Chapter 7C - Taste and smell
Taste and smell are considered to be "special visceral" sensations. These are chemical sensations with receptors being activated by odorants and tastants that interact with specific membrane molecules. Olfaction is highly complex due to the number of molecules that are detected and, to this point, there are no recognized number of "primary olfactory" stimulants from which all odors are created. This has limited the ability to analyze odors mechanically. The primary olfactory epithelium is in the upper part of the nasal cavity. Olfactory epithelial cells have processes containing membrane receptors for a specific molecular structure. These epithelial cells are neural structures that have axons traversing the cribriform plate of the ethmoid bone to reach the olfactory bulb. In the olfactory bulb, olfactory epithelial cells synapse with processes of mitral cells in glomeruli. Each glomerulus includes only axons from olfactory epithelial cells that detect the same odorant. The mitral cells give rise to axons comprising the olfactory tract, which are located immediately ventral to the frontal lobes. The olfactory tract trifurcates, with some fibers terminating directly in the anterior perforated substance of the ventral forebrain, and others splitting into lateral and medial olfactory stria. These stria terminate on the ipsilateral and contralateral olfactory regions of the temporal lobes, respectively. In the temporal lobes, there are terminations in the olfactory cortex (prepiriform part of the parahippocampal gyrus covering the uncus) and the amygdala. This latter structure participates in visceral and behavioral responses to odorants.
Taste, representing the primary sensations of sweet, salty, sour and bitter, are conveyed along fibers of the facial, glossopharyngeal and vagus nerves (the latter is minimal) from anterior to posterior on the tongue. These fibers terminate in the gustatory nucleus of the brain stem. Relay from taste is through the parabrachial nucleus and part of the VPM of the thalamus to the inferior part of the postcentral gyrus.