To smell, the nose must first have an odor source, usually volatile chemical molecules in the air. When chemical molecules are inhaled into the nasal cavity, they combine with olfactory receptors. Olfactory receptors are composed of G protein-coupled receptors, which are distributed on the surface of olfactory cells. Olfactory cells are located in the olfactory epithelium at the upper end of the nasal cavity, about seven centimeters away from the nostrils, with an area of about one square centimeter, such as the size of a nail.
Olfactory cells are a special kind of bipolar neuron. The tips of cells near the nasal cavity are specialized into cilia to increase the contact surface area with odor molecules. The top of olfactory cells is covered by nasal mucus, which contains odor-binding protein (OBP). OBP will first capture odor molecules in the air, then combine with olfactory receptors on olfactory cells, change the protein configuration of olfactory receptors, and produce a series of information transmission reactions, thus activating olfactory cells, stimulating potential changes on cell membranes, and then transmitting electrical signals to the brain.
Humans can recognize and remember 1000 kinds of smells, while cats and dogs can recognize and remember 40,000 to 50,000 kinds. Scientists believe that genes determine the sense of smell. Two American scientists discovered a large gene family. This gene family consists of 1000 different genes (accounting for 3% of our total genes), which constitute a large number of olfactory receptor types. These receptors are located in olfactory receptor cells at the upper end of nasal epithelium and can detect inhaled odor molecules. Each olfactory receptor is only particularly sensitive to one or several odors, while 1000 olfactory receptors can detect more than 1000 odors.
A professor at Washington University in St. Louis explained that "odorous" molecules in the air enter through nostrils, combine with olfactory receptors, activate olfactory cells and send electric waves to miniature olfactory bulbs, enter coronary cells, and finally reach brain nerves. The cerebral cortex sorts out and files every smell it receives, and keeps the memory of the smell in its mind. This is why when we smell something, we can remind you of it or remind you of something.
Humans will divide smells into "fragrant" and "smelly" and consciously distinguish them. This is because the sense of smell has resolution. In ancient times, ancestors who lived by hunting may have as sensitive noses as wild animals, but the process of evolution will make each animal have a different sense of smell. At the same time, with the evolution of human brain, the function of nose will gradually deteriorate, and the smell that human beings can recognize and remember will be less and less. Just like nerve cells in the brain, brain cells that have not been used for a long time will slowly shrink or even die without any stimulation. So is the sense of smell. Increasing the stimulation of the sense of smell may be the best way to keep the sense of smell sensitive.