MUSCULAR SYSTEM

1. MUSCULAR SYSTEM Chapter 10

2. Functions of Muscle Tissue • Producing body movements : Total body movements such as walking and running and localized movements such as grasping a pencil or nodding the head etc. • Stabilizing body positions : muscle contractions stabilize joints and help maintain body positions. • Regulating organ volume (entrances and exits) : smooth muscles called sphincters. • Moving substances within the body: cardiac muscle contractions pump blood to the rest of the body. • Producing heat : as muscle tissue contracts it also generates heat. Much of this heat is used to maintain normal body temperature.

3. Properties of Muscle Tissue • Electrical excitability-respond to certain stimuli by producing electrical signals. • Contractility-ability of muscle tissue to contract forcefully when stimulated by an action potential. Isometric and isotonic contraction. • Extensibility-ability to stretch without being damaged. This allows a muscle to contract even if it is already stretched. • Elasticity-ability of muscle to return to its original length and shape.

4. Skeletal Muscle Tissue • Each skeletal muscle is a separate organ composed of hundreds to thousands of cells called fibers. • Connective tissues surround muscle fibers and whole muscles. • Blood vessels and nerves penetrate into muscles.

5. Connective tissue components • Fascia is a sheet or broad band of fibrous connective tissue-deep in the skin and surrounds muscles and other organs of the body. • Superficial fascia separates muscle from skin. • Deep fascia-dense irregular connective tissue-lines the body wall and limbs and holds muscles together.

6. Connective Tissue Components • Outermost layer-epimysium • Fascicles-surrounded by perimysium • Penetrating the interior of each fascicle-endomysium. • All three may extend beyond the muscle fibers to form a tendon : a cord of dense regular connective tissue that attaches a muscle to the periosteum of a bone. • When it extends as a broad flat layer-aponeurosis : example is galea aponeurotica on top of the skull.

7. Nerve and Blood Supply • Neurons that stimulate skeletal muscle are -somatic motor neurons. • At the point of contact between the motor neuron and the muscle fiber-neuromuscular junction (NMJ). • Capillaries are plentiful in the muscle tissue-bring in oxygen and nutrients (glucose, fatty acids) and remove heat and waste.

8. Microscopic Anatomy • Arise from mesodermal cells, called myoblasts-has hundred or more nuclei. • Myoblasts exist as satellite cells. • Multiple nuclei are just below the plasma membrane-sarcolemma. Has thousands of invaginations called T tubules. • Within the sarcolemma is the sarcoplasm. • Sarcoplasm contains myoglobin-can bind oxygen.

9. Myofibrils • These are about 2 um in diameter and extend the entire length of the muscle fiber-the contractile elements of skeletal muscle. Their prominent striations make the whole muscle fiber look striated. • A fluid-filled system of membranous sacs called the sarcoplasmic reticulum, SR encircles each myofibril. • In a relaxed muscle the SR stores calcium ions. Release of calcium from the cisterns triggers muscle contraction.

10. Filaments and Sarcomere • Within the myofibrils are two types of filaments-thin and thick. • These are arranged in compartments called-sarcomere-the functional unit of a myofibril. • The thick and thin filaments overlap to create a variety of zones and bands. • The darker middle portion of the sarcomere is called the A band-the thin and thick filaments lie side by side. • Overall there are two thin filaments for every thick filament. The I band is a lighter, less dense area.

11. Bands and Zones • A band • I band • H zone • Z disc • M line

12. Muscle Proteins • Myofibrils are built from three kinds of proteins-contractile proteins, regulatory proteins and structural proteins. • The two contractile proteins are actin (22%)and myosin (44%)-thin and thick filaments. • Regulatorytropomyosin,(5%)troponin(5%. • Structural-titin (9%)myomesin,nebulin,(3%) dystrophin.

13. Contractile Proteins • Myosin-form shaft of thick filaments;myosin heads bind to myosin binding site on actin during muscle contraction. Functions as motor protein. • Actin-forms backbone of thin filament;contains sites where myosin heads bind during muscle contraction.

14. Regulatory Proteins • Tropomyosin (5%)-part of thin filament;blocks myosin-binding sites when muscle is relaxed. • Troponin (5%)-part of thin filament;holds tropomyosin in position.

15. Structural Proteins • Titin (9%)-extends from Z disc to M line and attaches to myosin. • Myomesin -forms the M line; helps stabilize position of thick filaments. • Nebulin (3%)-attaches into Z disc and lies alongside thin filaments. • Dystrophin-links thin filaments to integral membrane proteins of sarcolemma.

16. Contraction and Relaxation of Muscle Fibers • SLIDING FILAMENT MECHANISM • muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere. As the thin filaments slide inward, the Z discs come closer together and the sarcomere shortens. • At the onset of contraction, calcium ions are released which bind to troponin-tropomyosin complexes to move away from the myosin-binding sites on actin. Once the binding sites are free, the contraction cycle begins.

17. Contraction Cycle • Step 1-ATP hydrolysis. At the myosin head, ATP is hydrolyzed into ADP and a phosphate group. • Step 2-attachment of myosin to actin to form crossbridges. The energized myosin head attaches to the myosin-binding site on actin and release the phosphate group.

18. Contraction Cycle • Step 3-Power Stroke. Sliding of the thin filament past the thick filament toward the M line. • Step 4-detachment of myosin from actin. At the end of the power stroke, the myosin head is firmly attached to actin until it binds another molecule of ATP. The contraction cycle begins as the myosin ATPase again hydrolyzes ATP.

19. Excitation-Contraction Coupling • An increase in Ca+2 concentration in the cytosol starts muscle contraction, whereas a decrease stops it. • As a muscle action potential propagates along the sarcolemma and into the T tubules, it causes Ca+2 release channels to open and release the ions. • The released calcium combine with troponin, causing it to change shape.

20. Excitation-Contraction Coupling • This change in shape moves the troponin-tropomyosin complex away from the myosin-binding sites on actin. • Once the binding sites are free, the contraction cycle begins. • All these steps connect excitation(a muscle action potential propagating through the T tubules) to contraction of the muscle fiber.

21. Length-Tension Relationship • A muscle fiber develops its greatest tension when there is an optimal zone of overlap between thin and thick filaments. • The maximum tension during contraction occurs when the resting sarcomere length is 2.0-2.4 um. Decreasing or increasing reduce the tension. • Normally, resting muscle fiber length is kept very close to the optimum by firm attachments of skeletal muscles to bonesand to other inelastic tissues so there is no overstetching.

22. Muscle Metabolism • Muscle contraction requires tremendous amount of energy, for pumping Ca+2 into the SR to achieve muscle relaxation and for other metabolic reactions. • Additional ATP is synthesized by three ways:1)creatine phosphate 2)anaerobic cellular respiration 3) aerobic cellular respiration.

23. Muscle Metabolism • Creatine Phosphate: this is an energy-rich molecule that is unique to muscle fibers. The enzyme creatine kinase transfers one phosphate to creatine. • Aerobic Respiration:a series of oxygen requiring reactions. Muscle has two sources of oxygen1) direct diffusion2)oxygen released by myoglobin. • Anaerobic Respiration:absence of oxygen. Lactic acid formation.

24. Muscle Fatigue • The inability of muscle to contract forcefully after prolonged activity is called muscle fatigue. • Factors-inadequate release of calcium from SR, depletion of creatine phosphate, insufficient amount of oxygen, depletion of glycogen and other nutrients, buildup of lactic acid and ADP.

25. Oxygen Debt • During prolonged periods of muscle contraction, there is increased breathing and increased uptake of oxygen. • After muscle contraction has ceased, the breathing continues and oxygen consumption remains elevated-this is “oxygen debt”. • This extra oxygen was a “pay back” for:1. To convert lactic acid back to glycogen.2. To resynthesize creatine phosphate and ATP. 3. To replace oxygen from myoglobin.

26. Control of Muscle Tension

27. Types of Skeletal Muscle Fibers • Red muscle fibers: high myoglobin content. More mitochondria and more blood capillaries. • White muscle fibers: low content of myoglobin. • Slow oxidative fibers (SO)-smallest in diameter and least powerful.in endurance type activities. • Fast oxdative-glycolytic (FOG)-intermediate.walking and sprinting. • Fast Glycolytic fibers(FG):are largest in diameter. Most powerful contractions.weight-lifting, throwing a ball.fatigue easily.

28. Cardiac Muscle Tissue • This is found only in the heart. Have the same arrangement of filaments as skeletal muscle fibers. • The fibers connect through intercalated discs-contain desmososmes and gap junctions. • This remains contracted 10-15 times longer due to prolonged delivery of calcium ions into the sarcoplasm. • This contracts when stimulated by its own autorhythmic fibers. Depends on aerobic cellular respiration.

29. Smooth Muscle Tissue • This is nonstriated and involuntary. • Fibers contain intermediate filaments and dense bodies. • The duration of contraction and relaxation is longer than skeletal muscle. • They contract in response to nerve impulses, hormones, and local factors. • Found in the walls of hollow viscera, blood vessels, airways to lungs, eye, arrector pilli.

30. Aging and Muscle Tissue • Skeletal muscles fibers cannot divide and have limited powers of regeneration. Cardiac tissue-neither divide nor regenerate.Smooth muscle-limited power for division and regeneration. • Beginning about 30 years of age there is a progressive loss of skeletal muscle. This is replaced by fibrous connective tissue and fat. • Decreased muscle strength. • Diminished muscle reflexes.