Muscle contraction (accidentally made two of the same decks ;(

Question 1

Define sarcomere

Answer 1

The basic contractile unit of muscle fibre- each sarcomere is composed of actin and myosin and lots of sarcomeres grouped together would be myofibrils

Question 2

What is the function of muscle?

Answer 2

Provides movement to skeleton and hollow organs

Provides structure to skeleton and hollow organs when under pressure

Question 3

What are the 2 types of muscle classes based on appearance?

Answer 3

• Striated
• Smooth

Question 4

What are the proteins present in both types of muscle that produce contractions?

Answer 4

• Actin and myosin

Question 5

What are myofibrils?

Answer 5

Bundles of protein filaments, made up of actin and myosin

Question 6

What are myofibrils surrounded by?

Answer 6

Sacroplasmic reticulum- which stores calcium

Question 7

What muscle types are striated?

Answer 7

Skeletal and cardiac

Question 8

Describe striated skeletal muscle

Answer 8

• Voluntary (motor nerves controlled)
• Single, very long cylindrical, multinucleate cells with obvious striations

Question 9

Describe Striated cardiac muscle

Answer 9

• Involuntary (ANS controlled)
• Branching chains of cells
• Uni or binucleate
• Striations

Question 10

Describe smooth muscle

Answer 10

• Involuntary (ANS controlled)
• Single
• Fusiform (spindle-shaped, wide in middle, tapers at ends)
• Uninucleate
• No striations

Question 11

How is striated muscle organised?

Answer 11

• Sarcomere contains Z band, which is where actin attaches to. The Z bands are like a zig-zag line separating the sarcomeres in skeletal and cardiac muscles
• Light or I band- non-superimposed length of actin (this is when actin only slightly overlaps the myosin
• Dark or A band - Entire length of myosin
• H band- shows myosin but not actin

During contraction, myosin length stays same, actin moves across myosin

Question 12

How is smooth muscle organised?

Answer 12

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

Question 13

Describe the 2 types of actin

Answer 13

• G actin- globular protein, binds to ATP, contains ATPase activity
• F actin- Helical protein, uses ATP to make filaments

Question 14

What do the filaments produced by F actin contain?

Answer 14

• Active actin binding site allowing interactions with myosin

Question 15

What muscle class contain the tropomyosin-troponin system?

Answer 15

Striated

Question 16

What does tropomyosin do?

Answer 16

-Covers active actin binding sites at rest, preventing myosin interactions

Question 17

In striated muscle, what contains the troponin system?

Answer 17

Actin

Question 18

What does myosin II contain?

Answer 18

• 2 heavy chains
• 4 light chains

Question 19

Describe the 2 myosin II heavy chains

Answer 19

• Forms head domain at N-terminus
• Binds ATP and ADP via ATPase activity
• Binds to active actin binding sites

Question 20

Describe the 4 myosin II light chains?

Answer 20

• 2 per head
• Modulates myosin-actin interactions
• Especially in smooth muscle

Question 21

How is muscle contraction initiated?

Answer 21

• Rise of Ca2+ central to initiating muscle contraction
• Rise of Ca2+ leads to removal of tropomyosin from active actin binding sites,
• It does this because the Ca2+ binds to troponin, leading to a change in troponin conformation which allows it to remove tropomyosin from the actin binding site
• Allows myosin heads to interact with actin (in the case of striated muscle, as smooth muscle doesn’t have tropomyosin-troponin system)

Question 22

Describe the sliding filament hypothesis

Answer 22

• At rest, ATP combined to myosin heavy heads, so myosin currently has low affinity for actin, so they don’t interact
• Myosin heads have ATPase activity, break down ATP into ADP + pi via hydrolysis, causes an energised state of myosin heavy head chains, they become reorientated and energised
• ADP/Pi bound to myosin head has high affinity for actin, so myosin heads easily able to bind to actin
• Rise in calcium means actin active binding sites exposed, so they can come together
  Now, the heads of myosin and active actin binding sites are interacting
• There’s a 90 degree cocking motion, actin and myosin are bound together, they are perpendicular to each other, forming cross bridges
• ADP is now released, which releases further motion, causing a 45 degree cocking of the myosin heads, a rowing motion.
• This causes the shortening of the sarcomeres
• The myosin cross-bridges rotate towards centre of the sarcomere
  -> this is the power stroke
• As ADP has detached, myosin head binds to ATP, cross bridges detach from actin, process begins again