Human Olfactory Pathway

Chuah M. and Zheng D. (1992). The human primary olfactory pathway fine structural and cytochemical aspects during development, and in adults. Micros Res Tech 23, 76-85. 

Figure 5.9 The human olfactory system. (A) Section through the nose. (B) Section through the cribriform plate. (C) The olfactory pathway to the cerebrum (forebrain). This shows the pathway of olfactory sensation. Nasal stimulation begins at the cilia of the olfactory receptor cells located at the ends of the olfactory nerves. The olfactory nerves then carry the impulse to the cerebrum, resulting in the sense of smell.

McCabe C, Rolls ET. Umami a delicious flavor formed by convergence of taste and olfactory pathways in the human brain. Eur. J. Neurosci. 2007 25 1855-1864. 

Humans are highly sensitive to the influence of odors in their living environment, and human emotions such as relaxation, exhilaration, sensuality, happiness, and well-being are closely related to odors that stimulate the olfactory pathways in the brain and, in particular, the limbic system. Throughout the world, the application of fragrances to mask the presence of unpleasant odors has been a common method employed to refresh a domestic environment. Personal hygiene products and cosmetics are frequently scented in order to mask natural odors emanating from the body. Perfumed substances in the form of an aerosol, liquid, or solid are often sprayed or rubbed directly on to the skin. 

It is important to identify and measure the concentrations of a number of compounds in a mixture simultaneously for several reasons. First, among related compounds there may exist precursors of active ones, and pathways of pheromone synthesis may be elucidated. This is true for steroids in the human axilla. Nixon etal. (1988) determined the concentration of five steroids extracted from axillary hair of adult men aged 18 to 40 years. The relationships in concentrations between the two ketones 5Q -androst-16-en-3-one and 4,16-androstadien-3-one suggest that axillary bacteria reduce the former to the latter with the aid of the enzyme 4-ene-5a-reductase. Humans have a low olfactory threshold for several 16-androstenes, and the fact that some men have large quantities of 16-androstenes (Nixon etal., 1988) is biologically suggestive. 

Fig. 2. A. Forebrain dopamine projection system in rodents and primates. The nigrostriatal pathway projects from the A8 and A9 groups of the substantia nigra (SN) via the medial forebrain bundle (mfb) to the neostriatum (NS). The mesocorticolimbic pathway projects from the more medially located A10 cell group of the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) and olfactory tubercle (OT) of the ventral striatum (VS) and limbic forebrain areas including prefrontal cortex (Ctx), septum (Se) and amygdala (A). B. Striatal projection areas in the rodent brain are divided into the more dorsal neostriatum, and ventral striatum. C. In the primate brain, including human and illustrated for the marmoset, the neostriatum is divided by the fibers of the internal capsule into caudate nucleus (CN) and putamen (Pu). Correspondingly, the neostriatum of rats is sometimes designated the caudate-putamen (CPu) complex.

---