When we look at an object, we can see them because of the reflection of light from them. The light reflected passes through cornea. It then bends the light in a way so that it can smoothly go through the pupil which is in the middle of the iris. After entering the eye, light goes through a lens and finally reaches the retina.
The retina has photoreceptor cells which can detect and respond to light and help us to see objects. There are two specialised photoreceptors – rod and cone cells.
The sections below will discuss their function as well as significant difference between rods and cones.
What is Rod and Cone Cells?
The rod cells present in the outer boundaries of retina of vertebrates enable them to perceive the size and shape of objects even in low light conditions. These cells are stimulated when light enters our eye. Then, they convert it to chemical and electrical stimuli to be processed by the central nervous system in brain. Rod cells are susceptible to photon particles, but they cannot differentiate between various wavelengths of light, which is why they are unable to detect colours or any other small details. There are about 130 million rods in our retina.
Structure of a Rod Cell
These cells have an elongated or rod-like shape. The cells can be divided into four main areas – the outer part, an inner portion, connecting cilium and synaptic region. The outer part which facilitates transduction is made of tightly assembled membrane disks and contains a pigment called rhodopsin. The inner part contains organelles and the nucleus and synaptic region of a rod cell help in transmitting the information from photoreceptor cells to bipolar cells. The inner and outer part is connected via the connecting cilium.
Cone cells are one of the photoreceptive cells present in retina along with rod cells. Unlike rod cells, however, cone cells function in daylights and well-lit areas. These photoreceptive cells are less sensitive to photon particles but can detect various wavelengths of light.
Consequently, they can detect finer details such as differentiating between various colours of objects around us. Cone cells, therefore, facilitate photopic vision.
Notably, these are conical shaped cells located in the centre of the retina called the macula. Moreover, compared to rods, there are fewer cone cells present in humans. Rod and cone cells both contain a specific pigment. The pigment present in cone cells is called iodopsin.
Based on colour vision, Cone cells can be differentiated into three groups. These are: S- cones(blue), M-cones(green) and L-cones(blue). The photopigments react differently to various wavelengths of light. Consequently, humans respond to colour variations in different ways which is why they have trichromatic vision.
The formation of a rod and cone cells are the same. Both of them have an inner segment, outer segment connected by cilium.
What is Photoreceptive Degeneration?
This term is used to refer to the loss of photoreceptive cells such as rod and cones in eye. This phenomenon can have serious consequences, such as loss of vision or impairment. Some eye-related diseases caused by photoreceptive degeneration are retinitis.
The photoreceptor cells present in retina perform a similar function, i.e., they convert light rays to electrical signals that are sent to the brain. Nonetheless, there are some differences in the tasks performed by each of them. The table below shows the major differences between rod and cone cells which you can go through quickly to gain an idea of the same.
For medical aspirants, NEET is very significant as it is the gateway for admission to important medical colleges within India. Hence, thorough preparation is crucial to achieve a good rank. A systematic approach will go a long way towards that. Before you jump into the revision process, make a routine to divide your time equally between all the subjects. It will enable you to focus on all important topics such as rods and cones function.
Apart from studying rod and cone cells, however, students should also remember to take care of themselves and stay healthy.
1. What are the pigments found in rod and cone cells, respectively?
Ans. Rods have a pigment called rhodopsin, and cone cell contain iodopsin.
2. How do we see colours?
Ans. When light reflected off a surface of an object enters our eye, it stimulates the cone cells, which are a type of photoreceptors that detect colours. Once they are stimulated, they send a signal through the optic nerve to the brain, which then sends back the right information.
3. What are scotopic vision and photopic vision?
Ans. When we perceive objects and detect colours in normal or well-lit conditions, it is known as photopic vision. In scotopic view, we only see objects but cannot see colours.
4. What is phototransduction?
Ans. It is a process whereby light entering our eye is converted to an electrical signal in the photoreceptors present in retina.