The Nervous System Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2013 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Com- mons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: December 21, 2013 www.ck12.org Concept 1. The Nervous System CONCEPT 1 The Nervous System Lesson Objectives • Identify the functions of the nervous system. • Describe neurons and explain how they carry nerve impulses. • Describe the structures of the central nervous system. • Outline the divisions of the peripheral nervous system. Check Your Understanding • If groups of cells are called tissues and groups of tissues are called organs, what are groups of organs called? • What are examples of human organ systems? • Which organ system controls all the others? Vocabulary • autonomic nervous system • axon • brain • brain stem • cell body • central nervous system • cerebellum • cerebrum • dendrite • motor division • motor neuron • nerve • nerve impulse • nervous system • neuron • neurotransmitter • parasympathetic division • peripheral nervous system • sensory division • sensory neuron • somatic nervous system • spinal cord • sympathetic division • synapse What Does the Nervous System Do? Groups of organs are called organ systems. Examples of human organ systems are the skeletal, digestive, and respiratory systems. The nervous system controls all the others. 1 www.ck12.org Michelle was riding her scooter when she hit a hole in the road and started to lose control. She thought she would fall, but in the blink of an eye, she shifted her weight and kept her balance. Her heart was pounding, but at least she didn’t get hurt. How was she able to react so quickly? Michelle can thank her nervous system for that (Figure 1.1). FIGURE 1.1 Staying balanced when riding a scooter requires control over the body’s muscles. The nervous system controls the muscles and maintains balance. The nervous system controls all of the systems of the body. Controlling muscles and maintaining balance are just two of its roles. The nervous system also lets you: • Sense your surroundings with your eyes and other sense organs. • Sense the environment inside of your body, including temperature. • Control your internal body systems and keep them in balance. • Prepare your body to fight or flee in an emergency. • Think, learn, remember, and use language. The nervous system works by sending and receiving electrical signals. The signals are carried by nerves in the body, similar to the wires that carry electricity all over a house. For example, when Michelle started to fall off her scooter, her nervous system sensed that she was losing her balance. It responded by sending messages to muscles in her body. Some muscles tightened while others relaxed. As a result, Michelle’s body became balanced again. How did her nervous system do all that in a split second? To answer this question, you need to know how the nervous system carries messages. Neurons and Nerve Impulses The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron (Figure 1.2). The messages carried by neurons are called nerve impulses. Nerve impulses can travel very quickly because they are electrical impulses. Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside the body. What Does a Neuron Look Like? A neuron has a special shape that lets it pass signals from one cell to another. As shown in Figure 1.2, a neuron has three main parts: 1. The cell body. 2. Many dendrites. 3. One axon. 2 www.ck12.org Concept 1. The Nervous System FIGURE 1.2 The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin sheath. The sheath covers the axon, like the plastic covering on an electrical wire, and allows nerve impulses to travel faster along the axon. The cell body contains the nucleus and other organelles. Dendrites and axons connect to the cell body, similar to rays coming off of the sun. Dendrites receive nerve impulses from other cells. Axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells. Types of Neurons Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons. • Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system. • Motor neurons carry nerve impulses from the central nervous system to organs, glands, and muscles - the opposite direction. Both types of neurons work together. Sensory neurons carry information about the environment found inside or outside of the body to the central nervous system. The central nervous system uses the information to send messages through motor neurons to tell the body how to respond to the information. The Synapse The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found between neurons and other type of cells, such as muscle cells. The axon of the sending neuron does not actually touch the dendrite of the receiving neuron. There is a tiny gap between them, the synapse, as shown in Figure 1.3. FIGURE 1.3 This diagram shows a synapse between neurons. When a nerve impulse arrives at the end of the axon, neurotransmitters are released and travel to the dendrite of another neuron, carrying the nerve impulse from one neuron to the next. The following steps describe what happens when a nerve impulse reaches the end of an axon. 3 www.ck12.org 1. When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters. 2. Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron. 3. Neurotransmitters bind to the membrane of the dendrite. 4. The binding allows the nerve impulse to travel through the receiving neuron. Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline. You can watch an animation of nerve impulses and neurotransmitters at: http://www.mind.ilstu.edu/curriculum/neu rons_intro/neurons_intro.php. Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives. The Central Nervous System The central nervous system (CNS) is the largest part of the nervous system. As shown in Figure 1.4, it includes the brain and the spinal cord. The bony skull protects the brain. The spinal cord is protected within the bones of the spine, which are called vertebrae. FIGURE 1.4 The brain and spinal cord make up the central nervous system. The Brain What weighs about 3 pounds and contains up to 100 billion cells? The answer is the human brain. The brain is the control center of the nervous system. It’s like the pilot of a plane. It tells other parts of the nervous system what to do. The brain is also the most complex organ in the body. Each of its 100 billion neurons has synapses connecting it with thousands of other neurons. All those neurons use a lot of energy. In fact, the adult brain uses almost a quarter of the total energy used by the body. The developing brain of a baby uses an even greater amount of the body’s total energy. The brain is the organ that lets us understand what we see, hear, or sense in other ways.