The senses of gustation (taste) and olfaction (smell) differ from the other senses in that they respond to chemicals. The chemical senses tell us about the things we eat, drink, and breathe.
Taste
We are able to taste food and other things because of the 10,000 taste buds on the tongue. Each taste bud contains approximately a dozen sensory receptors, called taste cells , grouped together much like the segments of an orange ( fi gure 5.19 ).
These taste cells are sensitive to chemicals in our food and drink (Bartoshuk, 1988).
The taste buds are further bunched together in bumps, called papillae , that can easily be seen on the tongue.
Collectively, the taste buds respond to thousands of chemicals, but all our sensa- tions of taste apparently result from the stimulation of different combinations of a small number of different types of taste receptors, which are most responsive to only one class of chemicals each. There are taste buds that respond primarily to chemicals
gustation (gus-ta¯ ´ -shun) Sense of taste.
olfaction (o¯l-fak´ -shun) Sense of smell.
taste cells Sensory receptor cells for gustation located in the taste buds.
papillae (pah-pil ´ eˉ) Clusters of taste buds on the tongue.
There are no right or wrong answers to the following questions. They are presented to help you become an active reader and think critically about what you have just read.
1. In what ways is the experience of pain both a psychological and a physical event?
2. Have you had experiences in your life that are consistent with the gate-control theory of pain?
To be sure that you have learned the key points from the preceding section, cover the list of correct answers and try to answer each question. If you give an incorrect answer to any question, return to the page given next to the correct answer to see why your answer was not correct.
1. Sensory receptors located in the muscles, joints, and skin provide the brain with messages about movement, posture, and orientation of the body. These are called receptors.
a) vestibular c) ciliary
b) semicircular d) kinesthetic
2. Which is not one of the four types of receptors in the skin?
a) vestibular cells c) tactile discs b) basket cells d) specialized end bulbs
3. The sensation of intense heat is created by the stimulation of on the skin.
a) warm spots c) both warm and cold spots
b) cold spots d) vestibular cells
4. According to the theory of pain, placing a sore foot in warm water blocks pain signals on the slow-pain pathway by closing neural gates in the spinal cord and brain stem.
Thinking Critically about Psychology Check Your Learning
Correct Answers: 1. d (p. 137),
2. a (p. 138), 3. c (p. 139),
4. gate-control (p. 140)
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that give rise to the sensations of sweetness (mostly sugars), sourness (mostly acids), saltiness (mostly salts), and bitterness (in response to a variety of chemicals that have no food value or are toxic). In addition, there is evidence that there is a fi fth type of taste bud, which gives rise to the sensation of fattiness in response to fats (Schiffman
& others 1998). Some scientists believe that there is yet another kind of taste bud that give rise to the sensation called umami (the savory taste of meat stock, cheese, and mushrooms), but this has been shown to arise from the same taste buds that give rise to the sensation of sweetness (Li, & others, 2002).
Interestingly, the taste buds that are most sensitive to these fi ve classes of chemi- cals are not evenly distributed over the tongue. They are bunched together in different parts of the tongue, as shown in fi gure 5.20 . This means that different parts of the tongue are sensitive to different tastes. We do not usually notice this, because the differences in sensitivity are not great and because our food usually reaches all parts of the tongue during the chewing process anyway. But, if you ever have to swallow a truly bitter pill, try placing it in the exact middle of the tongue, where there are no taste receptors at all.
We lose taste buds as we age, especially over 45 years of age. Babies have the most taste buds and are very sensitive, whereas older adults are less sensitive to the chemicals that give rise to taste sensations (Schiffman & others, 1998).
Our perception of food also includes sensations from the skin surfaces of the tongue and mouth: touch (food texture and thickness), temperature (cold coffee tastes very different from hot coffee), and pain (as in jalapeño peppers). The sight and aroma of food also greatly affect our perception of food.
Smell
Chemicals in the air we breathe pass by the olfactory receptors on their way to the lungs. These receptor cells are located in a dime-sized, mucous-coated sheet at the top of the nasal cavity called the olfactory epithelium (see fi gure 5.21 on p. 146 ).
Until recently, scientists believed that we could smell only a limited number of pri- mary odors (Ackerman, 1991; Amoore, Johnston, & Rubin, 1964). In 2004, however, Linda Buck and Richard Axel received the Nobel Prize in Medicine and Physiology for demonstrating that hundreds—perhaps thousands—of different receptors detect a multitude of different chemicals in the air we breathe (Buck, 1996; Dulac & Axel, 1998; Liberles & others, 2009).
Interestingly, nearly all the chemicals that humans can detect as odors are organic compounds, meaning they come from living things. In contrast, we can smell very few inorganic compounds, such as rocks and sand. Thus, our noses are useful tools for sensing the qualities of plants and animals—necessary, among other things, to distin- guish between poisonous and edible things.
The sense of smell is important in and of itself, of course, sometimes bringing messages of sweet perfumes to the brain and other times warning of dangerous and foul odors. But the sense of smell contributes to the sense of taste as well. Not only do we smell foods as they pass beneath our noses on the way to our mouths, but odors also rise into the nasal passage as we chew. We are usually unaware of the impact of smell on the sense of taste, until a head cold makes everything taste like paste. The contribution of smell to taste is important because of the greater sensitivity of the sense of smell. The nose can detect the aroma of cherry pie in the air that is 1/25,000th of the amount required for the taste buds to identify it (Ackerman, 1991).
Pheromone Detection
There is growing evidence that an additional chemical sense plays important roles in the regulation of reproductive behavior in many species, including humans. Many mammals have receptors for detecting chemicals known as pheromones . Pheromones
olfactory epithelium (oˉl-fak ´ to-reˉ ep´´i-theˉ ´ leˉ-um) Sheet of receptor cells at the top of the nasal cavity.
pheromones Chemicals that
stimulate receptors in the vomeronasal organ in some animals, infl uencing some aspects of reproductive behavior.
Figure 5.19
Taste buds contain clusters of taste (gustatory) receptor cells.
Receptor cells
Pore Surface of tongue
Sensory nerve fiber
Figure 5.20
Areas of the tongue that are most sensitive to the fi ve primary qualities of taste: sweet, fatty, salty, sour, and bitter.
Bitter Sour Salty Sweet and fatty
released in the sweat, saliva, or urine of one animal are sensed by these chemical receptors. The pheromone receptor cells send neural messages to several areas of brain, includ- ing the hypothalamus, which stimulates the release of sex hormones, initiates ovulation in some nonhuman species, and infl uences sexual motivation and behavior (Savic, 2002).
There is evidence that when women inhale adrostadi- enone, a male sex hormone found in high concentrations on the skin, their attention is shifted to emotion-relevant stimuli, such as facial expressions (Hummer & McClintock, 2009) even when the women are unaware of the hormone. There also is evidence that smelling chemicals in human breast milk increases the sexual motivation and sexual fantasies of women (Spencer & others, 2004). When men smell T-shirts worn by women who are fertile (ovulating) they experi- ence a greater increase in testosterone than when the woman who wore the T-shirt was not ovulating (Miller & Maner, 2010). So when we say that sexual attraction is partly based on “chemistry,” maybe we are literally correct. Chemical pheromones may be part of sexual attraction. We discuss this topic more when we discuss sexual motivation in chapter 11.
In addition, researchers at the University of Chicago have discovered pheromones that infl uence human female reproductive cycles (Stern & McClintock, 1998). It has been known for some time that women who live together, such as in military barracks, soon fi nd that they are on similarly timed menstrual cycles. It was not clear until recently, however, that this was due to the infl uence of pheromones. Psy- chologists Kathleen Stern and Martha McClintock (1998) collected sweat from women at different times in their menstrual cycles and identifi ed at least two distinct pheromones.
When other women inhaled sweat collected during the early phase of the menstrual cycle, lutenizing hormone was secreted by most of the women, and their menstrual cycles were accelerated. When sweat collected during the later stages of the menstrual cycle was inhaled by other women, it had the opposite effect. Because these pheromones have no odor (that is, they do not stimulate receptors in the olfactory epithelium itself), and because the receptors for pheromones send their neural messages to subcortical structures rather than to the cortex, the women were not consciously aware of their infl uence.
Olfactory nerve to brain Olfactory epithelium Nasal cavity
Figure 5.21
Olfactory receptor cells are located in the olfactory epithelium at the top of the nasal cavity.
Chemicals in the air we breathe and in the things we eat and drink are sensed by the gustatory receptors (taste buds) on the tongue and the olfactory receptors in the nose.
For both chemical senses, combinations of a relatively small number of primary sensations apparently make up the entire variety of our experiences of taste and smell. Pheromones infl uence reproductive behavior in many animal species. Increasing evidence suggests that pheromones also infl uence human reproductive behavior, menstrual cycles, and mood.
Review
To be sure that you have learned the key points from the preceding section, cover the list of correct answers and try to answer each question. If you give an incorrect answer to any question, return to the page given next to the correct answer to see why your answer was not correct.
1. Approximately a dozen sensory receptors called taste cells are found on each of the 10,000 on the tongue.
2. All our sensations of taste appear to result from five basic sensations of taste:
sweetness, fattiness, sourness, saltiness, and .
Check Your Learning
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