Transcript Ch. 4 S. 4
Chapter 4 Section 4 Other Senses Obj: Identify the chemical, skin, and body senses. Smell People do not have as strong a sense of smell as many animals. Dogs use seven times as much of the cerebral cortex for smell as people do. Some dogs are used to sniff out drugs or explosives in suitcases or to track lost children or objects. But smell is important to people too. Without smell, you would not be able to taste as much as you do. For example, if your sense of smell were not working, an onion and an apple would taste very much alike to you. Odors of substances are detected by receptor neurons high in each nostril. Receptor neurons react when molecules of the substance in the form of a gas come into contact with them. The receptors send information about the odors to the brain via the olfactory nerve. Our sense of smell adapts quickly. We adapt rapidly even to annoying odors. This may be fortunate if we are in a locker room. It may not be fortunate if harmful fumes, such as from cars, are present-we may lose awareness of the smoke or fumes even though danger remains. One odor can also be masked by another, which is how air fresheners work. Taste Why would your dog gobble up a piece of a candy bar, but your cat turn up its nose at it? Dogs can taste sweetness, but cats cannot. The four basic taste qualities are sweetness, sourness, saltiness, and bitterness. Do you think that you taste more than these four qualities? What you are experiencing is the flavor of food, which involves odor, texture, and temperature as well as taste. If you have a cold, for example, food tastes flat because you cannot smell it. The reason older people sometimes complain that their food has little “taste” is usually that they have experienced a loss of the sense of smell, not taste. Thus, they perceive less of the flavor of their food. Older people often spice their food heavily to enhance its flavor. Taste is sensed through receptor neurons located on taste buds on the tongue. Some people have low sensitivity for sweetness and may have to use twice as much sugar to sweeten their food as others who are more sensitive to sweetness. People who claim to enjoy very bitter foods may actually be “taste blind” to them. Sensitivities to different tastes can be inherited. By eating hot foods and scraping your tongue, you regularly kill off many taste cells. But you need not be alarmed. Taste cells reproduce rapidly enough to completely renew themselves in a week. The taste system is thus one of the most resilient of all the body’s sensory systems. It is very rare for anyone to suffer a complete permanent taste loss. The Skin Senses What we normally call touch is better called the skin senses because touch is a combination of pressure, temperature, and pain. Humans have distinct sensory receptors for pressure, temperature, and pain, but some nerve endings may receive more than one type of sensory input. Our skin senses are vitally important to us. Studies have shown that premature infants grow more quickly and stay healthier if they are touched. And older people seem to do better if they have a dog or cat to stroke and cuddle. • Pressure – Your body is covered with hairs, some of them very tiny. Sensory receptors located around the roots of hair cells fire where the skin is touched. Other structures beneath the skin are also sensitive to pressure. Different parts of the body are more sensitive to pressure than others. The fingertips, lips, nose, and cheeks are more sensitive than the shoulders, thighs, and calves. • Temperature – Sensations of temperature are relative. When your body temperature is at a normal 98.6 F, you might perceive another person’s skin as being warm. When you are feverish, though, the other person’s skin might seem cool. The receptors for temperature are neurons just beneath the skin. When skin temperature increases because you touch something warm, receptors for warmth fire. Decreases in skin temperature, such as those that occur when you put a cool, moist cloth on your forehead, cause receptors for cold to fire. We adapt to differences in temperature. Have you ever walked out of an air-conditioned building into the hot sunshine? At first, the heat really hit you, but soon the sensation faded as you adapted to the warmth. In the same way, when you first jump into a swimming pool, the water may seem cold. Yet, after a few moments, the water feels warmer as your body adjusts to it. • Pain – Pain tells you something is wrong. It is also adaptive-it motivates us to do something to stop it. Headaches, backaches, toothaches-these are only a few of the types of pain most of us experience from time to time. More serious health problems-such as arthritis, cancer, or wounds-also cause pain. Not all areas of the body are equally sensitive to pain. The more pain receptors located in a particular area of our skin, the more sensitive that area is. Once a person gets hurt, everything happens very quickly. Pain originates at the point of contact. The pain message is sent from the point of contact to the spinal cord to the thalamus in the brain. Then it is projected to the cerebral cortex, where the person registers the location and severity of the pain. Chemicals called prostaglandins help the body transmit pain messages to the brain. Aspirin and ibuprofen are common pain-fighting drugs that work by curbing production of prostaglandins. Simple remedies like rubbing and scratching an injured area sometimes help relieve pain. Gate theory suggests that only a certain amount of information can be processed by the nervous system at a time. Rubbing or scratching the area transmits sensations to the brain that compete with the pain messages for attention. Thus, many neurons cannot get their pain messages to the brain. It is as if too many calls are flooding a switchboard. The flooding prevents many or all of the calls from getting through. One of the more fascinating facts in psychology is that many people experience pain in limbs that are no longer there. More than half of combat veterans with amputated limbs report feeling pain in the missing, or “phantom,” limbs. In such cases, there is no current tissue damage, but the pain is real. The pain appears to involve activation of nerves in the stump of the missing limb, along with activation of neural circuits that have memories connected with the limb. Body Senses Body senses are the senses that people are least aware of. But do not let that fool you. Without them, you would have to pay attention just to stay upright, to lift your legs to go downstairs, or even to put food in your mouth. • The Vestibular Sense – Stand up. Now close your eyes. Do you have to look in a mirror to be certain that you are still upright? No of course not. Your vestibular sense tells you whether you are physically upright without your having to use your eyes. Sensory organs located in the ears monitor your body’s motion and position in relation to gravity. Your vestibular sense enables you to keep your balance. It tells you whether you are upside down or not and lets you know when you are falling. It also informs you of whether your body is changing speeds, such as in accelerating automobiles. • Kinesthesis – Ask some friends to close their eyes. Then ask them to touch their noses with their index fingers. How close to their noses did they come? Many of them were probably right on the mark, while others could only come close. How did they locate their noses? Their eyes were closed, so they could not see their hands. They touched their noses through kinesthesis. Kinesthesis is the sense that informs people about the position and motion of their bodies. In kinesthesis, sensory information is fed to the brain from sensory organs in the joints, tendons, and muscles.