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Muscle Contraction Douglas Wilkin, Ph.D. Jean Brainard, Ph.D. 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 © 2012 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/NonCommercial/Share Alike 3.0 Unported (CC BY-NC-SA) License (http://creativecommons.org/licenses/by-nc-sa/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 21, 2012 AUTHORS Douglas Wilkin, Ph.D. Jean Brainard, Ph.D. www.ck12.org C ONCEPT 1 1 Muscle Contraction • Explain how muscles contract according to the sliding filament theory. What makes a muscle contract? It starts with a signal from the nervous system. So it starts with a signal from your brain. The signal goes through your nervous system to your muscle. Your muscle contracts, and your bones move. And all this happens incredibly fast. Muscle Contraction Muscle contraction occurs when muscle fibers get shorter. Literally, the muscle fibers get smaller in size. To understand how this happens, you need to know more about the structure of muscle fibers. Structure of Muscle Fibers Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines (see Figure 1.1). The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments. Concept 1. Muscle Contraction 2 www.ck12.org FIGURE 1.1 Sarcomere. A sarcomere contains actin and myosin filaments between two Z lines. Sliding Filament Theory The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to “walk” along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere. You can watch this occurring in a video animation at the link below. http://www.youtube.com/watch?v=7V-zFVnFk Wg#38;feature=related MEDIA Click image to the left for more content. When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn’t contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater. Actin, myosin and muscle contraction are discussed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC 5110CF64/45/zopoN2i7ALQ(9:38). MEDIA Click image to the left for more content. www.ck12.org 3 Additional information about muscle contraction is available at http://www.youtube.com/user/khanacademy#p/c/ 7A9646BC5110CF64/46/LiOfeSsjrB8 (9:22) and http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110 CF64/47/SauhB2fYQkM (14:42). MEDIA Click image to the left for more content. MEDIA Click image to the left for more content. Muscles and Nerves Muscles cannot contract on their own. They need a stimulus from a nerve cell to “tell” them to contract. Let’s say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves. Vocabulary • actin: The monomeric subunit of microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the myofibril in muscle cells. • cross bridge: Structure of myosin filament that attaches to actin filament. • motor neuron: Type of neuron that carries nerve impulses from the central nervous system to muscles and glands. • myofibril: Organelle of muscle fiber; composed of actin and myosin filaments. • myosin: Filamentous protein involved in muscle contraction; forms thick filaments of myofibril in muscle cells. • sacromere: Region of myofibril between two Z-lines. • sliding filament theory: Theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers. • Z line: Region of sarcomere where actin filaments are attached. Summary • According to the sliding filament theory, a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber. • When all the sarcomeres in a muscle fiber shorten, the fiber contracts. Concept 1. Muscle Contraction 4 www.ck12.org Practice Use this resource to answer the questions that follow. • http://www.hippocampus.org/Biology → Biology for AP* → Search: Skeletal Muscle Contraction 1. 2. 3. 4. 5. 6. Draw a sacromere. What happens when a sarcomere contracts? Describe the molecular events that occur during skeletal muscle contraction. What is creatine phosphate? Describe the role of tropomyosin. What is the role of calcium ions? Review 1. Explain how muscles contract according to the sliding filament theory. 2. A serious neck injury may leave a person paralyzed from the neck down. Explain why.