Olfaction olfactory bulb

Meisami E., Mikhail L Bairn D. and Bhatnagar K.R (1998). Human olfactory bulb aging of glomeruli and mitral cells and a search for the accessory olfactory bulb. In Olfaction and Taste, Xll (Murphy C., ed.), Ann NY Acad Sci 855, 708-715. 

Sanchez-Barcelo E., Mediavilla M.D., Sanchez-Criado J.E., Cos S., et al. (1985). Anti-gonadal actions of olfaction and light deprivation effects of blindness and main olfactory bulb — deafferentation and transection of vomeronasal nerve or bulbectomy. J Pineal Res 2, 177-190. 

When a molecule binds with its receptor site the olfactory cells become stimulated and send an impulse along the olfactory nerve. The olfactory nerve is the first cranial nerve. Cranial nerves that carry impulses into the brain are called sensory, while those that carry impulses away are called motor. Sensory information from the olfactory receptors of the nose is carried as a sensory impulse in the olfactory nerve to an area of the brain called the olfactory bulb. It is the olfactory regions of the brain that interpret this sensory information and distinguish different smells. Structures associated with the sense of smell are located in an area of the fore-brain (at the front) called the rhinencephalon. The rhinencephalon is not fully understood and its function is not restricted to olfaction or smelling. The olfactory tract then connects with another area called the neocortex that allows us to be aware of and to recognise odours or smells... 

The clue to this transition seems to be found in the sense of olfaction. Early in the course of AD, degeneration occurs in the entorhinal cortex-hippocampal-subicular complex (Price and Morris, 1999). The olfactory bulb, particularly the anterior olfactory nucleus, shows numerous neurofibrillary tangles (NFTs). Odor identification deficits during life may be associated with NFTs in the hippocampus (Wilson et al., 2007). 

Most olfaction models in the literature are far too simplistic and too mechanical in nature, and none of them have succeeded in accounting for all of the observations about olfaction. As described, recent advances in our understanding have confirmed that odor perception, as predicted by Polak (19), starts with a combinatorial mechanism at the receptor level (1) and involves pattern recognition in the higher brain (4). No single odorant-receptor interaction will be the sole determinant of odor percept, and even knowledge of the pattern elicited at the olfactory bulb is insufficient to enable prediction of the cortical image of odor. Therefore, structure/odor models are and, for the foreseeable future, will remain statistical tools rather than mechanistic indicators. 

The sense of smell, or olfaction, is remarkable in its specificity—it can, for example, discern stereoisomers of small organic compounds as distinct aromas. The 7TM receptors that detect these odorants operate in conjunction with a G protein that activates a cAMP cascade resulting in the opening of an ion channel and the generation of a nerve impulse. An outstanding feature of the olfactory system is its ability to detect a vast array of odorants. Each olfactory neuron expresses only one type of receptor and connects to a particular region of the olfactory bulb. Odors are decoded by a combinatorial mechanism—each odorant activates a number of receptors, each to a different extent, and most receptors are activated by more than one odorant. 

Kyle AL, Sorensen PW, Stacey NE, Dulka JG (1987) Medial olfactory tract pathways controlling sexual reflexes and behavior in teleosts. Ann N Y Acad Sci 519 97-107 Laberge F, Hara TJ (2001) Neurobiology of fish olfaction a review. Brain Res Rev 36 41-59 Laberge F, Hara TJ (2003) Non-oscillatory discharges of an F-prostaglandin responsive neuron population in the olfactory bulb-telencephalin transition area in lake whitefish. Neuroscience 116 1089-1095... 

Axons of the receptor cells form bundles (glomeruli). Between 30 and 50 such bundles carry olfaction information to the olfactory bulbs. 

Olfaction represents a subset of the sense of smell, which this chapter seeks to circumscribe by means of four propositions put forth as guidelines. Of these guidelines, the first is chemical, and the second is behavioral. The third guideline addresses the relationship between anatomy and behavior. Applying it requires that the organism possess a well developed eentral nervous system (CNS). The CNS of terrestrial vertebrates includes a spinal cord and a brain, from which emanates a set of cranial nerves. The first cranial nerve is often called the olfactory bulb. If the connections between the nose and the olfactory bulb are completely severed, the ability to sense and to discriminate among volatile stimuli do not necessarily vanish utterly. Those capabilities that are completely lost, however, include olfaction. 

If the odors of specific objects translate into unitary percepts, which constitute the basic entities in linguistic descriptions of olfaction, then the question follows as to whether these unitary percepts take shape at the level of the receptor neurons or in the olfactory bulb or elsewhere in the brain. That question remains unanswered, as of this writing. Because the sense of smell does not correlate perfectly with externally monitored patterns of electrical response from the receptor neurons or the olfactory bulb, the nature of olfactory coding remains unknown. Outside the laboratory unitary percepts rarely equate to pure compounds. Two vocabularies coexist, one of smells (which varies from individual to individual, and which refers to other inputs besides olfaction) and the other of chemical structures. 

Firestein, S. How the olfactory system makes sense of scents. Nature 413, 211-218 (2001) Friedrich, R.W., Korsching, S.I. Combinatorial and chemotopic odorant coding in the zebraf-ish olfactory bulb visualized by optical imaging. Neuron. 18, 737-752 (1997) Gntierrez-Osuna, R. Pattern analysis for machine olfaction A review. IEEE Sensors Journal 2(3), 189-202 (2002)... 

Laurent, G. A systems perspective on early olfactory coding. Science 286(22), 723-728 (1999) Leffingwell, J.C. Olfaction A review. Leffingwell Reports 2(1), 1-34 (2002) (accessed April 22, 2007), http //www. leffingwell. com Leon, M., Johnson, B. Olfactory coding in the mammalian olfactory bulb. Brain Research Review 42, 23-32 (2003)... 

There are three problems in particular that complicate interpretation of much of the data on structure-activity relations in olfaction. First, the different techniques used often yield data that are not strictly comparable. Recordings from a single or a few receptors, for example, are more reliable indicators of the odorant-receptor interaction than are recordings of the massed action of many neural elements in the olfactory bulb. 

Hanson, L.R., Sorensen, P.W. Cohen, Y. 1998. Sex pheromones and amino acids evoke distinctly different patterns of electrical activity in the goldfish olfactory bulb. Proceedings of the International Symposia on Olfaction and Taste XII, (Ed. by C. Murphy), New York New York Academy of Sciences (in press)... 

Because insectivores possess many plesiomorphic traits and are nocturnal, a number of studies have centered around the premise that olfaction is the sense that most characterizes Insectivora (Larochelle and Baron, 1989). Thus, olfactory structures, like the main olfactory bulbs, should be proportionately larger and are used as a basis of comparison to mammalian orders that might be expected to show a reduction in olfaction in favor of some other sense (e.g., vision in primates or echolocation in vespertilionid chiropterans Stephan, 1985 Frahm, 1985 Stephan, et al., 1988). We compared the olfactory system of short-tailed shrews Blarina brevicaudd) to white-footed mice (Peromyscus leucopus. Order Rodentia), another macrosmatic species of similar size but very distantly related to shrews. 

Experimental work has revealed a complex relationship between the vomeronasal and olfactory systems. Although the vomeronasal system mediates most of the classical primer effects, olfaction appears to regulate, in part, the expression of at least one. Normally, the reproductive systems of rats do not respond to shortened daylengths however, removal of the olfactory and accessory olfactory bulbs or intranasal zinc sulfate treatment, but not removal of the vomeronasal organ, induces photoperiodism in rats (Nelson et al., 1985). Furthermore, many behavioral responses to chemical cues are affected by lesion of the vomeronasal system (see Table 1). Hence, the vomeronasal system and olfaction modulate both primer and signaller effects, and may interact depending upon past experience. This latter contention has been pursued in our laboratories in studies of animals lacking the sensory afferents of the vomeronasal system. 

In the preceeding chapters it was discussed that removal or block of the olfactory bulbs may change the expected stereotyped response. However, the effect of anosmia, i.e., the loss of the sense of smell, on the social behavior of vertebrates is not well established and is a matter of controversy 119, 412-416). While Michael and Keverne 412) found that anosmic male rhesus monkeys show no interest in females receiving oestrogen until their olfaction was restored Goldfoot et al. 413) observed that such males display typical cycles of copulation. 

The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. It is accepted that the sensation of odor is triggered by highly complex mixtures of volatile molecules, mostly hydrophobic, and usually occurring in trace-level concentrations (ppm or ppb). These volatiles interact with odorant receptors of the olfactive epithelium located in the nasal cavity. Once the receptor is activated, a cascade of events is triggered to transform the chemical-structural information contained in the odorous stimulus into a membrane potential [58,59], which is projected to the olfactory bulb and then transported to higher regions of the brain [60] where the translation occurs. 

Farther upstream in the olfactory system, the projection neurons from the olfactory bulb (mitral and tufted cells) distribute themselves to myriad destinations [19], whieh in turn project elsewhere and, in most cases, send a return link to the originating location [19]. These distributed projection pathways allow for numerous influences on behavior e.g., the odorant can be spoken about, presumably because of connections to cortex [19] odors may influence mood and emotion, even at subconscious levels [20], because of diverse eonneetions to limbic structures [19] physiological, e.g., autonomic and neuroendocrine, responses may be modulated by odors, because olfaction communicates rather directly with the hypothalamopitui-tary gonadal axis [21]. 

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