Lecture 8: Skeletal Muscle - Force, Work, and Energetics

Question 1

How does muscles obtain ATP?

Answer 1

From catabolism of glucose and other nutrients, using either anaerobic or aerobic metabolism

Question 2

Trade-Off

Answer 2

A trade-off exists between the total amount of ATP generated and the rate at which it can be obtained

Question 3

Aerobic Metabolism

Answer 3

• Needs steady oxygen supply
• Is slow
• > 30 ATP molecules from a single glucose molecule
• Can also use fatty acids

Question 4

Anaerobic Metabolism

Answer 4

• Doesn’t need oxygen
• fast
• only 2 ATP molecules from a glucose molecule
• cannot use fatty acids

Question 5

Phosphocreatine

Answer 5

• doesn’t need oxygen
• is VERY fast
• limited stores in muscle (rapidly depleted)

Question 6

How do muscles use these mechanisms to obtain ATP?

Answer 6

Muscles switch how they obtain ATP depending on their activity levels

Question 7

Resting muscles

Answer 7

• Aerobic respiration of FFAs (slowly) produces an ATP surplus build up stores of glycogen and creatine phosphate (CP) levels

Question 8

Moderately active muscles

Answer 8

• Aerobic respiration of FFAs and glucose
• Meet current ATP requirements
• Use glycogen stores

Question 9

Peak Activity

Answer 9

• Most myofibrils active
• Aerobic and Anaerobic respiration of glucose and CP conversion
• Use glycogen and CP stores
• Waste products (lactate + creatine) build up

Question 10

Fatigue

Answer 10

• Occurs when rapid ATP production affects the ability of muscles to initiates or maintain the contraction cycle

Question 11

Muscle Fatigue

Answer 11

Reduce contractile tension for the same (excitation) stimulus
- Depletion of ACh vesicles in MN axon terminal
- Accumulation of K+ in the T-tubules (ECF) due to repeated action potentials

Question 12

Recovery from fatigue

Answer 12

• Takes time and requires coordination with other organ systems, especially liver
• In rest, lactate can be used in aerobic metabolism or shipped to the liver for gluconeogenesis

Question 13

Muscle fibre properties

Answer 13

• Timing (fast and shirt twitches)
• Protein composition (different subtypes of myosin, calcium pumps)
• Organelle/tissue composition (amount of mitochondria and capillaries)

Question 14

Type I (slow-oxidative)

Answer 14

Slow twitch or slow-oxidative
- Slow myosins and slow pumps
- Fewer myofibrils but more mitochondria per fibre
- not powerful but fatigue resistant

Question 15

Type II

Answer 15

• Fast twitch
• Fast myosins and fast pumps
• More myofibrils per fibre but fewer mitochondria
• produce high force and rapid contractions, but fatigue easily

Question 16

Type IIB (Fast-Glycolytic)

Answer 16

• Fastest

Question 17

Type IIA (Fast- oxidative)

Answer 17

• Fast contraction speeds
• intermediate twitch durations
• intermediate size and power
• intermediate resistance and recovery from fatigue

Question 18

How do motor units interact with different fibres?

Answer 18

All the fibres in a motor unit are of the dame fibre type and fibre type correlates with number of fibres per unit
- Largest motor unit recruited by the strongest stimuli

Question 19

Sustained contraction

Answer 19

motor unit recruitment is asynchronous so no unit is constantly activated

Question 20

Hyperplasia

Answer 20

• More myofibres per fascicle (stops after embryonic development)

Question 21

Hypertrophy

Answer 21

More myofibrils per myofibre (result of exercise)