Finding the Nerve CHALLENGE A-25. r e a d i n g

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1 6 Finding the Nerve r e a d i n g You probably found that some parts of your arm were more sensitive than others. In this reading you will learn about some of the reasons for these differences. CHALLENGE Why do different parts of your body have different sensitivities to touch? Reading You use your senses to gather information about your environment. Your senses include sight, hearing, taste, smell, and touch. To understand how you feel objects you touch, you need to know a little about your nervous system. Your nervous system includes your brain, spinal cord, and nerves. The diagram below shows how these parts of your nervous system are connected. Nerves are made up of nerve cells or neurons and are found throughout your body. Your nerves take in information from the world. For example, your nerves have helped you collect information about the weight of your backpack when you pick it up and think, It s heavy! The nerves that sense this information are called sensory neurons. Sensory neurons do more than just sense pressure; they also sense sound waves in your ear, light waves in your eyes, and chemicals in your nose and on your tongue. Human Nervous System A-25

2 Activity 6 Finding the Nerve Stopping to Think 1 a. Why are the nerves that take information from our world called sensory neurons? Responding to the Environment Signals travel from nerves in your body to your brain. These signals can travel up to several hundred miles per hour! b. What kind of information do sensory neurons detect? Information travels from sensory neurons all over your body to your spinal cord before continuing to your brain. There are nerves in your spinal cord and brain called interneurons that connect to the sensory neurons. Your brain analyzes all of the information from the interneurons and helps you understand what your sensory neurons have detected. For example, the interneurons help you identify the object at your feet as a dog. Your brain can then provide directions to other neurons called motor neurons. Motor neurons send signals to your muscles that cause you to move. For example, they might signal your muscles to reach down and pet the dog. Sometimes there is a need for a reflex action, such as when you touch a hot stove. In this case, the message never reaches the brain. The interneurons in your spinal cord signal the motor neurons of your arm and hand to move your hand away fast. Interneuron Sensory neuron Motor neuron Muscle Path of a Nerve Impulse Stopping to Think 2 a. Someone accidentally bumps into you in the hallway. What part of your body detected the feeling of being bumped? b. Where in your body is this feeling analyzed? c. Inter means between. Why are the neurons in the spinal cord and brain called interneurons? A-26

3 Finding the Nerve Activity 6 Sometimes your nervous system responds to messages that you provide and sometimes your nervous system responds to messages from your brain that you are not aware of. For example, you decide to eat a sandwich. Your brain sends signals along nerves to direct your muscles to pick up the sandwich and take a bite. But what happens after you swallow? Do you have to think about directing your body to process the food? Your body automatically moves food through your digestive system and processes it. For this to happen, muscles inside your body must move. These muscles are controlled by the automatic, or involuntary, part of your nervous system. You don t even have to think about it! Stopping to think 3 a. Is breathing completely involuntary? Explain. b. Explain how motor neurons can be part of both the voluntary and the involuntary nervous systems. Even when you are not consciously thinking about what your body is doing, your brain is controlling your body s movements. Your brain only weighs 3 pounds and is made up mainly of water and fat, and yet it: processes information from the world around you through your senses. controls and coordinates all of your movements. Human Brain functions controls basic life processes such as body temperature, blood pressure, heart rate and breathing. allows you to think, reason, dream, and experience emotions. Motor control Speech Smell Hearing Touch and pressure Taste Vision The parts of your brain have various functions. The largest, wrinkled part of the brain that you see is called the cerebrum. The folds or wrinkles increase the surface area of the brain, allowing more room for neurons and making it more efficient. This is where all of the thinking and reasoning you do takes place. Specific parts of the cerebrum control certain types of thought processes and actions. The two smaller parts of the brain are the cerebellum and the brain stem. The cerebellum controls the coordination of movement, posture, and balance. A-27

4 Activity 6 Finding the Nerve Structure of the Human Brain Cerebrum Cerebellum Brain stem During complex voluntary movements, such as those involved in learning to ride a bike, many parts of your brain work together to perform the sequence of movements necessary to ride the bike successfully. Deep within the brain is the brain stem. This is where the basic life functions such as heartbeat, blood pressure, and breathing are controlled. Stopping to Think 4 1. When you decide what to wear in the morning, which of the three parts of your brain do you use? 2. What is a benefit of the cerebrum having folds and wrinkles? 3. What part of the brain enables you to breathe, even when you are asleep? Now you know how you sense information from the outside world and react to it. But why are some parts of your body more sensitive than others? If you are touched with two closely spaced points, some parts of your body will be able to detect that there are two, while other parts of your body will only be able to detect one point. When something touches your arm, you feel it if it is detected by the nerve endings in your skin. You have nerves in your body that detect pressure, heat, sounds, smells, and light. The nerve endings that detect pressure on your skin are called touch receptors (ree SEP-tors). They help carry a message from your skin to your brain. Touch Receptors on a Human Hand Some parts of your body have more touch receptors than others. When two points stimulate the same touch receptors, you feel the points as one touch. When they stimulate different touch receptors, you feel two different touches. The illustration (left) shows the concentration of touch receptors on a hand. Notice that the tip of the finger has more touch receptors than the rest of the finger. When you reach out to touch something, you often A-28

5 Finding the Nerve Activity 6 use your fingertips. You may have heard of people with limited or no vision reading Braille. Braille is a written language that uses raised dots instead of letters. Braille is read with fingertips. Not surprisingly, you have the greatest number of touch receptors right at your fingertips just where they are needed. Analysis 1. a. Where would you expect to have more touch receptors: on the palm of your hand or on the back of your hand? Explain your ideas. b. Explain how you could test your answer to Question 1a. 2. When your partner touched you with the toothpick points in Activity 5, how did you sense it? Describe or draw the path through the nervous system that enabled you to identify if it was a one- or two-point touch. 3. Review your results from Activity 5, Can You Feel the Difference? Based on what you now know, where on your arm fingers, palm, or forearm do you have the fewest touch receptors? 4. Describe the structures that make up the nervous system. extension For links to more information on the human nervous system, go to the Issues and Life Science page of the SEPUP website. A-29