Structure and Function of 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.