Olfactory Bulb
   HOME

TheInfoList



OR:

The olfactory bulb (
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
: ''bulbus olfactorius'') is a
neural structure In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...
of the
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
forebrain In the anatomy of the brain of vertebrates, the forebrain or prosencephalon is the Anatomical terms of location#Directional terms, rostral (forward-most) portion of the brain. The forebrain (prosencephalon), the midbrain (mesencephalon), and hin ...
involved in
olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...
, the sense of smell. It sends olfactory information to be further processed in the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
, the
orbitofrontal cortex The orbitofrontal cortex (OFC) is a prefrontal cortex region in the frontal lobes of the brain which is involved in the cognitive process of decision-making. In non-human primates it consists of the association cortex areas Brodmann area 11, 12 ...
(OFC) and the
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
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 The piriform cortex, or pyriform cortex, is a region in the brain, part of the rhinencephalon situated in the cerebrum. The function of the piriform cortex relates to the sense of smell. Structure The piriform cortex is part of the rhinencepha ...
of the
primary olfactory cortex The primary olfactory cortex (POC) is a portion of the cerebral cortex. It is found in the inferior part of the temporal lobe of the brain. It receives input from the olfactory tract. It is involved in the sense of smell (olfaction). Structure ...
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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
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. Anosmia can be due to a nu ...
, while irritative
lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classifi ...
s of the
uncus The uncus is an anterior extremity of the parahippocampal gyrus. It is separated from the apex of the temporal lobe by a slight fissure called the incisura temporalis (also called rhinal sulcus). Although superficially continuous with the hippo ...
can result in olfactory and
gustatory The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
hallucinations.


Structure

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 In mammalian anatomy, the cribriform plate (Latin for lit. ''sieve-shaped''), 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 supp ...
of the
ethmoid bone The ethmoid bone (; from grc, ἡθμός, hēthmós, sieve) 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 ...
, which in mammals separates it from the
olfactory epithelium The olfactory epithelium is a specialized epithelial 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 ...
, and which is perforated by
olfactory nerve The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell. The afferent nerve fibers of the olfactory receptor neurons tr ...
axons. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb.


Layers

The main olfactory bulb has a multi-layered
cellular architecture A cellular architecture is a type of computer architecture prominent in parallel computing. Cellular architectures are relatively new, with IBM's Cell microprocessor being the first one to reach the market. Cellular architecture takes mul ...
. In order from surface to the center the layers are: * Glomerular layer *External plexiform layer *
Mitral cell 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 ca ...
layer *Internal plexiform layer *Granule cell layer 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 A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. Neural circuits interconnect to one another to form large scale brain networks. Biological neural networks have inspired the ...
, the glomerular layer receives direct input from
afferent nerves A sensory nerve, or afferent nerve, is a general anatomic term for a nerve which contains predominantly somatic afferent nerve fibers. Afferent nerve fibers in a sensory nerve carry sensory information toward the central nervous system (CNS) from ...
, made up of the axons from approximately ten million
olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are b ...
s in the
olfactory mucosa 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 ax ...
, a region of the
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 c ...
. The ends of the axons cluster in spherical structures known as
glomeruli ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...
such that each glomerulus receives input primarily from olfactory receptor neurons that express the same
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 (for example, compounds that have an odor) which give ri ...
. The glomeruli layer of the olfactory bulb is the first level of synaptic processing. The glomeruli layer represents a spatial odor map organized by chemical structure of
odorants An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vol ...
like
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
and carbon chain length. This spatial map is divided into zones and clusters, which represent similar glomeruli and therefore similar odors. One cluster in particular is associated with rank, spoiled smells which are represented by certain chemical characteristics. This classification may be evolutionary to help identify food that is no longer good to eat. The spatial map of the glomeruli layer may be used for perception of odor in the olfactory cortex. The next level of synaptic processing in the olfactory bulb occurs in the external plexiform layer, between the glomerular layer and the mitral cell layer. The external plexiform layer contains
astrocytes Astrocytes (from Ancient Greek , , "star" + , , "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of endo ...
,
interneurons Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. In ...
and some mitral cells. It does not contain many
cell bodies The soma (pl. ''somata'' or ''somas''), perikaryon (pl. ''perikarya''), neurocyton, or cell body is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus. The word 'soma' comes from the Greek '' σῶ ...
, rather mostly
dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
of mitral cells and GABAergic granule cells are also permeated by dendrites from neurons called
mitral cell 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 ca ...
s, which in turn output to the
olfactory cortex 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 ac ...
. Numerous interneuron types exist in the olfactory bulb including
periglomerular cell Periglomerular cells mediate lateral inhibition in the olfactory system together with granule cells. They have inhibitory synapses on mitral cells and tufted cell Tufted cells are found within the olfactory glomeruli. They receive input from the ...
s which synapse within and between glomeruli, and granule cells which synapse with mitral cells. The granule cell layer is the deepest layer in the olfactory bulb. It is made up of dendrodendritic granule cells that synapse to the mitral cell layer.


Function

This part of the 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 Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
, 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
olfactory cortex 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 ac ...
,
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
,
neocortex The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...
,
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
,
locus coeruleus The locus coeruleus () (LC), also spelled locus caeruleus or locus ceruleus, is a nucleus in the pons of the brainstem involved with physiological responses to stress and panic. It is a part of the reticular activating system. The locus coerule ...
, and
substantia nigra The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. ''Substantia nigra'' is Latin for "black substance", reflecting the fact that parts of the substantia nigra app ...
. Its potential functions can be placed into four non-exclusive categories: *discriminating among 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 The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
, 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. Interneurons in the external plexiform layer are responsive to pre-synaptic action potentials and exhibit both
excitatory postsynaptic potentials In neuroscience, an excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of postsynaptic membrane potential, caused by the ...
and inhibitory postsynaptic potentials. Neural firing varies temporally, there are periods of fast, spontaneous firing and slow modulation of firing. These patterns may be related to sniffing or change in intensity and concentration of odorant. Temporal patterns may have effect in later processing of spatial awareness of odorant. For example, synchronized mitral cell spike trains appear to help to discriminate similar odors better than when those spike trains are not synchronized. A well known model is that the bulbar neural circuit transforms the odor information in the receptors to a population pattern of neural oscillatory activities in the mitral cell population, and this pattern is then recognized by the associative memories of olfactory objects in the olfactory cortex. Top-down feedback from the olfactory cortex to the olfactory bulb modulates the bulbar responses, so that, for example, the bulb can adapt to a pre-existing olfactory background to single out a foreground odor from an odor mixture for recognition, or can enhance sensitivity to a target odor during odor search. Destruction to the olfactory bulb results in
ipsilateral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
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. Anosmia can be due to a nu ...
while irritative
lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classifi ...
of the
uncus The uncus is an anterior extremity of the parahippocampal gyrus. It is separated from the apex of the temporal lobe by a slight fissure called the incisura temporalis (also called rhinal sulcus). Although superficially continuous with the hippo ...
can result in olfactory and
gustatory The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
hallucinations.


Lateral inhibition

;External plexiform layer The interneurons in the external plexiform layer perform feedback inhibition on the mitral cells to control back propagation. They also participate in
lateral inhibition In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction ...
of the mitral cells. This inhibition is an important part of olfaction as it aids in odor discrimination by decreasing firing in response to background odors and differentiating the responses of olfactory nerve inputs in the mitral cell layer. Inhibition of the mitral cell layer by the other layers contributes to odor discrimination and higher level processing by modulating the output from the olfactory bulb. These hyperpolarizations during odor stimulation shape the responses of the mitral cells to make them more specific to an odor. There is a lack of information regarding the function of the internal plexiform layer which lies between the mitral cell layer and the granule cell layer. ;Granule cell layer The basal
dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
of mitral cells are connected to interneurons known as
granule cells A granule is a large particle or grain. It can refer to: * Granule (cell biology), any of several submicroscopic structures, some with explicable origins, others noted only as cell type-specific features of unknown function ** Azurophilic granul ...
, which by some theories produce
lateral inhibition In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction ...
between mitral cells. 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 Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (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). More specifically, the granule cell layer receives excitatory glutamate signals from the basal dendrites of the mitral and tufted cells. The granule cell in turn releases GABA to cause an inhibitory effect on the mitral cell. More neurotransmitter is released from the activated mitral cell to the connected dendrite of the granule cell, making the inhibitory effect from the granule cell to the activated mitral cell stronger than the surrounding 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. This in turn aids in odor discrimination. Other research suggest that the lateral inhibition contributes to differentiated odor responses, which aids in the processing and perception of distinct odors. There is also evidence of cholinergic effects on granule cells that enhance depolarization of granule cells making them more excitable which in turn increases inhibition of mitral cells. This may contribute to a more specific output from the olfactory bulb that would closer resemble the glomerular odor map. Olfaction is distinct from the other
sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved i ...
s where peripheral
sensory receptor 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 ...
s have a relay in the
diencephalon The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in ol ...
. Therefore, the olfactory bulb plays this role for the
olfactory system 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 ac ...
.


Accessory olfactory bulb

In vertebrates, the accessory olfactory bulb (AOB), which resides on the dorsal-posterior region of the main olfactory bulb, forms a parallel pathway independent from the main olfactory bulb. The
vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapods. T ...
sends projections to the accessory olfactory bulb making it the second processing stage of the
accessory olfactory system The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...
. As in the main olfactory bulb, axonal input to the accessory olfactory bulb forms synapses with mitral cells within glomeruli. The accessory olfactory bulb receives axonal input from the
vomeronasal organ The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapods. T ...
, a distinct sensory epithelium from the main
olfactory epithelium The olfactory epithelium is a specialized epithelial 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 ...
that detects chemical stimuli relevant for social and reproductive behaviors, but probably also generic odorants. It has been hypothesized that, in order for the vomernasal pump to turn on, the main olfactory epithelium must first detect the appropriate odor. However, the possibility that the vomeronasal system works in parallel or independently from generic olfactory inputs has not been ruled out yet. Vomeronasal sensory neurons provide direct excitatory inputs to AOB principle neurons called mitral cells which are transmitted to the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
and
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
and therefore are directly involved in sex hormone activity and may influence aggressiveness and mating behavior. Axons of the vomeronasal sensory neurons express a given receptor type which, differently from what occurs in the main olfactory bulb, diverge between 6 and 30 AOB glomeruli. Mitral cell dendritic endings go through a dramatic period of targeting and clustering just after presynaptic unification of the sensory neuron axons. The connectivity of the vomernasal sensorglomery neurons to mitral cells is precise, with mitral cell dendrites targeting the
glomeruli ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...
. There is evidence against the presence of a functional accessory olfactory bulb in humans and other higher primates. The AOB is divided into two main subregions, anterior and posterior, which receive segregated synaptic inputs from two main categories of vomeronasal sensory neurons, V1R and V2R, respectively. This appears as a clear functional specialization, given the differential role of the two populations of sensory neurons in detecting chemical stimuli of different type and molecular weight. Although it doesn't seem to be maintained centrally, where mitral cell projections from both sides of the AOB converge. A clear difference of the AOB circuitry, compared to the rest of the bulb, is its heterogeneous connectivity between mitral cells and vomeronasal sensory afferents within neuropil glomeruli. AOB mitral cells indeed contact through apical dendritic processes glomeruli formed by afferents of different receptor neurons, thus breaking the one-receptor-one-neuron rule which generally holds for the main olfactory system. This implies that stimuli sensed through the VNO and elaborated in the AOB are subjected to a different and probably more complex level of elaboration. Accordingly, AOB mitral cells show clearly different firing patterns compared to other bulbar projection neurons. Additionally, top down input to the olfactory bulb differentially affects olfactory outputs.


Further processing

The olfactory bulb sends olfactory information to be further processed in the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
, the
orbitofrontal cortex The orbitofrontal cortex (OFC) is a prefrontal cortex region in the frontal lobes of the brain which is involved in the cognitive process of decision-making. In non-human primates it consists of the association cortex areas Brodmann area 11, 12 ...
(OFC) and the
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
where it plays a role in emotion, memory and learning. The main olfactory bulb connects to the amygdala via the
piriform cortex The piriform cortex, or pyriform cortex, is a region in the brain, part of the rhinencephalon situated in the cerebrum. The function of the piriform cortex relates to the sense of smell. Structure The piriform cortex is part of the rhinencepha ...
of the
primary olfactory cortex The primary olfactory cortex (POC) is a portion of the cerebral cortex. It is found in the inferior part of the temporal lobe of the brain. It receives input from the olfactory tract. It is involved in the sense of smell (olfaction). Structure ...
and directly projects from the main olfactory bulb to specific amygdala areas. The amygdala passes olfactory information on to the
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
. The orbitofrontal cortex, amygdala, hippocampus,
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
, 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.


Amygdala

Associative learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machines; there is also evidence for some kind of learnin ...
between odors and behavioral responses takes place in the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
. The odors serve as the reinforcers or the punishers during the associative learning process; odors that occur with positive states reinforce the behavior that resulted in the positive state while odors that occur with negative states do the opposite. Odor cues are coded by neurons in the amygdala with the behavioral effect or emotion that they produce. In this way odors reflect certain emotions or physiological states. Odors become associated with pleasant and unpleasant responses, and eventually the odor becomes a cue and can cause an emotional response. These odor associations contribute to emotional states such as fear. Brain imaging shows amygdala activation correlated with pleasant and unpleasant odors, reflecting the association between odors and emotions.


Hippocampus

The
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
aids in olfactory memory and learning as well. Several olfaction-memory processes occur in the hippocampus. Similar to the process in the amygdala, an odor is associated with a particular reward, i.e. the smell of food with receiving sustenance. Odor in the hippocampus also contributes to the formation of
episodic memory Episodic memory is the memory of everyday events (such as times, location geography, associated emotions, and other contextual information) that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred ...
; the memories of events at a specific place or time. The time at which certain neurons fire in the hippocampus is associated by neurons with a stimulus such as an odor. Presentation of the odor at a different time may cause recall of the memory, therefore odor aids in recall of episodic memories.


Olfactory coding in Habenula

In lower vertebrates (lampreys and teleost fishes), mitral cell (principal olfactory neurons) axons project exclusively to the right hemisphere of Habenula in an asymmetric manner. It is reported that dorsal Habenula (Hb) are functional asymmetric with predominant odor responses in right hemisphere. It was also shown that Hb neurons are spontaneous active even in absence of olfactory stimulation. These spontaneous active Hb neurons are organized into functional clusters which were proposed to govern olfactory responses. (Jetti, SK. et al. 2014, Current Biology) ;Depression models Further evidence of the link between the olfactory bulb and emotion and memory is shown through animal depression models. Olfactory bulb removal in rats effectively causes structural changes in the amygdala and hippocampus and behavioral changes similar to that of a person with depression. Researchers use rats with olfactory bulbectomies to research antidepressants. Research has shown that removal of the olfactory bulb in rats leads to
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
reorganization, disrupted cell growth in the hippocampus, and decreased
neuroplasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of Neural circuit, neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that diffe ...
in the hippocampus. These hippocampal changes due to olfactory bulb removal are associated with behavioral changes characteristic of depression, demonstrating the correlation between the olfactory bulb and emotion. The hippocampus and amygdala affect odor perception. During certain physiological states such as hunger a food odor may seem more pleasant and rewarding due to the associations in the amygdala and hippocampus of the food odor stimulus with the reward of eating.


Orbitofrontal cortex

Olfactory information is sent to the primary olfactory cortex, where projections are sent to the
orbitofrontal cortex The orbitofrontal cortex (OFC) is a prefrontal cortex region in the frontal lobes of the brain which is involved in the cognitive process of decision-making. In non-human primates it consists of the association cortex areas Brodmann area 11, 12 ...
. The OFC contributes to this odor-reward association as well as it assesses the value of a reward, i.e. the nutritional value of a food. The OFC receives projections from the
piriform cortex The piriform cortex, or pyriform cortex, is a region in the brain, part of the rhinencephalon situated in the cerebrum. The function of the piriform cortex relates to the sense of smell. Structure The piriform cortex is part of the rhinencepha ...
, amygdala, and parahippocampal cortices. Neurons in the OFC that encode food reward information activate the reward system when stimulated, associating the act of eating with reward. The OFC further projects to the
anterior cingulate cortex In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involved ...
where it plays a role in appetite. The OFC also associates odors with other stimuli, such as taste. Odor perception and discrimination also involve the OFC. The spatial odor map in the glomeruli layer of the olfactory bulb may contribute to these functions. The odor map begins processing of olfactory information by spatially organizing the glomeruli. This organizing aids the olfactory cortices in its functions of perceiving and discriminating odors.


Adult neurogenesis

The olfactory bulb is, along with both the
subventricular zone The subventricular zone (SVZ) is a region situated on the outside wall of each lateral ventricle of the vertebrate brain. It is present in both the embryonic and adult brain. In embryonic life, the SVZ refers to a secondary proliferative zone ...
and the subgranular zone of the dentate gyrus of the hippocampus, one of only three structures in the brain observed to undergo continuing
neurogenesis Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs) ...
in adult mammals. In most mammals, new neurons are born from neural stem cells in the sub-ventricular zone and migrate rostrally towards the main and accessory olfactory bulbs. Within the olfactory bulb these immature neuroblasts develop into fully functional granule cell interneurons and periglomerular cell interneurons that reside in the granule cell layer and glomerular layers, respectively. The olfactory sensory neuron axons that form synapses in olfactory bulb glomeruli are also capable of regeneration following regrowth of an olfactory sensory neuron residing in the olfactory epithelium. Despite dynamic turnover of sensory axons and interneurons, the projection neurons (mitral and tufted neurons) that form synapses with these axons are not structurally plastic. The function of adult neurogenesis in this region remains a matter of study. The survival of immature neurons as they enter the circuit is highly sensitive to olfactory activity and in particular associative learning tasks. This has led to the hypothesis that new neurons participate in learning processes. No definitive behavioral effect has been observed in loss-of-function experiments suggesting that the function of this process, if at all related to olfactory processing, may be subtle.


Clinical significance

The olfactory lobe is a structure of the vertebrate forebrain involved in olfaction, or sense of smell. Destruction of the olfactory bulb results in ipsilateral
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. Anosmia can be due to a nu ...
.


Other animals


Evolution

Comparing the structure of the olfactory bulb among vertebrate species, such as the
leopard frog Leopard frog is a generic name used to refer to various species in the true frog genus Lithobates. They all have similar coloration: brown or green with spots that form a leopard pattern. They are distinguished by their distribution and behavior ...
and the
lab mouse The laboratory mouse or lab mouse is a small mammal of the order Rodentia which is bred and used for scientific research or feeders for certain pets. Laboratory mice are usually of the species '' Mus musculus''. They are the most commonly ...
, 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. A similar structure is shared by the analogous olfactory center in the fruit fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'', the
antennal lobe The antennal lobe is the primary (first order) olfactory brain area in insects. The antennal lobe is a sphere-shaped deutocerebral neuropil in the brain that receives input from the olfactory sensory neurons in the antennae and mouthparts. Function ...
. One possibility is that vertebrate olfactory bulb and insect antennal lobe structure may be similar because they contain an optimal solution to a
computational problem In theoretical computer science, a computational problem is a problem that may be solved by an algorithm. For example, the problem of factoring :"Given a positive integer ''n'', find a nontrivial prime factor of ''n''." is a computational probl ...
experienced by all olfactory systems and thus may have evolved independently in different phyla – a phenomenon generally known as
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
.
"The increase of brain size relative to body size—
encephalization Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...
—is intimately linked with human evolution. However, two genetically different evolutionary lineages,
Neanderthals Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an Extinction, extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ag ...
and
modern humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, an ...
, have produced similarly large-brained human species. Thus, understanding human brain evolution should include research into specific cerebral reorganization, possibly reflected by brain shape changes. Here we exploit developmental integration between the brain and its underlying skeletal base to test hypotheses about brain evolution in ''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...
''. Three-dimensional geometric morphometric analyses of endobasicranial shape reveal previously undocumented details of evolutionary changes in ''
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
''. Larger olfactory bulbs, relatively wider orbitofrontal cortex, relatively increased and forward projecting temporal lobe poles appear unique to modern humans. Such brain reorganization, beside physical consequences for overall skull shape, might have contributed to the evolution of ''H. sapiens learning and social capacities, in which higher olfactory functions and its cognitive, neurological behavioral implications could have been hitherto underestimated factors."


See also

*
Olfactory ensheathing glia Olfactory ensheathing cells (OECs), also known as olfactory ensheathing glia or olfactory ensheathing glial cells, are a type of macroglia (radial glia) found in the nervous system. They are also known as olfactory Schwann cells, because they ens ...
*
Phantosmia Phantosmia (phantom smell), also called an olfactory hallucination or a phantom odor, is smelling an odor that is not actually there. It can occur in one nostril or both. Unpleasant phantosmia, cacosmia, is more common and is often described as s ...
* Nobiletin


References


Further reading

*Shepherd, G. ''The Synaptic Organization of the Brain'', Oxford University Press, 5th edition (November, 2003). * *


External links

* {{Authority control Olfactory system Cerebrum Limbic system