Instruct: Given a question, retrieve passages that can help answer the question.
Query: How many olfactory sensors do humans have?
There are a large number of different odor receptors, with as many as 1,000 in the mammalian genome which represents approximately 3% of the genes in the genome. However, not all of these potential odor receptor genes are expressed and functional. According to an analysis of data derived from the Human Genome Project, humans have approximately 400 functional genes coding for olfactory receptors, and the remaining 600 candidates are pseudogenes.[26]
The Bloodhound's physical characteristics account for its ability to follow a scent trail left several days in the past. The olfactory bulb in dogs is roughly forty times bigger than the olfactory bulb in humans, relative to total brain size, with 125 to 220 million olfactory receptors. Consequently, dogs have an olfactory sense 40 times more sensitive than that of a human. In some dog breeds, such as bloodhounds, the olfactory sense has nearly 300 million receptors.
This part of brain receives sensations of smell.
As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons). As a result, it is generally assumed that it functions as a filter, as opposed to an associative circuit that has many inputs and many outputs. However, the olfactory bulb also receives "top-down" information from such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra.
Its potential functions can be placed into four non-exclusive categories:While all of these functions could theoretically arise from the olfactory bulb's circuit layout, it is unclear which, if any, of these functions are performed exclusively by the olfactory bulb. By analogy to similar parts of the brain such as the retina, many researchers have focused on how the olfactory bulb filters incoming information from receptor neurons in space, or how it filters incoming information in time. At the core of these proposed filters are the two classes of interneurons; the periglomerular cells, and the granule cells. Processing occurs at each level of the main olfactory bulb, beginning with the spatial maps that categorize odors in the glomeruli layer.
Some cats rub their faces on humans, apparently as a friendly greeting or indicating affection. This tactile action is combined with olfactory communication as the contact leaves scent from glands located around the mouth and cheeks. Cats also sometimes "head-bump" humans or other cats with the front part of the head; this action is referred to as "bunting". Again this communication might have an olfactory component as there are scent glands in this area of the body, and is possibly for seeking attention when the cat turns their head down or to the side while doing so.
A domestic cat's sense of smell is about fourteen times as strong as humans'. Cats have twice as many receptors in the olfactory epithelium (i.e. smell-sensitive cells in their noses) as people do, meaning that cats have a more acute sense of smell than humans. In fact cats have 200 million odor-sensitive cells in their noses where as humans only have 5 million odor-sensitive cells. Cats also have a scent organ in the roof of their mouths called the vomeronasal (or Jacobson's) organ. When a cat wrinkles its muzzle, lowers its chin, and lets its tongue hang a bit, it is opening the passage to the vomeronasal. This is called "gaping", "sneering", "snake mouth", or "flehming". Gaping is the equivalent of the Flehmen response in other animals, such as dogs, horses and big cats.
In most vertebrates, the olfactory bulb is the most rostral (forward) part of the brain, as seen in rats. In humans, however, the olfactory bulb is on the inferior (bottom) side of the brain. The olfactory bulb is supported and protected by the cribriform plate of the ethmoid bone, which in mammals separates it from the olfactory epithelium, and which is perforated by olfactory nerve axons. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb.
In vertebrates, smells are sensed by olfactory sensory neurons in the olfactory epithelium. The olfactory epithelium is made up of at least six morphologically and biochemically different cell types. The proportion of olfactory epithelium compared to respiratory epithelium (not innervated, or supplied with nerves) gives an indication of the animal's olfactory sensitivity. Humans have about of olfactory epithelium, whereas some dogs have . A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.
Olfactory receptor, family 6, subfamily C, member 75 is a protein in humans that is encoded by the OR6C75 gene.
Olfactory receptors interact with odorant molecules in the nose to initiate a neuronal response that triggers the perception of a smell. The olfactory receptor proteins are members of a large family of G-protein-coupled receptors (GPCR) arising from single coding-exon genes. Olfactory receptors share a 7-transmembrane domain structure with many neurotransmitter receptors and hormone receptors and are responsible for the recognition and G protein-mediated transduction of odorant signals. The olfactory receptor gene family is the largest in the genome. The nomenclature assigned to the olfactory receptor genes and proteins for this organism is independent of other organisms.
Olfactory memory has also been developed throughout evolution to help animals recognize other animals. It is suggested that smell allows for young infants to identify with their mothers or for humans to identify between males and females. Olfaction cues were also used, and are still used, by many animals to mark territory, protecting themselves from other threats to their survival. While the development of other sensory systems, such as the visual system and auditory systems, has decreased how reliant some animals are on the olfactory system, there is still evidence that shows these animals’ olfactory systems still have a strong influence on their social interactions. The memory for specific odorants gives the animal an opportunity to communicate with members of the same species and allows for lack of communication between species that do not have the proper receptor systems for the odor. These chemical signals can also be sensed in the dark or even under water.
Many animals, including most mammals and reptiles, but not humans, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an accessory olfactory system, which detects fluid-phase stimuli. Behavioral evidence suggests that these fluid-phase stimuli often function as pheromones, although pheromones can also be detected by the main olfactory system. In the accessory olfactory system, stimuli are detected by the vomeronasal organ, located in the vomer, between the nose and the mouth. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a facial expression called flehmen to direct stimuli to this organ.
Humans have about 10 million olfactory receptor neurons. In vertebrates, ORNs are bipolar neurons with dendrites facing the external surface of the cribriform plate with axons that pass through the cribriform foramina with terminal end at olfactory bulbs. The ORNs are located in the olfactory epithelium in the nasal cavity. The cell bodies of the ORNs are distributed among all three of the stratified layers of the olfactory epithelium.
The Human Olfactory Data Explorer (HORDE) is a database of human olfactory receptors. The database provides information of the human olfactory receptor families, as well for dogs, platypuses, opossums and chimpanzees. The database is hosted at the Weizmann Institute of Science.
Two major classes of olfactory receptors have been identified in humans:[34]
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. 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. The main olfactory bulb's pulses in the amygdala are used to pair odors to names and recognize odor to odor differences.
Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite into the mucus covering the surface of the olfactory epithelium. The surface of these cilia is covered with olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell expresses only one type of olfactory receptor (OR), but many separate olfactory receptor cells express ORs which bind the same set of odors. The axons of olfactory receptor cells which express the same OR converge to form glomeruli in the olfactory bulb.
Human behaviour genetics is a subfield of the field of behaviour genetics that studies the role of genetic and environmental influences on human behaviour. Classically, human behavioural geneticists have studied the inheritance of behavioural traits. The field was originally focused on testing whether genetic influences were important in human behavior (e.g., do genes influence human behavior). It has evolved to address more complex questions such as: how important are genetic and/or environmental influences on various human behavioral traits; to what extent do the same genetic and/or environmental influences impact the overlap between human behavioral traits; how do genetic and/or environmental influences on behavior change across development; and what environmental factors moderate the importance of genetic effects on human behavior (gene-environment interaction). The field is interdisciplinary, and draws from genetics, psychology, and statistics. Most recently, the field has moved into the area of statistical genetics, with many behavioral geneticists also involved in efforts to identify the specific genes involved in human behavior, and to understand how the effects associated with these genes changes across time, and in conjunction with the environment.
The olfactory tubercle differs in location and relative size between humans, non-human primates, rodents, birds, and other animals. In most cases, the olfactory tubercle is identified as a round bulge along the basal forebrain anterior to the optic chiasm and posterior to the olfactory peduncle. In humans and non-human primates, visual identification of the olfactory tubercle is not easy because the basal forebrain bulge is small in these animals. With regard to functional anatomy, the olfactory tubercle can be considered to be a part of three larger networks. First, it is considered to be part of the basal forebrain, the nucleus accumbens, and the amygdaloid nuclei because of its location along the rostral ventral region of the brain, that is, the front-bottom part. Second, it is considered to be part of the olfactory cortex because it receives direct input from the olfactory bulb. Third, it is also considered to be part of the ventral striatum based on anatomy, neurochemical, and embryology data.
The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate, a sieve-like structure of the ethmoid bone.
The olfactory bulb 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 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 amygdala passes olfactory information on to the hippocampus. The orbitofrontal cortex, amygdala, hippocampus, thalamus, and olfactory bulb have many interconnections directly and indirectly through the cortices of the primary olfactory cortex. These connections are indicative of the association between the olfactory bulb and higher areas of processing, specifically those related to emotion and memory.
The glomerulus process in dogs is divided in 3 parts. signal acquisition, signal transduction, and signal processing.Some dogs have as many as 100 times more ORNs than humans do, producing a correspondingly sharpened ability to detect and discriminate among millions of odors. Characteristics of these stages include: the role of olfactory plume and sniffing, the continuous renewal of Glomerulus receptors throughout the life cycle, and the relationship between the olfactory neuron and the glomerulus, and finally, the synthetic nature of glomerulus coding.
All glomeruli are located near the surface of the olfactory bulb. The olfactory bulb also includes a portion of the anterior olfactory nucleus, the cells of which contribute fibers to the olfactory tract. They are the initial sites for synaptic processing of odor information coming from the nose. A glomerulus is made up of a globular tangle of axons from the olfactory receptor neurons, and dendrites from the mitral and tufted cells, as well as, from cells that surround the glomerulus such as the external tufted cells, periglomerular cells, short axon cells, and astrocytes. In mammals, glomeruli typically range between 50-120 µm in diameter and number between 1100 and 2400 depending on the species, with roughly between 1100 and 1200 in humans. The number of glomeruli in a human decreases with age; in humans that are over 80 they are nearly absent. Each glomerulus is composed of two compartments, the olfactory nerve zone and the non-olfactory nerve zone. The olfactory nerve zone is composed of preterminals and terminals of the olfactory nerve and is where the olfactory receptor cells make synapses on their targets. The non-olfactory nerve zone is composed of the dendritic processes of intrinsic neurons and is where dendrodendritic interactions between intrinsic neurons occur.
The multi-sensory nature of the olfactory tubercle and the many innervations it receives from other brain regions, especially the direct input from the olfactory bulb and innervations from the ventral tegmental area, makes it likely to be involved in several psychiatric disorders in which olfaction and dopamine receptors are affected. Many studies have found reduced olfactory sensitivity in patients with major depressive disorders (MDD) and dementia and schizophrenia. Patients with MDD have been shown to have reduced olfactory bulb and olfactory cortex as compared to normal people. In dementias, especially of the Alzheimer's disease type, the olfactory bulb, anterior olfactory nucleus, and orbitofrontal cortex, all areas of the brain that process olfaction are affected. The deficits observed in dementia include decrease in odor threshold sensitivity, odor identification and odor memory. Patients with schizophrenia exhibit deficits in olfactory discrimination that are not seen in patients with other psychiatric disorders not mentioned here. Rupp, et al. found that in patients with schizophrenia olfactory sensitivity and discrimination as well as higher order identification abilities are reduced. As mentioned earlier, the olfactory tubercle may be involved in the perception of odors due to the inputs received from the bulb and thus, by extension, may play a role in these psychiatric disorders.
Olfactory receptor 52M1 is a protein that in humans is encoded by the "OR52M1" gene.
Olfactory receptors interact with odorant molecules in the nose, to initiate a neuronal response that triggers the perception of a smell. The olfactory receptor proteins are members of a large family of G-protein-coupled receptors (GPCR) arising from single coding-exon genes. Olfactory receptors share a 7-transmembrane domain structure with many neurotransmitter and hormone receptors and are responsible for the recognition and G protein-mediated transduction of odorant signals. The olfactory receptor gene family is the largest in the genome. The nomenclature assigned to the olfactory receptor genes and proteins for this organism is independent of other organisms.
The fundamental principle of Sardar's thought is that 'there is more than one way to be human'. 'I do not regard "the human" either as "the" or as a priori given', he has said. 'The western way of being human is one amongst many. Similarly, the Islamic way of being human is also one amongst many. The Australian aboriginal way of being human is also another way of being human. I see each culture as a complete universe with its own way of knowing, being and doing – and hence, its own way of being human'. The corollary is that there are also different ways of knowing. The question that Sardar has always asked is: 'how do you know? The answer depends a great deal on who 'you' are: 'how you look at the world, how you shape your inquiry, the period and culture that shapes your outlook and the values that frame how you think'.
Olfactory adult stem cells have been successfully harvested from the human olfactory mucosa cells, which are found in the lining of the nose and are involved in the sense of smell.[20] If they are given the right chemical environment these cells have the same ability as embryonic stem cells to develop into many different cell types. Olfactory stem cells hold the potential for therapeutic applications and, in contrast to neural stem cells, can be harvested with ease without harm to the patient. This means they can be easily obtained from all individuals, including older patients who might be most in need of stem cell therapies.