Fig. 1 The touch sensitive plant . (Mimosa pudica)

      Irritability or sensitivity is the ability of an organism to receive and respond to external stimuli.

      This may look like an ordinary plant, but it is a mysterious plant. Whenever a herbivore tries to eat it, the leaves get folded exposing thorns. That means it is sensitive to touch. It is also sensitive to certain chemicals like ammonia.

       It is a characteristic of all living things to respond to external stimuli. In biology this is known as ‘irritability’. Let us consider and compare our senses with that of other living things.

      The 5 senses such as seeing, hearing, tasting, smell and contact .  We believed as an absolute fact is an over simplification. Now many scientists are of opinion that humans are sensitive to a multitude of stimuli using very complicated systems. Click : Vision,,     Hearing,   taste,  smell. skin.

      There are different ways of conveying messages too. Although fast signalling is performed by the nerves using electrical charges , messages are sent by secreting hormones into the blood stream.

 Fig.2. Feeling of a mosquito bite.

Let us consider the case of a mosquito, in perfect darkness, biting your leg. Most probably your immediate response will be to slam the creature. 99% of the time you will get it absolutely right.   What organ helped you to get your hand right on to the spot of the mosquito bite?

    The creature can land so lightly, that your mechanoreceptors are not alerted. It pierces the skin and injects saliva. This conveys a signal to the brain which gives a slight pain and an itchy feeling. The brain at once, through experience recognises it as a mosquito bite and sends a signal to the hand. Even without opening your eyes, you can aim your shot crushing the creature.

    What are the senses involved in this action?

    Vision is out; Hearing may not be the response.

    It must be the receptors on the skin that sent the message to brain. But the problem is , how did the hand aim the shot without seeing?

    We have heard kids singing “ Head, shoulders and knees” touching their body parts without looking at them.

    There must be another sense not included in the five. This has been named as ‘proprioception’.    

    This is mainly governed by the ‘ stretch receptors’ in the hand muscles. Attached to skeletal muscles are the stretch receptors that detect  the state of the muscle fibres.

 

Fig.3. Mechanoreceptors. (placed in the skin, muscles, joints and internal cavities).

Most people can balance on a very thin surface. If you take a plank as long as your height, do you think you will be able to balance it on a surface, leave alone an edge?

So what helps us to perform this sort of balance acts?

It has to be an another sense. When our body tilts to a side we feel it. This is mainly controlled by the semilunar canals in the inner ear(Vestibular system). This sense has been termed Equilibrioception. Semilunar canals also detect change of velocity.

  There are many more receptors in our body. They often overlap in duties and work in unison to perceive the environment. Sensitivity to chemicals such as carbon dioxide and changes in temperature are not easy to analyse.

    There are sensory mechanisms to feel the changes in the body too. For example wen there is dehydration of the muscles we feel thirsty. Felling such as thirst, hunger and sleep are governed by the Hypothalamus which is inside the brain. If the body becomes too cold, the blood flow to fingers and skin is reduced. 

All living things can avoid extreme temperatures. If we inhale too much CO₂ , the rate of breathing increases.