7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
Gustation (taste) and olfaction (smell) are called chemical senses because the stimuli are molecules. Your chemical senses are important systems for warning and attraction. You won’t eat rotten eggs or drink sour milk, and you can smell smoke before a sensitive household smoke detector. Evolutionarily, these adaptations increased chances of survival.
Taste receptor cells are most concentrated not only on your tongue in taste buds embedded in tissue called fungiform papillae, but are also on the roof of your mouth and the opening of your throat. Tasters have an average number of taste buds, nontasters have fewer taste buds, and supertasters have the most. You can taste only molecules that dissolve in your saliva or a liquid you drink. Scientists have identified five types of taste receptors for sweet, salty, sour, bitter, and, most recently, umami or glutamate. Babies show a preference for sweet and salty, both necessary for survival, and disgust for bitter and sour, which are characteristic of poisonous and spoiled substances. Supertasters are more sensitive than others to bitter, spicy foods and alcohol, which they find unpleasant. Each receptor is sensitive to specific chemicals that initiate an action potential. The pathway for taste messages passes to the brainstem, thalamus, and primary gustatory cortex. Receptors for different tastes activate different regions of the primary taste cortex. Our tongues also have receptors for touch, pain, cold, and warmth. The sensory interaction of taste, temperature, texture, and olfaction determine flavor.
Odor molecules reach your moist olfactory epithelium high in your nasal cavity through the nostrils of your nose and the nasal pharynx linking your nose and mouth. Dissolved odor molecules bind to receptor sites of olfactory receptors, triggering an action potential. Research has not uncovered basic odors. Axons of olfactory sensory neurons pass directly into the olfactory bulbs of the brain. Sensory information about smell is transmitted to the hypothalamus and structures in the limbic system associated with memory and emotion as well as the primary cortex for olfaction on the underside of the frontal lobes, but not the thalamus. The primary olfactory cortex is necessary for making fine distinctions among odors and using those distinctions to consciously control behavior.