Muscle Physiology PDF

MUSCLE PHYSIOLOGY 3. Skeletal muscle Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Give examples of muscle functionlocomotio...

MUSCLE PHYSIOLOGY 3. Skeletal muscle Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Give examples of muscle functionlocomotionsupport Describe the 4 specific muscle properties elasticityexcitability contractility extensibility Describe the types of muscles and muscle fibers Describe the levels of organization in skeletal muscle Know what are the types of myofilaments and how they are organized in the sarcomere Describe the importance of sarcoplasmic reticulum, mitochondria, T- tubules, and myoglobin for the skeletal muscle fiber  Muscle functions L O Muscle is a contractile tissue and accomplishes diverse functions Locomotion Respiration Digestion Parturition Blood and lymph circulation Swallowing Generation of body heat  Muscular properties LO CONTRACTILITY The ability to contract or shorten. EXCITABILITY The capacity to receive and respond to a stimulus EXTENSIBILITY The ability to be stretched ELASTICITY The ability to return to its original shape after being stretched  10 Types of muscle SKELETAL MUSCLE 40% of the body mass SMOOTH MUSCLE 10% of the body mass CARDIAC MUSCLE TYPE SKELETAL SMOOTH CARDIAC Contractions Fast Slow Fast Location Trunk, Viscera, Heart extremities, blood vessels head, and neck Control Voluntary Involuntary Involuntary  Skeletal muscle Crucial for body movement Attached to bones Usually by tendons Moves and supports the skeleton Is stimulated by a motor nerve Under voluntary (conscious) control Body movement is the result of the contraction of skeletal muscle across a movable joint Most joints have one or more muscles on both sides either to increase (extension) or decrease (flexion) their angle  Organization of skeletal muscle LO EPIMYSIUM Sheath of connective tissue surrounding the muscle FASCICLE A small bundle or cluster of muscle fibers (cells) PERIMYSIUM Connective tissue extensions from the epimysium that surrounds each fascicle ENDOMYSIUM Connective tissue extensions from the perimysium that surround the individual muscle fibers and are attached to the sarcolema  Organization of skeletal muscle SARCOLEMMA Thin cell membrane enclosing a skeletal muscle fiber (cell) At each end of the muscle fibers, sarcolemma fuses with tendon fibers – collects into bundles to form muscle tendons, that connect the muscles to the bones MUSCLE FIBERS Muscle cells – long, cylindrical, and multinucleated Contain the basic contractile units which issarcomere The pull is transmitted by endomysium, perimysium, and epimysium to tendon or aponeurosis that is attached to the bones contain vs the basic contractile  Organization of skeletal muscle LO TYPES OF MUSCLE FIBERS Mammalian muscles are composed of muscle fibers of different contractile properties – physiologically different TYPE 1 – red fibers TYPE 2 – white fibers TYPE 2a TYPE 1 TYPE 2b The proportion of each fiber type varies from muscle to muscle TYPE 2B  Organization of skeletal muscle TYPE 1 FIBER (Slow twitch) Slow-contracting and fatigue-resistant fibers Rich in mitochondria, myoglobin, and capillaries, which give them a reddish color Rely heavily on oxidative metabolism Plentiful in muscles that the main function is slow, prolonged activity Ex: Muscles to maintain posture  Organization of skeletal muscle TYPE 2 FIBER (Fast twitch) Fast-contracting and more easily fatigable fibers Have fewer mitochondria Rely more on glycolytic metabolism In most species can be divided into: TYPE 2A - are mixed oxidative-glycolytic and, although fast-contracting, are also fatigue- resistant (intermediate between type 1 and type 2B) I.e.: Dogs muscles have only Type 2A fibers (strong oxidative capacity) TYPE 2B – fast contracting, fast-fatiguing glycolytic fibers. Depend on glycogen for their energy supply I.e.: Muscles designed for sprinting contain more type ATHLETIC TRAINING CAUSES SOME 2B 2B fibers FIBERS TO BE CONVERTED TO 2A FIBERS  Organization of skeletal muscle MYOFIBRILS Each muscle fiber contains several hundred to several thousand myofibrils Each myofibril is composed of a linear series of repeating sarcomeres SARCOMERES Basic contractile unit of striated muscle fibers Found between Z LINES or Z disks Contain protein MYOFILAMENTS (thin and thick) Their arrangement gives rise to striations  Organization of skeletal muscle Myofilaments are responsible for actual muscle contraction THIN FILAMENTS Actin, Troponin, Tropomyosin THICK FILAMENT Myosin  Thick filament Composed of multiple MYOSIN molecules Myosin molecule contains a TAIL of intertwined helices and 2 globular HEADS that can bind both ATP and ACTIN Functions as an ATPase enzyme – uses ATP as an energy source for contraction Approximately 500 myosin heads of a thick myosin filament form cross-bridges that interact with actin to shorten the sarcomere Myosin heads flex and relax cross bridge theory  Thin filaments Composed of ACTIN, TROPOMYOSIN, and TROPONIN 2 intertwined, helical strands of actin protein All wound together as a 2 strands of tropomyosin protein large helical complex Troponin is a complex globular protein Located intermittently along the tropomyosin-actin strand Bind to those proteins and have an affinity for Calcium ions  Titin filamentous molecules Keep the myosin and actin filaments in place Very elastic filamentous molecule Crucial for the contractile machinery One end attaches to the Z disk The other part connects to myosin  Myofilaments organization The myosin and actin filaments partially interdigitate Cause myofibrils to have alternate light and dark bands I BAND = light band (Isotropic to polarized light) A BAND = dark band (anisotropic to polarized light) 1111 THIS IS THE REASON WHY SKELETAL MUSCLE IS CALLED STRIATED  Myofilaments organization The end of actin filaments are attached to a Z DISK or Z LINE Forms the periphery of the sarcomere Transversely bisects the I BAND Other zones and lines of the sarcomere: H-ZONE kick Light zone in the center made from tick filaments M-LINE Found inside the H-zone and forms the middle of the sarcomere Made from tick filaments and accessory proteins Myofilaments organization  Organelles of the muscle cell SARCOPLASMIC RETICULUM Specialized endoplasmic reticulum Very important for muscle contraction Regulates calcium storage, release and reuptake Bigger in fast contracting fibers (type 2 – white) MITOCHONDRIA Present in a large number Lie in parallel to the myofibrils Supply contracting myofibrils with large amounts of energy in the form of ATP More numerous in slow fibers (type 1 – red)  Organelles of the muscle cell T TUBULES Tubules arranged transversely to the myofibril Periodic invaginations of the sarcolemma Contain extracellular fluid Allow the plasma membrane of the muscle fiber to carry the depolarization of the action potential to the interior of the fiber In mammals, they are located at the junctions between the A band and the I band of the sarcomere (two per sarcomere, in longitudinal view) Each T-tubule is flanked by two terminal cisternae of the SR, forming structures known as TRIADS  HEME GROUP Myoglobin It is a protein located primarily in the striated muscles of vertebrates Found exclusively in skeletal and cardiac muscle cells Myoglobin constitutes up to 5 - 10% of all the cytoplasmic proteins It is a member of the globin superfamily, which also includes GLOBIN CHAIN hemoglobin Myoglobin is a single polypeptide chain with ONE oxygen binding site (HEME) Hemoglobin has 4 chains and 4 oxygen-binding sites  HEME GROUP Myoglobin Serves as a local oxygen reservoir that can temporarily provide oxygen Whenever blood oxygen delivery is insufficient during periods of intense muscular activity The body uses myoglobin as an oxygen-storage protein in muscle It can bind and release oxygen depending on the oxygen concentration in the cell GLOBIN CHAIN Myoglobin also functions in the homeostasis of nitric oxide and detoxification of reactive oxygen species The total amount of myoglobin in an animal depends on bod weight, degree of muscle development, and the myoglobin concentration in muscle. Red muscle is rich in myoglobin, and white muscle is myoglobin poor Overview Questions?