What kind of neurons are olfactory receptors

However, the impressively large number of genes and the wide distribution in their expression support a cardinal role for ORs and TASRs in homeostasis, possibly with the complementary action of different receptors. Even considering that almost half of the human OR repertoire is probably non-functional Rouquier and Giorgi, ; Go and Niimura, , alterations in gene expression which are differentially regulated in a number of neurodegenerative and mental human diseases make the scenario a subject of attention.

This is further driven by the fact that observations in other organs and systems have shown that ORs located in non-olfactory organs are chemoreceptors with unexpected functions. The presence of a variable number of ORs in a single neuron, the variety of putative odorant ligands and the different feasible responses dependent on the region where the neuron is located and operates make elucidating the function of ORs in the nervous system an apparently tremendous task.

The study of single receptors expressed in neural cell lines and combined receptors in particular primary neuron cultures in combination with screening of putative agonists of ORs and suspected OR ligands, most of them probable intrinsic metabolites, will facilitate the task. Design and isolation of small molecules to act on selected ORs functioning as cell-type specific agonists or antagonists seems a promising tool.

Reconstruction of the tri-dimensional structure of receptors will help in the analysis of mechanisms of activation and in the recognition and design of functional ligands, and specific agonists and antagonists Persuy et al. The activation of ORs and immune cellular responses by common molecules in several systems Jang et al.

The fact that the CNS is not an isolated immune organ but rather interacts with the systemic immune system, together with the expression of ORs and related OR factors in brain and choroid plexus directly interacting with blood, reinforces the notion that ORs in the CNS play novel functions unrelated to olfaction.

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Taste buds are located on papillae and distributed on the surface of the tongue. Taste buds are also found on the oral mucosa of the palate and epiglottis. These pear-shaped structures contain about 80 cells arranged around a central taste pore Figure 9. Taste receptor cells are spindle shaped, modified neuro-epithelial cells that extend from the base to the apex of the taste buds.

Synaptic vesicles are present near the apex and the basal region in many taste cells. Microvilli from each taste cell project into the taste pore which communicate with the dissolved solutes on the surface of the tongue. These receptor cells are innervated by afferent nerve fibers penetrating the basal lamina.

The nerve fibers branch extensively and receive synaptic input from the taste receptor cells. A group of non-receptor columnar cells and basal cells are present within taste buds.

The basal cells migrate from adjacent lingual epithelium into the buds and differentiate into taste receptor cells which are replaced about every days.

Taste solutes are transported to the taste pore and diffuse through the fluid layer to make contact with membrane receptor proteins on the microvilli and apical membrane. Taste sensitivity is dependent upon the concentration of the taste molecules as well as their solubility in saliva.

Taste sensation can be evoked by many diverse taste solutes. The pattern of membrane potential change include depolarization, depolarization followed by hyperpolarization, or only hyperpolarization. In response to this cation, neurotransmitter is released, which produces synaptic potentials in the dendrites of the sensory nerves and action potentials in afferent nerve fibers Figure 9. Anions such as Cl - contribute to the salty taste, but anions are transported into these cells by a paracellular route.

The influx of these ions of salt evokes a depolarization in the apical membrane Figure 9. Sweet tasting solutes, sugars and related substances, bind to membrane receptor proteins which are coupled to a G-s protein gustducin , which activates adenylyl cyclase AC.

Some sweet solutes and non-sugar sweeteners interact with a receptor membrane protein through a G protein, which activates phospholipase C.

Bitter tasting solutes include many non-toxic and toxic alkaloids, hydrophilic quinine and some divalent ions. These mechanisms for taste transduction were identified in laboratory animals and are probably present in the microvilli and apical membrane of taste receptor cells in humans. Taste stimuli produce depolarizing and hyperpolarizing potentials in individual taste cells.

These transmitters diffuse across the synaptic cleft and lead to the initiation of action potentials in the afferent nerve fibers. However, taste studies conducted on the neural response of whole cranial nerves demonstrate that a pattern of activity is produced by foods that are similar in taste. These patterns of activity are a clue to a taste code that occurs in many different taste cells and neurons responding to a particular taste stimulus.

This finding indicates that no single fiber conducts only one taste quality i. Recognition that branches of nerve fibers innervate several cells within and between taste buds indicates that a population of sensory nerve fibers activated by a taste stimulus transmits a neural code of the taste quality.

Both the vagus and glossopharyngeal nerves innervate the pharynx and epiglottis. Axons of these three cranial nerves terminate on 2 nd order sensory neurons in the nucleus of the solitary tract. From this site in the rostral medulla, axons project into the parabrachial nucleus in lower animals but not in humans. In humans, fibers of the 2 nd order neurons travel through the ipsilateral central tegmental tract to the 3 rd order sensory neurons in the ventroposterior medial nucleus VPM of the thalamus.

The VPM projects to the ipsilateral gustatory cortex located near the post-central gyrus representing the tongue or to the insular cortex. See Figures 9. The olfactory system in humans is an extremely discriminative and sensitive chemosensory system. Humans can distinguish between 1, to a predicted high of 4, odors. All of these odors can be classified into six major groups; floral, fruit, spicy, resin, burnt, and putrid Refer back to Figure 9.

The perception of odors begins with the inhalation and transport of volatile aromas to the olfactory mucosa that are located bilaterally in the dorsal posterior region of the nasal cavity. Cochrane Collaboration on Olfactory receptor neuron. Bandolier on Olfactory receptor neuron. TRIP on Olfactory receptor neuron. Ongoing Trials on Olfactory receptor neuron at Clinical Trials. Trial results on Olfactory receptor neuron. Clinical Trials on Olfactory receptor neuron at Google.

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