Olfaction is the special sense of smell via chemoreceptors in the olfactory epithelium; the only sensory system that bypasses the thalamus en route to cortex.
The olfactory epithelium occupies about 2.5 sq.cm in each nostril and contains 10 to 20 million receptor cells.
Olfactory Epithelium and Receptor Cells
Roof of nasal cavity, superior concha to upper nasal septum; covered by mucus from Bowman's glands.
Olfactory receptor cell: bipolar neuron with short dendrite (olfactory rod) bearing 10 to 20 unmyelinated cilia contacting odorants in mucus.
Humans are microsmatic (small olfactory area); animals such as dogs, in which smell is well developed, are macrosmatic.
Receptor cells arise from progenitor (stem) cells and regenerate about every 60 days; bone morphogenic protein (BMP) inhibits this renewal.
Olfactory mucosa is the only site in the body where the nervous system is directly exposed to the external environment.
In rodents and other mammals, the vomeronasal organ (along the nasal septum) senses pheromones; its axons project to the accessory olfactory bulb, mainly to the amygdala and hypothalamus.
These vomeronasal-linked areas are concerned with reproduction and feeding behaviour.