Chapter 9 – MUSCLE TISSUE

1. Chapter 9 – MUSCLE TISSUE Structure, Function & Metabolism Alireza Ashraf, M.D. Associate Professor of Physical Medicine & Rehabilitation Shiraz Medical school

2. Summary • General • Types of muscle tissue • Functional characteristics • Functions • Structural organization of skeletal muscle • Gross and microscopic anatomy • Excitation/contraction sequence • Muscle metabolism

3. General • Skeletal muscle represents about 40% of our body mass, including smooth and cardiac muscle the figure may be as high as 50% • Muscles transform chemical energy into mechanical energy, i.e. exert a force. • Muscles pull they do not push. • Skeletal and smooth muscle cells are called fibers. • Myo-, mys- and sarco- refer to muscles

4. Types of Muscle Tissue, Table 9.3 • Skeletal • Smooth

5. Types of Muscle Tissue • Cardiac

6. Functional Characteristics • Excitability (irritability) – receive and respond to stimuli. • Stimuli = neurotransmitters, extracellular pH • Response = generation of an electrical impulse (AP) • Contractility = ability to shorten, requires energy • Extensibility = ability to stretch, does not require energy • Elasticity = ability to recoil, i.e. return to resting length

7. Functions of Muscle Tissue • Produces movement • Locomotion • Propulsion • Manipulation • Maintains posture • Stabilizes joints • Generates heat – primarily skeletal m.

8. Structural Organization of Skeletal Muscle, Table 9.1 • Organ = • Fascicles = • Cell = • Myofibrils • Myofilaments

9. Skeletal Muscle Gross Anatomy • Tissues: • Blood vessels • Nerves – branches to each fiber • Connective Tissue (Fig 9.2, Table 9.1) • Endomysium –wraps each fiber • Perimysium –wraps fibers into fascicles • Epimysium –wraps fascicles into a muscle • All are continuous with each other and the tendons.

10. Skeletal Muscle Microscopic Anatomy, fig 9.3 • Cell membrane = sarcolemma • Cell interior gel = sarcoplasm with myoglobin • Organelles • Mitochondria, multiple nuclei, etc. squeezed between myofibrils. • Myofibrils aligned in such a way as to produce alternating light (I) and dark (A) bands or striations.

11. Skeletal Muscle Microscopic Anatomy

12. Skeletal Muscle Microscopic Anatomy • Myofibrils – hundreds to thousands per cell contain the contractile proteins = myofilaments • Actin – thin filaments • Myosin – thick filaments

13. Bands A bands = actin & myosin overlap I bands = actin only H zone in A band – myosin only Z disc – attachment of actin and myosin; distance between Z discs = sarcomere Skeletal Muscle Microscopic Anatomy

14. Closer look at a sarcomere Skeletal Muscle Microscopic Anatomy

15. Skeletal Muscle Microscopic Anatomy • Ultrastructure of Sarcomere, fig 9.4 • Myosin – 2 globular heads whose tails are intertwined. Heads are the “business” end, i.e. form cross bridges with actin. • Actin – globular proteins arranged like 2 strands of beads twisted together in a helix. • Tropomyosin – protein filaments give strength and cover active sites on actin. • Troponin – controls position of tropomyosin.

17. Skeletal Muscle Microscopic Anatomy • Sarcoplasmic reticulum – smooth ER, regulates intracellular calcium; forms paired terminal cisternae at A-I junctions. • T-tubules – invaginations of sarcolemma that reach each A and I band junction, traveling between paired terminal cisternae = triad. • Communicate with external environment, carry electrical impulses into muscle mass.

18. Skeletal Muscle

19. Contraction – Sliding Filament Model • Sliding filament theory • Hugh Huxley 1950’s • Contraction (shortening) - the thin filaments slide past the thick filaments and overlap increases. • Relaxation (lengthening) - thin filaments return to their original position. • Occurs simultaneously in sarcomeres throughout the fiber = muscle shortening.

21. Physiology of Skeletal Muscle Fiber • Neuromuscular junction – chemical synapse (Fig 9.7)

22. Physiology of Skeletal Muscle Fiber • Neural Stimulation • Motor neuron generates electrical impulse (AP) that travels down the axon to the synapse. • The impulse opens Ca2+ channels, Ca2+ moves in and causes vesicles, filled with ACh, to empty the ACh into the synaptic cleft. • ACh diffuses across the cleft to the Motor End Plate on the muscle fiber and binds to ACh receptors. • Acetylcholinesterase destroys remaining ACh quickly to confine stimulation locally.

23. Neuromuscular Junction

24. Physiology of Skeletal Muscle Fiber • Skeletal Muscle Excitation, fig 9.8 • Sarcolemma is polarized at rest, inside negative relative to the outside. RMP = -65mV • ACh binds to ACh receptors at motor end plate and opens LG Na+ channels, Na ions move into the cell causing a small depolarization.

25. Physiology of Skeletal Muscle Fiber • Excitation cont’d • At threshold potential, VG Na+ channels open. • Na+ rushes into the cell and an Action Potential is generated.

27. Physiology of Skeletal Muscle Fiber • Excitation cont’d • AP is propagated along entire sarcolemma. • Repolarization follows closely behind depolarization as Na channels close and VG K channels open to restore the membrane potential back to normal. • During this period, the muscle fiber cannot produce another action potential = Refractory Period.

29. Contraction Sequence, fig 9.10 • Latent Period (excitation/contraction coupling) • Action potential (AP) travels across sarcolemma down the T-tubules to Terminal Cisternae. • Terminal Cisternae – Ca2+ channels open and release stored Ca2+ into sarcoplasm.

30. Contraction Sequence • Contraction • Ca2+ binds to troponin that then pulls tropomyosin out of groove to expose the active sites on actin. • As calcium levels increase, the myosin heads are activated and alternately attach/detach from actin filaments, moving them toward the center of the sarcomere. The sarcomere shortens.

32. Contraction • Relaxation • Ca-ATPase (calcium pump) moves calcium back into the terminal cisternae, tropomyosin moves back to cover active sites on actin. • Myosin heads detach and actin filaments move back to resting position = relaxation

33. Contraction • Role of ATP: • Cross-bridge formation: ATPase on head hydrolyses ATP to ADP and Pi and head “cocks” to attach to actin. • Power stroke: head rotates downward and pulls actin toward center of sarcomere, ADP and Pi are released • Cross bridge detachment – head binds a new ATP

34. Contraction

35. Muscle Metabolism • Stored ATP – 4-6 seconds but is regenerated by 3 mechanisms: • Direct phosphorylation of ADP from creatine phosphate (CP) with the help of creatine kinase. SUPER FAST gives another 6-10 seconds of activity. • Anaerobic glycolysis – glucose is broken down into 2 pyruvate molecules and 2 ATP molecules. Fast but short term – supports another 30-40 seconds of activity. Problem - lactic acid build-up

36. Muscle Metabolism • Energy • Aerobic glycolysis (respiration) or Kreb’s Cycle • Glucose + O2 yields CO2 + H2 O + 30 ATP’s • Pyruvate, amino acids, and fatty acids can also enter this cycle. Slow process, more useful for endurance exercise.

37. Fatigue • Physiological inability of a muscle to contract – not enough ATP; different from psychological fatigue. ATP production lags behind use – contractures (no ATP to release cross bridges. • Accumulation of lactic acid decreases pH and inhibits ATP production. • Restoration of ionic balance of Na and K requires ATP also – impaired with intense exercise.

38. Fatigue • Prolonged exercise leads to SR damage and lack of control of intracellular Ca. • Oxygen debt = amount of oxygen needed to restore muscle anaerobic fuel stores.

39. Heat • 60% of the energy released by muscle contraction is in the form of heat – 40% is in the form of work. Shivering is muscle contraction used to warm a cold body. • When you exercise strenuously your body heats up. How is the heat dissipated?????