Olfactory system nerve

In neurons of the olfactory system (nerves of smell) and in the cerebellum there are relatively high concentrations of the constitutive heme oxygenase-2. There appears to be a locally limited region of CO biosynthe-... 

Odors are perceived via the olfactory system, which is composed of two organs in the nose the olfactory epithelium, a very small area in the nasal system, and the trigeminal nerve endings, which are much more widely distributed in the nasal cavity (11). The olfactory epithelium is extremely sensitive, and humans often sniff to bring more odorant in contact with this area. The trigeminal nerves initiate protective reflexes, such as sneezing or interruption of irrhalation, with exposure to noxious odorants. 

Because the chemical signals (semiochemicals) are normally produced in minute amounts and diluted in the environment with a complex mixture of chemical compounds derived from a myriad of sources, the olfactory system in insects evolved as a remarkably selective and sensitive system, which approaches the theoretical limit for a detector. For example, it has been estimated that the male silkworm moth is able to distinguish within 1 s 170 nerve impulses generated by the female silkworm moth s sex pheromone from 1700 spontaneous nervous impulses [ 1 ], thus, operating on a remarkably low S/N ratio ... 

Apart from taste, vertebrates have five different chemoreceptor systems for airborne chemosignals the main olfactory system, the vomeronasal organ (VNO), the trigeminal nerve, the septal organ of Masera, and the nervus termi-nalis. They each will be discussed in turn. All five are fully functional in most mammals (Fig. 5.1). 

By contrast, bony fish have their olfactory organs on the dorsal side of the snout at some distance from the mouth. The olfactory system in fish involves the first (olfactory) cranial nerve, while the ninth (glossopharyngeal) and other nerves serve the sense of taste. 

The vomeronasal system, also known as the accessory olfactory system, consists of chemoreceptors, organized into the VNO, the vomeronasal nerve, its terminal, the accessory olfactory bulb, and more central pathways. First described by Jacobson in 1811, the VNO has been studied intensely. We now know how stimuli reach it and what behaviors it mediates. The VNO occurs in amphibians, reptiles, and mammals. Among mammals, it is best developed in marsupials and monotremes. In birds it only appears during embryogenesis. The VNO and its function are best known for squamate reptiles, particularly snakes, and rodents and ungulates among the mammals. 

Nerve transection experiments have shown that normal estrus cyclicity and behavioral estrus in mice relies on sensory input through the main olfactory bulbs and does not require the accessory olfactory system (Rajendren and Dominic, 1986). 

The olfactory system is another area where the density of H3 receptors is not correlated with histaminergic nerve fibres. H3 receptors are more abundant in the olfactory nuclei than in the main olfactory formation, a distribution opposite to that of... 

Figure 5.9 The human olfactory system. (A) Section through the nose. (B) Section through the cribriform plate. (C) The olfactory pathway to the cerebrum (forebrain). This shows the pathway of olfactory sensation. Nasal stimulation begins at the cilia of the olfactory receptor cells located at the ends of the olfactory nerves. The olfactory nerves then carry the impulse to the cerebrum, resulting in the sense of smell.

Olfactory Term relating to the sense of smell. The olfactory system is made up of a pair of small patches of epithelial tissue at the top of the nose called the olfactory organs. Microscopic threads called olfactory hairs detect the presence of odorous molecules and send impulses along the olfactory nerve to the brain for interpretation. 

Vertebrates possess three primary chemosensory systems gustation ( taste ), trigeminal, and olfaction ( smell ) but only one of these, the olfactory system, mediates responses to pheromones. Chemicals that stimulate the olfactory system are known as odorants and comprise one type of biological cue (any entity that stimulates a sensory system). Bouquets of odorants that can be discriminated as specific entities are termed odors. The olfactory system contains olfactory receptor neurons (ORNs) that comprise cranial nerve I and project directly to the forebrain. ORNs are now known to express only one to a few olfactory receptor proteins ( receptors ), which means that the chemoreceptive range of each neuron can be very narrow. The olfactory system also has several subcomponents including the vomeronasal organ, which is described below. 

Another way that animals eould provide an early warning is via their sense of smell. Most animals have much more sensitive olfactory systems when eompared to humans. Animals may be able to be trained to sense low nerve gas concentrations (Dalton, 2003). 

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. 

Heimer, L. (1968) Synaptic distribution of centripetal and centrifugal nerve fibres in the olfactory system of the rat. An experimental anatomical study. J. Anat., 103, 413 32. 

About 60% of odorants also trigger receptors in the trigeminal nerve fibres, which run around the entire surface of the nasal cavity. For example, nicotine fires both the olfactory and trigeminal nerves whereas vanillin triggers only the olfactory system. Indeed, it is suggested that our ability to distinguish between the odours of the enantiomers of nicotine is due to the trigeminal rather than the olfactory system (Hummel et al., 1992 Thurauf et al., 1995). 

Korsching, S. I., Argo, S., Campenhausen, H., Friedrich, R. W., Rummrich, A., and Weth, F. (1997), The ontogenesis of stereotyped neuronal networks in the olfactory system of zebrafish. Molecular Basis of Axon Growth and Nerve Pattern Formation, H. Fujisawa, ed., JSSP, Tokyo, 133—142. 

The once controversial concept of functional olfaction in birds is now firmly established on the basis of anatomical, behavioral, and physiological evidence. Olfactory structures are present in all birds examined and their general characteristics are the same as those of reptilian and mammalian olfactory systems. A notable difference in chemoreceptive systems appears to be the absence in birds of both the vomeronasal system and the terminal nerve. 

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