1. Structure and Function of Exercising Muscle
Chapter 1
Structure and Function of Exercising Muscle

2. Three Types of Muscle Tissue
Smooth muscle: involuntary, hollow organs
Cardiac muscle: involuntary, heart
Skeletal muscle: voluntary, skeleton

3. Figure 1.1

4. Skeletal Muscle Anatomy
Entire muscle
Surrounded by epimysium
Consists of many bundles (fasciculi)
Fasciculi
Surrounded by perimysium
Consists of individual muscle cells (muscle fibers)
Muscle fiber
Surrounded by endomysium
Consists of myofibrils divided into sarcomeres

5. Structure of Muscle Fibers
Plasmalemma (cell membrane)
Fuses with tendon
Conducts action potential
Maintains pH, transports nutrients
Satellite cells
Muscle growth, development
Response to injury, immobilization, training

6. Structure of Muscle Fibers
Sarcoplasm
Cytoplasm of muscle cell
Unique features: glycogen storage, myoglobin
Transverse tubules (T-tubules)
Extensions of plasmalemma
Carry action potential deep into muscle fiber
Sarcoplasmic reticulum (SR): Ca2+ storage

7. Figure 10.1, Marieb & Mallett (2003). Human Anatomy. Benjamin Cummings.

8. Figure 1.3

9. Myofibrils and Sarcomeres
Muscle  fasciculi  muscle fiber  myofibril
Hundreds to thousands per muscle fiber
Sarcomeres
Basic contractile element of skeletal muscle
End to end for full myofibril length

10. Sarcomere: Protein Filaments
Used for muscle contraction
Actin (thin filaments)
Show up lighter under microscope
I-band contains only actin filaments
Myosin (thick filaments)
Show up darker under microscope
A-band contains both actin and myosin filaments
H-zone contains only myosin filaments

11. Myosin (Thick Filaments)
Two intertwined filaments with globular heads
Globular heads
Protrude 360° from thick filament axis
Will interact with actin filaments for contraction
Stabilized by titin

12. Actin (Thin Filaments)
Actually composed of three proteins
Actin: contains myosin-binding site
Tropomyosin: covers active site at rest
Troponin: anchored to actin, moves tropomyosin
Anchored at Z-disk
Equally spaced out by titin

13. Figure 1.5

14. Motor Units • a-Motor neurons innervate muscle fibers Motor unit
Single a-motor neuron + all fibers it innervates
More operating motor units = more contractile force
Neuromuscular junction
Site of communication between neuron and muscle
Consists of synapse between a-motor neuron and muscle fiber

15. Figure 1.6

16. Skeletal Muscle Contraction (Excitation-Contraction Coupling)
1. Action potential (AP) starts in brain
2. AP arrives at axon terminal, releases acetylcholine (ACh)
3. ACh crosses synapse, binds to ACh receptors on plasmalemma
4. AP travels down plasmalemma, T-tubules
5. Triggers Ca2+ release from sarcoplasmic reticulum (SR)
6. Ca2+ enables actin-myosin contraction

17. Figure 1.7

18. Role of Ca2+ in Muscle Contraction
AP arrives at SR from T-tubule
SR sensitive to electrical charge
Causes mass release of Ca2+ into sarcoplasm
Ca2+ binds to troponin on thin filament
At rest, tropomyosin covers myosin-binding site, blocking actin-myosin attraction
Troponin-Ca2+ complex moves tropomyosin
Myosin binds to actin, contraction can occur

19. Sliding Filament Theory: How Muscles Create Movement
Process of actin-myosin contraction
Relaxed state
No actin-myosin interaction at binding site
Myofilaments overlap a little
Contracted state
Myosin head pulls actin toward sarcomere center (power stroke)
Filaments slide past each other
Sarcomeres, myofibrils, muscle fiber all shorten

20. Figure 1.8

21. Sliding Filament Theory: How Muscles Create Movement
After power stroke ends
Myosin detaches from active site
Myosin head rotates back to original position
Myosin attaches to another active site farther down
Process continues until
Z-disk reaches myosin filaments or
AP stops, Ca2+ gets pumped back into SR

22. Figure 1.9

23. Energy for Muscle Contraction
Adenosine triphosphate (ATP)
Binds to myosin head
ATPase on myosin head
ATP  ADP + Pi + energy
Necessary for muscle contraction

24. Muscle Relaxation AP ends, electrical stimulation of SR stops
Ca2+ pumped back into SR
Stored until next AP arrives
Requires ATP
Without Ca2+, troponin and tropomyosin return to resting conformation
Covers myosin-binding site
Prevents actin-myosin cross-bridging

25. Muscle Fiber Types Type I Type II ~50% of fibers in an average muscle
Peak tension in 110 ms (slow twitch)
Type II
Peak tension in 50 ms (fast twitch)
Type IIa (~25% of fibers in an average muscle)
Type IIx (~25% of fibers in an average muscle)

26. Figure 1.10

27. Type I Versus Type II Sarcoplasmic reticulum Motor units
Type II fibers have a more highly developed SR
Faster Ca2+ release, 3 to 5 times faster Vo
Motor units
Type I motor unit: smaller neuron, <300 fibers
Type II motor unit: larger neuron, >300 fibers

28. Type I Versus Type II: Peak Power
Peak power: type IIx > type IIa > type I
Effects of different SR, motor units, etc.
Single muscle fiber recording
Regardless of fiber type, all muscle fibers reach peak power at ~20% peak force

29. Single Muscle Fiber Peak Power

30. Table 1.1

31. Distribution of Fiber Types: Type I:Type II Ratios
Each person has different ratios
Arm and leg ratios are similar in one person
Endurance athlete: type I predominates
Power athlete: type II predominates
Soleus: type I in everyone

32. Type I Fibers During Exercise
High aerobic endurance
Can maintain exercise for prolonged periods
Require oxygen for ATP production
Low-intensity aerobic exercise, daily activities
Efficiently produce ATP from fat, carbohydrate

33. Type II Fibers During Exercise
Type II fibers in general
Poor aerobic endurance, fatigue quickly
Produce ATP anaerobically
Type IIa
More force, faster fatigue than type I
Short, high-intensity endurance events (1,600 m run)
Type IIx
Seldom used for everyday activities
Short, explosive sprints (100 m)

34. Table 1.2

35. Fiber Type Determinants
Genetic factors
Determine which a-motor neurons innervate fibers
Fibers differentiate based on a-motor neuron
Training factors
Endurance versus strength training, detraining
Can induce small (10%) change in fiber type
Aging: muscles lose type II motor units

36. Muscle Fiber Recruitment
Also called motor unit recruitment
Method for altering force production
Less force production: fewer or smaller motor units
More force production: more or larger motor units
Type I motor units smaller than type II
Recruitment order: type I, type IIa, type IIx

37. Orderly Recruitment and the Size Principle
Recruit minimum number of motor units needed
Smallest (type I) motor units recruited first
Midsized (type IIa) motor units recruited next
Largest (type IIx) motor units recruited last
Recruited in same order each time
Size principle: order of recruitment of motor units directly related to size of a-motor neuron

38. Fiber Type and Athletic Success
Endurance athletes—type I predominates
Sprinters—type II predominates
Fiber type not sole predictor of success
Cardiovascular function
Motivation
Training habits
Muscle size

39. Types of Muscle Contraction
Static (isometric) contraction
Muscle produces force but does not change length
Joint angle does not change
Myosin cross-bridges form and recycle, no sliding
Dynamic contraction
Muscle produces force and changes length
Joint movement produced

40. Dynamic Contraction Subtypes
Concentric contraction
Muscle shortens while producing force
Most familiar type of contraction
Sarcomere shortens, filaments slide toward center
Eccentric contraction
Muscle lengthens while producing force
Cross-bridges form but sarcomere lengthens
Example: lowering heavy weight

41. Generation of Force Motor unit recruitment
Type II motor units = more force
Type I motor units = less force
Fewer small fibers versus more large fibers
Frequency of stimulation (rate coding)
Twitch
Summation
Tetanus

42. Generation of Force Length-tension relationship
Optimal sarcomere length = optimal overlap
Too short or too stretched = little or no force develops
Speed-force relationship
Concentric: maximal force development decreases at higher speeds
Eccentric: maximal force development increases at higher speeds

43. Figure 1.12

44. Figure 1.13

45. Figure 1.14