Physiology of Smell
Breathing. [online image]
Air moves fairly quickly throught the nasal cavity (250 mL/s).  Therefore only a small amount of air actually comes in contact with the olfactory epithelium.  A sniff allows more air to come in contact with it.
In order to activate the olfactory receptors, the odorant must be in the gaseous state and it must dissolve in the mucus coating the olfactory epithelium.
activation [online image]
Once the odorant has dissolved it stimulates a G protien on the cilia of the receptor cell.  This stimulation causes Adenylyl cyclase (an enzyme) to be activated.  This enzyme speeds up the converison of ATP to cAMP.  Cyclic adenosine monophosphate (cAMP) binds to action channels in the membrane of the cilia.  The channel open allowing Ca++ and Na+ to enter the cilia.  The presence of Ca++ and Na+ cause the membrane to depolarize (become more positive) and an action potential is created.
G protien activation [online image]
The action potential allows the impulse to be sent to the glomeruli.  Each glomeruli receives only one type of odor signal.  Different odors excite a variety of different glomeruli.  Humans are able to smell about 10,000 different smells.  The mitral cells take the signal from the glmoeruli, refine it, and then send it via the olfactory tract to the brain. 
Neuronal connections [online image]
The olfactory tract travels to the olfactory cortex and then to the frontal lobe.  The olfactory tract also connects to other regions of the brain, that produce an emotional response to the smell.  Some smells activate a flight-or-fight response while others can trigger reflexes like sneezing and choking.  Appetizing odors can cause someone to salivate and it stimulates their digestive tract.
Olfactory projections [online image]
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