Muscle Contractile Mechanisms

Question 1

What is the function of muscle?

Answer 1

-Provides movement to skeleton and hollow organs
-Provides structure to skeleton and hollow organs when under pressure
-Allows for voluntary and involuntary control of muscle and associated functions over different time scales

Question 2

What is included in the organised apparatus in striated muscle?

Answer 2

-z band
-Light or I band
-Dark or A band

Question 3

What is a z band?

Answer 3

Attachment site for actin

Question 4

What is a light or I band?

Answer 4

-Non-superimposed length of actin

Question 5

What is a dark or A band?

Answer 5

Entire length of myosin

Question 6

What is the organisation level of myosin and actin in striated muscle?

Answer 6

-Myosin and actin filaments are highly organised

Question 7

What is a unit of striation called? What line to what line is it?

Answer 7

-unit of striation is called the sarcomere
-Z line to Z line

Question 8

What happens to striated sarcomere during contraction?

Answer 8

-When relaxed, little overlap
-When contracted, increased overlapped
-During contraction:
-Myosin length stays the same
-Actin moves across myosin

Question 9

What is the organisation like in smooth muscle and what does this organisation allow?

Answer 9

-myosin and actin filaments are disorganised:
-interact at dense bodies
-Disorganisation allows for more 3D contraction like in hollow organs

Question 10

What is G-actin?

Answer 10

A globular protein

Question 11

What does actin bind and contain?

Answer 11

-Binds ATP
-Contains ATPase activity

Question 12

What is F-actin?

Answer 12

A helical protein

Question 13

What does F-actin use to make filaments?

Answer 13

-Uses ATP to make filaments

Question 14

What sites do actin filaments contain that allow interactions with myosin?

Answer 14

-Filaments contain active actin binding sites which allow interactions with myosin

Question 15

what type of muscle contain tropomyosin?

Answer 15

-Striated muscle not smooth muscle contains tropomyosin

Question 16

What does tropomyosin do and what does this prevent?

Answer 16

-Tropomyosin covers active actin binding sites at rest
-Prevents myosin interactions

Question 17

What makes up the troponin system in striated muscle and what is this system essential for?

Answer 17

-In striated muscle, actin also contains the troponin system
-Tnl, TnC, TnT
-Essential for modulating actin-myosin interactions

Question 18

Where is myosin II found?

Answer 18

-Myosin II found in muscle

Question 19

How many heavy chains does myosin have and what do they form?

Answer 19

-2 heavy chains
-Both intertwined
-Forms head domain

Question 20

What do the heavy chains of myosin bind?

Answer 20

-Binds ATP and ADP, ATPase activity

Question 21

What do the heavy chains of myosin bind to?

Answer 21

-Binds to active actin binding sites

Question 22

How many light chains does myosin have?

Answer 22

-4 light chains-2 per head

Question 23

What do light chains in myosin modulate?

Answer 23

Modulates myosin-actin interactions
-Especially in smooth muscle(smooth muscle contraction

Question 24

What initiates muscle contraction? And what does this lead to?

Answer 24

-Rise in concentration of Ca2+ in cytosol of muscle cells is central of initiating muscle contraction
-Rise in Ca2+ concentration leads to removal of tropomyosin from active actin binding sites allowing myosin heads to interact with actin

Question 25

What is the extracellular concentration of Ca2+?

Answer 25

-Extracellular concentration in Ca2+ is high(1-2mM)

Question 26

What is the resting intracellular cytosolic concentration of Ca2+?

Answer 26

-Resting intracellular cytosolic concentration of Ca2+ is kept very low(100nM)

Question 27

What is the concentration of Ca2+ in intracellular SR?

Answer 27

-Concentration of Ca2+ in intracellular SR Ca2+ stores is high (0.5-1mM)

Question 28

What is a rise in Ca2+ concentration produced by?

Answer 28

-Rise in Ca2+ concentration are produced by Ca2+ influx + Ca2+ release from SR I

Question 29

How do actin and myosin interact to produce muscle contraction following rise in cytosolic Ca2+ concentration?

Answer 29

1. Myosin heads hydrolyse ATP and become reoriented and energised
   a. ADP/Pi alongside the myosin head have a high affinity for actin
2. Myosin heads bind to actin, forming crossbridges
   a. Myosin head-actin is 90 degrees cocked
3. Myosin crossbridge rotate toward centre of the sarcomere(power stroke)
   a. Myosin head-acting is 45 degrees cocked
4. As myosin heads bind ATP, the crossbridges detach from actin
   a. ATP and myosin head have a low affinity for actin

-All is like a rowing action

Question 30

What are the 3 sources of ATP for the sliding filament hypothesis?

Answer 30

1. Aerobic
2. Anaerobic
3. Phosphocreatine

Question 31

What happens in rigor mortis?

Answer 31

1. Rise in Ca2+ causes removal of tropomyosin from actin-myosin binding site
2. Loss of ATP production prevents detachment of actin-myosin filaments which causes stiffness of muscles (Rigor)