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  2. FUNCTIONS OF MUSCULAR SYSTEM Body movement Maintain posture Respiration Produce body heat Communication Constriction of organs and blood vessels Heartbeat

  3. Connective Tissue Sheaths in Skeletal Muscle Figure 10.1a

  4. Connective Tissue Sheaths • The MUSCLE FASCIA is loose connective tissue on the outside of the muscle. It creates a slippery surface for muscles to rub against each other. Superficial to the fascia is fat. Deep to the fascia is the EPIMYSIUM, (dense irregular fibrous connective tissue), and which eventually becomes the tendon (which is connected to bone). The epimycium extends into the muscle belly to form compartments called FASICLES. This tissue surrounding the fascicles is now called the PERIMYCIUM. Each fascicle contains MUSCLE FIBERS, which are individual muscle cells, each one surrounded by ENDOMYCIUM. When you eat steak and find it’s stringy, each string is a fascicle.


  6. PARALLEL MUSCLE The fascicles are parallel. They are long fibers, which can contract to 75% of their length. They contract a long way, but they are relatively weak, because there are relatively few fascicles. E.g. Sternocleidomastoid.

  7. Arrangement of Fascicles in Muscles Figure 11.3

  8. PENNATE PENNATE (means “feather shape”) MUSCLES: three types: • UNIPENNATE; looks like half a feather. The fascicles are short, but there are more of them. They are stronger, but do not have the same length contraction ability of the parallel muscles. • BIPENNATEare fascicles that insert into the tendon from both sides; they are stronger than unipennate (quadriceps). • MULTIPENNATE are the strongest (deltoid). The fascicles are in multiple bundles inserting on one tendon


  10. CONVERGENT CONVERGENT MUSCLE has more fibers than parallel, but contracts a greater distance than pinnate. E.g. Pectoralis major.

  11. CIRCULAR MUSCLE CIRCULAR MUSCLE (Sphincter) is arranged in a circle, with a small area of tendon on the sides. It allows closure of the eyes, mouth, etc. They are not very strong, but they don’t need to be.

  12. TERMS: • ORIGIN = The region which usually doesn’t move when the muscle contracts. Look at the biceps brachii; does the shoulder move when I bend my arm? No; the shoulder = origin. • INSERTION= The point of attachment that moves; bend arm, radial tuberosity = attachment. • AGONIST = The main muscle for a particular action; bend arm, biceps = agonist. • ANTAGONIST = Does the opposite action; bend elbow, antagonist extends. Every muscle in the body has to have an antagonist. • SYNERGIST = The muscle that helps the agonist. There are several muscles that assist when the arm is bent.

  13. Muscle Attachments

  14. Muscle Types • Skeletal: elongated • Moves the skeleton • Voluntary • striated • Smooth: spindle shaped • Found in organs and lining of blood vessels • Involuntary • no striations • Cardiac: cylindrical shaped • involuntary (only responds to direct electrical stimulation) • striated

  15. Skeletal Muscle Characteristics • Contractility • The ability to shorten with force • However, they lengthen passively, by gravity or by the contraction of an opposing muscle. • Excitability • Capacity to respond to a stimulus (nerves) • Extensibility • Can be stretched • After a contraction, they can be stretched to their normal resting length and beyond to a limited degree. • Elasticity • Can recoil to their original resting length after they have been stretched • Has thousands of nuclei per cell, unlike smooth and cardiac muscle

  16. SKELETAL MUSCLE • They have thousands of nuclei because they start from many stem cells that fuse together into one skeletal muscle fiber. • Theses are very long fibers (biceps muscle can be 8-10 cm).

  17. Skeletal Muscle • Myoblasts exist in adults, so muscle heals well. • A muscle cell torn in half can regenerate. • There are almost no muscle diseases for this reason (muscular dystrophy is the main muscle disease). • Muscles Overview Video •

  18. Skeletal Muscle: Longitudinal section In skeletal muscle fibers, there are light and dark stripes called striations, which can be seen under a microscope.

  19. Skeletal Muscle ON CROSS SECTION

  20. A cross section of skeletal muscle looks like bundles of circles because you are looking at cut fascicles.

  21. Skeletal Muscle • The plasma membrane of muscles is called a SARCOLEMMA. • The cytoplasm of muscle cells is called SARCOPLASM. • Muscle cells contain many mitochondria and other organelles. • One type of unusual organelle found only in muscle cells is called a myofibril. They are packed in bundles and fill up most of the cell.

  22. MUSCLE MYOFIBRILS • Cylindrical organelles found within muscle cells • Extend from one end of the muscle fiber (muscle cell) to the other • Contain sarcomeres joined end to end. • The sarcomeres are made of actin and myosin myofilaments

  23. Skeletal Muscle: Longitudinal section These striations (stripes) are caused by dark and light bands. The dark band is called an A band. (There is an “A” in dark) The light band is called an I band. There is an “I” in light)

  24. This is all part of one muscle cell that has many nuclei.

  25. Every dark band + light band is one sarcomere In the center of each light I band is a Z disc One sarcomere is the area from one Z disc to the next Z disc. So, each sarcomere extends from the middle of one light band to the middle of the next light band. In the center of the dark band is a lighter colored area called the H zone. It is the area of the myosin without heads.

  26. SARCOMERES • The striations result from the internal structure of SARCOMERES within the sarcoplasm. • The sarcomere is the basic structural and functional unit of skeletal muscle. The sarcomere is what contracts. • Each sarcomere • Extends from one Z disc to the next Z disc • Has a light colored H zone in the center (found in the middle of the dark band, which is in the center of the sarcomere. It is the area of myosin in the center that does not have myosin heads). • Contains parts of two I (light) bands and all of one A (dark) band • Contains overlapping actin and myosin myofilaments.

  27. Note: the I band consists only of actin myofilaments. The A band consists of both actin and myosin.

  28. Actin and Myosin Sarcomere model video 1 Sarcomere model video 2 • Sarcomeres consist of two types of myofilaments made out of protein: • thin (ACTIN) myofilaments • Look like two strands of beads twisted together. • Actin myofilaments are attached to the Z disc at one end. • thick (MYOSIN) myofilaments. • Both ends of a thick filament are studded with knobs called myosin heads (look like little golf clubs). • Myosin is NOT attached to the Z disc.

  29. Actin Myosin Actin Myosin

  30. Don’t confuse these terms! MUSCLE FASCICLE: a group of muscle fibers, surrounded by perimysium. MUSCLE FIBER: a single muscle cell MYOFIBRIL: a long organelle inside a muscle fiber, contains actin and myosin myofilaments. MYOFILAMENTS: these are proteins, and there are two types: actin (with troponin and tropomyosin) and myosin. The myofilament is the lowest level of organization that is composed of actin, myosin, troponin, and tropomyosin proteins. Therefore, a myofilament is part of a myofibril, which is inside a muscle fiber, which is inside a muscle fascicle.

  31. MECHANISM OF CONTRACTION The Sliding Filament Theory • Contraction results as the myosin heads of the thick filaments attach like hooks to the thin actin filaments at both ends of the sarcomere and pull the thin filaments toward the center of the sarcomere. • The myosin head is like a hook with a hinge. After a myosin head pivots at its hinge, it draws the actin closer, then lets go, springs up again to grab the actin filament again, pulls it closer, and it keeps repeating this until the entire actin filament has been drawn in as far as it can go. • The sites where the myosin heads hook onto the actin are called cross-bridges.

  32. Sarcomere Contraction • The complete process of contraction of the sarcomere takes only a fraction of a second. • The actin and myosin filaments do not shorten; they merely slide past each other. • The energy required is ATP. • The A band (dark stripe) in a sarcomere does not change length in a contraction. • This sliding filament mechanism begins whenever calcium ions bind to the thin filament. • Where does the calcium come from?

  33. SARCOPLASMIC RETICULUM AND T TUBULES • Within the cytoplasm of all body cells is an endoplasmic reticulum. • The endoplasmic reticulum in muscle cells is called the SACROPLASMIC RETICULUM. • It surrounds each sarcomere like the sleeve of a loosely crocheted sweater.

  34. Sarcoplasmic reticulum is in blue T tubules are in yellow

  35. Calcium is needed for muscle contraction The sarcoplasmic reticulum stores a lot of calcium ions, which are released when the muscle is stimulated to contract. The calcium diffuses out of the sarcoplasmic reticulum and land on the actin filaments, where they trigger the sliding filament mechanism of contraction. After the contraction, the calcium ions are pumped back into the sarcoplasmic reticulum for storage.

  36. Calcium is needed for muscle contraction • ACTIVE TRANSPORT is required to return the calcium ions to the sarcoplasmic reticulum. • It also requires energy to break the cross-bridge so the myosin head can cock back again, ready to spring onto the next binding site. • Therefore, ATP is used. • ATP is used to return calcium to the sarcoplasmic reticulum • ATP is used to cock back the myosin heads

  37. ATP is required for contraction • ATP attaches to the myosin myofilaments • Provides energy for the movement of the cross bridges • ATP is required for muscle relaxation • ATP releases part of its energy as heat. • That is why we get hot when we exercise • When we are cold, we shiver (muscle contraction) to warm up. • In order for the mitochondria to produce enough ATP, it needs oxygen and the sugars that are in storage.

  38. For contraction to take place, you need a nerve signal and calcium • For skeletal muscle to contract, the synaptic knob of a neuron must first release a chemical called ACETYLCHOLINE onto the region where it sits on the muscle cell, known as the ENDPLATE. • Calcium is also needed for muscle contraction. • The nerve signal is called an ACTION POTENTIAL. • It causes a release of calcium from the sarcoplasmic reticulum, which causes contraction.

  39. Muscle Contraction In a muscle fiber, an action potential results in muscle contraction. How does this happen? The action potential continues to travel along the sarcolemma (cell membrane of the muscle). Part of this electrical impulse breaks away from the sarcolemma and travels down the T-tubules, while the rest of the electrical impulse continues longitudinally down the muscle cell to the next sarcomere and T-tubule.

  40. T tubules are in yellow

  41. T TUBULES • T TUBULES (“T” stands for “transverse”) are continuations of the sarcolemma (cell membrane) which invaginate to the deepest regions of the muscle cell. • Since the T tubules conduct the nerve impulse throughout the muscle cell, all the sarcomeres of that cell contract at the same time.

  42. Muscle Contraction • The action potential of the nerve goes down the T-tubules and causes calcium to leak out of the sarcoplasmic reticulum. • The calcium causes the muscle fibers to contract. • After a while, the calcium gets pumped back where it came from, the muscle fibers relax, although it requires gravity or another muscle to pull the sarcomere back to its original length. • How does the calcium cause the muscle fibers to contract?

  43. TROPOMYOSIN is a single long protein strand like a piece of yarn that winds around the actin filament. • Tropomyosin blocks actin’s attachment site for the myosin head, so the myosin “hook” has nothing to grab onto, thus preventing contraction. TROPONIN is a globular complex of three proteins, and is found in clumps around the tropomyosin protein. • Troponin is the specific molecule that provides the calcium binding site on actin. • Calcium binds to troponin and causes troponin to move a little, taking the tropomyosin thread with it, so the attachment sites on the actin molecule are now exposed. The myosin heads can now hook into the exposed sites on the actin myofilament. Both troponin and tropomyosin cover the actin filament when the muscle is relaxed.

  44. This is an illustration of an actin molecule. You can see the thready tropomyosin and the globular troponin proteins wrapping around the double-stranded actin.