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Nerve Cells and Nerve Impulses 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 © 2014 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 Commons 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: July 27, 2014 www.ck12.org C HAPTER Chapter 1. Nerve Cells and Nerve Impulses 1 Nerve Cells and Nerve Impulses • Describe neurons and explain how they carry nerve impulses. What do nerve cells look like? Note that like most other cells, these nerve cells have a nucleus. They also have other organelles. However, the long, threadlike extensions of the nerve cells are unique. This is where the nerve impulses are transmitted. 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 1.1). 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. A neuron has three main parts ( Figure 1.1): 1. The cell body. 1 www.ck12.org FIGURE 1.1 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 node of Ranvier, shown in this diagram, is any gap in the myelin sheath; it allows faster transmission of a signal. 2. Many dendrites. 3. One 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 but only one axon. The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The node of Ranvier is any gap within the myelin sheath exposing the axon, and it allows even faster transmission of a signal. 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 types 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 synaptic cleft ( Figure 1.2). The following steps describe what happens when a nerve impulse reaches the end of an axon. 1. When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters. 2 www.ck12.org Chapter 1. Nerve Cells and Nerve Impulses FIGURE 1.2 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. 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. Vocabulary axon: Long, threadlike part of a neuron that transmits nerve impulses to other cells. cell body: Central part of a neuron that contains the nucleus and other cell organelles. dendrite: Extension of the cell body of a neuron that receives nerve impulses from other neurons. motor neuron: Nerve cell that carries nerve impulses from the brain and spinal cord to organs, glands, and muscles. • myelin sheath: Fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. • nerve: Bundle of nerve cells. • nerve impulse: Electrical signal transmitted by the nervous system. • • • • 3 www.ck12.org neuron: Nerve cell. neurotransmitter: Chemical that carries a nerve impulse from one nerve cell to another at the synapse. node of Ranvier: Any gap within the myelin sheath exposing the axon. sensory neuron: Nerve cell that carries nerve impulses from the sense organs and internal organs to the brain and spinal cord. • synapse: Place where the axon of one neuron meets the dendrite of another. • • • • Summary • Neurons, or nerve cells that carry nerve impulses, are made up of the cell body, the axon, and several dendrites. • Signals move across the synapse, the place where the axon of one neuron meets the dendrite of another, using chemicals called neurotransmitters. Practice Use the resource below to answer the questions that follow. • Neuroscience For Kids at http://faculty.washington.edu/chudler/cells.html 1. 2. 3. 4. 5. What are the three types of neurons? What neurons are most abundant in the central nervous system? What is the function of sensory neurons? What is the function of motor neurons? What is the role of interneurons? Review 1. 2. 3. 4. Describe a neuron and identify its three main parts. Distinguish between dendrites and the axon. Distinguish between sensory and motor neurons. Explain how one neuron transmits a nerve impulse to another neuron. References 1. Zachary Wilson, modifying image by Mariana Ruiz Villarreal. The axons of many neurons are covered in a myelin sheath to allow for faster signal transmission. CC BY-NC 3.0 (original image in public domain) 2. Laura Guerin. At a synapse, neurotransmitters are released to carry a nerve impulse from one neuron to the next. CC BY-NC 3.0 4