Responding to stimuli or sensitivity to the surrounding environment is one of the main criteria of being alive. There are six main characteristics of living organisms and responding to stimuli is one of them. Other characteristics are respiration, requiring food and nourishment, growth, movement, reproduction and excretion. These are essential for the survival of living organisms.
Have you ever wondered how life would be without the sense organs? What would happen if we can’t see, hear or feel anything? All the living organisms, animals as well as plants respond to their surroundings and it helps them in reproduction, finding food, avoiding any danger, etc. Responding to the environmental stimulus is an essential process for the survival of living organisms.
Flowers blooming during sunlight or bending towards the sun, dogs salivating with the smell of food, bats can locate distant objects, communication between animals are some of the examples of the responses to various stimuli. Sharks can even detect the electric field surrounding them. Sense organs help us in recognising and responding to any climatic changes.A
Our body not only responds to external stimuli but also detects any changes in the internal environment. Let’s now discover the world of senses.
What Makes Us Respond To Stimuli?
We all are familiar with the five sense organs, which detect any changes and send the signals to the brain or Central nervous system, where it is processed, analysed and sent back to the different organs to elicit a response.
There are sensory receptors present all over the body, which detects all types of stimuli and sends appropriate signals to the brain to respond. Apart from the five main senses, we can also perceive pressure, pain, balance, muscle tension, etc.
These sensory receptors are nothing but the neuronal endings or specialised cells, e.g. rods and cones in our eyes, which are present in close proximity to neurons.
Types Of Sensory Receptors
Sensory receptors can be categorised based on their location, structure and the stimuli they perceive.
Sensory receptors can be free neuronal endings or dendrites mostly present in the epidermis or dermis e.g. receptor for temperature. Receptors for pressure are encapsulated, e.g. Meissner and Pacinian corpuscles. Specialised cells are present in ears, eyes, e.g. rods and cones.
Exteroreceptors respond to external stimuli. They are essential for defence, feeding, movement and reproduction. Interoceptors detect and respond to internal changes, e.g. any change in the ionic concentration of blood, temperature, etc. They are essential to maintain homeostasis.
On the basis of types of stimuli they respond to, sensory receptors are categorised as:
Thermoreceptors – They respond to any change in the external or internal temperature. E.g. blood-sucking insects find their host by perceiving temperature.
Chemoreceptors – They respond to chemicals. Examples include olfactory receptors in the nose and gustatory receptors on the tongue, which provide sensitivity to smell and taste, respectively. Many animals communicate among themselves using chemicals, e.g. pheromones.
Mechanoreceptors – They respond to mechanical stimuli such as touch (tactile receptors), body movement and balance (proprioceptors). E.g. lateral line organs found in fish help them in navigation, vestibular apparatus in the inner ear.
Nociceptors – They induce pain. They detect any stimuli, which can be damaging.
Photoreceptors – They detect light. Rods and cones are present in the retina of our eyes and are photoreceptors.
A group of sensory receptors, with other associate specialised tissues, which together perform a specific function constitute a sense organ.
Sensory receptors convert energy from stimuli into electrical signals. If the potential generated by the signal is high enough, an action potential is generated and transmitted to the central nervous system through afferent or sensory neurons. The information is processed and interpreted in the central nervous system and the response is generated. It is important to learn about neuron physiology to understand the mechanism of neural communication.
The receptor potential generated by stimuli leads to a release of neurotransmitters at the synapse, which binds to sensory neurons and action potential or nerve impulse is conducted to the neurons for further transmission. In a neuron, conduction of nerve impulse is done by the wave of depolarisation and nerve impulse is transmitted from one neuron to another through the synapse.
Sensitivity to stimuli is a basic property of all the living organisms. It helps in maintaining homeostasis internally and better adapt to various environmental stress, reproduction, navigation, defence and predation.
This was in brief about how we perceive senses. For more information related to sense organs, sensory receptors and nervous systems, subscribe to BYJU’S YouTube Channel for more information.