Humans have approximately 350 different olfactory receptors, each of which has a specific odor molecule docked to its cilia, triggering activation of the cell. Via the collected messages of the olfactory receptors, the brain creates the conscious olfactory impression. The olfactory receptors, numbering several million, are located mainly in the olfactory mucosa, a small area in the upper nasal cavity.
What is an olfactory receptor?
Olfactory receptors, also called olfactory cells, belong to the group of chemoreceptors. Chemoreceptors perform a variety of tasks to subconsciously regulate and maintain homeostasis. Olfactory cells are highly selective sensors, each specialized to detect a particular odorant molecule. Up to ten million olfactory receptors are located in an area of about four square centimeters in the upper nasal cavity, the so-called olfactory mucosa. They can be subdivided into about 320 different cell types, each of which is able to dock a specific odor molecule to one of its ten to twenty cilia. For example, German shepherds, with about 1,200 different olfactory cell types, have a much finer and more differentiated sense of smell than humans. After docking of a specific odor molecule to a cilia of the matching receptor cell, the conversion of the chemical stimulus into an electrical potential already takes place in the cilia. The action potentials of identical olfactory receptors are first collected in the olfactory bulb before being transmitted to the brain.
Anatomy and structure
Olfactory cells are found not only in the olfactory mucosa but also, for example, in the liver and testes, where they may exert influence on homeostasis as unconscious chemoreceptors. The functional principle of olfactory receptors corresponds to that of G protein-coupled receptors. The principle is based on membrane proteins that, according to the lock-and-key principle, trap specific molecules in a kind of pocket and insert them through the membrane into the cytosol of the cell or into a lysosome or into another organelle. Olfactory receptors in the olfactory mucosa of the nose are surrounded by supporting cells. A dendritic process of the olfactory nerve pierces the mucosa outward and forms a small vesicle (vesicula olfactoria) at the end, from which 5 to 20 cilia extend into the mucus of the olfactory mucosa. In the thin layer of mucus, the “odor molecules” are released, which can dock at the olfactory cell that is suitable for them and initiate the cascade of signal transduction into an electrical nerve impulse. On the tissue side, olfactory receptors are directly connected to the olfactory bulb via an axon, where signals from the same types of olfactory cells are collected and transmitted to corresponding centers in the CNS. Some of the axons of the olfactory receptors are slightly bundled before they pass through the finest pores of the ethmoid bone as olfactory fibers (fila olfactoria) into the skull. The fila olfactoria are not myelinated and thus correspond to the slow-conducting nerves of fiber type C. Their conduction velocity is 0.5 to 2 m/sec. Because of the short distances from the olfactory mucosa to the CNS of only a few centimeters, the velocity is absolutely sufficient.
Function and tasks
The main task and function of olfactory receptors is to provide downstream centers in the CNS with information about the presence and abundance of about 350 different odorant or scent molecules. Each individual cilia that comes into contact with its specific odor molecule in the mucus of the olfactory epithelium and docks the molecule results in an electrical impulse that is transmitted. The processing of the millions of odor or scent impulses into a kind of “scent layer” occurs only at the downstream centers of the CNS. The first recipients of the electrical nerve impulses, which have already been presorted by the glomeruli according to the type of odor molecules, are the two olfactory bulbs (Sg. Bulbus olfactorius). They transmit the messages without additional processing power via so-called mitral cells to structures in the olfactory cortex, where the actual processing takes place and decisions about unconscious and conscious responses are made. The individual sensor messages can be very important for immediate survival, for example, to recognize spoiled food or dangerous toxins already by smell.Smells and scents independent of food intake can also warn of danger and also something of people’s state of mind. For example, fear sweat, which is produced by apocrine sweat glands in the armpits, smells distinctly different from sweat that serves exclusively for thermoregulation and is secreted by the eccrine sweat glands. Scent messages from the olfactory receptors also play an important role in the sexual sphere. During ovulation, a woman’s hormone levels change, which she unknowingly signals through olfactory secretion of pheromones called copulins. Men respond by producing more testosterone, although copulins cannot be consciously perceived at low concentrations.
Diseases
A number of causes are possible as triggers for dysfunction or complete loss of the sense of smell (anosmia). For example, the olfactory sensors themselves may become diseased, or the olfactory epithelium may be altered so that odor molecules cannot reach the cilia of the olfactory receptors. In some cases, signal transmission or signal processing in the CNS is also disturbed. By far the most common cause of impaired or even complete loss of olfaction is chronic inflammation of the sinuses (sinusitis). Severe colds that lead to swelling of the mucous membranes of the respiratory tract are often accompanied by a temporary impairment of the ability to smell, which usually improves on its own after the cold has healed. Another complex of causes for the occurrence of anosmia lies at the neuronal level. A traumatic brain injury (SHT) can lead to damage in the olfactory center, or the olfactory fibers are severed by an accident. Similarly, anosmia can be triggered by a brain tumor, or by progressive Alzheimer’s dementia or Parkinson’s disease. Very rarely, genetic abnormalities or mutations are responsible for loss of the sense of smell.
