Olfaction neuronal connections

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 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. 

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.

It has been observed that the discriminatory capabilities of human olfaction are tremendous It was estimated that an untrained person could differentiate up to ten million odors, perhaps even significantly more than that. Information theory then shows that in order to encode the qualities of ten million odors in a simple binary mode (Monoosmatic components on or off, their intensity, albeit important, is in this connection disregarded) only 2h to 27 specific profiles, disregarding possible and probable redundancies, and therefore the same number of complementary receptor sites would be required. Assuming furthermore that said redundancy, in which the informational modalities of two different specific receptor sites of two different olfactory neurons are confluent in one collector cell and therefore contribute to the expression of only one monoosmatic component is indeed operational it becomes necessary to increase the total number of types of specific receptor sites to 2k-30. This means that only 2U-30 specific detector proteins are required for structure recognition in the transduction process. This compares to about UOOO enzyme systems in different stages of activity estimated to be present in a cell any time. 

Farther upstream in the olfactory system, the projection neurons from the olfactory bulb (mitral and tufted cells) distribute themselves to myriad destinations [19], whieh in turn project elsewhere and, in most cases, send a return link to the originating location [19]. These distributed projection pathways allow for numerous influences on behavior e.g., the odorant can be spoken about, presumably because of connections to cortex [19] odors may influence mood and emotion, even at subconscious levels [20], because of diverse eonneetions to limbic structures [19] physiological, e.g., autonomic and neuroendocrine, responses may be modulated by odors, because olfaction communicates rather directly with the hypothalamopitui-tary gonadal axis [21]. 

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). 

---