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Skeletal Muscle Physiology

Skeletal Muscle Physiology. Muscular System Functions. Body movement (Locomotion) Maintenance of posture Respiration Diaphragm and intercostal contractions Communication (Verbal and Facial) Constriction of organs and vessels Peristalsis of intestinal tract

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Skeletal Muscle Physiology

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  1. Skeletal Muscle Physiology

  2. Muscular System Functions • Body movement (Locomotion) • Maintenance of posture • Respiration • Diaphragm and intercostal contractions • Communication (Verbal and Facial) • Constriction of organs and vessels • Peristalsis of intestinal tract • Vasoconstriction of b.v. and other structures (pupils) • Heart beat • Production of body heat (Thermogenesis)

  3. Properties of Muscle • Excitability: capacity of muscle to respond to a stimulus • Contractility: ability of a muscle to shorten and generate pulling force • Extensibility: muscle can be stretched back to its original length • Elasticity: ability of muscle to recoil to original resting length after stretched

  4. Types of Muscle • Skeletal • Attached to bones • Makes up 40% of body weight • Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement • Voluntary in action; controlled by somatic motor neurons • Smooth • In the walls of hollow organs, blood vessels, eye, glands, uterus, skin • Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, • In some locations, autorhythmic • Controlled involuntarily by endocrine and autonomic nervous systems • Cardiac • Heart: major source of movement of blood • Autorhythmic • Controlled involuntarily by endocrine and autonomic nervous systems

  5. Connective Tissue Sheaths • Connective Tissue of a Muscle • Epimysium. Dense regular c.t. surrounding entire muscle • Separates muscle from surrounding tissues and organs • Connected to the deep fascia • Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle • Contains b.v and nerves • Endomysium. Loose connective tissue that surrounds individual muscle fibers • Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue • Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

  6. Nerve and Blood Vessel Supply • Motor neurons • stimulate muscle fibers to contract • Neuron axons branch so that each muscle fiber (muscle cell) is innervated • Form a neuromuscular junction (= myoneural junction) • Capillary beds surround muscle fibers • Muscles require large amts of energy • Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

  7. Muscle Tissue Types

  8. Skeletal Muscle • Long cylindrical cells • Many nuclei per cell • Striated • Voluntary • Rapid contractions

  9. Basic Features of a Skeletal Muscle • Muscle attachments • Most skeletal muscles run from one bone to another • One bone will move – other bone remains fixed • Origin – less movable attach- ment • Insertion – more movable attach- ment

  10. Basic Features of a Skeletal Muscle • Muscle attachments (continued) • Muscles attach to origins and insertions by connective tissue • Fleshy attachments – connective tissue fibers are short • Indirect attachments – connective tissue forms a tendon or aponeurosis • Bone markings present where tendons meet bones • Tubercles, trochanters, and crests

  11. Skeletal Muscle Structure • Composed of muscle cells (fibers), connective tissue, blood vessels, nerves • Fibers are long, cylindrical, and multinucleated • Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length • Develop from myoblasts; numbers remain constant • Striated appearance • Nuclei are peripherally located

  12. Muscle Attachments

  13. Antagonistic Muscles

  14. Microanatomy of Skeletal Muscle

  15. Muscle Fiber Anatomy • Sarcolemma - cell membrane • Surrounds the sarcoplasm(cytoplasm of fiber) • Contains many of the same organelles seen in other cells • An abundance of the oxygen-binding protein myoglobin • Punctuated by openings called the transverse tubules (T-tubules) • Narrow tubes that extend into the sarcoplasm at right angles to the surface • Filled with extracellular fluid • Myofibrils -cylindrical structures within muscle fiber • Are bundles of protein filaments (=myofilaments) • Two types of myofilaments • Actin filaments (thin filaments) • Myosin filaments (thick filaments) • At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma • When myofibril shortens, muscle shortens (contracts)

  16. Sarcoplasmic Reticulum (SR) • SR is an elaborate, smooth endoplasmic reticulum • runs longitudinally and surrounds each myofibril • Form chambers called terminal cisternae on either side of the T-tubules • A single T-tubule and the 2 terminal cisternae form a triad • SR stores Ca++ when muscle not contracting • When stimulated, calcium released into sarcoplasm • SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

  17. Sarcoplasmic Reticulum (SR)

  18. Parts of a Muscle

  19. Sarcomeres: Z Disk to Z Disk • Sarcomere - repeating functional units of a myofibril • About 10,000 sarcomeres per myofibril, end to end • Each is about 2 µm long • Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance. • A bands: a dark band; full length of thick (myosin) filament • M line - protein to which myosins attach • H zone - thick but NO thin filaments • I bands: a light band; from Z disks to ends of thick filaments • Thin but NO thick filaments • Extends from A band of one sarcomere to A band of the next sarcomere • Z disk: filamentous network of protein. Serves as attachment for actin myofilaments • Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

  20. Structure of Actin and Myosin

  21. Myosin (Thick) Myofilament • Many elongated myosin molecules shaped like golf clubs. • Single filament contains roughly 300 myosin molecules • Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. • Myosin heads • Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site) • Attached to the rod portion by a hinge region that can bend and straighten during contraction. • Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

  22. Actin (Thin) Myofilaments • Thin Filament: composed of 3 major proteins • F (fibrous) actin • Tropomyosin • Troponin • Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. • Composed of G actin monomers each of which has a myosin-binding site (see yellow dot) • Actin site can bind myosin during muscle contraction. • Tropomyosin: an elongated protein winds along the groove of the F actin double helix. • Troponin is composed of three subunits: • Tn-A : binds to actin • Tn-T :binds to tropomyosin, • Tn-C :binds to calcium ions.

  23. Now, putting it all together to perform the function of muscle: Contraction

  24. Z line Z line

  25. H Band

  26. Sarcomere Relaxed

  27. Sarcomere Partially Contracted

  28. Sarcomere Completely Contracted

  29. Tropomyosin Binding Site Troponin Ca2+

  30. Myosin

  31. Excitation-Contraction Coupling • Muscle contraction • Alpha motor neurons release Ach • ACh produces large EPSP in muscle fibers (via nicotinic Ach receptors • EPSP evokes action potential • Action potential (excitation) triggers Ca2+ release, leads to fiber contraction • Relaxation, Ca2+ levels lowered by organelle reuptake

  32. Excitation-Contraction Coupling

  33. Excitation-Contraction Coupling

  34. Sliding Filament Model of Contraction • Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree • In the relaxed state, thin and thick filaments overlap only slightly • Upon stimulation, myosin heads bind to actin and sliding begins

  35. How striated muscle works: The Sliding Filament Model The lever movement drives displacement of the actin filament relative to the myosin head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN). Thick and thin filaments interdigitate and “slide” relative to each other.

  36. Neuromuscular Junction

  37. Neuromuscular Junction • Region where the motor neuron stimulates the muscle fiber • The neuromuscular junction is formed by : 1. End of motor neuron axon (axon terminal) • Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine(ACh) 2. The motor end plate of a muscle • A specific part of the sarcolemma that contains ACh receptors • Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

  38. Neuromuscular Junction

  39. Motor Unit: The Nerve-Muscle Functional Unit • A motor unit is a motor neuron and all the muscle fibers it supplies • The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500) • Muscles that control fine movements (fingers, eyes) have small motor units • Large weight-bearing muscles (thighs, hips) have large motor units

  40. Motor Unit: The Nerve-Muscle Functional Unit • Muscle fibers from a motor unit are spread throughout the muscle • Not confined to one fascicle • Therefore, contraction of a single motor unit causes weak contraction of the entire muscle • Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

  41. Motor UnitAll the muscle cells controlled by one nerve cell

  42. Acetylcholine Opens Na+ Channel

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