Olfaction sensory neurons

The detection of light, smells, and tastes (vision, olfaction, and gustation, respectively) in animals is accomplished by specialized sensory neurons that use signal-transduction mechanisms fundamentally similar to those that detect hormones, neurotransmitters, and growth... 

The anosmic phenotypes of mice strains which are deficient in G0if (Belluscio et al., 1998), ACIII (Wong et al., 2000) and functional relevant channel subunits (Brunet et al., 1996 Baker et al., 1999) emphasize the central role of the AC/ cAMP pathway for signal transduction in vertebrate olfactory sensory neurons. These features may be considered as evidence for cAMP as the sole relevant second messenger in vertebrate olfaction. 

PELZ, D., ROESKE, C. C., GALIZIA, C. G., Functional response spectrum of genetically identified olfactory sensory neurons in the fruit fly Drosophila melanogaster. Conference Abstract, 8th European Symposium in insect taste and olfaction, 2003. 

Many odor theories have been proposed in the past, attempting to explain the multitude of often very complex phenomena observed in human olfaction. Most of them were only partially, if at sill, successful. Nevertheless, slowly a consensus developed and today it is generally assumed that the primary process of chemorecep-tion takes place at the cell membrane of a sensory neuron and involves physical contact of the stimulant with potentieil or actual receptor sites which could be either specialists - reacting only with one structural class - or generalists which would react with a multitude of structural classes. 

Analogous events occur in the nose when many odorants interact with the epithelia therein. The odor component is conveyed via the odorant s interaction with molecular receptors on olfactory sensory neurons (first cranial nerve) in the olfactory epithelium [9] irritation is initiated by interactions with receptors or other mechanisms that stimulate the trigeminal (fifth cranial) nerve [10]. The term chemesthesis has been applied to distinguish this sensory experience from olfaction or, in the oral cavity, taste [11,12]. Importantly, chemesthesis is a bodywide experience. It is only on some portions of the head, e.g., eyes, nose, mouth, and some other facial areas, where information is conveyed by the trigeminal nerve (Fig. 1). 

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... 

The fact is that sensory-induced behavior changes can have many different ultimate explanations. All different neural levels in the sensory systems are theoretically open to the influence of neuroactive processes. In the case of insect olfaction, plasticity in AL neurons has been investigated mainly in three different... 

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. 

The time delay between sensory signaling and the subjective conscious perception of sensation which has so carefully been demonstrated by neurosurgeon Benjamin Li bet and other experimenters may also have a simpler explanation than those proposed so far. Li bet and others have shown that a pin-prick of the finger, for example, transmits a signal to the cortex of the brain via the thalamus, which arrives in a few thousandths of a second. All sensory signaling except olfaction is similarly transmitted first to the thalamus which acts as a sort of relay-station distributing the signals to the appropriate domains of the cortex. Yet conscious perception of the pin-prick can by various experimental techniques be shown to be delayed by up to a half-second, while "cerebral neuronal adequacy" is achieved. 

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... 

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