Muscle Structure and Function - I Flashcards
What % of total body mass is skeletal muscle?
40-50%
What are the 4 functions of skeletal muscle?
- Force production for locomotion and breathing.
- Force production for postural support.
- Heat production during cold stress.
- Acts as an endocrine organ
What are the 5 different types of CT surrounding skeletal muscle?
- Epimysium - surrounds entire muscle
- Perimysium - surrounds fascicles
- Endomysium - Surrounds muscle fibre
- Basement membrane - Just below endomysium.
- Sarcolemma - muscle cell membrane
What do satellite cells do?
Play a key role in muscle growth and repair.
- During muscle growth, satellite cells increase the number of nuclei in mature muscle fibers.
What is myonuclear domain?
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.
How is the NMJ trainable?
Increased size of NMJ, increased number of synaptic vesicles (ACh), increase number of ACh receptors on post-synaptic membrane.
Describe the ‘energy for muscle contraction’, with regards to actin and myosin.
- 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.
Describe (in detail) the steps in muscular contraction (E-C coupling).
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.
What is muscle contraction mediated by?
Calcium
Fatigue is defined as a decline in muscle power output. What does this occur due to?
- Decrease in muscle force production at cross-bridge level.
- Decrease in muscle shortening velocity
Mechanisms of fatigue during heavy, very heavy, and severe exercise (1-10 min). What are possible causes of fatigue?
- 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
How do the free radicals that accumulate during heavy exercise cause muscle fatigue?
- 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
Mechanisms of fatigue during moderate intensity exercise (>60 min). What are possible causes of fatigue?
- 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.
What is exercise-associated muscle cramps (EAMS) likely caused by?
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.
What are strategies to alleviate EAMS?
- 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.
