Muscle contractile mechanisms Flashcards
- Outline anatomical differences between striated and smooth muscle, including organisation of actin, myosin, tropomyosin - Describe the different phases of the sliding filament hypothesis - Outline the process of rigor mortis, and how it relates to the sliding filament hypothesis
What is each sarcomere composed of?
- Actin AND myosin, the proteins responsible for muscular contraction
What is a sarcomere?
The basic contractile unit of muscle fibre
What is the function of muscle?
- Provides movement to skeleton + hollow organs e.g. heart, blood vessels, GI tract
- Provides structure to skeleton and hollow organs when under pressure e.g. chambers of heart, circulation through vessels, bladder
What are the 2 types of muscle?
- Striated- skeletal and cardiac muscle
- Smooth - blood vessels, GI tract, internal organs bar heart
What 2 proteins are essential for contraction of both types of muscle?
- Actin
- Myosin
Describe skeletal muscle
- Voluntary
- Single, very long cylindrical, multinucleate cells with obvious striations (stripes)
Describe cardiac muscle
- Involuntary
- Striated
- Branching chains of cells, uni or binucleate; less organised striations than skeletal
Describe smooth muscle
- Involuntary
- Single, fusiform, uninucleate, NO striations
Why are muscles termed striated or smooth?
- Due to the structure of the myosin or actin filaments
- Striated muscles are striated as the A and I bands are aligned
What are voluntary contractions controlled by?
- Motor nerves
What are involuntary contractions controlled by?
- ANS (autonomic nervous system)
How is striated muscle organised?
Describe it in terms of the Z, A, I and H bands
- Unit of striation- sarcomere
- Z band- attachment sites for actin, separate each sarcomere
- Light or I band- non- superimposed length of actin,
- Dark or A band- entire length of myosin, and shows slight overlap of actin with myosin
- H band - only myosin component of sarcomere, middle of A band
What happens to the striated sarcomere during contraction?
- Myosin length stays the same, but actin moves across myosin, so the contraction occurs due to the increased overlap, whereas when the muscle is relaxed, there’s only a very small overlap
How is smooth muscle organised?
- Myosin and actin filaments are disorganised interaction at dense bodies
- This disorganisation allows for a more 3D contraction e.g. in hollow organs
What are the 2 types of actin?
- G actin
- F actin
Describe G actin
- Globular protein, binds to ATP, contains ATPase activity
Describe F actin
- Helical protein, uses ATP to make filaments
What do the actin filaments contain?
- Active actin binding sites- allow interactions with myosin
What do striated muscles contain that smooth muscle does not?
- Tropomyosin
What does tropomyosin do?
- Covers the actin binding sites at rest, preventing actin-myosin interactions
What does myosin II contain?
- 2 myosin HEAVY chains- intertwined, forms head domain, binds ATP and ADP, ATPase activity, binds to active actin binding site
- 4 myosin LIGHT chains- 2 per head- especially in smooth muscle, modulates myosin-actin interactions
What is the sliding filament hypothesis?
Cycling of 90 degrees to 45 degrees motion of myosin head-actin interactions, rowing action
Describe the different phases of the sliding filament hypothesis
1 - Myosin heads hydrolyse ATP and become reoriented and energised
2 - ADP/Pi + myosin head have a high affinity for actin, the myosin head binds to actin, forming crossbridges at 90 degrees, releasing the ADP, which ultimately causes the contraction
3 - Myosin crossbridges rotate toward centre of the sarcomere (power stroke) at 45 degrees
4 - ATP + myosin head have low affinity for actin- as myosin heads bind to ATP, the crossbridges detach from actin
What are the consequences of the sliding filament hypothesis?
- Myosin heads must be ‘primed’ with ATP for muscular contraction + movement, hence need plentiful sources of ATP
What are the main sources of ATP for muscle contraction?
- Aerobic respiration - glycolysis and oxidative phosphorylation
- Anaerobic respiration - production of lactate into ATP in absence of O2
- Phosphocreatine- source of ATP
What initiates striated muscle contraction?
- Rise in Ca2+ leads to removal of tropomyosin from actin actin binding sites, allowing myosin heads to interact with actin
- Rise is produced by influx of Ca2+ and Ca2+ release from sarcoplasmic reticulum (SR)
What happens to the muscle if there’s a decrease in Ca2+?
- Muscle relaxation
What is needed for the detachment of myosin-actin?
- Need ATP-myosin head binding
Outline the process of rigor mortis following death
- Rise in ca2+, removal of tropomyosin from actin-myosin binding sites
- Loss of ATP production (due to no longer respiring aerobically), preventing detachment of actin-myosin filaments, this is what causes the stiffness of muscles known as ‘rigor’
- This starts 2-6 hours after death (anaerobic respiration still produces some ATP, so stops the process from beginning immediately), peaks after approx 12 hours
- Rigor stops after 24-48 hours due to decomposition of myosin/actin
