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
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. Skeletal Muscle Microscopic Anatomy
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

14. Skeletal Muscle Microscopic Anatomy
Closer look at a sarcomere

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 = mV
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 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?????