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CHAPTER 11 - CHAPTER 11 MUSCULAR TISSUE Types and...

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CHAPTER 11 MUSCULAR TISSUE Types and Characteristics of Muscle Tissue Universal characteristics of Muscle. 1. All muscle tissue exhibits responsiveness (excitability) conductivity, contractility, extensibility and elasticity. Skeletal Muscle 1. Skeletal muscle is voluntary striated muscle usually attached to bone. 2. A typical cell (fiber) is about 100 um long, but can be up to 30 cm long. 3. Skeletal muscle exhibits striations, reflecting the overlapping nature of the internal contractile proteins. Series-Elastic Components 1. The series-elastic components of muscular tissue include the stretched endomysium, perimysium, epimysium, fascia, and tendon, all of which are not excitable but do stretch and recoil. 2. During muscle contraction, the muscle generates internal tension, followed by external tension that moves the load. Microscopic Anatomy of Skeletal Muscle A. The Muscle Fiber 1. The plasma membrane of the muscle fiber is the sarcolemma. It has transverse (T) tubules. 2. The cytoplasm (sarcoplasm) contains myofibrils made up of myofilaments. 3. Sarcoplasmic reticulum (a reservoir for calcium ions) joins with T tubules to form terminal cisternae. 4. The sarcoplasm contains abundant glycogen and myoglobin. B. Myofilaments. 1. 1. Myofilaments are central to muscle contraction. Three kinds exist: thick filaments, thin filaments, and elastic filaments. 2. Thick myofilaments are made up of myosin and shaped somewhat like a gold club.
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3. Thin myofilaments are made up of fibrous actin with beadlike subunits of globular actin, each of which has an active site that can bind the head of a myosin molecule. 4. Elastic filaments are made up of titin and run through the thick filament, connecting it to the Z disc. 5. Within the fibrous actin lies another protein called tropmyosin, which itself has smaller proteins called troponin. Tropomysin and troponin are the regulatory proteins of muscle contraction. Striations 1. Striated muscle has dark A bands (thick filaments) alternating with lighter I bands. In the middle of the A band is a lighter region called the H band, into which the thin filaments do not extend. 2. Each I band has a narrow dark line called the Z disc. Each segment of myofibril from one Z disc to the next is the sarcomere, the unit of contraction of a muscle fiber. The NERVE-MUSCLE Relationship A. Motor Neurons 1. Skeletal muscle is innervated by somatic motor neurons whose cell bodies are in the brainstorm and spinal cord. 2. The axons of motor neurons are called somatic motor fibers, and their distal branches lead to the fibers of skeletal muscle. 3. Each muscle fiber is supplied by only one motor neuron. B. The Motor Unit 1. A motor unit consists of one nerve fiber and all the muscle fibers it innervates. 2. A motor unit behaves as a single functional unit and contracts as one.
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