Skeletal Muscle Fibre

Question 1

There are 3 main stages of myogenesis:

Answer 1

1. Myoblast determination
2. Myoblast proliferation/ migration
3. Differentiation into muscle

Question 2

Myoblast determination

Answer 2

Signals from surrounding tissues transform somites (collections of embryonic mesodermal cells) into myoblasts (precursor skeletal muscle cells)

Question 3

Myoblast proliferation/ migration

Answer 3

Myoblasts are committed but not yet differentiated
Signals from surrounding tissues transform them into premuscle masses. 1st signs of commitment: appearance of Ach receptors spread over the sarcolemma and formation of actin + myosin filaments but no sarcomeres

Question 4

Differentiation into muscle overview

Answer 4

Fusion of myoblasts and activation of myogenes causes differentiation of pre muscle masses into myotube -> multinucleate skeletal muscle fibre

Question 5

Differentiation into muscle mechanism

Answer 5

1. Transient extracellular signals
2. Expression of intracellular factors
3. Change in the pattern of gene expression
   - activation of four genes = muscle-regulatory factors = transcription factors
4. Maintenance of the myogenic program

Question 6

What happens to ACh receptors after innvervation?

Answer 6

after innervation extra-junctional ACh receptors disappear and ACh receptors restricted to neuromuscular junctions

Question 7

What are the two types of muscle?

Answer 7

Slow muscle - long duration contraction - great endurance

Fast muscle - high contraction velocities - less endurance

Question 8

The two main criteria for muscle fibre characteristics are

Answer 8

myosin ATPase activity and oxidative capacity

Question 9

Myosin characteristics

Answer 9

Myosin is an actin dependent ATPase

• myosin isoforms: different rates of splitting ATP
• different rates of muscle shortening
• different optimal conditions

Question 10

Oxidative capacity =

Answer 10

microlitres of oxygen consumed per gram of muscle per hour

Question 11

What does oxidative capacity depend on?

Answer 11

1. the rate of oxygen delivery to working muscles (capillary density)
2. the oxygen-carrying capacity of the blood
3. the amount of oxygen extracted from the blood into muscle (enhanced by the presence of myoglobin = oxygen-binding protein)
4. the amount of oxygen utilised by muscles (depends on the number of mitochondria)

Question 12

Types of skeletal muscle fibres:

Answer 12

Slow oxidative (red), Intermediate Fast Oxidative-Glycolytic/Red (FOG) and Fast Glycolytic/White

Question 13

Slow Oxidative/Red is a

Answer 13

Type 1

low myosin ATPase activity + high oxidative capacity

Question 14

Intermediate Fast Oxidative-Glycolytic/Red

Answer 14

Type II A and Type II D

high myosin ATPase activity + high oxidative capacity

Question 15

Fast Glycolytic/White

Answer 15

Type II B

high myosin ATPase activity + low oxidative capacity

Question 16

There are 4 myosin heavy chain isoforms:

Answer 16

1. I = MHC I = Slow
2. IIA = MHCIIA =Intermediate
3. IID(X) = MHCIID(X) = Intermediate
4. IIB = MHCIIB = fast

Question 17

Pure fibre types express

Answer 17

only one MHC isoform

Question 18

Which type of myofibril stains darker with NADH reductase and why?

Answer 18

Type 1 due to greater use of aerobic metabolism

Question 19

What is the time course of muscle twitch defined by?

Answer 19

The different isoforms of myosin present

Question 20

Why does a slow myosin fibre have a slow speed of interaction?

Answer 20

slow rate of ATP hydrolysis -> slow rate of cross-bridge cycling -> slow softening velocity -> slow speed of interaction