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The olfactory bulb is a structure of the vertebrate
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
 forebrain involved in olfaction
Olfaction

Olfaction refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates....
, the perception of odors.

ost vertebrates, the olfactory bulb is the most rostral
Rostral

Rostral can refer to:* Anatomical terms of location#Directions, Top or cephaled, as opposed to caudal or down.* Rostral bone* Rostral column, a monumental scaled column, frequently decorated with ship bows...
 (forward) part of the brain. 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
Ethmoid bone

The ethmoid bone is a bone in the skull that separates the nasal cavity from the brain. As such, it is located at the roof of the nose, between the two Orbit s....
 which in mammals, separates it from the olfactory epithelium
Olfactory epithelium

The olfactory epithelium is a specialized epithelium tissue inside the nasal cavity that is involved in olfaction. In humans, it measures about 2 cm by 5 cm long and lies on the roof of the nasal cavity about 3 inches above and behind the nostrils....
, and which is perforated by olfactory nerve
Olfactory nerve

The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity....
 axons.






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The olfactory bulb is a structure of the vertebrate
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
 forebrain involved in olfaction
Olfaction

Olfaction refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates....
, the perception of odors.

Anatomy

In most vertebrates, the olfactory bulb is the most rostral
Rostral

Rostral can refer to:* Anatomical terms of location#Directions, Top or cephaled, as opposed to caudal or down.* Rostral bone* Rostral column, a monumental scaled column, frequently decorated with ship bows...
 (forward) part of the brain. 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
Ethmoid bone

The ethmoid bone is a bone in the skull that separates the nasal cavity from the brain. As such, it is located at the roof of the nose, between the two Orbit s....
 which in mammals, separates it from the olfactory epithelium
Olfactory epithelium

The olfactory epithelium is a specialized epithelium tissue inside the nasal cavity that is involved in olfaction. In humans, it measures about 2 cm by 5 cm long and lies on the roof of the nasal cavity about 3 inches above and behind the nostrils....
, and which is perforated by olfactory nerve
Olfactory nerve

The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity....
 axons. The bulb is divided into two distinct structures, the main olfactory bulb, and the accessory olfactory bulb.

Main olfactory bulb

The main olfactory bulb has a multi-layered cellular architecture. In order from the surface to the center of the bulb the layers are
  • Glomerular layer
    Glomerulus (olfaction)

    The glomerulus in olfaction is a structure in the olfactory bulb. It is made up of a globular tangle of axons from the olfactory receptor neurons in the olfactory epithelium and dendrites from the mitral cells, tufted cells, and other cell types....
  • External plexiform layer
  • Mitral cell
    Mitral cell

    Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammal central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomerulus ....
     layer
  • Internal plexiform layer
  • Granule cell
    Granule cell

    In neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the Cerebellum#Granular_Layer of the cerebellum, layer 4 of cerebral cortex, the dentate gyrus of the hippocampus, and in the olfactory bulb....
     layer
The glomerular layer receives direct input from olfactory nerves, made up of the axons from approximately ten million olfactory receptor neuron
Olfactory receptor neuron

An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell in the olfactory system....
s in the olfactory mucosa
Olfactory mucosa

The olfactory mucosa is an organ made up of the olfactory epithelium and the lamina propria, or mucus secreting glands, behind the epithelium. The mucus protects the olfactory epithelium and allows odors to dissolve so that they can be detected by olfactory receptor neurons....
, a region of the 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 ends of the axons cluster in spherical structures known as glomeruli
Glomerulus (olfaction)

The glomerulus in olfaction is a structure in the olfactory bulb. It is made up of a globular tangle of axons from the olfactory receptor neurons in the olfactory epithelium and dendrites from the mitral cells, tufted cells, and other cell types....
 such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor
Olfactory receptor

Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain....
. Glomeruli are also permeated by dendrites from neurons called mitral cell
Mitral cell

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammal central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomerulus ....
s, which in turn output to the olfactory cortex
Piriform cortex

In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain. The piriform cortex is part of the rhinencephalon situated in the telencephalon....
. Numerous interneuron types exist in the olfactory bulb including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells.

Accessory olfactory bulb

The accessory olfactory bulb, which resides on the dorsal-posterior region of the main olfactory bulb, forms a parallel pathway independent from the main olfactory bulb. It is the second processing stage of the accessory olfactory system
Accessory olfactory system

The Accessory olfactory system is one of the two olfactory systems commonly found in vertebrates. Like the Olfaction , the accessory olfactory system is a chemosensory system, which transduction chemicals into neural activity....
. It receives axonal input from the vomeronasal organ
Vomeronasal organ

The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Ludvig Jacobson in 1813....
, a distinct sensory epithelium from the main olfactory epithelium
Olfactory epithelium

The olfactory epithelium is a specialized epithelium tissue inside the nasal cavity that is involved in olfaction. In humans, it measures about 2 cm by 5 cm long and lies on the roof of the nasal cavity about 3 inches above and behind the nostrils....
 that detects pheremones, among other chemical stimuli. Like the main olfactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. However, mitral cells in the accessory olfactory bulb project their axons to targets in the amygdala
Amygdala

The are almond-shaped groups of neurons located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotions, the amygdalae are considered part of the limbic system....
 and hypothalamus
Hypothalamus

The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
 where they may influence aggressive and mating behavior.

Function

Gray858
The olfactory bulb transmits smell information from the nose to the brain, and is thus necessary for a proper sense 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
Electronic filter

Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal and/or to enhance wanted ones....
, 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
Amygdala

The are almond-shaped groups of neurons located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotions, the amygdalae are considered part of the limbic system....
, neocortex
Neocortex

The neocortex is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI ....
, hippocampus
Hippocampus

The hippocampus is a brain structure located inside the medial temporal lobe of the cerebral cortex, and therefore is part of the telencephalon ....
, locus coeruleus, and substantia nigra
Substantia nigra

The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in dopaminergic neurons....
. With this in mind, its potential functions can be placed into four non-exclusive categories:
  • enhancing discrimination between odors.
  • enhancing sensitivity of odor detection.
  • filtering out many background odors to enhance the transmission of a few select odors.
  • permitting higher brain areas involved in arousal and attention to modify the detection or the discrimination of odors.


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
Retina

The vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera....
, 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.

The basal dendrites of mitral cells are connected to interneurons known as granule cells, which by some theories produce lateral inhibition
Lateral inhibition

In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors....
 between mitral cells. It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non-activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro-dendritic" which means that both sides of the synapse are dendrites that release neurotransmitter. In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid
Gamma-aminobutyric acid

γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system....
 (GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto-inhibition), as well as neighboring mitral cells (lateral inhibition).

Olfaction is distinct from the other sensory system
Sensory system

A sensory system is a part of the nervous system responsible for processing sense information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception....
s where peripheral sensory receptor
Sensory receptor

In a sensory system, a sensory receptor is a sensory nerve ending that recognizes a stimulus in the internal or external environment of an organism....
s have a relay in the diencephalon
Diencephalon

The diencephalon is the region of the brain that includes the thalamus, hypothalamus, epithalamus, prethalamus or subthalamus and pretectum. The diencephalon is located at the midline of the brain, above the mesencephalon of the brain stem....
. Therefore the olfactory bulb plays this role for the olfactory system
Olfactory system

The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system....
.

Evolution

Comparing the structure of the olfactory bulb between species such as the fruit fly
Drosophila melanogaster

Drosophila melanogaster is a two-winged insect that belongs to the Diptera, the Order of the Fly. The species is commonly known as the Drosophilidae or vinegar fly, and is one of the most commonly used model organisms in biology, including studies in genetics, physiology and Life history theory....
 (where it is called the antennal lobe
Antennal lobe

Antennal lobe is the deutocerebral neuropil of the insect which receives the input from the olfactory sensory neurons on the antenna . Functionally, it shares some similarities with the olfactory bulb in vertebrates....
), the Leopard Frog and the lab mouse reveals that they all share the same fundamental layout (five layers containing the nuclei of three major cell types, see "Anatomy" for details), despite being dissimilar in shape and size. It is quite possible, however, that function has not been conserved with structure, indicating that olfactory bulbs of distant species may be paralogs of each other. An intriguing alternative possibility states olfactory bulb structure is conserved because it contains an optimal solution to a computational problem experienced by all olfactory systems, and thus may have evolved independently in different species.

Additional images


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