Sarcomere, Muscle Contraction, and motor unit types

Question 1

Events during Muscle Contraction

Answer 1

o Action potential enters muscle through T-tubule
o Ca2+ is released when “triad” region depolarizes
o Ca2+ binds to troponin

Question 2

6 Steps leading to muscular contraction (FOUNDATION FOR PHYSIOLOGIST)

Answer 2

1) Binding site on actin exposed in presence of Ca++; muscle contraction begin
o 2) ATP bound to myosin; myosin weakly bound to actin
o 3) ATP is split into ADP + Pi (inorganic phosphate), myosin head reset to “ready” position
o 4) Myosin strongly bound to actin forming cross-bridge
o 5) Release of Pi and initiation of power stroke; ADP released
o 6) Myosin head returns to uncocked position; ATP rebinds to myosin
o Cycle can repeat in the presence of Ca++ and ATP
o Cross bridge cycling- when muscular contraction is repeated

Question 3

Application of muscle contraction physiology

Answer 3

length-tension relationship, force-velocity relationship

Question 4

Length-tension relationship

Answer 4

o 1) Overly shortened muscle length, missing H zone
o 2) Normal resting length, optimal length for force or tension
o 3) Overly lengthened muscle length, limited ability to create force or tension, also crossover is limited (few sites for myosin heads to bind to actin)
o Optimal length; 100-120% resting length

Question 5

Force-velocity relationship

Answer 5

o Ability to generate force related to velocity of contraction
o High velocities limit cross-bridging
o Low velocities increase cross-bridging

Question 6

3 Types of motor units

Answer 6

o 1) slow oxidative- Type1
o 2) fast oxidative glycolytic- Type 2a
o 3) fast glycolytic- Type 2x

Question 7

Contractile properties

Answer 7

• Contraction velocity
o Type 1- slow
o Type 2a- fast
o Type 2x- fastest (3-5 times faster than type 1)
• Relaxation Time
o Type 1- slow
o Type 2a- fast
o Type 2x- fast
• Force Production
o Type 1- low
o Type 2a- high
o Type 2x- high (10-20% more force than Type 1)
• Fatiguability
o Type 1- resistant
o Type 2a- fatiguable
o Type 2x- most

Question 8

Biochemical properties

Answer 8

• CP (Creatine Phosphate) stores
o Type 1- low
o Type 2a- high
o Type 2x- high
• Glycogen Stores
o Type 1- low
o Type 2a- high
o Type 2x- high
• Lipid Stores
o Type 1- high (beta oxidation for break down of lipids in oxidative phosphorylation)
o Type 2a-medium
o Type 2x- low

Question 9

Structural Properties

Answer 9

• Fiber diameter
o Type 1- small (actin and myosin are in a smaller diameter)
o Type 2a- largest (Storing CP, Glycogen, Lipids)
o Type 2x- large
• # of Mitochondria
o Type 1- high (need oxygen to be utilized)
o Type 2a- med-high
o Type 2x- low (not dependent on oxygen)
• Capillary Density
o Type 1- high (angiogenesis- creating more capillary beds, remember why)
o Type 2a- medium
o Type 2x- low
• Recruitment order: low to high intensity
o Progressive and additive
o “Size principle” in recruitment order
o Based on size of motor neuron, smaller neurons depolarize faster

Question 10

Sport

Answer 10

• Endurance (Fiber types)
o % Type 1- 60-70%
o % Type 2- 30-40%
• Sprinters
o % Type 1- 20-25%
o % Type 2- 75-80%
• Wt/ Power lifters
o % Type 1- 40-45%
o % Type 2- 55-60%
• Non—athletes
o % Type 1- 47-53%
o % Type 2- 47-53%