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Muscles

Muscles. 13.8 Muscles are effectors which enable movement to be carried out. Muscle. Is responsible for almost all the movements in animals 3 types. Muscles & the Skeleton. Skeletal muscles cause the skeleton to move at joints They are attached to skeleton by tendons.

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Muscles

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  1. Muscles 13.8 Muscles are effectors which enable movement to be carried out

  2. Muscle • Is responsible for almost all the movements in animals • 3 types

  3. Muscles & the Skeleton • Skeletal muscles cause the skeleton to move at joints • They are attached to skeleton by tendons. • Tendons transmit muscle force to the bone. • Tendons are made of collagen fibres & are very strong & stiff

  4. Antagonistic Muscle Action • Muscles are either contracted or relaxed • When contracted the muscle exerts a pulling force, causing it to shorten • Since muscles can only pull (not push), they work in pairs called antagonistic muscles • The muscle that bends the joint is called the flexor muscle • The muscle that straightens the joint is called the extensor muscle

  5. Elbow Joint • The best known example of antagonistic muscles are the bicep & triceps muscles

  6. Muscle Structure Bicep Muscle • A single muscle e.g. biceps contains approx 1000 muscle fibres. • These fibres run the whole length of the muscle • Muscle fibres are joined together at the tendons

  7. Muscle Structure • Each muscle fibre is actually a single muscle cell • This cell is approx 100 m in diameter & a few cm long • These giant cells have many nuclei • Their cytoplasm is packed full of myofibrils • These are bundles of protein filaments that cause contraction • Sarcoplasm (muscle cytoplasm) also contains mitochondria to provide energy for contraction

  8. Muscle Structure • The E.M shows that each myofibril is made up of repeating dark & light bands • In the middle of the dark band is the M-line • In the middle of the light band is the Z-line • The repeating unit from one Z-line to the next is called the sarcomere

  9. Muscle Structure • A very high resolution E.M reveals that each myofibril is made up of parallel filaments. • There are 2 kinds of filament called thick & thin filaments. • These 2 filaments are linked at intervals called cross bridges, which actually stick out from the thick filaments

  10. The Thick Filament (Myosin) • Consists of the protein called myosin. • A myosin molecule is shaped a bit like a golf club, but with 2 heads. • The heads stick out to form the cross bridge • Many of these myosin molecules stick together to form a thick filament

  11. Thin Filament (Actin) • The thin filament consists of a protein called actin. • The thin filament also contains tropomyosin. • This protein is involved in the control of muscle contraction

  12. Sarcomere = the basic contractile unit

  13. The Sarcomere

  14. I Band = actin filaments

  15. Anatomy of a Sarcomere • The thick filaments produce the dark A band. • The thin filaments extend in each direction from the Z line. • Where they do not overlap the thick filaments, they create the light I band. • The H zone is that portion of the A band where the thick and thin filaments do not overlap. • The entire array of thick and thin filaments between the Z lines is called a sarcomere

  16. Sarcomere shortens when muscle contracts • Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril • And, in turn, of the muscle fibre of which it is a part

  17. Mechanism of muscle contraction • The above micrographs show that the sarcomere gets shorter when the muscle contracts • The light (I) bands become shorter • The dark bands (A) bands stay the same length

  18. The Sliding Filament Theory • So, when the muscle contracts, sarcomeres become smaller • However the filaments do not change in length. • Instead they slide past each other (overlap) • So actin filaments slide between myosin filaments • and the zone of overlap is larger

  19. What makes the filaments slide past each other? • Energy for the movement comes from splitting ATP • ATPase that does this is located in the myosin cross bridge head. • These cross bridges attach to actin. • The energy from the ATP causes the angle of the myosin head to change. • So they are able to cause the actin filament to slide relative to the myosin. • This movement reduces the sarcomere length.

  20. The Cross Bridge Cycle • The cross bridge cycle has 4 steps • It is analogues to 4 steps in rowing a boat

  21. Step 1 • The Cross bridge swings out from the myosin filament & attaches to the actin filament. • Put Oars in water

  22. Step 2 – The Power Stroke • The cross bridge changes shape & rotates through 45 degrees • Causes the filaments to slide. • Energy from ATP is used for this power stroke • ADP + Pi are released • Pull oars through water

  23. Step 3 • A new ATP molecule binds to myosin • The Cross bridge detaches from the thin filament • Push oars out of water

  24. Step 4 • The Cross bridge changes back to its original shape • This occurs while it is detached (to ensure the actin filament is not pushed back again). • It is now ready for a new cycle, but further along the actin filament • Push oars into starting position

  25. Repetition of the cycle • One ATP molecule is split by each cross bridge in each cycle. • This takes only a few milliseconds • During a contraction 1000’s of cross bridges in each sarcomere go through this cycle. • However the cross bridges are all out of synch, so there are always many cross bridges attached at any one time to maintain force. http://199.17.138.73/berg/ANIMTNS/SlidFila.htm

  26. Control of Muscle Contraction • How is the cross bridge cycle switched off in a relaxed muscle? • This is where the regulatory protein on the actin filament, tropomyosin is involved. • Actin filaments have myosin binding sites. • These binding sites are blocked by tropomyosin in relaxed muscle. • When Ca2+ bind tropomyosin is displaced and the myosin binding sites are uncovered. • So myosin & actin can now bind together to start the cross bridge cycle

  27. Tropomyosin, Ca2+ & ATP • Ca2+ causes tropomyosin to be displaced. • No longer blocks myosin binding site • Power stoke can begin. • Ca2+ also active myosin molecules to breakdown ATP • So energy is released to begin contraction

  28. Neuromuscular junction: Note Ach = Acetylcholine

  29. Sarcoplasmic Reticulum

  30. Sequence of events • 1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction. • 2. This causes the release of the neurotransmitter acetylcholine. • 3 This initiates an action potential in the muscle cell membrane (Sarcolemma). • 4. This action potential is carried quickly into the large muscle cell by invaginations in the cell membrane called T-tubules.

  31. Sequence of events • 5. The action potential causes the sarcoplasmic reticulum to release its store of calcium into the myofibrils. • 6. Ca2+ causes tropomoysin to be displaced uncovering myosin binding sites on actin. • 7. Myosin cross bridges can now attach and the cross bridge cycle can take place. • Relaxation is the reverse of these steps

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