Smell is an additional stimulus that plays a fundamental role, along with taste, in the nutritional process. In fact, smell and taste are difficult to separate. It is estimated that 80% of what is considered taste is in fact smell. The smell of a substance is closely related to its chemical structure. Since the human being is capable of precisely identifying hundreds of different smells, it is thought that the neurons in charge of identifying smells can contain different codes that allow remembering distinct smells and identify them in other moments.
The cells that contain the chemoreceptors for smell are located in the upper part of the nasal cavity, in the olfactory epithelium. These cells are specialized afferent neurons, which possess one dendrite with many cilia that are in contact with the mucus and contain the chemoreceptors. The axons of these neurons combine to form the olfactory nerve, also known as cranial nerve I. These cells have the highest proliferation rate of all neurons, although in humans it is much lower compared to other mammals.
In order to detect a smell, the molecules of a substance must be present in the air and reach the nasal cavity, dissolve in the mucus and bind to the cilia where the receptors are located. There are approximately 1000 different chemoreceptors, and each neuron expresses several of them, although they all share certain regions called consensus regions. On the other hand, other hypervariable regions exist that allow the identification of many different olfactory molecules. One receptor is only capable of recognizing a region of the molecule, and it is possible that several receptors can bind to different regions of the same molecule. In this manner, olfactory molecules of the same chemical group can be recognized by the same receptors. Also, there are various molecules in the air that cannot be detected by the olfactory receptors. In other words, the combination of smell molecules give rise to the large variety of smells that the olfactory system can detect.
The glomeruli of the olfactory bulb are thousands of spherical structures that produce the olfactory nerve synapse, which contacts with a population of neurons call mitral cells of the primary olfactory cortex. The mitral cells present fewer synaptic receptors, since it converges the peripheral influx. There are also interneurons that connect the different glomeruli in order to modulate the activity of the mitral cells. In this manner, when a very intense stimulus is present, the interneurons inhibit the activity of certain mitral cells, allowing a better discrimination of the smell over other less intensive smells. Therefore, olfaction begins in the olfactory bulbs, which form part of the limbic system, that pass the information to the primary olfactory cortex and connects with the areas of the brain associated with emotional states, food and sexual behavior. In this manner, the smells activate different areas of the cortex, thalamus and hypothalamus, allowing the identification of more than 10,000 smells using 1000 chemoreceptors.
It is easy to imagine the close relation between the sense of smell and taste, since the chemical substances that are released while chewing are rapidly dissolved in the oral cavity and easily pass to the nasal cavity. Also, olfactory discrimination increases in certain circumstances, such as when an individual is hungry. Gender differences also exist, as women are more sensitive to smells than men. However, sensitivity decreases in smokers, elderly and when certain pathologies are present, especially those that include nasal congestion. In this latter case, the affected individuals do not perceive taste, further proving the close relation between smell and taste.