Editing Olfactory system (section)

===Central===
[[File:1403 Olfaction.jpg|thumb|Details of olfaction system]]
The main olfactory bulb transmits pulses to both mitral and tufted cells, which help determine odor concentration based on the time certain neuron clusters fire (called 'timing code'). These cells also note differences between highly similar odors and use that data to aid in later recognition. The cells are different with mitral having low firing-rates and being easily inhibited by neighboring cells, while tufted have high rates of firing and are more difficult to inhibit.<ref>{{Cite journal |last1=Schoenfeld |first1=Thomas A. |last2=Marchand |first2=James E. |last3=Macrides |first3=Foteos |date=1985-05-22 |title=Topographic organization of tufted cell axonal projections in the hamster main olfactory bulb: An intrabulbar associational system |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.902350408 |journal=The Journal of Comparative Neurology |language=en |volume=235 |issue=4 |pages=503–518 |doi=10.1002/cne.902350408 |pmid=2582006 |s2cid=5544527 |issn=0021-9967|url-access=subscription }}</ref><ref>{{Cite journal |last1=Igarashi |first1=K. M. |last2=Ieki |first2=N. |last3=An |first3=M. |last4=Yamaguchi |first4=Y. |last5=Nagayama |first5=S. |last6=Kobayakawa |first6=K. |last7=Kobayakawa |first7=R. |last8=Tanifuji |first8=M. |last9=Sakano |first9=H. |last10=Chen |first10=W. R. |last11=Mori |first11=K. |date=2012-06-06 |title=Parallel Mitral and Tufted Cell Pathways Route Distinct Odor Information to Different Targets in the Olfactory Cortex |journal=Journal of Neuroscience |language=en |volume=32 |issue=23 |pages=7970–7985 |doi=10.1523/JNEUROSCI.0154-12.2012 |issn=0270-6474 |pmc=3636718 |pmid=22674272}}</ref><ref>{{Cite journal |last1=Friedrich |first1=Rainer W. |last2=Laurent |first2=Gilles |date=2001-02-02 |title=Dynamic Optimization of Odor Representations by Slow Temporal Patterning of Mitral Cell Activity |url=https://www.science.org/doi/10.1126/science.291.5505.889 |journal=Science |language=en |volume=291 |issue=5505 |pages=889–894 |doi=10.1126/science.291.5505.889 |pmid=11157170 |bibcode=2001Sci...291..889F |issn=0036-8075|url-access=subscription }}</ref><ref>{{Cite journal |last=Shepherd |first=G. M. |date=1963-08-01 |title=Neuronal systems controlling mitral cell excitability |journal=The Journal of Physiology |language=en |volume=168 |issue=1 |pages=101–117 |doi=10.1113/jphysiol.1963.sp007180 |pmc=1359412 |pmid=14056480}}</ref> How the bulbar neural circuit transforms odor inputs to the bulb into the bulbar responses that are sent to the olfactory cortex can be partly understood by a mathematical model.<ref>{{Cite journal|last1=Li|first1=Zhaoping|last2=Hopfield|first2=J. J.|date=1989-09-01|title=Modeling the olfactory bulb and its neural oscillatory processings|url=https://doi.org/10.1007/BF00200803|journal=Biological Cybernetics|language=en|volume=61|issue=5|pages=379–392|doi=10.1007/BF00200803|pmid=2551392|s2cid=7932310|issn=1432-0770|url-access=subscription}}</ref>

The [[uncus]] houses the '''olfactory cortex''' which includes the [[piriform cortex]] (posterior [[orbitofrontal cortex]]), [[amygdala]], [[olfactory tubercle]], and [[parahippocampal gyrus]].

The olfactory tubercle connects to numerous areas of the amygdala, [[thalamus]], [[hypothalamus]], [[hippocampus]], [[brain stem]], [[retina]], [[auditory cortex]], and olfactory system. In total it has 27 inputs and 20 outputs. An oversimplification of its role is to state that it:<ref>{{Cite journal |last=Ikemoto |first=Satoshi |date=November 2007 |title=Dopamine reward circuitry: Two projection systems from the ventral midbrain to the nucleus accumbens–olfactory tubercle complex |journal=Brain Research Reviews |language=en |volume=56 |issue=1 |pages=27–78 |doi=10.1016/j.brainresrev.2007.05.004 |pmc=2134972 |pmid=17574681}}</ref><ref>{{Cite journal |last1=Newman |first1=Richard |last2=Winans |first2=Sarah Schilling |date=1980-05-15 |title=An experimental study of the ventral striatum of the golden hamster. II. Neuronal connections of the olfactory tubercle |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.901910204 |journal=The Journal of Comparative Neurology |language=en |volume=191 |issue=2 |pages=193–212 |doi=10.1002/cne.901910204 |pmid=7410591 |s2cid=7019544 |issn=0021-9967|hdl=2027.42/50010 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Wesson |first1=Daniel W. |last2=Wilson |first2=Donald A. |date=January 2011 |title=Sniffing out the contributions of the olfactory tubercle to the sense of smell: Hedonics, sensory integration, and more? |journal=Neuroscience & Biobehavioral Reviews |language=en |volume=35 |issue=3 |pages=655–668 |doi=10.1016/j.neubiorev.2010.08.004 |pmc=3005978 |pmid=20800615}}</ref>
* checks to ensure odor signals arose from actual odors rather than villi irritation,
* regulates motor behavior (primarily social and stereotypical) brought on by odors,
* integrates auditory and olfactory sensory info to complete the aforementioned tasks, and
* plays a role in transmitting positive signals to reward sensors (and is thus involved in addiction).

The amygdala (in olfaction) processes [[pheromone]], [[allomone]], and [[kairomone]] (same-species, cross-species, and cross-species where the emitter is harmed and the sensor is benefited, respectively) signals. Due to [[cerebrum]] evolution this processing is secondary and therefore is largely unnoticed in human interactions.<ref>{{Cite journal |last1=Monti-Bloch |first1=L. |last2=Grosser |first2=B.I. |date=October 1991 |title=Effect of putative pheromones on the electrical activity of the human vomeronasal organ and olfactory epithelium |url=https://linkinghub.elsevier.com/retrieve/pii/0960076091902554 |journal=The Journal of Steroid Biochemistry and Molecular Biology |language=en |volume=39 |issue=4 |pages=573–582 |doi=10.1016/0960-0760(91)90255-4|pmid=1892788 |s2cid=46330425 |url-access=subscription }}</ref> Allomones include flower scents, natural herbicides, and natural toxic plant chemicals. The info for these processes comes from the [[vomeronasal organ]] indirectly via the olfactory bulb.<ref>{{Cite journal |last=Keverne |first=Eric B. |date=1999-10-22 |title=The Vomeronasal Organ |url=https://www.science.org/doi/10.1126/science.286.5440.716 |journal=Science |language=en |volume=286 |issue=5440 |pages=716–720 |doi=10.1126/science.286.5440.716 |pmid=10531049 |issn=0036-8075|url-access=subscription }}</ref> The main olfactory bulb's pulses in the amygdala are used to pair odors to names and recognize odor to odor differences.<ref>{{Cite journal |last1=Zald |first1=David H. |last2=Pardo |first2=José V. |date=1997-04-15 |title=Emotion, olfaction, and the human amygdala: Amygdala activation during aversive olfactory stimulation |journal=Proceedings of the National Academy of Sciences |language=en |volume=94 |issue=8 |pages=4119–4124 |doi=10.1073/pnas.94.8.4119 |issn=0027-8424 |pmc=20578 |pmid=9108115|bibcode=1997PNAS...94.4119Z |doi-access=free }}</ref><ref>{{Cite journal |last1=Krettek |first1=J. E. |last2=Price |first2=J. L. |date=1977-04-15 |title=Projections from the amygdaloid complex and adjacent olfactory structures to the entorhinal cortex and to the subiculum in the rat and cat |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.901720409 |journal=The Journal of Comparative Neurology |language=en |volume=172 |issue=4 |pages=723–752 |doi=10.1002/cne.901720409 |pmid=838896 |s2cid=24976754 |issn=0021-9967|url-access=subscription }}</ref>

The [[Stria terminalis#Bed nucleus of the stria terminalis|bed nuclei of the stria terminalis]] (BNST) act as the information pathway between the amygdala and hypothalamus, as well as the hypothalamus and [[pituitary gland]]. BNST abnormalities often lead to sexual confusion and immaturity. The BNST also connect to the septal area, rewarding sexual behavior.<ref>{{Cite journal |last1=Dong |first1=Hong-Wei |last2=Petrovich |first2=Gorica D |last3=Swanson |first3=Larry W |date=December 2001 |title=Topography of projections from amygdala to bed nuclei of the stria terminalis |url=https://linkinghub.elsevier.com/retrieve/pii/S0165017301000790 |journal=Brain Research Reviews |language=en |volume=38 |issue=1–2 |pages=192–246 |doi=10.1016/S0165-0173(01)00079-0|pmid=11750933 |s2cid=21122983 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Dong |first1=Hong-Wei |last2=Swanson |first2=Larry W. |date=2004-04-12 |title=Projections from bed nuclei of the stria terminalis, posterior division: Implications for cerebral hemisphere regulation of defensive and reproductive behaviors |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.20002 |journal=The Journal of Comparative Neurology |language=en |volume=471 |issue=4 |pages=396–433 |doi=10.1002/cne.20002 |pmid=15022261 |s2cid=24651099 |issn=0021-9967|url-access=subscription }}</ref>

Mitral pulses to the hypothalamus promote/discourage feeding, whereas accessory olfactory bulb pulses regulate reproductive and odor-related-reflex processes.

The hippocampus (although minimally connected to the main olfactory bulb) receives almost all of its olfactory information via the amygdala (either directly or via the BNST). The hippocampus forms new memories and reinforces existing ones.

Similarly, the parahippocampus encodes, recognizes and contextualizes scenes.<ref>{{Cite journal |last1=Moser |first1=May-Britt |last2=Moser |first2=Edvard I. |date=1998 |title=Functional differentiation in the hippocampus |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1098-1063(1998)8:6%3C608::AID-HIPO3%3E3.0.CO;2-7 |journal=Hippocampus |volume=8 |issue=6 |pages=608–619 |doi=10.1002/(sici)1098-1063(1998)8:6<608::aid-hipo3>3.0.co;2-7 |pmid=9882018 |s2cid=32384692 |issn=1050-9631|url-access=subscription }}</ref> The parahippocampal gyrus houses the topographical map for olfaction.

The orbitofrontal cortex (OFC) is heavily correlated with the cingulate gyrus and septal area to act out positive/negative reinforcement. The OFC is the expectation of reward/punishment in response to stimuli. The OFC represents the emotion and reward in decision making.<ref>{{Cite journal |last1=O'Doherty |first1=J. |last2=Kringelbach |first2=M. L. |last3=Rolls |first3=E. T. |last4=Hornak |first4=J. |last5=Andrews |first5=C. |date=January 2001 |title=Abstract reward and punishment representations in the human orbitofrontal cortex |url=http://www.nature.com/articles/nn0101_95 |journal=Nature Neuroscience |language=en |volume=4 |issue=1 |pages=95–102 |doi=10.1038/82959 |issn=1097-6256 |pmid=11135651|s2cid=52848707 |url-access=subscription }}</ref>

The [[anterior olfactory nucleus]] distributes reciprocal signals between the olfactory bulb and piriform cortex.<ref>{{Cite journal |last1=Davis |first1=Barry J. |last2=Macrides |first2=Foteos |date=1981-12-10 |title=The organization of centrifugal projections from the anterior olfactory nucleus, ventral hippocampal rudiment, and piriform cortex to the main olfactory bulb in the hamster: An autoradiographic study |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.902030310 |journal=The Journal of Comparative Neurology |language=en |volume=203 |issue=3 |pages=475–493 |doi=10.1002/cne.902030310 |pmid=6274922 |s2cid=21901628 |issn=0021-9967|url-access=subscription }}</ref> The anterior olfactory nucleus is the memory hub for smell.<ref>{{Cite journal |last1=Scalia |first1=Frank |last2=Winans |first2=Sarah S. |date=1975-05-01 |title=The differential projections of the olfactory bulb and accessory olfactory bulb in mammals |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.901610105 |journal=The Journal of Comparative Neurology |language=en |volume=161 |issue=1 |pages=31–55 |doi=10.1002/cne.901610105 |pmid=1133226 |s2cid=46084419 |issn=0021-9967|url-access=subscription }}</ref>

When different odor objects or components are mixed, humans and other mammals sniffing the mixture (presented by, e.g., a sniff bottle)  are often unable to identify the components in the mixture even though they can recognize each individual component presented alone.<ref>{{Cite journal|date=1989-11-01|title=The capacity of humans to identify odors in mixtures|url=https://www.sciencedirect.com/science/article/abs/pii/0031938489900413|journal=Physiology & Behavior|language=en|volume=46|issue=5|pages=809–814|doi=10.1016/0031-9384(89)90041-3|issn=0031-9384|last1=Laing|first1=D.G.|last2=Francis|first2=G.W.|pmid=2628992|s2cid=2926752|url-access=subscription}}</ref> This is largely because each odor sensory neuron can be excited by multiple odor components. It has been proposed that, in an olfactory environment typically composed of multiple odor components (e.g., odor of a dog entering a kitchen that contains a background coffee odor),  feedback from the olfactory cortex to the olfactory bulb<ref>{{Cite journal|date=2012-12-20|title=Cortical Feedback Control of Olfactory Bulb Circuits|journal=Neuron|language=en|volume=76|issue=6|pages=1161–1174|doi=10.1016/j.neuron.2012.10.020|issn=0896-6273|last1=Boyd|first1=Alison M.|last2=Sturgill|first2=James F.|last3=Poo|first3=Cindy|last4=Isaacson|first4=Jeffry S.|pmid=23259951|pmc=3725136}}</ref> suppresses the pre-existing odor background (e.g., coffee)  via olfactory adaptation,<ref>{{Cite journal|last=Li|first=Z.|date=1990-02-01|title=A model of olfactory adaptation and sensitivity enhancement in the olfactory bulb|url=https://doi.org/10.1007/BF00201449|journal=Biological Cybernetics|language=en|volume=62|issue=4|pages=349–361|doi=10.1007/BF00201449|pmid=2310788|s2cid=6241381|issn=1432-0770|url-access=subscription}}</ref> so that the newly arrived foreground odor (e.g.,  dog) can be singled out from the mixture for recognition.<ref>{{Cite journal|date=2016-10-01|title=Olfactory object recognition, segmentation, adaptation, target seeking, and discrimination by the network of the olfactory bulb and cortex: computational model and experimental data|url=https://www.sciencedirect.com/science/article/pii/S2352154616300766|journal=Current Opinion in Behavioral Sciences|language=en|volume=11|pages=30–39|doi=10.1016/j.cobeha.2016.03.009|issn=2352-1546|last1=Zhaoping|first1=Li|s2cid=27989941|url-access=subscription}}</ref>   [[Image:Olfactory system.svg|thumb|280px|1: [[Olfactory bulb]] 2: [[Mitral cell]]s 3: Bone 4: [[Olfactory epithelium|Nasal epithelium]] 5: [[Glomerulus (olfaction)|Glomerulus]] 6: [[Olfactory receptor|Olfactory receptor cells]]]]