Muscle Structure and Adaptation

Question 1

Slow twitch fibres (type I)

Answer 1

Twitch contraction generates moderate peak force relatively slowly; resistant to fatigue with repeated stimulation.

• fatigue resistant
• moderate max force
• oxidative
• many mitochondria
• rich vascularisation
• small diameter
• high myoglobin (‘red muscle’)

Question 2

Fast twitch fibres (type II)

Answer 2

Twitch contraction generates high peak force relatively quickly; fatigue easily.

• fatigue rapidly
• high max force
• glycolytic or mixed
• fewer mitochondria
• sparser vascularisation
• larger diameter
• low myoglobin (‘white muscle’)

Question 3

Tetanus

Answer 3

When a motor unit fires action potentials at high frequency the individual twitches fuse and summate to generate a smooth contraction (temporal summation)

Question 4

Oxidative

Answer 4

Muscle fibres powered by ATP generated via oxidative phosphorylation of nutrients in the mitochondria (type I fibres have high oxidative capacity; some type II have moderate)

Question 5

Glycolytic

Answer 5

Muscle fibres powered by ATP generated via anaerobic breakdown of glucose to pyruvate through glycolysis (type II)

Question 6

Motor pool

Answer 6

All of the motor neurons (10s - 100s) innervating a single muscle. Cell bodies in ventral horn of spinal cord, axons exit via ventral root and travels to target muscle in peripheral nerve. Muscle force can be increased by increasing the fraction of active MNs within its motor pool

Question 7

Motor unit

Answer 7

A single motor neuron together with the subset of muscle fibres (10 - 100s) that it innervates within its target muscle. A single MN action potential will generate APs and twitch contractions in all the fibres it innervates. Increasing the number of active motor units will increase the force generated by the whole muscle (spatial summation)

Question 8

Size principle

Answer 8

Refers to the orderly recruitment of motor units from a motor pool (small to large; type I first, then more type I and II as force generation increases).

Question 9

Myoblasts

Answer 9

A cell commitment to a myogenic lineage, but not yet differentiated.

Question 10

Myogenic Regulatory Factors

Answer 10

Transcription factor required for commitment and terminal differentiation of muscle cells.

Question 11

Satellite cells

Answer 11

Divide and serve as a source of new myonuclei during postnatal growth. They contribute to the growth of the fibres and participate in the regeneration process.

Question 12

Muscle Hypertrophy

Answer 12

Increase in muscle mass due to increase in fibre size

Question 13

Muscle Hyperplasia

Answer 13

Increase in the number of muscle fibres.

Question 14

Sarcopenia

Answer 14

Age-related loss of muscle mass

Question 15

What’s the role of myoglobin?

Answer 15

In muscle cells, it facilitates the uptake of oxygen and makes oxygen more readily available.
= high oxidative metabolism capacity of slow twitch fibres is facilitated by high myoglobin, whereas fast twitch has less/low moderate myoglobin content

Question 16

What is red and white muscle?

Answer 16

This is due to the fibre composition of muscles.

High myoglobin levels, together with the rich capillary supply in slow twitch fibres gives a reddish appearance to muscle with many type 1 fibres.

Lower myoglobin content results in a paler and whiter appearance, which is seen by muscles with many type 2 fibres, with lower vascularisation supply.

Question 17

Myofibril proteins exist as multiple
isoforms with different functional
character

Answer 17

Vertebrate sarcomere structure essentially the same, but functional
tuning occurs via isoforms of muscle proteins
•Variable Ca sensitivity (troponin, tropomyosin)
•Rate of ATP hydrolysis (myosin isoforms)
• Slow twitch fibres (type I) express type I myosin heavy chain
• Fast twitch fibres (type II) express type II MHC

Question 18

Fibre type composition of muscles adapts

to function

Answer 18

Muscles vary in proportion of type II / type II fibres
•Lateral rectus (eye muscle) mainly type II fast twitch
•Gastrocnemius (calf muscle) mixed type I & II
•Soleus (calf) more type 1 slow twitch

Question 19

running fast or slow twitch?

Answer 19

• Long and middle distance runners: 60-70% slow

* Sprinters: 80% fast twitch

Question 20

Force generation in a
muscle is controlled at
the level of the motor
unit

Answer 20

• The motor unit consists of a motor neuron and the set of muscle fibres within a muscle that it innervates
• Motor unit size ranges from ~10 to many 100s of muscle fibres
• A muscle may be innervated by 10s to 100s of motor MNs
• Size of MN correlates with size of motor unit
• Muscle fibres of a motor unit are generally of the same type

Question 21

Temporal summation in muscle fibres

Answer 21

diagram

•Fusion of individual twitches generates tetanus

Question 22

Summation in fast and slow motor units

Answer 22

• Fast motor units(A) need higher firing rates to generate tetanic forces than slow motor units (B, C)
• Slow motor units are recruited first, followed by fast units for higher levels of force generation

Question 23

Skeletal muscle tone

Answer 23

•Most muscles at rest exhibit some 
low level of contractile activity
•Denervation leads to complete 
relaxation (flaccid)
•Driven by reflex arcs from muscle 
spindles (sectioning dorsal roots 
abolishes resting tone)

Question 24

Myogenesis

Answer 24

• Paracrine factors induce myogenic regulatory factors (MRFs) in mesodermal precursor cells – myogenic commitment (myoblasts)
• Myoblast proliferate under influence of growth factors
• Cell cycle exit, myogenin expression – terminal differentiation
• Structural proteins expressed and myotubes form from myoblasts
• Myotubes align and fuse, becoming multinucleated muscle fibres
• Satellite cells: regeneration and postnatal growth (muscle stem cells)

Question 25

Postnatal muscle growth:

hypertrophy

Answer 25

• After birth, increase in muscle mass due to increase in fibre size (hypertrophy)
• Muscle Stem Cells called satellite cells. Undifferentiated muscle precursors, self renewing
• Muscle fibre growth involves satellite cell proliferation and incorporation of nuclei into muscle fibres. Increased protein synthesis and muscle fibre size (hypertrophy)
• Muscle fibres are multinucleated. Maintain cytoplasm: nuclei ratio
• Satellite cells return to quiescence when not needed

Question 26

Postnatal muscle growth:

hyperplasia

Answer 26

•Increase in muscle mass due to formation of 
new muscle fibres
•Some evidence from animal models
•Uncertain whether this happens – main 
mechanism is probably hypertrophy

Question 27

Ageing muscle: sarcopenia

Answer 27

•Sarcopenia: reduction in muscle mass
•Part of ageing process
•Atrophy of muscle fibres
•May be due to disease or immobilization
•Associated with decreased satellite cell number and recruitment
•Anabolic resistance – reduced protein synthesis in response to
hormonal stimulation or resistance exercise
•Can be resisted – importance of resistance exercise