Losing senses

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There have been times when I’ve been asked, “if you could lose one of you senses, which one would it be? Sight, touch, hearing, smell, taste?” This is usually a random question that comes in a game or may come up in a conversation on a rare occasion. This is always a difficult question because I am very fortunate to have all these senses fully functioning and can’t really imagine living without even just one. Each one is vital in its own unique way and serves a purpose every day of my life. I guess if I had to choose, I would reluctantly say, smell and I’ll explain why. I know that smell and taste go hand-in-hand. These are chemical senses. They allow us to sample molecules of substances, and one doesn’t fully function to its true potential without the other. Without smell, although it is tied to taste, you could probably get away with experiencing the food you’re eating without it. I remember doing an experiment in elementary school where we had our noses plugged and were given different flavors of food to try and see if we could make out the flavors without our sense of smell. To an extent, this did work; some of us were able to taste things (if only slightly) with our nose plugged. Of course, some students may have been exaggerating or not telling the truth, but I do think it could be possible, although some might disagree completely. Either way, I would not want to give up the chance of maybe being able to taste my favorite foods. Our textbook says that odors are registered through receptor neurons transmitting information to the brain by what is called the ‘olfactory nerve.’ I guess a pro in this situation would be not having to smell bad odors anymore. Our textbook also says we can smell “1 one-millionth of a milligram of vanilla in a liter of air,” which is pretty impressive if you ask me. I do know individuals who are either super sensitive to smell or have lost their sense of smell altogether. Not everyone’s senses are totally alike, even if their senses are functioning. Maybe one or more senses have deteriorated over time or have been altered through an incident. Something as simple as a normal bodily function like our sense of smell is important. Even though we don’t notice it because it comes naturally, being able to smell is a sense I doubt anyone would actually want to give up. Smell allows us to experience great things like food, perfume, lovely candles or flowers. It’s such a basic human experience, but one that shouldn’t be taken for granted.

The Chemical Senses: Smell and Taste

Smell and taste are the chemical senses. In vision and hearing, physical energy strikes our sensory receptors. In smell and taste, we sample molecules of substances.

Smell

Smell has an important role in human behavior. It contributes to the flavor of foods, for example. If you did not have a sense of smell, an onion and an apple might taste the same to you. People’s sense of smell may be deficient when compared with that of a dog, but we can detect the odor of 1 one-millionth of a milligram of vanilla in a liter of air.

The sense of smell detects odors. An odor is a sample of molecules of a substance in the air. Odors trigger firing of receptor neurons in the olfactory membrane high in each nostril. Receptor neurons can detect even a few molecules of the substance in gaseous form. The receptor neurons transmit information about odors to the brain via the olfactory nerve.

Taste

As in the case of smell, the sense of taste samples molecules of a substance. Taste is sensed through taste cells—receptor neurons located on taste buds. You have about 10,000 taste buds, most of which are located near the edges and back of your tongue. Some taste buds are more responsive to sweetness, whereas others react to several tastes. Other taste receptors are found in the roof, sides, and back of the mouth, and in the throat. Buds in the mouth are evolutionarily adaptive because they can warn of bad food before it is swallowed (Brand, 2000).

Researchers historically agreed on four primary taste qualities: sweet, sour, salty, and bitter. They have recently added a new basic taste to the list: umami, which is pronounced ooh-mommy in Japanese and means “meaty” or “savory” (Kurihara, 2015). The flavor of a food is more complex than taste alone. Flavor depends on odor, texture, and temperature as well as on taste. Apples and onions are similar in taste, but their flavors differ greatly.

Believe it or not, apples and onions are quite similar in taste, but their flavors differ greatly. What is the difference between a taste and a flavor?

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Just as some people have better vision than others, some people are more sensitive to tastes than others—but their superiority may be limited to one or more basic tastes. Those of us with low sensitivity for sweetness may require twice the sugar to sweeten our food as those who are more sensitive. Those of us who claim to enjoy bitter foods may actually be taste-blind to them. Sensitivities to various tastes have a genetic component (Knaapila et al., 2012; Feeney & Hayes, 2014).

The Skin Senses

The skin senses include touch, pressure, warmth, cold, and pain. We have distinct sensory receptors for pressure, temperature, and pain, but some nerve endings may receive more than one type of sensory input. Let’s focus on touch, pressure, temperature, and pain.

Touch and Pressure

Sensory receptors embedded in the skin fire when the surface of the skin is touched. There may be several kinds of receptors for touch, some that respond to constant pressure, some that respond to intermittent pressure, as in tapping the skin. Active touching means continuously moving your hand along the surface of an object so that you continue to receive sensory input from the object. If you are trying to “get the feel of” a fabric or the texture of a friend’s hair, you must move your hand over it. Otherwise, the sensations quickly fade. If you pass your hand over the fabric or hair and then hold it still, the sensations of touching will fade. Active touching receives information concerning pressure, temperature, texture, and feedback from the muscles involved in movements of our hands.

Different parts of the body are more sensitive to touch and pressure than others. The parts of the body that “cover” more than their fair share of somatosensory cortex are most sensitive to touch. These parts include the hands, face, and some other regions of the body. Our fingertips, lips, noses, and cheeks are more sensitive than our shoulders, thighs, and calves. Why the difference in sensitivity? First, nerve endings are more densely packed in the fingertips and face than in other locations. Second, more sensory cortex is devoted to the perception of sensations in the fingertips and face (see Figure 2.10).

Temperature

The receptors for temperature are neurons located just beneath the skin. When skin temperature increases, the receptors for warmth fire. Decreases in skin temperature cause receptors for cold to fire.

Sensations of temperature are relative. When we are at normal body temperature, we might perceive another person’s skin as warm. When we are feverish, though, the other person’s skin might seem cool. We also adapt to differences in temperature. When we enter a swimming pool, the water may seem cold because it is below body temperature. Yet after a few moments an pool may seem quite warm. In fact, we may chide a newcomer for not diving right in.

Pain

For most people, pain is a frequent visitor. Headaches, backaches, toothaches—these are only a few of the types of pain that most of us encounter from time to time. According to national surveys (Arthritis Foundation, 2000; AAPM Facts and Figures on Pain, 2018), 89% of us experience pain at least once a month. About one American in three reports experiencing chronic pain. More than half (55%) of people aged 65 and above say they experience pain daily. People aged 65 and above are most likely to attribute pain to getting older (88%) and to assume they can do nothing about disabilities such as arthritis. By contrast, people aged 18 to 34 are more likely to attribute pain to tension or stress (73%), overwork (64%), or their lifestyle (51%). When we assume that there is nothing we can do about pain, we are less likely to try. Yet 43% of Americans say that pain curtails their activities, and 50% say that pain puts them in a bad mood.

Pain results when neurons called nociceptors in the skin are stimulated. Evolutionary psychologists would point out that pain is adaptive, if unpleasant, because it motivates us to do something about it. For some of us, however, chronic pain—pain that lasts once injuries or illnesses have cleared—saps our vitality and interferes with the pleasures of everyday life (Fenton & Pitter, 2010).

We can sense pain throughout most of the body, but pain is usually sharpest where nerve endings are densely packed, as in the fingers and face. Pain can also be felt deep within the body, as in the cases of abdominal pain and back pain. Even though headaches may seem to originate deep inside the head, there are no nerve endings for pain in the brain.

Pain usually originates at the point of contact, as when you bang a knee. But it reverberates throughout the nervous system. The pain message to the brain is facilitated by the release of chemicals such as prostaglandins, bradykinin, and P (yes, P stands for “pain”). Prostaglandins facilitate transmission of the pain message to the brain and heighten circulation to the injured area, causing the redness and swelling that we call inflammation. Inflammation attracts infection-fighting blood cells to the injury to protect it against invading germs. Pain-relieving drugs such as aspirin and ibuprofen help by inhibiting production of prostaglandins. The pain message is relayed from the spinal cord to the thalamus and then projected to the cerebral cortex, making us aware of the location and intensity of the damage. Ronald Melzack speaks of a “neuromatrix” that includes these chemical reactions but involves other aspects of our physiology and psychology in our reaction to pain (Gatchel et al., 2012). For example, visual and other sensory inputs tell us what is happening and affect our interpretation of the situation. Our emotional response affects the degree of pain, and so do the ways in which we respond to stress. If, for instance, the pain derives from an object we fear, perhaps a knife or a needle, we may experience more pain. If we perceive that there is nothing we can do to change the situation, perception of pain may increase. If we have self-confidence and a history of successful response to stress, perception of pain may diminish.

One of the more intriguing topics in the study of pain is phantom limb pain. About two out of three combat veterans with amputated limbs report feeling pain in such missing, or “phantom,” limbs (Schipper & Maurer, 2017). Although the pain occurs in the absence of the limb, it is real enough. The pain sometimes involves activation of nerves in the stump of the missing limb, but local anesthesia does not always eliminate the pain.

Researchers have found that many people who experience phantom limb pain have also undergone reorganization of the motor and somatosensory cortex that is consistent with the pain (Ng et al., 2018).

3-5 Truth or Fiction

Simple remedies like rubbing a banged knee frequently help relieve pain. Why? According to the gate theory of pain originated by Ronald Melzack and Patrick Wall (1965), the nervous system can process only a limited amount of stimulation at a time. Rubbing the knee transmits sensations to the brain that “compete” for the attention of neurons. Many nerves are thus prevented from transmitting pain messages to the brain. It is like shutting down a “gate” in the spinal cord, or like a website flooded with visitors.

Thousands of years ago, the Chinese began mapping the body to learn where pins might be placed to deaden pain. This practice is termed acupuncture. Traditional acupuncturists believe that the practice balances the body’s flow of energy, but research has shown that it stimulates nerves that reach the hypothalamus and may also cause the release of endorphins and cortisol (Stener-Victorin, 2013). Endorphins are neurotransmitters that are similar in effect to the narcotic morphine. Cortisol is a stress hormone.

Kinesthesis and the Vestibular Sense

Try an experiment. Close your eyes and then touch your nose with your finger. If you weren’t right on target, I’m sure you came close. But how? You didn’t see your hand moving, and you didn’t hear your arm swishing through the air. Humans and many other animals have senses that alert them to their movements and body position without relying on vision, including kinesthesis and the vestibular sense.

Kinesthesis

Kinesthesis is the sense that informs you about the position and motion of parts of the body. The term is derived from the ancient Greek words for “motion” (kinesis) and “perception” (aisthesis). In kinesthesis, sensory information is fed back to the brain from sensory organs in the joints, tendons, and muscles. You were able to bring your finger to your nose easily by employing your kinesthetic sense. When you make a muscle in your arm, the sensations of tightness and hardness are also provided by kinesthesis.

Good gymnasts and dancers can sense the location and movement of nearly every part of their body.

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Imagine going for a walk without kinesthesis. You would have to watch the forward motion of each leg to be certain you had raised it high enough to clear the curb. And if you had tried the brief nose-to-finger experiment without the kinesthetic sense, you would have had no sensory feedback until you felt the pressure of your finger against your nose (or cheek, or eye, or forehead), and you probably would have missed dozens of times.

The Vestibular Sense

It is your vestibular sense that provides your brain with information as to whether or not you are physically upright. Sensory organs located in the semicircular canals and elsewhere in the ears monitor your body’s motion and position in relation to gravity. They tell you whether you are falling and provide cues to whether your body is changing speed, such as when you are in an accelerating airplane or automobile.