Lecture 8 - Skeletal Muscle: Force, Work and Energetics

Question 1

What is the Basal metabolic rate?

Answer 1

The number of calories the entire body requires to produce enough ATP to maintain all basic functions at rest.

Question 2

3 features of resting muscles

Answer 2

1. Aerobic respiration of free fatty acids (FFAs)
2. Slowly produce an ATP surplus
3. Build up stores of glycogen and creatine phosphate (CP) levels

Question 3

3 features of moderately active muscles

Answer 3

1. Aerobic respiration of FFAs AND glucose
2. Meet current ATP requirements
3. Use glycogen stores

Question 4

3 features of Peak activity

Answer 4

1. (Aerobic) + anaerobic respiration of glucose + CP conversion
2. Use glucose and CP stores
3. Waste products (lactate + creatine) build up

Question 5

True or False. ATP is only required for the contraction cycle.

Answer 5

False. It is also required to maintain the processes that support myofibre excitation.

Question 6

What is fatigue and what does it result in?

Answer 6

Fatigue is when there is rapid ATP production affects the ability of muscles to initiate or maintain the contraction cycle

(for more info) More ATP = more myosin heads not attached to actin –> cannot contract

Muscle fatigue occurs and it results in reduced contractile tension for the same excitation stimulus

Question 7

2 fatigue factors that affect excitation processes in skeletal muscles

Answer 7

1. Depletion of ACh vesicles in MN axon terminal (Na+ channels don’t open, so no AP)
2. Accumulation of K+ in the T-tubules (ECF) due to repeated APs

Question 8

3 fatigue factors that affect Contraction Processes in skeletal muscles

Answer 8

1. Leakage of Ca2+ back into the sarcoplasm (mysoin heads can’t bind to actin)
2. Micro tears in myofibrils
3. Build-up of lactate and H+

Question 9

Which organ is required to coordinate for the muscle to recover from fatigue. Describe how the processes differ from short and medium/long term

Answer 9

Liver

Short term: lactate used in aerobic metabolism or shipped to liver for gluconeogenesis

Medium/long term: myosatellites cells are activated and proliferate, utilizing supplies of free amino acids (in blood) to rebuild torn myofibrils

Question 10

True or False. Muscle fibres have the same anatomical and physiological properties.

Answer 10

False! They can come in distinct types that differ across many properties

Question 11

3 ways to differentiate different types muscle fibres

Answer 11

1. Timing - fast, short, or slow, prolonged twitches
2. Protein composition - expressing of different subtypes of myosin, pumps, etc.
3. Organelle/tissue composition - lots/few mito, capillaries, etc.

Question 12

Type I fibres + features

Answer 12

slow twitch/slow oxidative

Feature: fewer myofibrils but more mitochondria per fibre - low maximal tension, more aerobic capacity, higher oxygen supply

Fatigue resistant!!

Question 13

Type II fibres + features

Answer 13

fast twitch fibres

Feature: more myofibrils per fibre, but fewer mitochondria - higher maximum tension (develops quickly, but doesn’t last long), lower aerobic capacity, lower oxygen supply but high CP

high forces and rapid contractions!! Fatigue easily :(

Question 14

2 types of Type II fibres

Answer 14

Type IIA - fast-oxidative fibres
Type IIB - fast-glycolytic fibres

Question 15

Properties of Type IIA fibres

Answer 15

Intermediate properties between Type IIB and Type I
1. Fast contraction speeds
2. Intermediate twitch durations
3. Intermediate size (and power)
4. Intermediate mitochondria and capillary density
5. Intermediate resistance/recovery from fatigue

Question 16

Motor unit

Answer 16

A single motor neuron can make NMJs between 1 and 1000 muscle fibres

Question 17

True or False. All fibres in a motor unit are of the same fibre type.

Answer 17

True.

Question 18

Fibre type associated with small motor units

Answer 18

Type I, few fibres

Question 19

Fibre type associated with large motor units

Answer 19

All Type IIB, many fibres

Question 20

True or False. # of myfibrils per fibre and the # of myofibres per motor unit are correlated

Answer 20

True. They both contribute to the tension generated when that unit is recruited

Question 21

Synchronous recruitment

Answer 21

Motor neuron activation (in spinal cord) is organized so largest motor units are recruited by the strongest stimuli.
ie. The Size Principle

Question 22

Asynchronous Recruitment and when does it occur?

Answer 22

Similarly sized motor units turn on and off so that each is active part of the time, and rests the other part - no unit is constantly active

Occurs when there is sustained contraction

Question 23

Hyperplasia

Answer 23

More myofibres per fascicles

Question 24

Hypertrophy

Answer 24

More myofibrils per myofibre

Question 25

Why can’t muscle increase in size through hyperplasia in adults?

Answer 25

After embryonic development, skeletal muscles can repair muscle fibres but cannot add new ones. Therefore, physical fitness increases fibre diameter and NOT fibre number

Question 26

What does aerobic training lead to?

Answer 26

Hypertrophy of Type I fibres

Question 27

What does anaerobic (high intensity) training lead to?

Answer 27

Hypertrophy of Type IIB fibres