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Muscle Tissue • Characteristics – Cells are referred to as fibers – Contracts or shortens with force when stimulated – Moves entire body and pumps blood • Types – Skeletal:attached to bones – Cardiac: muscle of the heart. – Smooth: muscle associated with tubular structures and with the skin. Nonstriated and involuntary. Muscular Tissue 10-2 Functions of Muscle Tissue • Producing body movements • Stabilizing body positions • Regulating organ volumes – bands of smooth muscle called sphincters • Movement of substances within the body – blood, lymph, urine, air, food and fluids, sperm • Producing heat – involuntary contractions of skeletal muscle 10-3 Special functional characteristics of muscle Contractility Only one action: to shorten Shortening generates pulling force Excitability Nerve fibers cause electrical impulse to travel Extensibility Stretch with contraction of an opposing muscle Elasticity Recoils passively after being stretched Muscle Tissue I. Striated Muscle - regularly arranged contractile units A. Skeletal Muscle - long, cylindrical multinucleated cells with peripherally placed nuclei. Contraction is typically quick and vigorous and under voluntary control. Used for locomotion, mastication, and phonation. B. Cardiac Muscle - elongated, branched cells with a single centrally placed nucleus and intercalated discs at the ends. Contraction is involuntary, vigorous, and rhythmic. II. Smooth Muscle - possesses contractile machinery, but it is irregularly arranged (thus, non-striated). Cells are fusiform with a central nucleus. Contraction is involuntary, slow, and long lasting. Muscle Regeneration and Growth Skeletal Muscle • Increase in size (hypertrophy) • Increase in number (regeneration/proliferation) • Satellite cells are proposed source of regenerative cells Smooth Muscle • Increase in size (hypertrophy) • Increase in number (regeneration/proliferation) • Smooth muscle cells are proliferative (e.g. uterine myometrium and vascular smooth muscle) • Vascular pericytes can also provide source of smooth muscle Heart Muscle • Increase in size (hypertrophy) • Formerly thought to be non-proliferative • Post-infarction tissue remodeling by fibroblasts (fibrosis/scarring) • New evidence suggests mitotic cardiomyocytes and regeneration by blood or vascular-derived stem cells Skeletal Muscle Investments Epimysium dense irr. c.t. Perimysium less dense irr. c.t. Endomysium basal lamina and reticular fibers ALL MUSCLE CELLS HAVE BASAL LAMINAE! FROM THE OUTSIDE IN MUSCLE STRUCTURE EPIMYSIUM FIBROUS CONNECTIVE TISSUE BINDS BUNDLES OF FIBERS TOGETHER PERIMYSIUM FIBROUS CONNECTIVE TISSUE COVERS FASCICLES FASCICLES BUNDLES OF MUSCLE FIBERS ENDOMYSIUM FIBROUS CONNECTIVE TISSUE COVERS MUSCLE FIBERS MUSCLE FIBER MUSCLE CELL ALSO KNOWN AS A MYOFIBER TENDON ALL FIBROUS CONNECTIVE TISSUE EXTENDS BEYOND THE MUSCLE FIBERS KNOWN AS A TENDON OR APONEUROSIS http://training.seer.cancer.gov/module_anatomy/unit4_2_muscle_structure.html Formation of a skeletal muscle fiber (muscle cell) Skeletal muscle cells (fibers) develop from the fusion of myoblasts, resulting in large, multinuclear cells. The cells then assemble their contractile machinery in the cytoplasm. These come in the form of myofibrils, which have an alternate dark-light banding pattern when viewed from the side. The fact that the cell is chock-full of these myofibrils pushes the nuclei to the periphery of the cell. Muscle Fiber or Myofibers • Muscle cells are long, cylindrical & multinucleated • Sarcolemma = muscle cell membrane • Sarcoplasm filled with tiny threads called myofibrils & myoglobin (red-colored, oxygen-binding protein) 10-11 Microscopic anatomy of a skeletal muscle fiber Nuclei Fiber (a) Sarcolemma Mitochondrion Myofibril (b) Dark A band Light I band Nucleus Z disc H zone Z disc Thin (actin) filament Thick (myosin) filament (c) Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Composition of thick and thin filaments Thick filament Tail Thin filament Heads Flexible hinge region (a) Myosin molecule (d) Longitudinal section of filaments within one sarcomere of a myofibril Thin filament (actin) Myosin heads Thick filament (myosin) Myosin head (b) Portion of a thick filament Troponin complex Tropomyosin (c) Portion of a thin filament Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Actin (e) Transmission electron micrograph of part of a sarcomere Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Filaments and the Sarcomere • Thick and thin filaments overlap each other in a pattern that creates striations (light I bands and dark A bands) • They are arranged in compartments called sarcomeres, separated by Z discs. • In the overlap region, six thin filaments surround each thick filament 10-14 Microscopic anatomy of a skeletal muscle fiber Z disc H zone Z disc Thin (actin) filament Thick (myosin) filament (c) I band Thin (actin) filament Z disc A band Sarcomere M line I band M line Z disc Elastic (titin) filaments Thick (myosin) filament (d) (e) Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn I band thin filaments only H zone M line Outer edge of thick filaments thick filaments linked A band only by accessory proteins thick and thin filaments overlap Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle I band A band I band Z disc H zone Z disc M line Part of a skeletal muscle fiber (cell) Sarcolemma Triad Mitochondrion Myofibrils Myofibril Tubules of sarcoplasmic reticulum Sarcolemma Terminal cisterna of the sarcoplasmic reticulum T tubule Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Connective tissue sheaths of skeletal muscle Epimysium Tendon Muscle fiber in middle of a fascicle Epimysium Endomysium Endomysium (between fibers) (b) Perimysium Muscle fiber (cell) Bone (a) Perimysium Blood Fascicle (wrapped by vessel perimysium) Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Blood vessel Endomysium Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Cardiac Muscle Tissue Features: • Striated (same contractile machinery) • Self-excitatory and electrically coupled • Rate of contractions modulated by autonomic nervous system – innervation is neuroendocrine in nature (i.e. no “motor end plates”) Cell Features: • 1 or 2 centrally placed nuclei • Branched fibers with intercalated discs • Numerous mitochondria (up to 40% of cell volume) • Sarcoplasmic reticulum & T-tubules appear as diads at Z lines – T tubules are about 2x larger in diameter than in skeletal muscle • transport Ca2+ into fibers Cardiac Muscle (longitudinal section) Cardiac muscle is composed of smaller, branched muscle cells, which are connected to each other by intercalated discs. These intercalated disks, which are unique to cardiac muscle tissue, include adherent junctions for cell-cell strength, as well as gap junctions to allow electrical synchrony (so the cells contract at the same time). Similar to skeletal muscle, cardiac muscle fibers are packed with myofibrils, which are in-register, and give the tissue a striated appearance. Each cardiac muscle cell has a single nucleus that is centrally located. Smooth Muscle • • • • Fusiform, non-striated cells Single, centrally-placed nucleus Contraction is non-voluntary Contraction is modulated in a neuroendocrine manner • Smooth muscle tissue is composed of many smooth muscle cells. Although there are connective tissue elements (e.g. collagen) between the cells, smooth muscle is much more cellular than connective tissue. In addition, smooth muscle cells are smaller than cardiac and skeletal muscle cells. These features result is a tissue that has lots of nuclei. Depending on the orientation of the cells, the nuclei are slightly elongated in longitudinally-oriented cells (L), or round in transverse (cross) -sections (T). 6 major locations: 1. inside the eye 2. walls of vessels 3. respiratory tubes 4. digestive tubes 5. urinary organs 6. reproductive organs , p. 296. Spinal cord Motor unit 1 Motor unit 2 Axon terminals at neuromuscular junctions Nerve Motor neuron cell body Muscle Motor neuron axon Muscle fibers Muscle fibers Branching axon to motor unit (a) Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn (b) Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. The cross bridge cycle Myosin head (high-energy configuration) ADP Pi 1 Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament ATP hydrolysis ADP ADP Thick filament Pi 4 As ATP is split into ADP and Pi, the myosin head is energized (cocked into the high-energy conformation). 2 Inorganic phosphate (Pi) generated in theprevious contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. ATP ATP Myosin head (low-energy configuration) 3 As new ATP attaches to the myosin head, the link between Myosin and actin weakens, and the cross bridge detaches. Human Anatomy and Physiology, 7e by Elaine Marieb & Katja Hoehn Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. --Skeletal muscle cells have the largest diameter, cardiac muscle cells are smaller, and smooth muscle cells have the smallest diameter. --The diameter of skeletal muscle is very consistent between cells. Cardiac muscle cells are somewhat consistent in diameter (but vary in shape), while the smooth muscle cell profiles vary depending on whether the cell is sectioned through the middle (with nucleus) or near the periphery of the cell (without nucleus). --Skeletal muscle nuclei are near the plasma membrane, while cardiac and smooth muscle nuclei are centrally located. --In cuts through the nucleus of a cell, there is significant cytoplasm in skeletal and, to a lesser extent, cardiac muscle, while smooth muscle has a small rim of cytoplasm around the nucleus. --In ideal sections, stippling can be seen in skeletal and cardiac muscle, but not smooth muscle. skeletal muscle 320X cardiac muscle 480X smooth muscle 320X