Muscle Structure and Function - I

Question 1

What % of total body mass is skeletal muscle?

Answer 1

40-50%

Question 2

What are the 4 functions of skeletal muscle?

Answer 2

• Force production for locomotion and breathing.
• Force production for postural support.
• Heat production during cold stress.
• Acts as an endocrine organ

Question 3

What are the 5 different types of CT surrounding skeletal muscle?

Answer 3

• Epimysium - surrounds entire muscle
• Perimysium - surrounds fascicles
• Endomysium - Surrounds muscle fibre
• Basement membrane - Just below endomysium.
• Sarcolemma - muscle cell membrane

Question 4

What do satellite cells do?

Answer 4

Play a key role in muscle growth and repair.
- During muscle growth, satellite cells increase the number of nuclei in mature muscle fibers.

Question 5

What is myonuclear domain?

Answer 5

Volume of sarcoplasm surrounding each nucleus. Each nucleus can support a limited myonuclear domain.

More myonuclei allow for greater protein synthesis.
*Muscle hypertrophy = increased myonuclei.
*Muscle atrophy = decreased myonuclei.

Question 6

How is the NMJ trainable?

Answer 6

Increased size of NMJ, increased number of synaptic vesicles (ACh), increase number of ACh receptors on post-synaptic membrane.

Question 7

Describe the ‘energy for muscle contraction’, with regards to actin and myosin.

Answer 7

• ATP molecules binds to myosin head
• ATP hydrolysed, myosin allowed to bind to actin
• Release of Pi + ADP causes power stroke where actin slides over myosin filament
• In order for myosin to release from actin, we need ATP to bind to myosin head.

Question 8

Describe (in detail) the steps in muscular contraction (E-C coupling).

Answer 8

1) the nerve impulse travels down the TT and reaches the SR, where Ca2+ is released.
2) Ca2+ binds to troponin causing tropomyosin to move away from the ‘active sites’ on actin, permitting a strong binding state between actin and myosin.
3) Muscular contraction occurs by multiple cycles of cross-bridge activity.
- Shortening continues as long as energy is available and Ca2+ is free to bind to troponin.
4) When neural activity ceases at NMJ, Ca2+ is removed from SR by the Ca2+ pump.
- Tropomyosin moves to cover active site on actin and muscle relaxes.

Question 9

What is muscle contraction mediated by?

Answer 9

Calcium

Question 10

Fatigue is defined as a decline in muscle power output. What does this occur due to?

Answer 10

• Decrease in muscle force production at cross-bridge level.
• Decrease in muscle shortening velocity

Question 11

Mechanisms of fatigue during heavy, very heavy, and severe exercise (1-10 min). What are possible causes of fatigue?

Answer 11

• Decreased Ca2+release from the sarcoplasmic reticulum
• Accumulation of metabolites that inhibit
  myofilament sensitivity to Ca2+.

Free metabolites contributing to fatigue = Pi, H+, free radicals

Question 12

How do the free radicals that accumulate during heavy exercise cause muscle fatigue?

Answer 12

• Pi + free radicals modify cross-bridge head and reduce number of cross-bridges bound to actin.
• H+ binds to Ca2+ binding sites on troponin, preventing Ca2+ binding

Question 13

Mechanisms of fatigue during moderate intensity exercise (>60 min). What are possible causes of fatigue?

Answer 13

• Increased radical production
• Glycogen depletion

Free metabolites contributing to fatigue =
- NOT Pi and H+ accumulation.
- Radical accumulation modifies cross-bridge head and reduces number of cross-bridges bound to actin.
- Depletion of muscle glyocgen reduces TCA cycle intermediates and decreases ATP production via ox. phos.

Question 14

What is exercise-associated muscle cramps (EAMS) likely caused by?

Answer 14

Hyperactive motor neurons in the spinal cord.
- HIE can alter muscle spindle and GTO function.
- Increased excitatory activity of muscle spindles and reduced inhibitory effect of GTO.

Question 15

What are strategies to alleviate EAMS?

Answer 15

• Passive stretching
• Possible that activating ion channels (transient receptor potential channels) in the mouth/throat could send inhibitory signals to the spinal cord, and thus inhibit overactive motor neurons.

Question 16

Why is EAMC not necessarily caused by electrolyte or hydration imbalance?

Answer 16

• Electrolyte or hydration imbalance is whole-body
• Similar blood conc. of electrolyte in those who cramps vs. those who don’t.