Olfactory bulb

The olfactory bulb (Latin: bulbus olfactorius) constitutes a neural structure within the vertebrate forebrain, primarily responsible for olfaction, the sense of smell. This structure relays olfactory data to various cerebral areas, such as the amygdala, orbitofrontal cortex (OFC), and hippocampus, thereby influencing emotional responses, memory formation, and learning processes.

The olfactory bulb comprises two distinct components: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb establishes connections with the amygdala through the piriform cortex, a component of the primary olfactory cortex, and also directly projects to particular regions of the amygdala. Positioned dorsally and posteriorly to the main olfactory bulb, the accessory olfactory bulb establishes a distinct, parallel neural pathway.

Lesions affecting the olfactory bulb lead to ipsilateral anosmia. Conversely, irritative lesions within the uncus may induce olfactory and gustatory hallucinations.

Structure

Across most vertebrate species, exemplified by rodents, the olfactory bulb represents the most rostral cerebral component. In contrast, the human olfactory bulb is situated on the inferior aspect of the brain. The cribriform plate of the ethmoid bone provides structural support and protection for the olfactory bulb; in mammals, this plate separates the bulb from the olfactory epithelium and is permeated by olfactory nerve axons. This bulb itself is further subdivided into two discrete entities: the main olfactory bulb and the accessory olfactory bulb.

Layers

The main olfactory bulb exhibits a complex, multi-layered cellular architecture. Proceeding from the superficial aspect towards the core, these layers include:

• Glomerular layer
• External plexiform layer
• Mitral cell layer
• Internal plexiform layer
• Granule cell layer

The olfactory bulb serves to relay olfactory information from the nasal cavity to the brain, thereby being indispensable for an intact sense of smell. Within its neural circuitry, the glomerular layer directly receives afferent input comprising axons from approximately ten million olfactory receptor neurons located in the olfactory mucosa, a specific region of the nasal cavity. These axonal terminals aggregate into spherical formations termed glomeruli, with each glomerulus predominantly receiving input from olfactory receptor neurons expressing an identical olfactory receptor. Consequently, the glomerular layer of the olfactory bulb represents the initial stage of synaptic processing.

The glomerular layer functions as a spatial odor map, systematically organized according to the chemical structures of odorants, including their functional groups and carbon chain lengths. This topographical representation is further segmented into zones and clusters, each corresponding to analogous glomeruli and, consequently, similar olfactory perceptions. Notably, a specific cluster is linked to the detection of pungent, spoiled odors, characterized by distinct chemical signatures. This categorization likely possesses an evolutionary basis, aiding in the identification of putrescent food items. The spatial organization within the glomerular layer potentially contributes to odor perception within the olfactory cortex.

Subsequent synaptic processing within the olfactory bulb transpires in the external plexiform layer, situated between the glomerular and mitral cell layers. This layer encompasses astrocytes, various interneurons, and a subset of mitral cells. Characterized by a paucity of cell bodies, this layer is predominantly composed of dendrites from mitral cells and GABAergic granule cells; mitral cell dendrites, in particular, project to the olfactory cortex. The olfactory bulb hosts a diverse array of interneuron types, such as periglomerular cells, which form synapses both within and between glomeruli, and granule cells, which synapse with mitral cells. The granule cell layer constitutes the deepest stratum of the olfactory bulb. It is composed of dendrodendritic granule cells that establish synaptic connections with the mitral cell layer.

Function

This cerebral region is responsible for processing olfactory sensations. Functionally, as a neural circuit, the olfactory bulb possesses a singular source of sensory input—axons originating from olfactory receptor neurons within the olfactory epithelium—and a singular output, consisting of mitral cell axons. Consequently, it is commonly hypothesized to operate as a sensory filter, contrasting with associative circuits characterized by multiple inputs and outputs. Nevertheless, the olfactory bulb also integrates "top-down" modulatory input from various brain regions, including the olfactory cortex, amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra. Its putative functions can be broadly categorized into four non-exclusive domains:

• Odor discrimination
• Enhancement of odor detection sensitivity
• The olfactory bulb filters out numerous background odors, thereby enhancing the transmission of a limited number of specific odorants.
• This mechanism allows higher brain regions, implicated in arousal and attention, to modulate the detection and discrimination of odors.

Although the olfactory bulb's intrinsic circuit architecture theoretically supports all these functions, the extent to which any are exclusively performed by it remains ambiguous. Drawing parallels with other brain regions like the retina, numerous investigators have explored the olfactory bulb's role in filtering afferent information from receptor neurons, both spatially and temporally. Central to these hypothesized filtering mechanisms are two distinct classes of interneurons: periglomerular cells and granule cells. Olfactory processing commences at every stratum of the main olfactory bulb, starting with the spatial mapping that categorizes odors within the glomeruli layer.

Interneurons situated within the external plexiform layer demonstrate responsiveness to presynaptic action potentials, manifesting both excitatory and inhibitory postsynaptic potentials. Neuronal firing exhibits temporal variability, characterized by phases of rapid, spontaneous discharge interspersed with slower modulations. These distinct patterns may correlate with sniffing behaviors or alterations in odorant intensity and concentration. Furthermore, temporal patterns could influence subsequent processing related to the spatial perception of odorants. For instance, synchronized spike trains from mitral cells appear to facilitate superior discrimination of similar odors compared to unsynchronized activity. A prominent model posits that the bulbar neural circuit converts olfactory information from receptors into a population-level pattern of neural oscillatory activities within the mitral cell population. This pattern is subsequently recognized by the associative memories of olfactory objects residing in the olfactory cortex. Descending feedback from the olfactory cortex to the olfactory bulb modulates bulbar responses, enabling the bulb to adapt to an existing olfactory background, thereby isolating a foreground odor from a complex mixture for recognition, or to heighten sensitivity to a specific target odor during an odor search.

Lateral Inhibition

External Plexiform Layer

Interneurons within the external plexiform layer exert feedback inhibition on mitral cells, thereby regulating back propagation. They also contribute to the lateral inhibition of mitral cells. This inhibitory mechanism is crucial for olfaction, as it facilitates odor discrimination by attenuating neuronal firing in response to background odors and by differentiating the responses of olfactory nerve inputs within the mitral cell layer. Inhibition of the mitral cell layer by other layers further contributes to odor discrimination and advanced processing through the modulation of olfactory bulb output. The hyperpolarizations observed during odor stimulation refine the responses of mitral cells, rendering them more specific to particular odorants.

Limited information is available concerning the functional role of the internal plexiform layer, which is anatomically positioned between the mitral cell layer and the granule cell layer.

Granule Cell Layer

Basal dendrites of mitral cells establish connections with interneurons, specifically granule cells, which are implicated in mediating lateral inhibition among mitral cells. The synaptic junctions between mitral and granule cells represent a distinctive "dendro-dendritic" classification, characterized by both pre- and post-synaptic elements being dendrites capable of neurotransmitter release. Within this unique synaptic arrangement, mitral cells discharge the excitatory neurotransmitter glutamate, while granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (GABA). Due to its bidirectional nature, the dendro-dendritic synapse facilitates both auto-inhibition of individual mitral cells and lateral inhibition of adjacent mitral cells. Specifically, the granule cell layer receives excitatory glutamatergic input originating from the basal dendrites of both mitral and tufted cells. Subsequently, the granule cell releases GABA, thereby exerting an inhibitory influence on the mitral cell. Enhanced neurotransmitter release from an activated mitral cell to the associated granule cell dendrite intensifies the inhibitory effect from the granule cell on the activated mitral cell, surpassing its influence on surrounding, less active mitral cells. The precise functional contribution of lateral inhibition remains undetermined; however, it is hypothesized to enhance the signal-to-noise ratio of olfactory signals by suppressing the baseline firing activity of quiescent surrounding neurons. This mechanism potentially contributes to improved odor discrimination. Alternative research indicates that lateral inhibition plays a role in generating differentiated odor responses, thereby assisting in the processing and perception of discrete olfactory stimuli. Furthermore, evidence suggests cholinergic modulation of granule cells, which promotes their depolarization, increases their excitability, and consequently augments the inhibition of mitral cells. Such effects could lead to an olfactory bulb output that more accurately reflects the glomerular odor map. Olfaction distinguishes itself from other sensory modalities because its peripheral sensory receptors do not relay through the diencephalon; instead, the olfactory bulb fulfills this crucial relay function for the olfactory system.

The Accessory Olfactory Bulb

In vertebrates, the accessory olfactory bulb (AOB) is situated in the dorsal-posterior region of the main olfactory bulb and constitutes a distinct parallel pathway, operating independently from the primary olfactory system. Projections from the vomeronasal organ terminate in the accessory olfactory bulb, establishing it as the secondary processing stage within the accessory olfactory system. Similar to the main olfactory bulb, axonal inputs to the accessory olfactory bulb form synapses with mitral cells located within glomeruli. The accessory olfactory bulb receives axonal input from the vomeronasal organ, a specialized sensory epithelium separate from the main olfactory epithelium, which is responsible for detecting chemical stimuli pertinent to social and reproductive behaviors, and potentially also general odorants. A hypothesis posits that activation of the vomeronasal pump necessitates prior detection of the relevant odor by the main olfactory epithelium. Nevertheless, the potential for the vomeronasal system to function either in parallel with or independently of generic olfactory inputs remains an open question.

Vomeronasal sensory neurons deliver direct excitatory inputs to the principal neurons of the AOB, known as mitral cells. These signals are subsequently transmitted to the amygdala and hypothalamus, thereby directly influencing sex hormone activity and potentially modulating aggressive and mating behaviors. Axons of vomeronasal sensory neurons express a specific receptor type, which, unlike the arrangement in the main olfactory bulb, projects divergently to between 6 and 30 AOB glomeruli. Following the presynaptic convergence of sensory neuron axons, mitral cell dendritic endings undergo a significant phase of targeting and clustering. The connectivity between vomeronasal sensory neurons and mitral cells is highly precise, characterized by mitral cell dendrites specifically targeting the glomeruli. Evidence suggests the absence of a functional accessory olfactory bulb in humans and other higher primates.

The accessory olfactory bulb (AOB) comprises distinct anterior and posterior subregions, each receiving segregated synaptic inputs from specific categories of vomeronasal sensory neurons, namely V1R and V2R, respectively. This anatomical segregation suggests a clear functional specialization, considering the distinct roles of these sensory neuron populations in detecting diverse chemical stimuli based on type and molecular weight. Nevertheless, this specialization does not appear to persist centrally, where mitral cell projections from both AOB sides converge. A notable distinction of the AOB circuitry, compared to the main olfactory bulb, is its heterogeneous connectivity between mitral cells and vomeronasal sensory afferents within neuropil glomeruli. AOB mitral cells establish connections via apical dendritic processes within glomeruli formed by afferents from various receptor neurons, thereby deviating from the 'one-receptor-one-neuron' principle typically observed in the main olfactory system. Consequently, stimuli processed through the vomeronasal organ (VNO) and elaborated within the AOB undergo a distinct and likely more intricate level of processing. Accordingly, AOB mitral cells exhibit unique firing patterns compared to other bulbar projection neurons. Furthermore, top-down input to the olfactory bulb exerts differential effects on olfactory outputs.

Subsequent Processing

The olfactory bulb transmits olfactory information for subsequent processing in several brain regions, including the amygdala, the orbitofrontal cortex (OFC), and the hippocampus, where it contributes to emotion, memory, and learning. The main olfactory bulb establishes connections with the amygdala both indirectly, via the piriform cortex within the primary olfactory cortex, and directly, through projections to specific amygdalar subregions. Subsequently, the amygdala relays olfactory information to the hippocampus. Extensive direct and indirect interconnections exist among the orbitofrontal cortex, amygdala, hippocampus, thalamus, and olfactory bulb, often mediated by the cortices of the primary olfactory cortex. These intricate connections underscore the significant association between the olfactory bulb and higher-order processing areas, particularly those involved in emotional responses and memory formation.

Amygdalar Processing

The amygdala is a critical site for associative learning, linking olfactory stimuli with specific behavioral responses. During this process, odors function as either reinforcers or punishers; those co-occurring with positive states strengthen the preceding behavior, whereas odors associated with negative states elicit the inverse effect. Neurons within the amygdala encode odor cues in conjunction with the behavioral effects or emotional responses they elicit. Consequently, odors can become representations of particular emotions or physiological conditions. Through repeated exposure, odors become associated with pleasant or unpleasant responses, eventually transforming into cues capable of triggering emotional reactions. These olfactory associations significantly contribute to the development of emotional states, including fear. Neuroimaging studies demonstrate amygdalar activation correlating with both pleasant and unpleasant odors, thereby substantiating the link between olfaction and emotion.

Hippocampal Involvement

The hippocampus plays a crucial role in olfactory memory and learning. This region facilitates several olfaction-memory processes. Analogous to amygdalar processing, an odor can become associated with a specific reward, such as the scent of food linked to receiving nourishment. Furthermore, olfactory input to the hippocampus contributes to the formation of episodic memories, which are recollections of events tied to specific spatial and temporal contexts. The precise timing of neuronal firing in the hippocampus is correlated with specific stimuli, including odors. Subsequent presentation of the associated odor at a later time can trigger memory recall, thus demonstrating the role of olfaction in retrieving episodic memories.

Olfactory Encoding in the Habenula

In lower vertebrates, such as lampreys and teleost fishes, axons from mitral cells (the principal olfactory neurons) project exclusively to the right hemisphere of the habenula. The dorsal habenulae (Hb) exhibit functional asymmetry, with the right hemisphere displaying predominant responses to olfactory stimuli. Habenular neurons demonstrate spontaneous activity even in the absence of external olfactory stimulation. These spontaneously active habenular neurons are organized into functional clusters, which may regulate olfactory responses.

Models of Depression

Animal models of depression provide further evidence for the intricate connection between the olfactory bulb, emotional states, and memory functions. In rats, the surgical removal of the olfactory bulb effectively induces structural alterations within the amygdala and hippocampus, alongside behavioral changes that closely resemble depressive symptoms observed in humans. Consequently, olfactory bulbectomized rats are frequently utilized by researchers to investigate the efficacy of antidepressant treatments. This ablation of the olfactory bulb in rats leads to significant dendritic reorganization, impaired cellular proliferation in the hippocampus, and a reduction in hippocampal neuroplasticity. These hippocampal modifications, resulting from olfactory bulb removal, are consistently associated with behavioral manifestations characteristic of depression, thereby underscoring the correlation between the olfactory bulb and emotional regulation. Furthermore, both the hippocampus and amygdala exert an influence on odor perception. During specific physiological conditions, such as hunger, a food odor may be perceived as more pleasant and rewarding, a phenomenon attributed to the associative links formed in the amygdala and hippocampus between the olfactory stimulus and the gratification derived from eating.

Orbitofrontal Cortex

Olfactory information is initially transmitted to the primary olfactory cortex, from which efferent projections extend to the orbitofrontal cortex (OFC). The OFC plays a critical role in establishing odor-reward associations and in evaluating the nutritional value of food. It receives afferent projections from the piriform cortex, the amygdala, and the parahippocampal cortices. Neurons within the OFC that encode information pertaining to food reward, upon stimulation, activate the brain's reward system, thereby associating the act of consumption with a rewarding experience. Additionally, the OFC projects to the anterior cingulate cortex, where it contributes to the regulation of appetite. The OFC also facilitates the association of odors with other sensory stimuli, such as taste. Its functions further encompass odor perception and discrimination. The spatial odor map located within the glomerular layer of the olfactory bulb may contribute to these OFC-mediated functions. This odor map initiates the processing of olfactory information by spatially organizing the glomeruli, an arrangement that assists the olfactory cortices in the perception and differentiation of various odors.

Adult Neurogenesis

The olfactory bulb is recognized as one of only three brain structures in adult mammals, alongside the subventricular zone and the subgranular zone of the hippocampal dentate gyrus, where continuous neurogenesis has been observed. In most mammalian species, new neurons originate from neural stem cells within the subventricular zone and subsequently migrate rostrally towards both the main and accessory olfactory bulbs. Within the olfactory bulb, these immature neuroblasts differentiate into fully functional granule cell interneurons and periglomerular cell interneurons, which reside in the granule cell layer and glomerular layers, respectively. Furthermore, the axons of olfactory sensory neurons that form synapses within the olfactory bulb glomeruli possess the capacity for regeneration following the regrowth of an olfactory sensory neuron originating in the olfactory epithelium. Despite this dynamic turnover of sensory axons and interneurons, the projection neurons, specifically mitral and tufted neurons that establish synapses with these axons, do not exhibit structural plasticity.

As of 2013, the precise function of adult neurogenesis within this region remains an active area of scientific inquiry. The survival of immature neurons as they integrate into the neural circuit is highly sensitive to olfactory activity, particularly during associative learning tasks. This observation has led to the hypothesis that these newly generated neurons participate in learning processes. However, loss-of-function experiments have not yet yielded a definitive behavioral effect, suggesting that the role of this process, if indeed related to olfactory processing, may be subtle.

Clinical Significance

Unilateral destruction of the olfactory bulb results in ipsilateral anosmia, whereas irritative lesions affecting the uncus can precipitate olfactory and gustatory hallucinations.

Other Animals

Evolution

The fundamental layout of the olfactory bulb, characterized by five layers containing the nuclei of three primary cell types, is consistently observed across diverse vertebrate species, including the leopard frog and lab mouse, despite variations in their overall shape and size. This structural commonality extends to the analogous antennal lobe, which serves as the olfactory center in the fruit fly Drosophila melanogaster. This shared architecture suggests a potential instance of convergent evolution, where both vertebrate olfactory bulbs and insect antennal lobes may have independently developed similar structures because they represent an optimal solution to a universal computational challenge inherent to all olfactory systems.

Encephalization, defined as the increase in brain size relative to body size, is fundamentally associated with human evolutionary processes. Despite this, both Neanderthals and modern humans, representing distinct genetic lineages, developed similarly large brains. Consequently, a comprehensive understanding of human brain evolution necessitates investigating specific cerebral reorganization, which may be evidenced by alterations in brain morphology. Research leveraging the developmental interplay between the brain and its underlying skeletal base has been employed to examine hypotheses concerning brain evolution within the genus Homo. Three-dimensional geometric morphometric analyses of endobasicranial shape have uncovered previously unrecorded evolutionary modifications in Homo sapiens. Distinctive features of modern humans include larger olfactory bulbs, a comparatively broader orbitofrontal cortex, and relatively enlarged, anteriorly projecting temporal lobe poles. This cerebral reorganization, in addition to its impact on overall skull morphology, may have played a role in the development of H. sapiens' learning and social capabilities, suggesting that advanced olfactory functions and their associated cognitive, neurological, and behavioral implications might have been previously undervalued factors.

Olfactory ensheathing glia

• Olfactory ensheathing glia
• Phantosmia
• Nobiletin

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