You are made aware of the world around you by your senses. At one time, it was believed that human beings had only five senses: sight, hearing, touch, smell, and taste. But modern scientists have added to the list the senses of hunger, pain, and thirst.
All your senses are divided into two groups, external senses and internal senses. External senses make us aware of things happening outside our body or things coming in contact with our body, such as hearing, sight, smell, taste, and touch. We are made aware of these things by special cells, called receptors, in our skin. Our internal senses make us aware of changes that take place in the organs and tissues inside our body, such as hunger, thirst, pain, and fatigue. Here, receptors inside our body respond to chemical and physical changes and notify the brain.
There are several steps in the process of sensing any of these external or internal senses. A stimulus acts on the nerves in tone of the sense organs. Nerve impulses from that sense organ travel to the brain. In the brain, the impulses are interpreted as a feeling or a sensation. For example, if you stick your finger with a needle, nerve endings in the skin of your finger are stimulated to send impulses to your brain, which interprets the impulses of pain. But the pain itself is not felt in the brain even though the brain interprets the impulses as pain. Pain is felt in the finger, which is the sense organ.