What are Photoreceptors?
Photoreceptors are specialized cells that are found in the retina of the eye. The structure and function of photoreceptors are so specialized that they have the ability to perform under different conditions.
The receptors of the eye are extremely specialized neuroepithelial cells. This is because the cells have both epithelial and neurological functions which make them capable of visual phot transduction.
These photoreceptors are biologically important as they have the ability to convert the light which is from the visible electromagnetic radiation into the signals that can stimulate the biological processes.
Receptors of Eye
There are three types of photoreceptor cells found in mammalian cells. They are rods, cones and intrinsically photosensitive retinal ganglion cells. Of these, the well-known and classic photoreceptors are rods and cones. Both the receptors of the eye contribute to the information which is used by the visual system to form the representation of the visual world which forms the sight. These photoreceptors are usually typically arranged in an irregular manner. Even though irregular the arrangement is an approximate hexagon which is known as retinal mosaic. The human retina is known to contain 120 million rod cells and 6 million cone cells.
All the types of photoreceptors are known to contain pigments that make the cells specialized for the visualization of the image. The photoreceptor cells are very tightly packed which allows them to attain high photopigment density because of which a large number of photoreceptors are able to absorb large amounts of light photons. This leads to better image processing by the brain. There are differences in-between the rods and cones of different species of organisms. The structure and function of photoreceptors allow them to be connected to a network of biological pathways as is observed in the case of invertebrates, and vertebrates although they might be morphologically different and differently packed.
Structure and Function of Photoreceptors
The mechanism by which the photoreceptors function in light detection through the eyes is defined is known as photoreception. As per this mechanism the light is absorbed by the specialized cells known as photoreceptors which convert the light stimulus into nerve impulses. As already mentioned that there are primarily two types of photoreceptors - rod cells and cones cells. To understand better the structure and function of photoreceptors - the rod cells and cones cells a diagram is given below:
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Given Below is the Function of Rod and Cone Cells and Their Characteristics:
Rod Cells
The rod photoreceptor cells are specialized cells that function very well under low light conditions. This is also known as the twilight vision. When stimulated with bright light, they get bleached. Hence, they are the cells responsible for both twilight and night vision.
To activate the rod cells very low light intensity is required and under high intensity, they cannot function much.
These photoreceptors contain a pigment called rhodopsin. All the rod cells contain rhodopsin only. Due to this pigment, these cells have the ability to absorb a very wide range of wavelengths.
Another peculiar feature of rod cells is that they are monochromatic. This means that the rod cells cannot differentiate in-between the different colours present in the wavelengths of the visible light spectrum.
Although present in large numbers, the rod cells are mostly present at the periphery of the eye. This makes them the primary photoreceptor cells for peripheral vision and thus help in widening the visual capacity to a certain extent.
The image formed by the stimulus generated from rod cells is usually blurry. The images are poorly resolved because a single bipolar neuron forms a connection or a synapse with multiple rod cells.
Cone Cells
The cone photoreceptors are the ones that have the ability to function very well under bright light conditions. This makes these photoreceptors suitable for daylight vision.
Large amounts of photons are required to activate these kinds of photoreceptor cells.
In humans, there are three different types of cone cells present. These cone cells are referred to as S-cones, M-cones, and L-cones. These three different types of cones are differentiated on the basis of the three different types of pigments present in them. The three pigments give a response when they absorb different types of narrow wavelengths in the visible light spectrum. This helps humans to identify the different colours of objects around them.
The different colours that are differentiated by the three different pigments in the three different types of cones help in identifying the three different colours which are red, blue and green.
These types of photoreceptors are present abundantly at the centre of the retina i.e. within the fovea region. Because of their location and ability, the cones are the ones responsible for visual focusing.
The quality of images formed by the cones is very well-defined. This is because, unlike the rods which form synapses with single bipolar neurons in multiples, only one cone forms the synapse with a single bipolar neuron, hence providing improved quality images.
Conclusion
Thus, from the given information the main function of rod and cone cells is clear which can be simply stated as - rods are responsible for night-time vision and the cones are responsible for day-time vision.
FAQs on Structure and Function of Photoreceptors
1. What exactly are photoreceptors in the human eye?
Photoreceptors are special nerve cells located in the retina, the light-sensitive layer at the back of the eye. Their main job is to convert light that enters the eye into electrical signals. The brain then interprets these signals as the images we see. The two main types of photoreceptors are rods and cones.
2. What is the main difference between rod and cone cells?
The main differences between rods and cones lie in their function and sensitivity to light:
- Rods: These are highly sensitive to light and are responsible for our vision in dim light or at night (scotopic vision). However, they cannot detect colour, which is why we see in shades of grey in the dark.
- Cones: These are less sensitive to light and work best in bright conditions (photopic vision). They are responsible for our ability to see sharp details and perceive different colours.
3. Can you explain the basic structure of a photoreceptor cell?
A typical photoreceptor cell, whether a rod or a cone, has three main parts: an outer segment containing light-absorbing photopigments, an inner segment with organelles like mitochondria to provide energy, and a synaptic terminal that passes the electrical signal to other neurons in the retina.
4. How do photoreceptors allow us to see in both bright daylight and dim light?
This is possible because of the two different types of photoreceptors working together. In bright daylight, the cones are active, providing sharp, colourful vision. When light levels drop, the cones stop functioning effectively, and the highly sensitive rod cells take over, allowing us to see shapes and movements in dim light, though without colour.
5. What happens inside a photoreceptor when light hits it?
When light strikes a photoreceptor, it is absorbed by a special molecule called a photopigment (like rhodopsin in rods). This absorption causes the photopigment molecule to change its shape. This single change triggers a chemical cascade that alters the electrical state of the cell, generating a nerve impulse that is then sent to the brain.
6. Why is our peripheral vision better for detecting motion at night?
Our peripheral vision is better for night-time motion detection because the outer edges of the retina are densely packed with rod cells. Since rods are extremely sensitive to low light levels and motion, any movement in our side view is quickly picked up by them, even when it's too dark for our central, cone-rich vision to see clearly.
7. Are photoreceptors the only cells in the retina needed for vision?
No, they are just the first step. While photoreceptors detect light, the retina contains other crucial nerve cells, such as bipolar cells and ganglion cells. These cells collect, process, and relay the signals from the photoreceptors, bundling them into the optic nerve which carries the complete visual information to the brain for final interpretation.
