Skin, Skeletal Tissue and Muscles - Part 3 PDF

Skin, skeletal tissue and muscles – part 3 Lecture – Biology, Physiology Part  Muscle Contraction Chapter 17 2 Introduction Muscular system is responsible for moving the framework of the body In addition to movement, muscle tissue performs various other functions 3 General...

Skin, skeletal tissue and muscles – part 3 Lecture – Biology, Physiology Part  Muscle Contraction Chapter 17 2 Introduction Muscular system is responsible for moving the framework of the body In addition to movement, muscle tissue performs various other functions 3 General Functions Movement of the body as a whole or movement of its parts Heat production Posture 4  Characteristics of Skeletal Muscle Cells Excitability (irritability): Ability to be stimulated Contractility: Ability to contract, or shorten, and produce body movement Extensibility: Ability to extend, or stretch, thereby allowing muscles to return to their resting length 5 Overview of the Muscle Cell Muscle cells are called fibers because of their threadlike shape Sarcolemma: Plasma membrane of muscle fibers Sarcoplasmic reticulum (SR) T tubules: Network of tubules and sacs found within muscle fibers Membrane of the SR continually pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release 6 T Tubules Transverse tubules extend across the sarcoplasm at right angles to the long axis of the muscle fiber Membrane has ion pumps that continually transport Ca++ ions inward from the sarcoplasm Allow electrical impulses traveling along the sarcolemma to move deeper into the cell 7 Structure of the Skeletal Muscle 8 Features of the Skeletal Muscle Cell 9 Overview of the Muscle Cell Muscle fibers contain many mitochondria and several nuclei Myofibrils: Numerous fine fibers packed close together in sarcoplasm Sarcomere Contractile unit of muscle fibers Each myofibril consists of many sarcomeres 10 Striated Muscle and Triad Striated muscle Dark stripes called A bands; light H band runs across the midsection of each dark A band Light stripes called I bands; dark Z disk extends across the center of each light I band Triad Triplet of tubules; a T tubule sandwiched between two sacs of sarcoplasmic reticulum 11 Skeletal Muscle Striations A: Courtesy Dr. J.H. Venable, Department of Anatomy, Colorado State University, Fort Collins, CO. B: Courtesy Dr. H.E. Huxley. 12 Myofilaments Each myofibril contains thousands of thick and thin myofilaments Four different kinds of protein molecules make up myofilaments Myosin Makes up almost all the thick filaments Myosin “heads” are known as cross bridges when attached to actin 13 Structure of Myofilaments 14 Cross Section of Myofilaments From Leeson CR, Leeson T, Paparo A: Text/atlas of histology, St Louis, 1988, Saunders. 15 Myofilaments Actin: Globular protein that forms two fibrous strands twisted around each other to form the bulk of the thin filament Tropomyosin: Protein that blocks the active sites on actin molecules Troponin: Protein that holds tropomyosin molecules in place 16 Excitation and Contraction of a Muscle Fiber A skeletal muscle fiber remains at rest until stimulated by a motor neuron Neuromuscular junction: Motor neurons connect to the sarcolemma at the motor endplate Acetylcholine (ACh): The neurotransmitter released into the synaptic cleft that diffuses across the gap, stimulates the receptors, and initiates an impulse in the sarcolemma 17 Neuromuscular Junction A: Courtesy of Don Fawcett, Harvard Medical School, Boston, Massachusetts. In Pollard TD: Earnshaw W: Cell biology, ed 2, St. Louis, 2007, Saunders 18 Excitation of a Muscle Fiber 19 Excitation and Contraction Nerve impulse travels over the sarcolemma and inward along the T tubules, which triggers the release of calcium ions Calcium binds to troponin, which causes tropomyosin to shift and expose active sites on actin 20  Molecular Basis of Muscle Contraction 21  Role of Calcium in Muscle Contraction From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman. 22 Cross Bridges From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman. 23 Sliding-Filament Model (Slide 1 of 2) When active sites on actin are exposed, myosin heads bind to them Myosin heads bend and pull the thin filaments past them Each head releases, binds to the next active site, and pulls again The entire myofibril shortens 24  Sliding-Filament Model (Slide 2 of 2) B: Courtesy H.E. Huxley, Brandeis University, Waltham, Ma. 25 Contracting Sarcomere 26 Muscle Relaxation Immediately after the Ca++ ions are released, the sarcoplasmic reticulum begins actively pumping them back into the sacs Ca++ ions are removed from the troponin molecules, thereby shutting down the contraction 27  Energy Sources for Muscle Contraction (Slide 1 of 2) Hydrolysis of adenosine triphosphate (ATP) yields the energy required for muscular contraction ATP binds to the myosin head and then transfers its energy to the myosin head to perform the work of pulling the thin filament during contraction Muscle fibers continually resynthesize ATP from the breakdown of creatine phosphate (CP) 28  Energy Sources for Muscle Contraction (Slide 2 of 2) 29 Energy Sources for Muscle Contraction: Catabolic Pathways Aerobic pathway Occurs when adequate O2 is available from blood Slower than anaerobic pathway Anaerobic pathway Very rapid, providing energy during first minutes of maximal exercise May occur when low levels of O2 are available Production of an “oxygen debt” is sometimes called excess postexercise oxygen consumption (EPOC) 30 Blood Supply of Muscle Fibers From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman. 31 Aerobic and Anaerobic Pathways 32 Energy Sources for Muscle Contraction: Skeletal Muscle Skeletal muscle contraction produces waste heat that can be used to help maintain the setpoint body temperature 33 Motor Units Some motor units consist of only a few muscle fibers, whereas others consist of numerous fibers Generally, the smaller the number of fibers in a motor unit, the more precise are the available movements; the larger the number of fibers in a motor unit, the more powerful the contraction available Myography: A method of graphing the changing tension of a muscle as it contracts 34 Motor Unit B: Courtesy Dr. Paul C. Letourneau, Department of Anatomy, 35 Medical School, University of Minnesota, MN. Myography 36 Twitch Contraction (Slide 1 of 2) A quick jerk of a muscle that is produced as a result of a single, brief threshold stimulus (generally occurs only in experimental situations) The twitch contraction has three phases Latent phase Contraction phase Relaxation phase 37 Twitch Contraction (Slide 2 of 2) 38 Treppe: The Staircase Phenomenon Gradual, steplike increase in the strength of contraction seen in a series of twitch contractions that occur 1 second apart Eventually, the muscle responds with less forceful contractions, and the relaxation phase becomes shorter If the relaxation phase disappears completely, a contracture occurs 39 Muscle Contractions (Slide 1 of 2) 40  Muscle Contractions (Slide 2 of 2) 41 Tetanus Contractions Multiple wave summation Incomplete tetanus Complete tetanus The availability of calcium determines whether a muscle will contract; if calcium is continuously available, a contraction will be sustained 42 Twitch and Tetanus Adapted from Pollard T, Earnshaw W: Cell biology, ed 2, Philadelphia, 2008, Saunders. 43 Muscle Tone Tonic contraction: Continual, partial contraction of a muscle Muscles with less tone than normal are flaccid Muscles with more tone than normal are spastic Muscle tone is maintained by negative feedback mechanisms 44 Graded Strength Principle (Slide 1 of 2) Factors that contribute to the phenomenon of graded strength Metabolic condition of individual fibers Number of muscle fibers contracting simultaneously Number of motor units recruited Intensity and frequency of stimulation 45 Strength of Contraction Compared With Strength of Stimulus 46  Graded Strength Principle (Slide 2 of 2) Maximal strength that a muscle can develop bears a direct relationship to the initial length of its fibers A shortened muscle’s sarcomeres are compressed; therefore, the muscle cannot develop much tension Strongest maximal contraction is possible only when the skeletal muscle has been stretched to its optimal length 47 Length-Tension Relationship 48 Stretch Reflex The load imposed on a muscle influences the strength of a skeletal contraction Stretch reflex: The body tries to maintain constancy of muscle length in response to increased load 49 Strength of Muscle Contraction 50 Isotonic and Isometric Contraction (Slide 1 of 2) Isotonic contraction Contraction in which the tone or tension in a muscle remains the same as the length of the muscle changes Isometric contraction Contraction in which muscle length remains the same while muscle tension increases 51 Isotonic and Isometric Contraction (Slide 2 of 2) 52 Cardiac Muscle (Slide 1 of 2) Found only in the heart Also known as striated involuntary muscle Cardiac muscle resembles skeletal muscle but has unique features related to its role in continuously pumping blood Each cardiac muscle contains parallel myofibrils Syncytium: Continuous, electrically coupled mass 53 Cardiac Muscle Fiber 54  Cardiac Muscle (Slide 2 of 2) T tubules are larger and form diads with a rather sparse sarcoplasmic reticulum Cardiac muscle sustains each impulse longer than does skeletal muscle; therefore, impulses cannot come rapidly enough to produce tetanus Cardiac muscle is self-stimulating 55 Structure of Cardiac Muscle 56 Smooth Muscle (Slide 1 of 2) Smooth muscle is composed of small, tapered cells that have a single nucleus Ca++ comes from outside the cell and binds to calmodulin instead of troponin to trigger a contraction No striations are seen because the thick and thin myofilaments are arranged differently than in skeletal or cardiac muscle fibers 57 Smooth Muscle Fiber Photos: Courtesy Dr. Frederic S. Fay, Department of Physiology, University of Massachusetts, Worcester, MA. 58 Two Types of Smooth Muscle Tissue Single-unit (visceral) smooth muscle Gap junctions join smooth muscle fibers into large, continuous sheets Exhibits autorhythmicity and produces peristalsis Multiunit smooth muscle Composed of many independent single-cell units Can form thin sheets, such as the walls of large blood vessels 59  Smooth Muscle (Slide 2 of 2) 60 Muscle Tissue and the Whole Body Function of all three major types of muscle is integral to function of the entire body All three types of muscle tissue provide the movement necessary for survival Muscles maintain the body in a relatively stable position 61 Major Muscular Disorders Muscle injury Muscle strains and sprains Muscle infections Tetanus and poliomyelitis Muscular dystrophy Myasthenia gravis Hernias 62