Smell:
In human the receptors for the sense of smell are called olfactory cells located high in the roof of nasal cavity.
Olfactory cells are actually neurons that synapse with the nerve fibres making up the olfactory nerve. Each olfactory cell has cilia, which are stimulated by many chemicals in the air. Research resulting in the stereochemical theory of smell has shown that different smells may be related to the various shapes of the molecules rather than to the atoms that make up the molecules. These shapes fit specific olfactory sites on the olfactory cells cilia.
The sense of smell is generally much more acute than the sense of taste. Human nose, for example can detect one 25 millionth of 1 mg of mercaptan, the odoriferous chemical given off by a skunk. This averages out to approximately one molecule per sensory ending. Yet humans have a weak sense of smell compared to other vertebrates, such as dogs.
Taste:
The receptors for the sense of taste in mammals are the taste buds. Most of these are located on the tongue but they also present on the surface of the soft palate, pharynx and epiglossis. Each taste bud contains number of elongated cells. These cells have microvilli which project through the taste pore an opening in the taste bud. It is the micro villi that stimulated by various chemicals in the environment.
Humans are believed to have four types of taste buds, each type stimulated by chemicals that result in a bitter, a sour, a salty or a sweet sensation.
The sense of taste and the sense of smell supplement each other, creating a combined effect when interpreted
by the cerebral cortex. For example when you have a cold, food seems to lose its taste but actually the ability to sense its smell is temporarily absent. This may work in reverse also. When we smell something, some of the molecules move from the nose down into the mouth and stimulate certain taste buds. Thus, part of what we refer to as smell is actually taste.
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