How does the sense of smell work?
See an overview of the olfactory (smell) system in action.
See the olfactory system in action below
Anatomical level1Odor molecules enter the nasal passage and sweep past the olfactory epithelium, where olfactory neurons reside. The odor signal is transmitted through the olfactory nerves into the olfactory bulb and is sent to the brain regions that process smell, including the amygdala, the hippocampus, and the orbitofrontal cortex.
Cellular level1,2Olfactory neurons in the olfactory epithelium present cilia into the nasal passage. Odor molecules bind to olfactory receptors on the cilia and trigger an action potential inside the olfactory neuron. This signal is then transmitted to synapses of tufted cells in the glomeruli in the olfactory bulb.
Healthy olfactory neurons have numerous olfactory receptors coupled to G proteins.
Upon binding an odor molecule, they activate adenylate cyclase III (ACIII), which generates cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP).
cAMP activates cAMP-gated calcium channels (CNG), leading to calcium influx, which in turn triggers an action potential.
Calcium influx also shuts the cAMP signal down by triggering phosphodiesterase (PDE) activity, leading to cAMP degradation to adenosine monophosphate (AMP).
References: 1. Hartevelt, T. J. van & Kringelbach, M. L. Brain Mapping. Anat Physiology 347–355 (2015) doi:10.1016/b978-0-12-397025-1.00235-9 2. Ronnett, G. V. & Moon, C. G proteins and olfactory signal transduction. Annu Rev Physiol 64, 189–222 (2002). 3. Kaupp, U. B. Olfactory signalling in vertebrates and insects: differences and commonalities. Nat Rev Neurosci 11, 188–200 (2010) 4. Pifferi S, Menini A, Kurahashi T. Signal Transduction in Vertebrate Olfactory Cilia. In: Menini A, editor. The Neurobiology of Olfaction. Boca Raton (FL): CRC Press/Taylor & Francis; 2010. Chapter 8. Available from: https://www.ncbi.nlm.nih.gov/books/NBK55986/. 5. Nakamura, T. Cellular and molecular constituents of olfactory sensation in vertebrates. Comp Biochem Physiology Part Mol Integr Physiology 126, 17–32 (2000).