Molly Ayala
The olfactory bulb is a structure located in the forebrain of vertebrates that is responsible for receiving and processing neural input about odours detected by cells in the nasal cavity. The olfactory nerve, which is the first cranial nerve, is responsible for transmitting olfactory information to the olfactory bulb. This nerve contains olfactory receptor cells, which are stimulated by tiny hairs in the olfactory mucosa. The olfactory bulb contains a layer of glomeruli, which are spherical structures formed by the branching ends of axons of receptor cells. These glomeruli are thought to respond selectively to distinct odorants. While the olfactory bulb is involved in filtering incoming information from receptor neurons, it is unclear if this is its exclusive function. So, does the olfactory bulb contain electrical receptors?
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What You'll Learn
Olfactory receptor neurons
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. Many tiny hair-like non-motile cilia protrude from the olfactory receptor cell's dendrites. The dendrites extend to the olfactory epithelial surface and each ends in a dendritic knob from which around 20 to 35 cilia protrude. The cilia have a length of up to 100 micrometres and with the cilia from other dendrites form a meshwork in the olfactory mucus. The surface of the cilia is covered with olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell expresses only one type of OR, but many separate olfactory receptor cells express ORs that bind the same set of odours.
The olfactory epithelium includes several distinct cell types. The most important of these is the ORN, a bipolar cell that gives rise to a small-diameter, unmyelinated axon at its basal surface that transmits olfactory information centrally. At its apical surface, the receptor neuron gives rise to a single process that expands into a knob-like protrusion from which several microvilli, called olfactory cilia, extend into a thick layer of mucus. The mucus that lines the nasal cavity and controls the ionic milieu of the olfactory cilia is produced by secretory specializations (called Bowman's glands) distributed throughout the epithelium. The olfactory epithelium also includes sustentacular cells (that detoxify potentially dangerous chemicals) and basal cells.
The axons of the ORNs converge onto a few specific glomeruli within the olfactory bulb. The glomeruli layer of the olfactory bulb is the first level of synaptic processing. The glomeruli layer represents a spatial odour map organized by the chemical structure of odorants like functional group and carbon chain length. This spatial map is divided into zones and clusters, which represent similar glomeruli and therefore similar odours.
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Olfactory nerve
The olfactory nerve is the first and shortest cranial nerve, responsible for the sense of smell. Embryologically, the olfactory nerve is derived from the olfactory placode, a thickening of the ectoderm layer, which also gives rise to the glial cells that support the nerve. The olfactory nerve is a special visceral afferent nerve, which transmits information relating to smell. The olfactory nerve is purely a sensory nerve and does not have a precortical connection to the thalamus.
The olfactory nerve contains only afferent sensory nerve fibres and, like all cranial nerves, is paired. It is the only cranial nerve that does not converge with the brainstem, along with the optic nerve. The olfactory nerve is not myelinated by Schwann cells but is ensheathed by olfactory ensheathing glia. The olfactory nerve fibres synapse with mitral cells, forming collections known as synaptic glomeruli. The olfactory nerve originates from the cell bodies of bipolar olfactory neurons in the olfactory epithelium, a specialised epithelial tissue found in the posterosuperior portion of each nasal cavity. The olfactory mucosa and olfactory neurons are located in the roof of the nasal cavity and receive an abundant blood supply from branches of the carotid arteries.
The olfactory bulb is a relay station for the transmission of impulses from the olfactory nerve to the olfactory tract and then to the cerebral cortex (olfactory cortex). The olfactory bulb is an ovoid structure that contains specialised neurons, called mitral cells. The olfactory bulb acts as a filter, as opposed to an associative circuit with many inputs and outputs. The olfactory bulb has a clear laminar structure, with the olfactory nerve layer being the innermost layer. The glomerular layer of the olfactory bulb is the first level of synaptic processing, with the glomeruli representing a spatial odour map. The spatial map is divided into zones and clusters, which represent similar glomeruli and, therefore, similar odours.
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Olfactory epithelium
The olfactory epithelium is a specialised epithelial tissue inside the nasal cavity that is involved in the sense of 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 odours.
The olfactory epithelium consists of four distinct cell types. The olfactory receptor neurons are sensory neurons of the olfactory epithelium. They are bipolar neurons and their apical poles express odourant receptors on non-motile cilia at the ends of the dendritic knob, which extend out into the airspace to interact with odourants. Olfactory receptor neurons are the most important cell type in the olfactory epithelium. The odourant receptors bind odourants in the airspace, which are made soluble by the serous secretions from olfactory glands located in the lamina propria of the mucosa. The axons of the olfactory sensory neurons congregate to form the olfactory nerve (CN I).
The sustentacular cells function as metabolic and physical support for the olfactory epithelium. They are analogous to neural glial cells and are located in the apical layer of the pseudostratified ciliated columnar epithelium. Microvillar cells are another class of supporting cells that are morphologically and biochemically distinct from the sustentacular cells, and arise from a basal cell population that expresses the c-KIT cell surface protein.
Basal cells are stem cells capable of division and differentiation into either supporting or olfactory cells. They can be divided on the basis of their cellular and histological features into two populations: the horizontal basal cells, which are slowly dividing reserve cells that express p63; and globose basal cells, which are a heterogeneous population of cells consisting of reserve cells, amplifying progenitor cells, and immediate precursor cells.
The olfactory epithelium derives from two structures during embryonic development: the nasal placodes and neural crest cells. The embryonic olfactory epithelium consists of fewer cell types than in adults, including apical and basal progenitor cells, as well as immature olfactory sensory neurons. Early embryonic neurogenesis relies mostly on the apical cells, while later stage embryonic neurogenesis and secondary neurogenesis in adults relies on basal stem cells.
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Olfactory cortex
The olfactory cortex is the portion of the cerebral cortex concerned with the sense of smell. It is part of the cerebrum and is structurally distinct, found on the ventral surface of the forebrain. The olfactory cortex includes the piriform lobe and the hippocampal formation. It is a component of the limbic system, which is involved in the processing of emotions, survival instincts, and memory formation. The olfactory cortex connects senses, such as odours, to memories and emotions.
The olfactory cortex is defined as the cortical region of the mammalian brain that receives direct input from the olfactory bulb. The olfactory bulb is a structure located in the forebrain of vertebrates that receives neural input about odours detected by cells in the nasal cavity. The olfactory bulb has been described as a cortical structure, and the olfactory cortex is, therefore, sometimes referred to as a secondary olfactory structure. The olfactory cortex receives mitral and tufted cell projections from the olfactory bulb. The olfactory bulb's output passes through the olfactory tract to the ipsilateral piriform cortex, the amygdala, and the rostral entorhinal cortex.
The olfactory cortex is involved in the perception of odours. It is also involved in sharpening the contrast between different odours, as the cells of the piriform cortex rapidly habituate to familiar odours but not to unfamiliar ones. The orbitofrontal cortex (OFC), traditionally considered a secondary olfactory cortex, is involved in establishing the reward value of odants. For example, neurons in this region decrease their response to the odour of food after satiety.
Smell training has been shown to improve olfactory function in older people, as well as their verbal function and overall well-being. This has implications for the treatment of neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease, where olfactory dysfunction is one of the initial symptoms.
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Olfactory functions
The olfactory bulb is a structure located in the forebrain of vertebrates that receives input about odours detected by cells in the nasal cavity. It is a neural circuit with one source of sensory input: axons from olfactory receptor neurons of the olfactory epithelium. The olfactory system serves multiple functions in humans, including influencing ingestive behaviour, increasing awareness of environmental hazards, and facilitating social communication.
The olfactory bulb contains a glomeruli layer, which is a spatial odour map organised by the chemical structure of odorants, such as functional groups and carbon chain length. This layer is the first level of synaptic processing, and it represents similar odours in zones and clusters. One specific cluster is associated with rank, spoiled smells, which may be an evolutionary mechanism to help identify food that is no longer edible.
The olfactory bulb also contains the granule cell layer, which is the deepest layer. It is made up of dendrodendritic granule cells that synapse to the mitral cell layer. This part of the brain receives sensations of smell. The mitral cells, which are a type of interneuron, output to the olfactory cortex. The olfactory bulb functions as a filter, processing information about odours.
The olfactory system is important for detecting food and ensuring good taste quality, as well as avoiding dangerous situations by identifying odours associated with fire and microbial threats. It also plays a role in mate choice and detecting emotions in others.
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Frequently asked questions
The olfactory bulb is a structure located in the forebrain of vertebrates that receives neural input about odours detected by cells in the nasal cavity.
The olfactory bulb contains discrete spheres of nerve tissue called glomeruli. They are formed from the branching ends of axons of receptor cells and from the outer (dendritic) branches of interneurons, known in vertebrates as mitral cells, that pass information to other parts of the brain.
The olfactory bulb processes olfactory signals and transmits olfactory information to the brain. It is also believed to function as a filter, as opposed to an associative circuit with multiple inputs and outputs.
The olfactory bulb does not contain electrical receptors. However, it does contain mitral cells, which are the principal projection neurons of the olfactory bulb. These mitral cells receive input from olfactory receptor neurons, which express a distinct odorant receptor molecule.
