16. Our other senses

16.1 The sense of smell

Your ability to smell comes from specialized sensory cells called olfactory sensory neurons. Smells reach the olfactory sensory neurons through your nostrils and a channel that connects the roof of the throat to the nose. The nasal cavities or nasal passages are wrinkled and protected by the mucosa.

Olfactory sensory neurons are found in a small patch of tissue high inside the nose. These cells connect directly to the brain. Each olfactory neuron has one odor receptor. Once the neurons detect molecules from the air, they send messages to your brain, where the smell is identified.

The sense of smell is one of the senses that take care of human safety. It helps us identify danger, for example.

Harmful substances produce smells that are perceived by us as unpleasant. This is why inedible food, faeces and smoke smell bad. By detecting harmful substances, the sense of smell keeps you away from potentially dangerous situations.

For the most part, the process of smelling takes place unconsciously. People can avoid some smells without even understanding why.

Smells are also said to have an effect on the development of human relationships. Indeed, the sense of smell often provides us with our first memories. A person can remember the smells they smelled when they were under the age of three when they are introduced to the particular smells again.

16.2 The sense of taste

Taste cells are located in the tongue and oral mucosa. They are located deep inside taste buds, so that food moving on the tongue does not damage them. When eating, chemical substances responsible for a particular taste are released in the mouth. These substances come into contact with a nerve cell. The nerve cell transmits messenger substances, which in turn activate further nerve cells. These nerve cells then pass information about a particular taste sensation to the brain.

Much of what we perceive as the sense of taste actually comes from the sense of smell. As we bite food, the chemical substances reach the olfactory sensory neurons of the nose through the roof of the throat. They are perceived as flavors.

Our taste buds are often said to be inferior compared to that of other animals. In fact, the problem is mainly that very few descriptive words for specific tastes have been developed in human languages. In the past, it was important to describe edibility with the simple terms such as "good" or "bad." A person can learn to distinguish even slight differences in flavor. 

16.3 The sense of touch

The sense of touch is a combination of many different senses. For example, our skin has receptors for pressure, coldness and warmth. There are cells in the joints and tendons that sense stretching. With the help of the first three senses, we get information about our environment and are able to react to it. With cells that perceive stretching, we get information about our own body and the positions of our own limbs.

Pressure-responsive receptors are located near the surface of the skin. They help us to sense touch. Most of these receptors are on the fingertips and lips, whereas the back has only a few of them. Indeed, human fingers are particularly advanced. With the sense of touch, we can do a lot of things with our hands without looking at what we are doing.

The skin also has warm and cold receptors. By sensing changes in temperature in the environment, a person not only protects themselves, but can also regulate their bodily functions. Like other warm-blooded mammals, humans must produce more heat than is evaporated from their bodies when the air outside grows colder. In order for this kind of thermal control to work, we need constant information about changes in the temperature around us.

The sense of touch allows for many functions without the sense of sight. For example, you can type on a computer keyboard even if you cannot see your fingers at all. When doing so, the receptors that sense the stretching of your finger joints tell your brain what positions the fingers are in. By using your memory, the fingers receive information about where they should be in order to find certain letters on the keyboard. In turn, the receptors that sense pressure on the skin tell your brain when you have pressed the keys firmly enough.

In braille lettering, the dots that appear as an embossing form their own alphabet. The height of the dots is about half a millimeter of the rest of the surface of the paper. For a long time, the braille alphabet was the only way for blind people to read texts by themselves. Today, computers also have immersive readers that read the text on the screen aloud. 


The layers of skin.

16.4 Pain

The purpose of pain is to protect the body by warning it of both internal and external damage. Pain is sensed by free nerve endings found in the skin, the membranes that surround the internal organs, the bladder and the muscles. There are no pain receptors in the brain or liver, but the membranes that cover them warn of sources of pain coming from outside.

Tight clothing can squeeze in the morning, but after a few hours the tightness is no longer noticeable. Pain, however, does not work the same way as other sensations do. Even if you do not get used to a particular kind of pain, you can learn to control the sensations caused by it. For example, if you think about pleasant things or keep yourself busy, the pain sensation is not as dominant in the brain as it is if you actively think about pain.

Often, pain is not felt at its actual source. For example, a headache is usually caused by tension in the muscles of the shoulders. When no blood circulates to the shoulder muscles, the resulting loss of oxygen causes pain that is sensed as a headache. Similarly, heart pain can be felt in your hand. Likewise, when you hit your elbow the nerve pain can be felt in your little finger. This happens because information from several areas of the body often travels through the same nerve pathways in the spinal cord and the brain.

Painkillers prevent nerve impulses caused by pain from reaching the brain, so that the pain message is not transmitted. Excessive intake of painkillers will reduce their effectiveness and may even cause poisoning. The cause of the pain should be treated first, whereas the pain itself should only be taken care of after its cause has been determined.