Olfactory bulb
Olfactory bulb | |
---|---|
olfactory tracts outlined in red | |
Details | |
System | Olfactory |
Identifiers | |
Latin | bulbus olfactorius |
MeSH | D009830 |
NeuroNames | 279 |
NeuroLex ID | birnlex_1137 |
TA98 | A14.1.09.429 |
TA2 | 5538 |
FMA | 77624 |
Anatomical terms of neuroanatomy] |
The olfactory bulb (
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 of the ethmoid bone, which in mammals separates it from the olfactory epithelium, and which is perforated by olfactory nerve 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. In order from surface to the center the layers are:
- Glomerular layer
- External plexiform layer
- Mitral cell 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
The spatial map of the glomeruli layer may be used for perception of odor in the olfactory cortex.
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, 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, amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra.[5] Its potential functions can be placed into four non-exclusive categories:[citation needed]
- 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, 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.[2]
Interneurons in the external plexiform layer are responsive to pre-synaptic action potentials and exhibit both
Lateral inhibition
- External plexiform layer
The interneurons in the external plexiform layer perform feedback inhibition on the mitral cells to control
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.[citation needed]
- Granule cell layer
The basal
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 sensory neurons provide direct excitatory inputs to AOB principle neurons called mitral cells[19] which are transmitted to the amygdala and hypothalamus and therefore are directly involved in sex hormone activity and may influence aggressiveness and mating behavior.[20] 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.[19] There is evidence against the presence of a functional accessory olfactory bulb in humans and other higher primates.[21]
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.[22] Additionally, top down input to the olfactory bulb differentially affects olfactory outputs.[23]
Further processing
The olfactory bulb sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas.[24] The amygdala passes olfactory information on to the hippocampus. The orbitofrontal cortex, amygdala, hippocampus, thalamus, 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.[24]
Amygdala
Hippocampus
The hippocampus 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.[26] Odor in the hippocampus also contributes to the formation of episodic memory; 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.[26]
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.[27] Research has shown that removal of the olfactory bulb in rats leads to dendrite reorganization, disrupted cell growth in the hippocampus, and decreased neuroplasticity 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.[28] 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.[25]
Orbitofrontal cortex
Olfactory information is sent to the primary olfactory cortex, where projections are sent to the orbitofrontal cortex. 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, amygdala, and parahippocampal cortices.[25] 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 where it plays a role in appetite.[29] The OFC also associates odors with other stimuli, such as taste.[25] 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.[2]
Adult neurogenesis
The olfactory bulb is, along with both the subventricular 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 in adult mammals.[citation needed] In most mammals, new neurons are born from neural stem cells in the sub-ventricular zone and migrate rostrally towards the main [30] and accessory[31] 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.[citation needed]
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.[32] 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.[citation needed]
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.
Other animals
Evolution
Comparing the structure of the olfactory bulb among vertebrate species, such as the
"The increase of brain size relative to body size—
Homo sapiens. 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."[35]
See also
- Olfactory ensheathing glia
- Phantosmia
- Nobiletin
References
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Further reading
- Shepherd, G. The Synaptic Organization of the Brain, Oxford University Press, 5th edition (November, 2003). ISBN 0-19-515956-X
- Halpern, M; Martínez-Marcos, A (2003). "Structure and function of the vomeronasal system: An update" (PDF). Progress in Neurobiology. 70 (3): 245–318. S2CID 31122845. Archived from the original(PDF) on 2017-11-07.
- Ache, BW; Young, JM (2005). "Olfaction: Diverse species, conserved principles". Neuron. 48 (3): 417–30. S2CID 12078554.
External links
- "Anatomy diagram: 13048.000-1". Roche Lexicon – illustrated navigator. Elsevier. Archived from the original on 2014-11-07.