When we think of smelling, most folks think of what is properly called the olfactory sense, arising from millions of receptors in the olfactory mucosa. These receptors, by the way, unlike other mammalian neurons, can regenerate after injury, and undergo continuous turnover (replacing themselves with new copies every few weeks).
The vomeronasal organ is an important chemoreceptor in many animals, but probably not in humans. Its neurons can regenerate too. Don't get me started talking about this organ or I will bore you to death (if I have not done so already).
The septal organ of Masera feeds into the olfactory bulb in the brain, but its connections do not fuse with or exchange fibers with those from the olfactory and the vomeronasal receptors, which also feed to the olfactory bulb. Just think of the olfactory bulb of the brain as the location where the sensation arises, once the message has been transmitted from the receptors. Of course, if the olfactory bulb does not work properly, or if the fibers transmitting the signal from the receptors to the brain do not work properly, then normal sensation will not take place.
The nervus terminalis may be involved in chemoreception, but the evidence is spotty.
The trigeminal nerve is distributed within the olfactory sensory epithelium and contributes to the detection of odours, notably those that are irritating.
Regretfully, little is know about the central (in the brain) pathways of these five chemosenses.
Primary source: Graziadei, P. P. C. (1977). Functional anatomy of the mammalian chemoreceptor system. In Müller-Schwarze, D., & Mozell, M. M. (Eds.), Chemical signals in vertebrates. New York: Plenum.
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This page most recently revised on
the 22nd of November, 2013.