Receptor neurons olfaction

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. 

The sensitivity and selectivity of olfaction and contact chemosensation are due (1) in the brain, to the existence of a neuronal network of neurons tuned to a specific chemical stimulus, and (2) in the periphery, to the existence of olfactory/ chemosensory receptor neurons housed in sensory microorgans called sensilla. The sensilla can best be viewed as simple cuticular porous extrusions that increase the surface that captures airborne odorants or chemicals dissolved in water droplets. They contain the receptive olfactory or chemosensory structures (Schneider, 1969). The olfactory sensilla are most numerous on the antennae and mediate the reception of sex pheromones and plant volatiles, as well as other odorants. Low volatility pheromones may also be detected by contact chemoreceptors on... 

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. 

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. 

Figure 32.16 Differing gene expression and connection patterns in olfactory and bitter taste receptors., In olfaction, each neuron expresses a single OR gene, and the neurons expressing the same OR converge to specific sites in the brain, enabling specific perception of different odorants. In gustation, each neuron expresses many bitter receptor genes, and so the identity of the tastant is lost in transmission.

The sense of smell long remained the most enigmatic of our senses. The genetic bases of biological olfaction were not understood until Richard Axel and Linda Buck discovered a family of about one thousand genes that encode olfactory receptor neurons [1], For their pioneering smdies Axel and Buck were awarded in 2004 with the Nobel Prize in Physiology or Medicine. 

The human oral cavity is potentially coimected to the nasal cavity by way of the buccopharynx (oropharynx), pharynx, and nasopharynx [1,2]. Under those circumstances in which this potential connection is open, the air movement of an exhalation that exits from the anterior nates (nostrils) can acquire odorants from the oral cavity and move them through the nasal cavity. If these odorants, while in the nasal cavity, reach the olfactory mucosa at a flow rate and concentration [3,4] that allow penetration to olfactory receptor neurons [5] and activation of these receptors such that sufficient central nervous system (CNS) responses develop, retronasal olfaction may occur. A limitation to the present understanding of retronasal olfaction is the absence of empirical or numerical models of retronasal odorant transport in adult humans. Such models have been published for orthonasal olfaction via the anterior nares [4,6] but are not presently available for retronasal olfaction (experimental airflow and odorant uptake analysis is in progress PW Scherer, personal communication, October 2002). 

Access from the oral cavity to the nasal cavity is necessary for retronasal olfaction, but it is not sufficient. As suggested, it is conceivable that odorants could reach the nasal cavity from the mouth but not arrive at the olfactory epithelium at all, or at least not in a quantity per unit time that would be sufficient to partition into the mucous covering of that sensory epithelium, activate relevant populations of olfactory receptor neurons, and... 

One of the lesser functions of the nose in man is that of olfaction. The olfactory region of the nose, a small patch of tissue containing the smell receptors, is located towards the roof of the nasal cavity and is lined with non-ciliated neuro-epithelium. Approximately 20% of the air flowing through the nasal cavity is directed upwards to the olfactory region. Here, bipolar neurones react to inspired air and initiate impulses in the olfactory nerves. 

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. 

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