1. Muscle Physiology

2. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

3. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

4. 1- Skeletal Muscle Structure
Muscle = group of fascicles
Muscle fibers extend length of muscle from tendon to tendon

5. Motor units
Motor unit: Composed of one motor neuron and all the muscle fibers that it innervates
There are many motor units in a muscle
The number of fibers innervated by a single motor neuron varies (from a few to thousand)
The fewer the number of fibers per neuron  the finer the movement (more brain power)
Which body part will have the largest motor units? The smallest?

6. Components of a muscle fiber

7. Muscle fiber components
Sarcolemma: muscle cell membrane
Sarcoplasma: muscle cell cytoplasm
Motor end plate: contact surface with axon terminal
T tubule: cell membrane extension into the sarcoplasm (to reach the myofibrils)
Cisternae: areas of the ER dedicated to Ca++ storage (located on each side of the T-tubules)
Myofibrils: organized into sarcomeres
Figure 12.2 (2 of 2)

8. The sarcomere
The myofibrils are organized into a repetitive pattern, the sarcomere
Myosin: thick filament
Actin: thin filament
Bands formed by pattern: A and I and H bands
Z line: area of attachment of the actin fibers
M line: Myosin fiber centers

9. The sarcomere
Figure 12.5d

10. Myosin structure Many myosin molecules per filament, golf club shape
Long tail topped by a thickening: the head  forms crossbridges with the thin filament
Presence of the enzyme, ATPase in the head  release energy for contraction

11. Actin structure Formed by 3 different proteins:
- globular (G) actins: bind to myosin heads
- tropomyosin: long, fibrous molecule, extending over actin, and preventing interaction between actin and myosin
- troponin: binds reversibly to calcium and able to move tropomyosin away from the actin active site
Figure 12.4

12. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

13. 2- Muscle contraction: Cell events
Figure 11.13

14. Synaptic events The AP reaches the axonal bulb
Voltage-gated calcium channels open
The influx of calcium in the bulb activates enzymes the vesicles containing the neurotransmitter molecule dock and release the neurotransmitter in the synapse
The neurotransmitter for skeletal muscles is always acetylcholine
The receptors on the muscle fiber are cholinergic receptors
These receptors are nicotinic (fast) acting receptors

15. 2- The Mechanism of Force Generation in Muscle

16. Figure 12.7

17.
Figure 12.6

18. Muscle relaxation
Ach is removed from the receptors by acetylcholinesterase
Ligand-gated Na+channels close
Na/K pumps reestablish the RMP
Ca++ ions leave troponin and are brought back into the cisternae (this process needs energy)
Tropomyosin moves back over the actin active site
The myosin heads release their binding to actin
The filaments passively move back into resting position

19. Applications
Myasthenia gravis: autoimmune disease where antibodies against the Ach receptors are produced. Which consequences do you expect?
Muscular dystrophy: some proteins forming the muscle fibers are abnormal. Which consequences do you expect?
Curare binds to the Ach receptor without activating them. What are the effect of curare on the skeletal muscle?
The botulism toxin prevents the release of the neurotransmitter into the synapse. What will be the consequence?
Nerve gas inhibits acetylcholinerestase present in the synapse. What will be the consequence?

20. Rigor mortis: why does the body stiffen shortly after death?

21. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

22. 3- Muscle contraction: Mechanical events
1 stimulation  1 twitch
Muscle twitch: 3 phases:
- latent phase
- contraction phase
- relaxation phase
☻ do not confuse the AP and the twitch!!!

23. Figure 12.16

24. Events during the twitch
Latent phase: Stimulus to beginning contraction: AP to myosin binding to actin active site
Contraction phase: beginning to end of muscle tension  myosin heads slide along the actin filaments
Relaxation phase: peak tension to no tension  Ca++ ions moved back into the cisternae, tropomyosin moves back over actin, myosin head release actin and the filaments move back into resting position

25. Figure 12.18

26. Isometric/isotonic contractions
Isometric: muscle contraction without movement  no muscle shortening
Isotonic: muscle contraction with movement  muscle shortens

27. Effect of consecutive stimuli: Treppe
Treppe: gradual increase in contraction intensity during sequential stimulation
Might be due to calcium ions accumulating in the cytoplasm with each stimulation
Figure 12.15

28. Summation and tetanus
Summation: Rapid sequence of stimuli muscle twitches fuse into each other, each subsequent one being stronger that its precedent (due to Ca++?)
Tetanus: very rapid sequence of stimuli: no relaxation
Figure 12.17

29. Recruitment
An increase in force is made possible by recruiting more motor units
Muscles have various sizes of motor units  allows them to adjust the size of the effort to be made
Activating motor units alternatively allows the muscle to sustain contraction with minimal fatigue
Figure 12.19

30. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

31. IV- Muscle metabolism
Muscle fibers use ATP (only first few seconds) for contraction
ATP must then be generated by the muscle cell:
- from creatine phosphate, first
- from glucose and glycogen
- from fatty-acids
ATP formation from the above compound is possible if oxygen is present (oxidative phosphorylation: 36 ATP per glucose)
Oxygen is delivered to the muscle by myoglobin, a molecule with high affinity to oxygen and related to hemoglobin

32. Consequence of anaerobic metabolism?
If the effort is strong and sustained, the muscle might not have enough oxygen delivered to it by myoglobin  anaerobic glycolysis with only 2 ATP formed per glucose and synthesis of lactic acid
Consequence of anaerobic metabolism?
Figure 12.11

33. Muscle fatigue
Muscle fatigue: a decline in the ability of the muscle to sustain the strength of contraction
Causes:
- rapid build-up of lactic acid
- decrease in oxygen supply
- decrease in energy supply (glucose, glycogen, fatty-acids)
Decreased neurotransmitter at the synapse
- psychological causes

34. Effects of exercise on the muscle
Aerobic exercises: long sustained exercises  promote increased oxidative capacity of the muscle fiber  increased blood vessel supply, increased mitochondria
High intensity, short burst exercise: increased glycolytic activity  increased synthesis of glycolytic enzymes, increased synthesis of myofibrils (increased muscle size)

35. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

36. V- Types of Muscle Fibers
Various muscles contract at different speed  composed of different types of muscle fibers
Figure 12.23

37. Basis for classification
Velocity of contraction: slow vs fast
Energy source: oxidative vs glycolytic

38. Oxydative Glycolytic Primary energy through oxidative phosphorylation
Many mitochondria
Myoglobin (red)
Small diameter
Resistant to fatigue
Primary energy through anaerobic glycolysis
Fewer mitochondria
Many glycolytic enzymes
High glycogen stores
Use little oxygen—anaerobic
Large diameter
Quick to fatigue

39. Which types of meat are chicken breast and duck breast?
Why the difference?

41. Outline Skeletal Muscle Structure Muscle Contraction: Cell Events
Muscle Contraction: Mechanical Events
Muscle Metabolism
Types of Skeletal Muscle Fibers
VI. Smooth and Cardiac Muscles

42. VI- Smooth and Cardiac Muscles

43. Skeletal Cardiac Smooth Appearance Control voluntary unvoluntary
Neural input
somatic
ANS
Hormone
Epi
Epi/others
Ca++ prot
Troponin
Calmodulin
Gap junctions No
Yes
Pacemaker

44. Readings
Chp. 12, p