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.
An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system.
The olfactory epithelium is a specialized epithellial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.
The olfactory mucosa is located in the upper region of the nasal cavity and is made up of the olfactory epithelium and the underlying lamina propria, connective tissue containing fibroblasts, blood vessels, Bowman's glands and bundles of fine axons from the olfactory neurons.
The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons and glial cells.
Odotope theory, also known as weak shape theory, is a theory of how olfactory receptors bind to odor molecules. The theory proposes that a combination of shape factors determine the coupling. The word itself is an analogy to epitopes.
Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs still a matter of controversy. One prominent hypothesis is the notion that mitral cells transform the strength of olfactory input into a timing code, where odor concentration is encoded in the phase of mitral cell firing relative to the sniff cycle. A second hypothesis is the idea of decorrelation in the olfactory bulb network, where the olfactory bulb network acts as a dynamical system whose action over time increases some (abstract) measure of distance between representations of highly similar odorants. Support for the second hypothesis comes primarily from research in zebrafish.
The olfactory tubercle (OT), also known as the tuberculum olfactorium, is a multi-sensory processing center that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. OT has also been shown to play a role in locomotor and attentional behaviors, particularly in relation to social and sensory responsiveness, and it may be necessary for behavioral flexibility. The OT is interconnected with numerous brain regions, especially the sensory, arousal, and reward centers, thus making it a potentially critical interface between processing of sensory information and the subsequent behavioral responses.
The olfactory tract is a bilateral bundle of afferent nerve fibers from the mitral and tufted cells of the olfactory bulb that connects to several target regions in the brain, including the piriform cortex, amygdala, and entorhinal cortex. It is a narrow white band, triangular on coronal section, the apex being directed upward.
The anterior olfactory nucleus is a portion of the forebrain of vertebrates.
Plateau potentials, caused by persistent inward currents (PICs), are a type of electrical behavior seen in neurons.
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.
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.
Olfaction is a chemoreception that forms the sense of smell. Olfaction has many purposes, such as the detection of hazards, pheromones, and food. It integrates with other senses to form the sense of flavor.
Gordon Murray Shepherd is a neuroscientist who has carried out basic experimental and computational research on how neurons are organized into microcircuits to carry out the functional operations of the nervous system. Using the olfactory system as a model that spans multiple levels of space, time and disciplines, his studies have ranged from molecular to behavioral, recognized by an annual lecture at Yale University on "integrative neuroscience". He is currently professor of neuroscience emeritus at the Yale School of Medicine.
Guidepost cells are cells which assist in the subcellular organization of both neural axon growth and migration. They act as intermediate targets for long and complex axonal growths by creating short and easy pathways, leading axon growth cones towards their target area.
Tufted cells are found within the olfactory glomeruli. They receive input from the receptor cells of the olfactory epithelium found in areas of the nose able to sense smell. Both tufted cells and mitral cells are called projection neurons. Projection neurons send the signals from the glomeruli deeper into the brain. The actual signal sent through these projection cells has been sharpened or filtered by a process called lateral inhibition. Both the periglomerular cells and the granule cells contribute to lateral inhibition. Projection neurons therefore transmit a sharpened olfactory signal to the deeper parts of the brain. Tufted cells project onto the anterior piriform cortex.
Retronasal smell, retronasal olfaction, or mouth smell, is the ability to perceive flavor dimensions of foods and drinks. Retronasal smell is a sensory modality that produces flavor. It is best described as a combination of traditional smell and taste modalities. Retronasal smell creates flavor from smell molecules in foods or drinks shunting up through the nasal passages as one is chewing. When people use the term "smell", they are usually referring to "orthonasal smell", or the perception of smell molecules that enter directly through the nose and up the nasal passages. Retronasal smell is critical for experiencing the flavor of foods and drinks. Flavor should be contrasted with taste, which refers to five specific dimensions: (1) sweet, (2) salty, (3) bitter, (4) sour, and (5) umami. Perceiving anything beyond these five dimensions, such as distinguishing the flavor of an apple from a pear for example, requires the sense of retronasal smell.
