Lecture 30 - Muscle Structure, Function, Neuromuscular Transmission Flashcards
Difference between myopathies and dystrophies
Myopathies are disorders of muscle contractile apparatus. Are normally static (don’t progress)
Dystrophies are disorders of supporting structures. Often progressive.
What leads to variable muscle colour?
Myoglobin content (more myoglobin –> deeper red colour)
Feature of skeletal muscle cells
Multinucleated
Skeletal muscle structure 1) 2) 3) 4) 5) 6)
1) Muscle made up of bundles of muscle fibres called fasciculi.
2) Fasciculi are wrapped in perimysium
3) Fasciculi are made up of muscle fibers wrapped in endomysium
4) Each fibre made up of myofibrils wrapped in sarcolemma
5) Myofibrils and sarcolemma are surrounded by sarcoplasm
6) Each myofibril is made up of sarcomeres, which are the smallest contractile unit in muscle
What is wrapped around fasciculi?
Perimysium
What is wrapped around muscle fibers?
Endomysium
What makes up muscle fibres?
Myofibrils wrapped in sarcolemma
In striated muscle, which band is light?
I band (actin)
In striated muscle, which band is dark?
A band (actin and myosin)
Thin filament
Actin
Thick filament
Myosin
Sarcoplasm
1)
2)
1) Liquid surrounding myofibrils
2) Contains glycogen, fat particles, enzymes, mitochondria
Which filament has globular heads that crosslink?
Myosin
Sarcomere structure 1) 2) 3) 4) 5)
1) Surrounded by Z discs. Where actin attaches
2) I band - Aligned actin filaments
3) A band - Aligned actin and myosin filaments
4) H zone - Aligned myosin filaments
5) M line - Centre of sarcomere. Where myosin anchor
Fibre present in I band
Only actin
Fibre present in H zone
Only myosin
Part of sarcomere that only contains myosin
H zone
Part of sarcomere that only contains actin
I band
Sliding filament muscle contraction 1) 2) 3) 4)
1) At rest, tropomyosin covers myosin binding sites on actin
2) Action potential depolarises T-tubule membrane, causes Ca2+ release into sarcoplasm
3) Ca2+ binds tropomyosin, makes it unbind actin, revealing myosin binding site
4) Myosin head binds actin, begins crossbridging
Is crossbridge formation synchronous?
No.
If it were, all would detach at the same time, and the muscle would return to uncontracted state
Crossbridge formation 1) 2) 3) 4)
1) ATP binds myosin head, myosin detaches from actin
2) ATP hydrolysed to ADP + Pi, myosin head is cocked
3) Myosin head attaches to actin
4) ADP and Pi detach from myosin head. This leads to a power stroke (pulls Z discs together)
What causes muscle contraction?
1)
2)
3)
1) Motor neurons release ACh into neuromuscular junction.
2) ACh binds nicotinic ACh receptors, influx of Na+ into cell –> depolarisation of sarcolemma
3) Depolarisation of T-tubule membrane leads to Ca2+ influx into sarcoplasm
T tubules
Transverse tubules
Invaginations in sarcolemma, aligned with I and A bands
Supporting proteins of muscle fibers 1) 2) 3) 4)
1) Dystrophin-associated protein complex (in sarcolemma membrane)
2) Sarcoglycans (a complex) (in sarcolemms)
3) Dystrophin
4) Proteins in the nuclear envelope - emerin and lamin a
ATP sources for muscle 1) 2) 3) 4)
1) Within the muscle fibre
2) Creatine phosphate
3) Glucose stored in the cell as glycogen
4) Glucose and fatty acids obtained from the bloodstream
How long can ATP stored within a muscle maintain contraction?
A few seconds
Creatine phosphate
Molecule in muscles that transfers a high-energy Pi to ADP to form ATP
How long can creatine phosphate maintain muscle contraction?
About 15 seconds
Where do the fatty acids and glucose taken from the bloodstream by muscles cone from?
1)
2)
1) Glucose from glycogen broken down in the liver
2) Fatty acids from adipose cells and the liver
Advantages of anaerobic respiraiton
Fast
Doesn’t require oxygen
Disadvantages of anaerobic respiration
Only generates 2 ATP
Generates lactic acid
Number of ATP produced by aerobic respiration
36 (including 2 from glycolysis)
Advantages of aerobic respiration
Produces a lot of ATP (36)
Disadvantages of aerobic respiration
Relatively slow
Requires oxygen
Timeline of cellular respiration in muscle 1) 2) 3) 4)
1) 0-30 seconds - ATP/Creatine phosphate used
2) 20-120 seconds - Anaerobic respiration
3) 110 seconds - Aerobic respiration, if contraction continues
4) Depletion of resources (ATP, oxygen, glycogen), lactic acid buildup, lead to cessation of contraction
How long can anaerobic respiration sustain muscle contraction at maximum capacity?
Around 30 seconds
Type I fibres
1)
2)
3)
1) Slow-twitch
2) Slow oxidative
3) Fatigue-resistant
Type I fibre features 1) 2) 3) 4)
1) Myoglobin-, mitochondria-rich
2) Many capillaries
3) Generate ATP aerobically
4) Split ATP at a slow rate –> slow contraction velocity
Where are many type I fibres found?
In the postural muscles of the trunk
Used in long-distance running
Type IIA fibres 1) 2) 3) 4)
1) Fast-oxidative fibres
2) Myoglobin-, mitochondria-rich
3) Fatigue resistant, but not as much as type I
4) Split ATP at a rapid rate –> rapid contraction velocity
5) Rich in capillaries
Type IIB fibres 1) 2) 3) 4) 5) 6) 7)
1) White fibres
2) Fast glycolytic fibres
3) Low in myoglobin, mitochondria
4) Few capillaries
5) Large amount of glycogen
6) Split ATP very quickly
7) Fatigue easily
Another name for type IIB fibres
Type X fibres
Age and sex differences in muscle fibre type distribution
None
