The senses of olfaction (smell) and taste are very closely associated. The receptors of both senses are chemoreceptors and, thus, are able to detect the presence of a chemical once the chemical has gone into solution in the liquid coating of the receptors' cell membranes. Vapors of foods can pass easily from the mouth to the nose so that taste is often a compound sensation detected by receptors in both the mouth and the nose. Vapors can also pass from the nose to the mouth and, thus, smell can be a compound sensation as well.
Yet, there are several important differences between the receptors of the nose and those of the mouth. The receptors of the nose, specialized to detect chemicals with origins distant from the organism, are much more sensitive than taste receptors which are specialized to detect chemicals from sources inside the mouth. Olfactory receptor cells, located in the upper portion of the nasal cavity, are true neurons. Each receptor cell's dendrite reaches to the surface of the nasal cavity's epithelial layer, in which the cell bodies of the receptors are embedded. At the epithelial lining's surface, the modified cilia of each receptor cell's dendrite protrudes into the layer of mucus covering the epithelium and act as the receptor sites for chemical stimulation. The receptor cells transmit impulses to the olfactory bulb of the brain. In contrast, taste receptor cells or taste buds, located primarily on the upper surface of the tongue but also on the palate and pharynx, are not neurons. The receptor cells have microvilli that are specialized to detect chemical presences; when
