Olfactory system number

In vertebrates the neurons for olfaction are located in the nose mucosa and consist of short neurons with a peripheral ending endowed with odorant receptors for a large number of molecules in the environment. Each receptor neuron only contains one odorant receptor and is connected directly with the olfactory lobe of the brain. The vertebrate olfactory system must cope with a staggering developmental problem how to connect millions of olfactory neurons expressing different odorant receptors to appropriate targets in the brain. 

Earlier experiments based on EAG and SSR highlighted the inordinate specificity and sensitivity of the insect olfactory system. While minimal structural modifications to pheromone molecules render them inactive [12], a single molecule of the native ligand is estimated to be sufficient to activate an olfactory neuron in male antennae [14]. The large number of detectors certainly contributes to the sensitivity of the olfactory system, but selectivity is a matter of... 

Remarkable are the relations between the chemical structure of aliphatic compounds and their odour threshold for the human olfactory system and hence for their importance in aroma chemistry. Thus, e.g. the odour threshold for alkanals decreases with increasing number of carbon atoms (C5 to C10), the introduction of a double bond generally lowering the threshold. In general, it can be assumed that alkenals... 

Olfaction, like visual and taste perception, is an ancient process. Olfaction plays a role in sexual arousal. The olfactory system in mammals is remarkable with respect to the number of receptors engaged in monitoring odours. There are several thousand hepta-helical G-protein-coupled receptors in the olfactory epithelium and the nasal organ of a dog, and still about 1000 receptors in the corresponding human organs. It has been estimated that nearly 1% of all genes code for olfactory receptors alone. 

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. 

Olfactory Marker Protein ORNs express high levels of olfactory marker protein (OMP), which is unique in the olfactory system to ORNs (Margolis, 1972 Keller and Margolis, 1975). OMP is found in a number of mammalian species and it appears to be expressed in all mature ORNs. Studies in mice containing a null mutation for OMP suggest that this protein may play a role in ORN adaptation to odors (Ivic et al., 2000), signal amplification, and transduction (Youngentob et al., 2004). 

Major connections of the main olfactory system. Axons of MOB mitral/tufted cells (circles in the EPL and MCL, respectively) project as the LOT to synapse in a number of structures collectively referred to as primary olfactory cortex (POC). Centrifugal inputs to MOB include feedback projections from POC as well as inputs from subcortical forebrain and brainstem neuromodulatory cell groups. Abbreviations AON, anterior olfactory nucleus DP, dorsal peduncular cortex Ent, entorhinal cortex IG-AHC, indusium griseum-anterior hippocampal continuation LC, locus coeruleus NDB, nucleus of the diagonal band PeCo, periamygdaloid cortex PC, piriform cortex RN, raphe nuclei (dorsal and median raphe) TT, taenia tecta Tu, olfactory tubercle... 

Meisami, E. and Safari, L. (1981) A quantitative study of the effects of early unilateral olfactory deprivation of the number and distribution of mitral and tufted cells and of the glomeruli in the rat olfactory system. Brain Res, 221. 81-107. 

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