Olfactory cells

The direct link between the nose and the reproductive system has been called a non-cognitive bridge (Stoddart, 1990). A part of VN projection is non-olfactory and reaches the limbic system via a direct and monosynaptic relay (Larriva-Sahd et al., 1993 and 1994 Matsumoto et al., 1994). In addition to the production of the peripheral olfactory cells (Sec. 4.1 above), the placode also gives rise to a group of important... 

The taste cells are situated in the lingual epithelium with the apical membrane exposed to the mucosal surface of the oral cavity and the basal surface in contact with the nerve [interstitial fluid] [FIGURE 10]. Within the basolateral surface are the nerves which respond to the chemestiietic stimulants, i.e. direct nerve stimulation. The microvilli at the apical membrane contain receptor proteins which respond to sweeteners, some bitters and possibly coolants. The olfactory cells are bipolar neurons with dendritic ends containing cilia exposed to the surface and axons linked to the brain, where they synapse in the olfactory bulb. The transfer of information from this initial stimulus-receptor interaction to the brain processing centers involves chentical transduction steps in the membrane and within the receptor cells. The potential chemical interactions at the cell membrane and within the cell are schematically outlined in FIGURE 10. 

A fine example of this problem Is the detection of an NP responsible for one species of insect finding its host plant. An analysis of the volatile chemicals made by the plant showed some chemicals made in large amounts, others in smaller amounts and some in minute amounts. By analysing the nerve output of the insect s olfactory cells to these chemicals, the insect was found to be responding to one of the very minor chemicals. 

Stratified squamous to mucociliated epithelium with olfactory cells Mucociliated epithelium (ciliated, mucous cells, basal cells) smooth muscle cells fibroblasts neuroendocrine cells immune cells Mucociliated epithelium with Clara cells in distal bronchioles and alveolar ducts... 

Boekhoff, I., Michel, W. C., Breer, H., and Ache, B. W., Single odors differentially stimulate dual second messenger pathways in lobster olfactory cells, J. Neurosci., 14, 3304, 1994. 

Kaissling K.-E., Klein U., de Kramer J. J., Keil T. A., Kanaujia S. and Hemberger J. (1985) Insect olfactory cells electrophysiological and biochemical studies. In Molecular Basis of Nerve Activity, eds J. P. Changeux and F. Hucho. Walter de Gruyter Co., Berlin. 

Asanuma N. and Nomura H. (1991) Cytochemical localization of adenylate cyclase activity in rat olfactory cells. Histochem. J. 23, 83-90. 

Den Otter C. J. and Van der Goes van Naters W. M (1993) Responses of individual olfactory cells of tsetse flies (Glossina M. morsitans) to phenols from cattle urine. Physiol. Entomol. 18, 43—49. 

The mechanism of olfaction has many theories but is not fully understood and is still the subject of research. The nose is the human organ that detects smell (Fig. 5.9). It extends from the face to the end of the palate. In its simplest explanation the two nasal cavities are lined with a mucous membrane, kept moist by the secreted substance mucus. Chemicals in the air entering the nose must dissolve in this mucus before they can be detected. A small area - about the size of a small postage stamp - in the upper part of the nasal cavity contains olfactory cells, which are sensitive to the chemicals in the mucus solution. For a molecule to be detected it must bind specifically to the sensitive cells that act as sensory receptors. The sensory receptors situated in the olfactory epithelium (epithelium is the name given to the outer layer of covering cells) are believed to bind specifically with substances according to the shape of their molecules. 

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 olfactory epithelium is composed of basal, neuronal (olfactory), and susten-tacular (support) cells (Figure 27.3). The portion of each olfactory cell that responds to the olfactory chemical stimuli is the cilia. The odorant substance first diffuses into the mucus that covers the cilia and then binds to specific receptor proteins in the membrane of each cilium. Next, receptor activation by the odorant activates a multiple molecules of the G-protein complex in the olfactory epithelial cell. This, in turn, activates adenylyl cyclase inside the olfactory cell membrane, which, in turn, causes formation of a greater multitude of cAMP molecules. Finally, the cAMP molecules trigger the opening of yet an even greater multitude of sodium ion channels. This amplification mechanism accounts for the exquisite sensitivity of the olfactory neurons to extremely small amounts of odorant. The olfactory epithelium is an important target of certain inhaled toxicants. Certain metals, solvents, proteins, and viruses are transported to the brain via transport from the olfactory epithelium to the olfactory tract and exert neurotoxicity. 

A potent aaivator of the cAMP, cyclic nucleotide-gated (CNG) Na+/K+ channel in olfactory cells is NO (nitric oxide), which is many times more potent than cAMP. However, NO is present only in developing olfactory neurons, where it seems to react directly with cysteine SH-groups in the P-subunit of the channel. It was speculated, therefore, that NO might be a channel activator early in development or in regenerating olfactory neurons (see ref. 91). 

CNG channels (cyclic nucleotide-gated ion channels) are activated by the binding of cyclic GMP. They mediate sensory signal transduction in photoreceptors and olfactory cells. Six mammahan CNG channel genes are known and some human visual disorders are caused by mutations in retinal rod or cone CNG genes. ... 

The olfactory cells run through the cribriform plate at the base of the skull and feed directly into the olfactory bulb. Neurons from the epithelium converge on areas of the olfactory bulb known as glomeruli. Evidence... 

Kaissling, K.-E. Elementary receptor potentials of insect olfactory cells. In Kurihara, K., Suzuki, N., Ogawa, H. (eds.) XI Int. Symp. Olfaction and Taste, pp. 812-815. Springer, Tokyo (1994)... 

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