The purpose of the eye in the human body is to focus light into the retina. The retina is the area at the back of the eye where the photoreceptors are. Photoreceptors are the cells in the retina that respond to light.
These photoreceptors include rod cells and cone cells. As they are types of receptors, both rod and cone cells act as transducers since they convert light energy into the electrical energy of a nerve impulse.
We refer to receptors as transducers because they transduce (convert) energy from one form to another.
Where is the retina located in the human body?
The human eye diagram below shows the location of the retina. It is a thin layer of tissue at the back of the eye. The other structures in the eye are also important, but this article will focus mainly on the retina, where the photoreceptors are.
Fig. 1 - The anatomy of the eye
When light enters our eye and lands on the retina, the optic nerve sends electrical signals to the brain. The point where the optic nerve connects to the retina has no photoreceptors. This part of the eye is called the blind spot, as we cannot see anything here.
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.
Table 1. Features of the eye.
Structure | Function |
Pupil | The hole that light passes through to enter the eye. |
Iris | Controls the amount of light that enters the eye by opening and closing the pupil. |
Cornea | Clear, protective outer layer of the eye that also refracts (bends) light as it enters, helping to focus it. |
Lens | Located behind the iris. Changes shape to refract light rays onto the retina, helping to focus it. |
Optic nerve | A sensory nerve that carries nerve impulses from the receptors on the retina to the brain’s visual centres. |
Conversion of electrical impulses in the human retina
Let’s check out the process of electrical impulse conversion in the retina.
This section will require previous knowledge about how action potentials work. For more information on this, check out our explanation on Action Potential.
- Light enters the eye through the pupil and the cornea and the lens focus it onto the retina. Here, the light is absorbed by light-sensitive optical pigments. (Remember that optical just refers to something related to sight.)
- The pigments are bleached, which results in a chemical change. This increases the permeability of the membrane to sodium ions. There is an influx of sodium ions into the membrane, causing depolarisation.
- This depolarisation eventually results in a generator potential if the threshold potential is reached. When this happens, the photoreceptor sends a nerve impulse (action potential.)
- The nerve impulse first goes through the bipolar neurone, which is connected to the optic nerve. The optic nerve will then take the nerve impulses to the brain.
A bipolar neurone is a neurone with two extensions attached to its cell body. One extension acts as an axon and one acts as a dendrite.
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
Rod cells are sensitive to light intensity. This means that they can detect light of very low intensity. Rod cells have this property because many of them are attached to a singular neurone. This property is called retinal convergence, and it allows for summation to occur.
Summation is the process where several electrical impulses can add together. Individually, they may be too weak to cause a response. But together, they have an increased chance of reaching the threshold potential, thereby creating a generator potential.
On the other hand, cone cells are less sensitive to light intensity because each of them is connected to its own separate bipolar cell. This means that summation cannot occur in cone cells. A generator potential will only occur if the stimulation of a singular cone cell is enough to reach the threshold.
Sensitivity to colour
Cone cells contain different types of pigment than those found in rod cells. The pigment in cone cells is called iodopsin. This pigment requires a higher light intensity to be broken down.
Our eyes have three different types of cone cells. Each one contains a specific type of iodopsin: green-sensitive, blue-sensitive, and red-sensitive. Each pigment is only sensitive to one colour because these colours all have a different set of specific wavelengths.
Rod cells, on the other hand, only contain one pigment called 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.
‘Blue’ pigment is most sensitive to shorter wavelengths between 400–500nm. ‘Green’ pigment is most sensitive to mid-length wavelengths between 450–630nm. And ‘red’ pigment is most sensitive to longer wavelengths between 500–700nm. The diagram below shows the visible light spectrum. The numbers at the bottom represent the wavelength of the colour in nm (nanometres).
Fig. 2 - Wavelengths of absorption of cone cells
Visual acuity
Visual acuity is the ability to tell apart points that are close together.
A high visual acuity means that you see a very detailed image, whilst a low visual acuity, means that you see fewer details as it's more difficult to tell apart points that are close together.
As discussed previously, many rod cells link to a single bipolar neurone. The consequence of this is that the light received by rod cells that share the same neurone will only generate one nervous impulse. This means that the brain doesn’t get separate information about two close points. Thus, the brain can’t tell apart the light from two points that are close together. As a result, rod cells give low visual acuity.
Cone cells, on the other hand, give high visual acuity. This is because cones are close together and each cone is connected to its own individual neurone. As a result, when light from two points hits two cones, each cone generates its own action potential. The brain, thus, receives two action potentials, which means that it has separate information about the two points. This allows us to distinguish between them.
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 deteriorate further to become a retinal detachment. This happens when fluid passes through a retinal tear, causing the retina to detach from the underlying tissue layers. It has similar symptoms to a retinal tear, and both conditions must be treated immediately with surgery.
Human Retina - Key takeaways
- The retina is a thin layer of tissue located at the back of the eye. The photoreceptors called rod cells and cone cells are in it.
- The light that passes into the eye causes the pigments in photoreceptors to undergo a chemical change. This information is then passed via the bipolar neurone and optic nerve to the brain in the form of nervous impulses.
- Rod cells are more sensitive to light, have lower visual acuity, and only allow us to see in black and white. Cone cells are less sensitive to light, have higher visual acuity, and allow us to see in colour.
- There are several retinal disorders that can cause issues with vision such as retinal detachment and tears.
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