Skeletal Muscle: Histology and Sliding Filament Theory

2. The action of muscles in the body Learning outcomes – A grade questions (n) describe, with the aid of diagrams, photographs and electron micrographs, the histology and ultrastructure of skeletal muscle; (o) describe the sliding filament theory of muscle contraction, to include the importance of the power stroke and the role of ATP and calcium ions. Key words/phrases Skeletal muscle Sliding filament theory Power stroke Calcium ions Ultrastructure

3. Success criteria

4. Types of muscles in the body • Three types of muscle • Cardiac • Skeletal • Smooth • All three types of muscle contain the proteins actin and myosin forming myofilaments that allow contraction to occur. When talking about muscle, say exactly which type you mean!

5. Skeletal Muscle • Stimuli from nerve impulses causes skeletal muscle to contract. • Contraction uses ATP to produce movement and also releases heat to warm the body. • Specialised muscle fibres that make up the structure of muscle enable it to carry out these important functions.

6. Skeletal Muscle • Muscles fibres are surrounded by a membrane called the sarcolemma. • The sarcolemma encases a specialised cytoplasm called the sarcoplasm. • This membrane has special finger-like projections called T-tubules that link up to the sarcoplasmicreticulum. • This creates a complete cellular network in the muscle.

7. Skeletal Muscle • The cytoplasm contains many mitochondria, plus stores of myoglobin and glycogen. • The cytoplasm also contains the proteins (filaments) actin and mysoin, arranged in bundles of myofibrils. • The sarcoplasmic reticulum contains calcium ions that are essential for muscle contraction.

8. Learning outcomes – A grade questions (n) describe, with the aid of diagrams, photographs and electron micrographs, the histology and ultrastructure of skeletal muscle; (o) describe the sliding filament theory of muscle contraction, to include the importance of the power stroke and the role of ATP and calcium ions. Task 1 • Make flash cards of the following words with definitions on the back. • Actin • Myosin • Sarcolemma • Sarcoplasm • T-tubules • Sarcoplasmic reticulum • Myofibrils End 12 minutes

9. Mini Plenary Actin and myosin form myofilaments True False

10. Mini Plenary Sodium ions are essential for muscle contraction True False

11. Mini Plenary The sarcoplasm is a specialised membrane that surrounds muscle fibres True False

12. Mini Plenary Muscle membrane has special finger-like projections called T-tubules that link up to the sarcoplasmic reticulum. True False

14. The structure of a sarcomere • Sarcomeres are made up of myofilaments, which are bundles of myofibrils. • The two types of myofilament are actin and myosin. • Actin in the thin band, whereas myosin in thicker. • Actin is actually made up of three different proteins bound together: • Actin • Tropomyosin • troponin

15. The structure of a sarcomere • Myosin filament is made of bundles of polypeptide molecules with a globular head for binding to ATP and to the actin filament.

16. Learning outcomes – A grade questions (n) describe, with the aid of diagrams, photographs and electron micrographs, the histology and ultrastructure of skeletal muscle; (o) describe the sliding filament theory of muscle contraction, to include the importance of the power stroke and the role of ATP and calcium ions. Task 2 • Make flash cards of the following words with definitions on the back. • Tropomyosin • Troponin • Sarcomeres • Myofilaments • Test each other using all your flashcards to ensure that you have a good understanding of what these words mean End 15 minutes

17. Sliding filament theory

18. Contraction of skeletal muscle is initiated by neurotransmitters (acetylcholine) which are released from motor neurones. A series of strictly controlled events happens as a result… Sliding filament theory

19. Sliding filament theory • Nerve impulses stimulate the neuromuscular junction (NMJ)– causes the release of acetylcholine (Ach), which is an excitatory neurotransmitter. • ACh travels across the synapse of the NMJ and binds to receptor sites on the motor end plate. • Causes impulses to spread quickly through the nerve fibre to the T tubule system and then to the sarcoplasmic reticulum.

20. Sliding filament theory • Ca2+ ions bind to troponin which causes tropomyosin to detach from actin. • This results in actin being exposed so myosin heads can now bind to it because binding sites have been exposed. • ADP is normally attached to mysoin heads. The enzyme mysoin kinase removes the ADP so that myosin can attach to the binding sites on actin.

21. http://www.sci.sdsu.edu/movies/actin_myosin_gif.html

22. Sliding filament theory • In the “power stroke”, the myosin head changes position and “pulls” the actin • This results in the filaments sliding past each other. • If ATP is available, myosin heads bind to it and hydrolyse it to release energy. • This causes myosin heads to detach from the binding sites on the actin and return to their original position. • This cycle will repeat as long as there is a supply of calcium ions and ATP

23. Sliding filament theory • As the filaments slide past each other, the attachment breaks and new attachments form further along the molecule. This can happen 100 times per second. • The contractions continue to shorten the fibre until nervous stimulation stops and the calcium ions return to the sarcoplasmic reticulum • If the concentration of Ca ions falls, actin filaments reattach to troponin and tropomyosin. The muscle fibre then returns to its original shape

26. Learning outcomes – A grade questions (n) describe, with the aid of diagrams, photographs and electron micrographs, the histology and ultrastructure of skeletal muscle; (o) describe the sliding filament theory of muscle contraction, to include the importance of the power stroke and the role of ATP and calcium ions. Task 3 • In your own words, write out a flow diagram to describe the stages of the sliding filament theory. • Do this in pairs. • Feedback your flow chart to the rest of the group. Try to describe it in detail, but make sure you understand the detail you are giving! End 15 minutes

29. The Banding Pattern of Skeletal Muscle A BAND I BAND I BAND z z SARCOMERE Under a light microscope the striated nature of skeletal muscle can be observed This is seen as a regular alternation of light and dark bands This banding pattern is due to the arrangement of the thick and thin filaments within the myofibrils Dark bands (A bands) appear where thick myosin filaments are located The edges of the A bands are very dark as thick and thin filaments are present together Light bands (I bands) appear where there are thin filaments only H ZONE Across the middle of each I-band is a dark line called the Z line: The section of myofibril between these Z lines is the SARCOMERE The centre of the A band contains thick filaments only and is slightly lighter (H Zone)

30. This electron micrograph of a longitudinal section of skeletal muscle shows the myofibrils and Z-lines of the sarcomeres (magnification X75 000) Z line IBAND A BAND Z line H ZONE Courtesy of Dr. H. Huddart – Department of Biological Sciences, The University of Lancaster

31. The Sarcomere Z LINE Z LINE SARCOMERE x 40,000 A BAND I BAND I BAND

32. SARCOMERE Thick Filament Thin Filament Longitudinal Section The Banding Pattern of Skeletal Muscle The arrangement of the myofilaments can also be seen in cross sections of the myofibrils Cross Sections Myosin and Actin (A band - very dark) Actin Only I band - light Myosin Only (A band - dark)

33. The Myofilaments The ends of each thick filament are surrounded by six thin filaments 3-D SIX ACTIN FILAMENTS Cross sections of myofibrils show the HEXAGONAL organisation of actin and myosin filaments MYOSIN

34. ELECTRON MICROGRAPH OF A CROSS SECTION OF SKELETAL MUSCLE SHOWING THE ARRANGEMENT OF THICK AND THIN FILAMENTS THICK FILAMENT HEXAGONAL ARRAY OF THIN FILAMENTS