The stimuli for taste
are chemical substances dissolved in water or other fluids. Taste can be described as four basic sensations, sweet, sour, salty,
, which can be combined in various ways to make all other taste sensations. Taste receptors (called taste buds
) for these sensations are located primarily on various areas of the tongue: front, sweet; sides, sour; sides and front, salty; and back, bitter (Figure
). There are about 10,000 taste buds, which are situated primarily in or around the bumps (papillae)
on the tongue. Each papilla
contains several taste buds, from which information is sent by afferent nerves
to the thalamus
and, ultimately, to areas in the cortex.
Smell (olfaction). The stimuli for smell are volatile chemical substances suspended in the air. These molecules stimulate the olfactory receptors, which are in the upper portions of the nasal passages. Neurons from these receptors bundle together to form the olfactory nerve, which travels to the olfactory bulb at the base of the brain. The theory of smell is not well understood (for example, how an odor of apple pie can evoked pleasant childhood memories).