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Olfactory system

From Wikipedia, in a visual modern way
Olfactory system
Head Olfactory Nerve Labeled.png
Components of the olfactory system
Identifiers
FMA7190
Anatomical terminology
This article is about the olfactory system in vertebrates, particularly humans. For the sense of smell in other lifeforms, see Olfaction. For machines, see Machine olfaction.

The olfactory system, or sense of smell, is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an accessory olfactory system. The main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli.

The senses of smell and taste (gustatory system) are often referred to together as the chemosensory system, because they both give the brain information about the chemical composition of objects through a process called transduction.

Structure

This diagram linearly (unless otherwise mentioned) tracks the projections of all known structures that allow for olfaction to their relevant endpoints in the human brain.
This diagram linearly (unless otherwise mentioned) tracks the projections of all known structures that allow for olfaction to their relevant endpoints in the human brain.

Peripheral

The peripheral olfactory system consists mainly of the nostrils, ethmoid bone, nasal cavity, and the olfactory epithelium (layers of thin tissue covered in mucus that line the nasal cavity). The primary components of the layers of epithelial tissue are the mucous membranes, olfactory glands, olfactory neurons, and nerve fibers of the olfactory nerves.[1]

Odor molecules can enter the peripheral pathway and reach the nasal cavity either through the nostrils when inhaling (olfaction) or through the throat when the tongue pushes air to the back of the nasal cavity while chewing or swallowing (retro-nasal olfaction).[2] Inside the nasal cavity, mucus lining the walls of the cavity dissolves odor molecules. Mucus also covers the olfactory epithelium, which contains mucous membranes that produce and store mucus, and olfactory glands that secrete metabolic enzymes found in the mucus.[3]

Transduction

Action potential propagated by olfactory stimuli in an axon.
Action potential propagated by olfactory stimuli in an axon.

Olfactory sensory neurons in the epithelium detect odor molecules dissolved in the mucus and transmit information about the odor to the brain in a process called sensory transduction.[4][5] Olfactory neurons have cilia (tiny hairs) containing olfactory receptors that bind to odor molecules, causing an electrical response that spreads through the sensory neuron to the olfactory nerve fibers at the back of the nasal cavity.[2]

Olfactory nerves and fibers transmit information about odors from the peripheral olfactory system to the central olfactory system of the brain, which is separated from the epithelium by the cribriform plate of the ethmoid bone. Olfactory nerve fibers, which originate in the epithelium, pass through the cribriform plate, connecting the epithelium to the brain's limbic system at the olfactory bulbs.[6]

Central

Details of olfaction system
Details of olfaction system

The main olfactory bulb transmits pulses to both mitral and tufted cells, which help determine odor concentration based on the time certain neuron clusters fire (called 'timing code'). These cells also note differences between highly similar odors and use that data to aid in later recognition. The cells are different with mitral having low firing-rates and being easily inhibited by neighboring cells, while tufted have high rates of firing and are more difficult to inhibit.[7][8][9][10] How the bulbar neural circuit transforms odor inputs to the bulb to the bulbar responses that are sent to the olfactory cortex can be partly understood by a mathematical model.[11]

The uncus houses the olfactory cortex which includes the piriform cortex (posterior orbitofrontal cortex), amygdala, olfactory tubercle, and parahippocampal gyrus.

The olfactory tubercle connects to numerous areas of the amygdala, thalamus, hypothalamus, hippocampus, brain stem, retina, auditory cortex, and olfactory system. *In total it has 27 inputs and 20 outputs. An oversimplification of its role is to state that it: checks to ensure odor signals arose from actual odors rather than villi irritation, regulates motor behavior (primarily social and stereotypical) brought on by odors, integrates auditory and olfactory sensory info to complete the aforementioned tasks, and plays a role in transmitting positive signals to reward sensors (and is thus involved in addiction).[12][13][14]

The amygdala (in olfaction) processes pheromone, allomone, and kairomone (same-species, cross-species, and cross-species where the emitter is harmed and the sensor is benefited, respectively) signals. Due to cerebrum evolution this processing is secondary and therefore is largely unnoticed in human interactions.[15] Allomones include flower scents, natural herbicides, and natural toxic plant chemicals. The info for these processes comes from the vomeronasal organ indirectly via the olfactory bulb.[16] The main olfactory bulb's pulses in the amygdala are used to pair odors to names and recognize odor to odor differences.[17][18]

Stria terminalis, specifically bed nuclei (BNST), act as the information pathway between the amygdala and hypothalamus, as well as the hypothalamus and pituitary gland. BNST abnormalities often lead to sexual confusion and immaturity. BNST also connects to the septal area, rewarding sexual behavior.[19][20]

Mitral pulses to the hypothalamus promote/discourage feeding, whereas accessory olfactory bulb pulses regulate reproductive and odor-related-reflex processes.

The hippocampus (although minimally connected to the main olfactory bulb) receives almost all of its olfactory information via the amygdala (either directly or via the BNST). The hippocampus forms new and reinforces existing memories.

Similarly, the parahippocampus encodes, recognizes and contextualizes scenes.[21] The parahippocampal gyrus houses the topographical map for olfaction.

The orbitofrontal cortex (OFC) is heavily correlated with the cingulate gyrus and septal area to act out positive/negative reinforcement. The OFC is the expectation of reward/punishment in response to stimuli. The OFC represents the emotion and reward in decision making.[22]

The anterior olfactory nucleus distributes reciprocal signals between the olfactory bulb and piriform cortex.[23] The anterior olfactory nucleus is the memory hub for smell.[24]

When different odor objects or components are mixed, humans and other mammals sniffing the mixture (presented by, e.g., a sniff bottle) are often unable to identify the components in the mixture even though they can recognize each individual component presented alone.[25] This is largely because each odor sensory neuron can be excited by multiple odor components. It has been proposed that, in an olfactory environment typically composed of multiple odor components (e.g., odor of a dog entering a kitchen that contains a background coffee odor), feedback from the olfactory cortex to the olfactory bulb[26] suppresses the pre-existing odor background (e.g., coffee) via olfactory adaptation,[27] so that the newly arrived foreground odor (e.g., dog) can be singled out from the mixture for recognition.[28]

1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus 6: Olfactory receptor cells
1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus 6: Olfactory receptor cells

Discover more about Structure related topics

Nostril

Nostril

A nostril is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation. Fish do not breathe through noses, but they do have two small holes used for smelling, which can also be referred to as nostrils.

Ethmoid bone

Ethmoid bone

The ethmoid bone is an unpaired bone in the skull that separates the nasal cavity from the brain. It is located at the roof of the nose, between the two orbits. The cubical bone is lightweight due to a spongy construction. The ethmoid bone is one of the bones that make up the orbit of the eye.

Nasal cavity

Nasal cavity

The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal cavity is the uppermost part of the respiratory system and provides the nasal passage for inhaled air from the nostrils to the nasopharynx and rest of the respiratory tract.

Olfactory epithelium

Olfactory epithelium

The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 5 cm2 (0.78 sq in) and lies on the roof of the nasal cavity about 7 cm (2.8 in) above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.

Mucus

Mucus

Mucus is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is a viscous colloid containing inorganic salts, antimicrobial enzymes, immunoglobulins, and glycoproteins such as lactoferrin and mucins, which are produced by goblet cells in the mucous membranes and submucosal glands. Mucus serves to protect epithelial cells in the linings of the respiratory, digestive, and urogenital systems, and structures in the visual and auditory systems from pathogenic fungi, bacteria and viruses. Most of the mucus in the body is produced in the gastrointestinal tract.

Olfactory glands

Olfactory glands

Olfactory glands, also known as Bowman's glands, are a type of nasal gland situated in the part of the olfactory mucosa beneath the olfactory epithelium, that is the lamina propria, a connective tissue also containing fibroblasts, blood vessels and bundles of fine axons from the olfactory neurons.

Afferent nerve fiber

Afferent nerve fiber

Afferent nerve fibers are axons of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections arrive at a particular brain region.

Metabolic pathway

Metabolic pathway

In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reactions catalyzed by enzymes. In most cases of a metabolic pathway, the product of one enzyme acts as the substrate for the next. However, side products are considered waste and removed from the cell. These enzymes often require dietary minerals, vitamins, and other cofactors to function.

Olfactory receptor

Olfactory receptor

Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants which give rise to the sense of smell. Activated olfactory receptors trigger nerve impulses which transmit information about odor to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs). The olfactory receptors form a multigene family consisting of around 800 genes in humans and 1400 genes in mice.

Sensory neuron

Sensory neuron

Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord.

Cribriform plate

Cribriform plate

In mammalian anatomy, the cribriform plate, horizontal lamina or lamina cribrosa is part of the ethmoid bone. It is received into the ethmoidal notch of the frontal bone and roofs in the nasal cavities. It supports the olfactory bulb, and is perforated by olfactory foramina for the passage of the olfactory nerves to the roof of the nasal cavity to convey smell to the brain. The foramina at the medial part of the groove allow the passage of the nerves to the upper part of the nasal septum while the foramina at the lateral part transmit the nerves to the superior nasal concha.

Clinical significance

Loss of smell is known as anosmia. Anosmia can occur on both sides or a single side.

Olfactory problems can be divided into different types based on their malfunction. The olfactory dysfunction can be total (anosmia), incomplete (partial anosmia, hyposmia, or microsmia), distorted (dysosmia), or can be characterized by spontaneous sensations like phantosmia. An inability to recognize odors despite a normally functioning olfactory system is termed olfactory agnosia. Hyperosmia is a rare condition typified by an abnormally heightened sense of smell. Like vision and hearing, the olfactory problems can be bilateral or unilateral meaning if a person has anosmia on the right side of the nose but not the left, it is a unilateral right anosmia. On the other hand, if it is on both sides of the nose it is called bilateral anosmia or total anosmia.[29]

Destruction to olfactory bulb, tract, and primary cortex (brodmann area 34) results in anosmia on the same side as the destruction. Also, irritative lesion of the uncus results in olfactory hallucinations.

Damage to the olfactory system can occur by traumatic brain injury, cancer, infection, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. These conditions can cause anosmia. In contrast, recent finding suggested the molecular aspects of olfactory dysfunction can be recognized as a hallmark of amyloidogenesis-related diseases and there may even be a causal link through the disruption of multivalent metal ion transport and storage.[30] Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy. Doctors must exclude other diseases that inhibit or eliminate 'the sense of smell' such as chronic colds or sinusitus before making the diagnosis that there is permanent damage to the olfactory system.

Prevalence of olfactory dysfunction in the general US population was assessed by questionnaire and examination in a national health survey in 2012-2014.[31] Among over a thousand persons aged 40 years and older, 12.0% reported a problem with smell in the past 12 months and 12.4% had olfactory dysfunction on examination. Prevalence rose from 4.2% at age 40-49 to 39.4% at 80 years and older and was higher in men than women, in blacks and Mexican Americans than in whites and in less than more educated. Of concern for safety, 20% of persons aged 70 and older were unable to identify smoke and 31%, natural gas.

Causes of olfactory dysfunction

Vesalius' Fabrica, 1543. Human Olfactory bulbs and Olfactory tracts outlined in red
Vesalius' Fabrica, 1543. Human Olfactory bulbs and Olfactory tracts outlined in red

The common causes of olfactory dysfunction: advanced age, viral infections, exposure to toxic chemicals, head trauma, and neurodegenerative diseases.[29]

Age

Age is the strongest reason for olfactory decline in healthy adults, having even greater impact than does cigarette smoking. Age-related changes in smell function often go unnoticed and smell ability is rarely tested clinically unlike hearing and vision. 2% of people under 65 years of age have chronic smelling problems. This increases greatly between people of ages 65 and 80 with about half experiencing significant problems smelling. Then for adults over 80, the numbers rise to almost 75%.[32] The basis for age-related changes in smell function include closure of the cribriform plate,[29] and cumulative damage to the olfactory receptors from repeated viral and other insults throughout life.

Viral infections

The most common cause of permanent hyposmia and anosmia are upper respiratory infections. Such dysfunctions show no change over time and can sometimes reflect damage not only to the olfactory epithelium, but also to the central olfactory structures as a result of viral invasions into the brain. Among these virus-related disorders are the common cold, hepatitis, influenza and influenza-like illness, as well as herpes. Notably, COVID-19 is associated with olfactory disturbance.[33] Most viral infections are unrecognizable because they are so mild or entirely asymptomatic.[29]

Exposure to toxic chemicals

Chronic exposure to some airborne toxins such as herbicides, pesticides, solvents, and heavy metals (cadmium, chromium, nickel, and manganese), can alter the ability to smell.[34] These agents not only damage the olfactory epithelium, but they are likely to enter the brain via the olfactory mucosa.[35]

Head trauma

Trauma-related olfactory dysfunction depends on the severity of the trauma and whether strong acceleration/deceleration of the head occurred. Occipital and side impact causes more damage to the olfactory system than frontal impact.[36] However, recent evidence from individuals with traumatic brain injury suggests that smell loss can occur with changes in brain function outside of olfactory cortex. [37]

Neurodegenerative diseases

Neurologists have observed that olfactory dysfunction is a cardinal feature of several neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Most of these patients are unaware of an olfactory deficit until after testing where 85% to 90% of early-stage patients showed decreased activity in central odor processing structures.[38]

Other neurodegenerative diseases that affect olfactory dysfunction include Huntington's disease, multi-infarct dementia, amyotrophic lateral sclerosis, and schizophrenia. These diseases have more moderate effects on the olfactory system than Alzheimer's or Parkinson's diseases.[39] Furthermore, progressive supranuclear palsy and parkinsonism are associated with only minor olfactory problems. These findings have led to the suggestion that olfactory testing may help in the diagnosis of several different neurodegenerative diseases.[40]

Neurodegenerative diseases with well-established genetic determinants are also associated with olfactory dysfunction. Such dysfunction, for example, is found in patients with familial Parkinson's disease and those with Down syndrome.[41] Further studies have concluded that the olfactory loss may be associated with intellectual disability, rather than any Alzheimer's disease-like pathology.[42]

Huntington's disease is also associated with problems in odor identification, detection, discrimination, and memory. The problem is prevalent once the phenotypic elements of the disorder appear, although it is unknown how far in advance the olfactory loss precedes the phenotypic expression.[29]

Discover more about Clinical significance related topics

Anosmia

Anosmia

Anosmia, also known as smell blindness, is the loss of the ability to detect one or more smells. Anosmia may be temporary or permanent. It differs from hyposmia, which is a decreased sensitivity to some or all smells.

Hyposmia

Hyposmia

Hyposmia, or microsmia, is a reduced ability to smell and to detect odors. A related condition is anosmia, in which no odors can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma. In 2012 an estimated 9.8 million people aged 40 and older in the United States had hyposmia and an additional 3.4 million had anosmia/severe hyposmia.

Dysosmia

Dysosmia

Dysosmia is a disorder described as any qualitative alteration or distortion of the perception of smell. Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. Dysosmia can be classified as either parosmia or phantosmia. Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. Phantosmia is the perception of an odor when no odorant is present. The cause of dysosmia still remains a theory. It is typically considered a neurological disorder and clinical associations with the disorder have been made. Most cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. Dysosmia tends to go away on its own but there are options for treatment for patients that want immediate relief.

Agnosia

Agnosia

Agnosia is the inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is not defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia only affects a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual information.

Hyperosmia

Hyperosmia

Hyperosmia is an increased olfactory acuity, usually caused by a lower threshold for odor. This perceptual disorder arises when there is an abnormally increased signal at any point between the olfactory receptors and the olfactory cortex. The causes of hyperosmia may be genetic, hormonal, environmental or the result of benzodiazepine withdrawal syndrome.

Brodmann area 34

Brodmann area 34

Brodmann area 34 is a part of the brain.

Cancer

Cancer

Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bleeding, prolonged cough, unexplained weight loss, and a change in bowel movements. While these symptoms may indicate cancer, they can also have other causes. Over 100 types of cancers affect humans.

Alzheimer's disease

Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation, mood swings, loss of motivation, self-neglect, and behavioral issues. As a person's condition declines, they often withdraw from family and society. Gradually, bodily functions are lost, ultimately leading to death. Although the speed of progression can vary, the typical life expectancy following diagnosis is three to nine years.

Human

Human

Humans are the most common and widespread species of primate in the great ape family Hominidae, and also the most common species of primate overall. Humans are broadly characterized by their bipedalism and high intelligence. Humans' large brain and resulting cognitive skills have allowed them to thrive in a variety of environments and develop complex societies and civilizations. Humans are highly social and tend to live in complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. As such, social interactions between humans have established a wide variety of values, social norms, languages, and rituals, each of which bolsters human society. The desire to understand and influence phenomena has motivated humanity's development of science, technology, philosophy, mythology, religion, and other conceptual frameworks.

Olfactory bulb

Olfactory bulb

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

Common cold

Common cold

The common cold or the cold is a viral infectious disease of the upper respiratory tract that primarily affects the respiratory mucosa of the nose, throat, sinuses, and larynx. Signs and symptoms may appear fewer than two days after exposure to the virus. These may include coughing, sore throat, runny nose, sneezing, headache, and fever. People usually recover in seven to ten days, but some symptoms may last up to three weeks. Occasionally, those with other health problems may develop pneumonia.

Hepatitis

Hepatitis

Hepatitis is inflammation of the liver tissue. Some people or animals with hepatitis have no symptoms, whereas others develop yellow discoloration of the skin and whites of the eyes (jaundice), poor appetite, vomiting, tiredness, abdominal pain, and diarrhea. Hepatitis is acute if it resolves within six months, and chronic if it lasts longer than six months. Acute hepatitis can resolve on its own, progress to chronic hepatitis, or (rarely) result in acute liver failure. Chronic hepatitis may progress to scarring of the liver (cirrhosis), liver failure, and liver cancer.

History

Linda B. Buck and Richard Axel won the 2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system.

Source: "Olfactory system", Wikipedia, Wikimedia Foundation, (2023, March 6th), https://en.wikipedia.org/wiki/Olfactory_system.

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Further Reading

Anosmia

Olfactory nerve

Olfactory bulb

Olfactory epithelium

Glomerulus (olfaction)

Olfactory tubercle

Olfactory tract

Sense of smell
See also
References
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