Muscle contractile mechanisms

Question 1

What is each sarcomere composed of?

Answer 1

• Actin AND myosin, the proteins responsible for muscular contraction

Question 1

What is a sarcomere?

Answer 1

The basic contractile unit of muscle fibre

Question 2

What is the function of muscle?

Answer 2

• 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

Question 3

What are the 2 types of muscle?

Answer 3

• Striated- skeletal and cardiac muscle
• Smooth - blood vessels, GI tract, internal organs bar heart

Question 4

What 2 proteins are essential for contraction of both types of muscle?

Answer 4

• Actin
• Myosin

Question 5

Describe skeletal muscle

Answer 5

• Voluntary
• Single, very long cylindrical, multinucleate cells with obvious striations (stripes)

Question 6

Describe cardiac muscle

Answer 6

• Involuntary
• Striated
• Branching chains of cells, uni or binucleate; less organised striations than skeletal

Question 7

Describe smooth muscle

Answer 7

• Involuntary
• Single, fusiform, uninucleate, NO striations

Question 8

Why are muscles termed striated or smooth?

Answer 8

• Due to the structure of the myosin or actin filaments
• Striated muscles are striated as the A and I bands are aligned

Question 9

What are voluntary contractions controlled by?

Answer 9

• Motor nerves

Question 10

What are involuntary contractions controlled by?

Answer 10

• ANS (autonomic nervous system)

Question 11

How is striated muscle organised?

Describe it in terms of the Z, A, I and H bands

Answer 11

• 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

Question 12

What happens to the striated sarcomere during contraction?

Answer 12

• Myosin length stays the same, but actin moves across myosin, so the contraction occurs due to the increased overlap, whereas when the muscle is released, there’s only a very small overlap

Question 13

How is smooth muscle organised?

Answer 13

• Myosin and actin filaments are disorganised interaction at dense bodies
• This disorganisation allows for a more 3D contraction e.g. in hollow organs

Question 14

What are the 2 types of actin?

Answer 14

• G actin
• F actin

Question 15

Describe G actin

Answer 15

• Globular protein, binds to ATP, contains ATPase activity

Question 16

Describe F actin

Answer 16

• Helical protein, uses ATP to make filaments

Question 17

What do the actin filaments contain?

Answer 17

• Active actin binding sites- allow interactions with myosin

Question 18

What do striated muscles contain that smooth muscle does not?

Answer 18

• Tropomyosin

Question 19

What does tropomyosin do?

Answer 19

• Covers the actin binding sites at rest, preventing actin-myosin interactions

Question 20

What does myosin II contain?

Answer 20

• 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

Question 21

What is the sliding filament hypothesis?

Answer 21

Cycling of 90 degrees to 45 degrees motion of myosin head-actin interactions, rowing action

Question 22

Describe the different phases of the sliding filament hypothesis

Answer 22

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
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

Question 23

What are the consequences of the sliding filament hypothesis?

Answer 23

• Myosin heads must be ‘primed’ with ATP for muscular contraction + movement, hence need plentiful sources of ATP

Question 24

What are the main sources of ATP for muscle contraction?

Answer 24

• Aerobic respiration - glycolysis and oxidative phosphorylation
• Anaerobic respiration - production of lactate into ATP in absence of O2
• Phosphocreatine- source of ATP

Question 25

What initiates muscle contraction?

Answer 25

• 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)

Question 26

What happens to the muscle if there’s a decrease in Ca2+?

Answer 26

• Muscle relaxation

Question 27

What is needed for the detachment of myosin-actin?

Answer 27

• Need ATP-myosin head binding

Question 28

Outline the process of rigor mortis following death

Answer 28

• 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