different pigments and different connections to bipolar neurones. This causes them to have different properties, as detailed below.

Sensitivity to light

Did you know that rod cells in our retina are amazing at detecting low-intensity light? This is because many of them are connected to a single neurone, a property called retinal convergence. This allows for summation to occur, where several electrical impulses can add together to reach the threshold potential, creating a generator potential. This is how our eyes can detect low levels of light, even in dimly lit areas. On the other hand, cone cells are less sensitive to light intensity because each one is connected to its own separate bipolar cell. As a result, summation cannot occur in cone cells, and a generator potential will only occur if the stimulation of a singular cone cell is enough to reach the threshold potential. It's amazing how our eyes have evolved to have different types of cells that work together to help us see in a variety of lighting conditions!

Sensitivity to colour

Our eyes have three different types of cone cells, each containing a specific type of iodopsin: green-sensitive, blue-sensitive, and red-sensitive. These pigments are only sensitive to one colour because they each have a different set of specific wavelengths. For example, the 'blue' pigment is most sensitive to shorter wavelengths between 400–500nm, the 'green' pigment is most sensitive to mid-length wavelengths between 450–630nm, and the 'red' pigment is most sensitive to longer wavelengths between 500–700nm.

Rod cells, on the other hand, only contain one pigment rhodopsin. Rhodopsin is not sensitive to colours because it is destroyed by bleaching on exposure to light. This is why rod cells only allow us to see in black and white.

Wavelengths of absorption of cone cells

Visual acuity

Visual acuity refers to our ability to distinguish between points that are close together. A high visual acuity means that we can see a very detailed image, while a low visual acuity means that we see fewer details as it's more difficult to distinguish points that are close together. Rod cells give us low visual acuity because many of them link to a single bipolar neurone. This means that the light received by rod cells that share the same neurone will only generate one nervous impulse. Therefore, the brain can't receive separate information about two close points, and it can't distinguish between them. As a result, rod cells give low visual acuity. Cone cells, on the other hand, give high visual acuity. is cones, and each cone is connected to its own individual neurone. light from two points hits two cones, each cone generates its own action potential. The brain, therefore, receives two action potentials, which means that it has separate information about the two points. This allows us to distinguish between them and have high visual acuity.

Frequency and distribution

The majority of cone cells are packed together in the area of the retina called the fovea. On the other hand, rod cells are mainly in the peripheral parts of the retina. There are a lot more rod cells in the human eye than there are cone cells. We have around 120 million rod cells in one eye, versus around 6 million cone cells.

Conditions of the retina

The retina is extremely important in allowing us to see. There are several disorders of the retina, which are collectively referred to as retinal diseases. Diabetic retinopathy, retinal tears, and retinal detachments are amongst the most common.

Diabetic retinopathy

Having diabetes can damage tiny blood vessels (capillaries) in the back of the eye due to high blood sugar levels. This causes them to leak fluid around and inside the retina. Fluid entering the retina will cause it to swell up, leading to blurred or distorted vision. If left untreated and undiagnosed, it can also cause blindness.

Retinal tear

we have a colourless gel-like substance called the vitreous humour inside our eyes. This gel-like substance can shrink and cause tugging on the retina. If the tugging is strong enough, it may break the tissue. This is called a retinal tear. The symptoms tend to be sudden and include seeing floaters and flashing lights.

Retinal detachment

A retinal tear can lead to a more serious condition called retinal detachment. This occurs when fluid passes through a tear in the retina, causing it to detach from the underlying tissue layers. Both conditions have similar symptoms and require immediate surgical treatment. The retina is a thin layer of tissue located at the back of the eye. Itoreceptors called rod cells and cone cells. When light enters the eye, the pigments in these cells undergo a chemical change, which is then transmitted to the brain through the bipolar neurone and optic nerve in the form of nervous impulses. Rod cells are more sensitive to light and have lower visual acuity, allowing us to see in black and white. Cone cells are less sensitive to light but have higher visual acuity, allowing us to see in colour. There are several retinal disorders that can affect vision, such as retinal detachment and tears. It is important to seek medical attention promptly if you experience any symptoms of these conditions.

Human Retina

What causes detached retina in humans?

This occurs when a colourless gel in the eye passes through a retinal tear, causing the retina to lift away and detach from the tissue layers that lie beneath it.

What does the retina do in the human eye?

The retina allows us to see. It has photoreceptors called cone and rod cells, that, together with other structures in the eye, respond to light. These receptors send electrical signals via the optic nerve to the brain, allowing us to interpret what we see.

What is the retina?

The retina is a thin layer of tissue at the back of the eye. 

How many rod cells does the human retina have?

120 million

Why do humans have a blind spot on their retina?

The blind spot is the part of the eye where the optic nerve connects to the retina. There are no photoreceptors, so we cannot see anything there.

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