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LO 9 Describe the process of olfaction.

Many people take olfaction (ōl-ˈfak-shən)—the sense of smell—for granted. But such individuals might think twice if they actually understood how losing olfaction would affect their lives. People with the rare condition of anosmia are unable to perceive odors. They cannot smell smoke in a burning building or a gas leak from a stove. They cannot tell whether the fish they are about to eat is spoiled. Nor can they savor the complex palates of pesto, curry, chocolate, coffee, or prime rib. Because, without smell, food doesn’t taste as good.

A CHEMICAL SENSE Olfaction and taste are called chemical senses because they involve sensing chemicals in the environment. For olfaction, those chemicals are odor molecules riding currents of air. For taste, they are flavor molecules surfing on waves of saliva. Odor molecules, which are emitted by a variety of sources (for example, spices, fruits, flowers, bacteria, and skin), make their way into the nose by hitchhiking on currents of air flowing into the nostrils or through the mouth. About 3 inches into the nostrils is a patch of tissue called the olfactory epithelium. Around the size of a typical postage stamp, the olfactory epithelium is home to millions of olfactory receptor neurons that provide tiny docking sites, or receptors, for odor molecules (much as a lock acts as a docking site for a key; Figure 3.3 below). When enough odor molecules attach to an olfactory receptor neuron, it fires, causing an action potential, illustrating how transduction occurs in the chemical sense of olfaction.

OLFACTION IN THE BRAIN Olfactory receptor neurons stimulate a part of the brain called the olfactory bulb, where they converge in clusters called glomeruli (Figure 3.3). Then the signal is passed along to higher brain centers, including the hippocampus, amygdala, and olfactory cortex (Firestein, 2001). The other sensory systems relay data through the thalamus before going to higher brain centers. But the wiring of the olfactory system is unique; olfaction is on a fast track to the limbic system, where emotions like fear and anger are processed.

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Humans have about 350 different types of odor receptors, yet we can distinguish over 1 trillion smells (Bushdid, Magnasco, Vosshall, & Keller, 2014). Each receptor type recognizes several odors, and each odor activates several receptors; a given scent creates a telltale pattern of electrical activity that the brain recognizes as lemon, garlic, or smelly feet. The types of receptors and the degree to which they are activated identify the odor (Firestein, 2001).

Scents have a powerful influence on human behavior and thought. Minutes after birth, babies use the scent of their mother’s breast to guide them toward the nipple, and they quickly learn to discriminate Mom’s milk from someone else’s (Makin & Porter, 1989; Porter & Winberg, 1999). Research suggests that odor-induced memories are not necessarily more accurate than those evoked by touching, hearing, or seeing (Herz, 1998), but they are more emotional and “associated with stronger feelings of being brought back in time” (Larsson & Willander, 2009, p. 318). Why are memories triggered by smell drenched in emotion? Remember that olfactory information is communicated directly to the limbic system. Evidence suggests that smells activate this emotion-processing hub more than other sensations. Brain-imaging research indicates that smelling a perfume remembered from one’s past activates the amygdala more than looking at its bottle (Herz, 2007; Herz, Eliassen, Beland, & Souza, 2004).

RECEPTOR REPLENISHMENT There is another reason smell is special. Olfactory receptor neurons are one of the few types of neurons that regenerate. Once an olfactory receptor neuron is born, it does its job for a minimum of 30 days, then dies and is replaced (Schiffman, 1997). This process slows with age, impairing odor sensitivity (Larsson, Finkel, & Pedersen, 2000). If it weren’t for this continual replenishment of olfactory neurons, we wouldn’t smell much of anything. Olfactory neurons are in direct contact with the environment, so they are constantly under assault by bacteria, dirt, and noxious chemicals. Some toxins—like the fumes and dust at New York City’s World Trade Center site after the terrorist attacks in 2001—can even cause long-term damage to the olfactory system. Many workers involved in the clean-up effort following 9/11 may be permanently impaired in their ability to smell (Dalton et al., 2010).

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Eating is as much an olfactory experience as it is a taste experience. Just think back to the last time your nose was clogged from a really bad cold. How did your meals taste? When you chew, odors from food float up into your nose, creating a flavor that you perceive as “taste,” when it’s actually smell. If this mouth–nose connection is blocked, there is no difference between apples and onions, Sprite and Coke, or wine and cooled coffee (Herz, 2007). Don’t believe us? Then do the Try This experiment below.

LO 10 Discuss the structures involved in taste and describe how they work.

ANOTHER CHEMICAL SENSE If the nose is so crucial for flavor appreciation, then what role does the mouth play? Receptors in the mouth are sensitive to five basic but very important tastes: sweet, salty, sour, bitter, and umami. You are likely familiar with all these tastes, except perhaps umami, which is a savory taste found in seaweed, aged cheeses, protein-rich foods, mushrooms, and monosodium glutamate (MSG; Chandrashekar, Hoon, Ryba, & Zuker, 2006). We call the ability to detect these stimuli our sense of taste, or gustation (gəs-ˈtā-shən; Figure 3.4).

Receptor cells for taste are found in the taste buds on the tongue, the roof of the mouth, and lining the cheeks. Somewhere between 5,000 to 10,000 taste buds are embedded in the papillae, those bumps you can see on a person’s tongue (Society for Neuroscience, 2012). Jutting from each of these buds are 50 to 100 taste receptor cells, and it is onto these cells that food molecules bind (similar to the lock-and-key mechanism of an odor molecule binding to a receptor in the nose).

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As you bite into a juicy orange and begin to chew, chemicals from the orange (sour acid and sweet sugar) are released into your saliva, where they dissolve and bathe the taste buds throughout your mouth. The chemicals find their way to matching receptors and latch on, sparking action potentials in sensory neurons, another example of transduction. Signals are then sent through sensory neurons to the thalamus, and then on to higher brain centers for processing.

Receptors for taste are constantly being replenished, but their life span is only about 10 days (Schiffman, 1997). If they didn’t regenerate, you would be in trouble every time you burned your tongue sipping hot coffee or soup. Even so, by age 20, you have already lost half of the taste receptors you had at birth. And as the years go by, their turnover rate gets slower and slower, making it harder to appreciate the basic taste sensations (Kaneda et al., 2000). Drinking alcohol and smoking worsen the problem by impairing the ability of receptors to receive taste molecules. Losing taste is unfortunate, and it may take away from life’s simple pleasures, but it’s probably not going to kill you—at least not if you are a modern human. We can’t be so sure about our primitive ancestors.

EVOLUTION AND TASTE The ability to taste has been essential to the survival of our species. Tastes push us toward foods we need and away from those that could harm us. We gravitate toward sweet, calorie-rich foods for their life-sustaining energy—an adaptive trait if you are a primitive human foraging in trees and bushes, not so adaptive if you are a modern human looking for something to eat at the food court in your local shopping mall. We are also drawn to salty foods, which tend to contain valuable minerals, and to umami, which signals the presence of proteins essential for cellular health and growth. Bitter and sour tastes we tend to avoid, on the other hand. This also gives us an evolutionary edge because poisonous plants or rancid foods are often bitter or sour. It is interesting to note that our absolute threshold for bitter is lower than the threshold for sweet. Can you see how this is advantageous?

Every person experiences taste in a unique way. You like cilantro; your friend thinks it tastes like bath soap. Taste preferences may begin developing before birth, as flavors consumed by a pregnant woman pass into amniotic fluid and are swallowed by the fetus. In one study, infants who had been exposed to the flavor of carrots before birth (through their mothers’ consumption of carrot juice in the last trimester) showed fewer disapproving facial expressions when fed carrot-flavored cereal compared to plain cereal. This suggests exposure to the carrot flavor before birth led to greater enjoyment of this taste compared to that of plain cereal. This preference was not evident in infants who had not been exposed to the flavor of carrots in utero (Mennella, Coren, Jagnow, & Beauchamp, 2001).

We have made some major headway in this chapter, examining four of the major sensory systems: vision, hearing, smell, and taste. Now it is time to get a feel for a fifth sense. Are you ready for tingling, tickling, and titillating touch?

If you were introduced to Zoe, she would probably explore your face for about 20 seconds, and then return to whatever she was doing. Emma would likely spend more time getting to know you. She might give you a hug, put her cheek against yours, and carefully examine your head with her fingers. At the dinner table, Emma is neat and tidy, and does not like to get food on her hands and face. Zoe loves to dig in and make a mess. These two girls, who are genetically identical, experience the world of touch in dramatically different ways. But both rely on touch receptors within the body’s main touch organ—the skin.

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Take a moment to appreciate your vast epidermis, the outer layer of your skin. Weighing around 6 pounds on the average adult, the skin is the body’s biggest organ and the barrier that protects our insides from cruel elements of the environment (bacteria, viruses, and physical objects) and distinguishes us from others (fingerprints, birthmarks). It also shields us from the cold, sweats to cool us down, and makes vitamin D (Bikle, 2004). And perhaps most importantly, skin is a data collector. Every moment of the day, receptors in our skin gather information about the environment. Among them are thermoreceptors that sense hot or cold, Pacinian corpuscles that detect vibrations, and Meissner’s corpuscles sensitive to the slightest touch, like a snowflake landing on your nose (Bandell, Macpherson, & Patapoutian, 2007; Figure 3.5).

Not all touch sensations are as pleasant as the tickle of a snowflake. Touch receptors can also communicate signals that are interpreted as pain. Nociceptive pain is caused by heat, cold, chemicals, and pressure. Nociceptors that respond to these stimuli are primarily housed in the skin, but they also are found in muscles and internal organs.

In very rare cases, a baby is born without the ability to feel pain. Children with this type of genetic disorder may face a greater risk of serious injury (Romero, Simón, Garcia-Recuero, & Romance, 2008). When scrapes and cuts are not felt, they may not receive the necessary protection and treatment. Pain can protect us from minor bangs and bruises as well as serious bodily harm. The “ouch” that results from the stub of a toe or the prick of a needle tells us to stop what we are doing and tend to our wounds. Let’s find out how this protective mechanism works.

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TWO PATHWAYS FOR PAIN Thanks to the elaborate system of nerves running up and down our bodies, we are able to experience unpleasant, yet very necessary, sensations of pain. Fast nerve fibers, made up of large, myelinated neurons, are responsible for conveying information about pain occurring in the skin and muscles, generally experienced as a stinging feeling in a specific location. If you stub your toe, your first perception is a painful sting where the impact occurred. Slow nerve fibers, made up of smaller, unmyelinated neurons, are responsible for conveying information about pain throughout the body. Pain conveyed by the slow nerve fibers is more like a dull ache, not necessarily concentrated in a specific region. The diffuse aching sensation that follows the initial sting of the stubbed toe results from activity in the slow nerve fibers. As the names suggest, fast nerve fibers convey information quickly (at a speed of 30–110 meters/sec) and slow nerve fibers convey information more slowly (8–40 meters/sec; Somatosensory Function, 2004). But where are they sending this information?

The axons of these fast and slow sensory nerve fibers band together as nerves on their way to the spinal cord and up to the brain (Figure 3.6). The bundled fast nerve fibers make their way to the reticular formation of the brain, alerting it that something important has happened. The information then goes to the thalamus and on to the somatosensory cortex, where sensory information from the skin is processed further (for example, indicating where it hurts most). The bundled slow nerve fibers start out in the same direction, toward the brain, with processing occurring in the brainstem, hypothalamus, thalamus, and limbic system. In the midbrain and amygdala, for example, emotional reactions to the pain are processed (Gatchel, Peng, Peters, Fuchs, & Turk, 2007). As with other neurons, the transmission of information, in this case about pain, occurs through electrical and chemical activities. Substance P and glutamate are two important neurotransmitters that work together to increase the firing of the pain fibers at the injury location.

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Understanding the mechanisms of pain at the neural level is important, but biology alone cannot explain how we perceive pain and why people experience it so differently. How can the same flu shot cause great pain in one person but mere discomfort in another? And why does our sensitivity to pain seem to fluctuate from one day to the next?

LO 11 Describe how the biopsychosocial perspective is used to understand pain.

As with most complex topics in psychology, pain is best understood using a multilevel method, such as the biopsychosocial perspective. Chronic pain, for example, can be explained by biological factors (the neurological pathways involved), psychological factors (distress, cognition), and social factors (immediate environment, relationships; Gatchel et al., 2007). Prior experiences, environmental factors, and cultural expectations influence how the pain is processed (Gatchell & Maddrey, 2004).

Researchers studying pain once searched for a direct path from pain receptors to specific locations in the brain, but this simple relationship does not exist (Melzack, 2008). Instead, there is a complex interaction between neurological pathways and psychological and social factors, and gates involved in the shuttling of information back and forth between the brain and the rest of the body.

GATE CONTROL The most influential theory of pain perception is the gate-control theory. It states that a person’s perception of pain is either increased or decreased by how the brain interprets pain through an interaction of biopsychosocial factors (Melzack & Wall, 1965). Once pain messages are received in the brain (through the mechanisms outlined above), the brain interprets the incoming information, and then sends signals back down through the spinal cord to either open or close the “gates” that control the neurological pathways for pain. Depending on the situation and psychological and social factors, the gates may open to increase the experience of pain, or close to decrease it. In situations where it’s important to keep going in spite of an injury (athletes in competitions, soldiers in danger), the gates might be instructed to close. In situations where feeling intense pain has value (when you are ill, and your body needs to rest, for instance), the gates may be instructed to remain open.

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Signals to shut the gate do not always come from the brain (Melzack, 1993, 2008). Information sent from the pain receptors in the body can also open and close the neurological gates from the bottom up, depending on which type of fiber is active. If the small unmyelinated fibers are active, the gates are more likely to open, sending the pain message up the spinal cord to the brain. If the large myelinated fibers are active, the gates are more likely to close, inhibiting pain messages from being sent on. Suppose you stub your toe. One way to ease the pain is by rubbing the injured area, stimulating the pressure receptors of the large fibers. This activity closes the gates, interfering with the pain message that would otherwise be sent to the brain.

It is clear that everyone experiences pain in a unique way. But are there factors that predispose a person to be more or less sensitive to pain?

THE PSYCHOLOGY OF PAIN Pain sensitivity can ebb and flow for an individual, with psychological factors exerting a powerful effect (Gatchel et al., 2007; Raichle, Hanley, Jensen, & Cardenas, 2007). Negative feelings such as fear and anxiety can amplify pain, whereas laughter and distraction can soften it, interfering with the ability to attend to pain. Certain types of stressors (for example, running in a marathon, or giving birth) trigger the release of endorphins. Endorphins reduce pain by blocking the transmission of pain signals to the brain and spinal cord, possibly through the inhibition of substance P, a neurotransmitter in the spinal cord and brain (Rosenkranz, 2007).

PHANTOM LIMB PAIN Perhaps the most striking illustration of pain’s complexity is phantom limb pain. Some 50–80% of people who have lost an arm or leg through amputation can experience burning, tingling, intense pain, cramping, and other sensations that they feel originate in their missing limb (Ramachandran & Brang, 2009; Ramchandran & Hauser, 2010; Rosenblum, 2010). Researchers are still trying to determine what causes phantom limb pain, but they have proposed a variety of plausible mechanisms, ranging from changes in the structure of neurons to reorganization of the brain in relation to sensations felt in different locations in the body (Flor, Nikolajsen, & Jensen, 2006). When the brain doesn’t receive normal signals from the limb, conflicting messages are sent to the brain from nerve cells and seem to be interpreted as pain.

LO 12 Illustrate how we sense the position and movement of our bodies.

You may have thought “touch” was the fifth and final sense, but there is more to sensation and perception than the traditional five categories. Closely related to touch is the sense of kinesthesia (ki-nəs-ˈthē-zhē-ə), which provides feedback about body position and movement. Kinesthesia endows us with the coordination we need to walk gracefully, dance the salsa, and put on our clothes without looking in the mirror. This knowledge of body location and orientation is made possible by specialized nerve endings called proprioceptors (proh-prē-uh-sep-tərs), which are primarily located in the muscles and joints. Proprioceptors monitor changes in the position of body parts and the tension in muscles. When proprioception is impaired, our ability to perform physical tasks like holding a book or driving a car is compromised.

You may recall from earlier in the chapter that Emma, Zoe, and Sophie began to curl up in the fetal position and vomit while riding in the car during the same period that they lost their hearing. This was because they were losing their vestibular sense (ve-ˈsti-byə-lər), which helps the body maintain balance as it deals with the effects of gravity, movement, and position. Astronauts often become nauseated during their first days in space because their vestibular systems are confused. The vestibular system comprises fluid-filled organs in the inner ear: the semicircular canals and the nearby vestibular sacs. When the head tilts, fluid moves the hairlike receptors in these structures, causing neurons to fire, initiating a signal that travels to the cerebellum (transduction once again). Motion sickness, which many people experience on boats and roller coasters, is caused by conflicting information coming from the eyes and vestibular system. This is known as sensory conflict theory (Johnson, 2005).

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Before we move on to the next section on perception, let’s take a moment to appreciate the sensory systems that allow us to know and adapt to the surrounding world. How would you function without vision, hearing, smell, taste, and touch, and what steps can you take to preserve them (Table 3.2)?

Question 1

Question 2

1. olfaction.

2. transduction.

3. sensory adaptation.

4. thermoreceptors.

Question 3

Question 4

1. the theory of evolution

2. an absolute threshold

3. the gate-control theory

4. gustation