Module 5 : Contraction at a Neuromuscular junction and Sliding Filament Theory

Question 1

How do you know , from a diagram , that a sarcomere is contracted?

3 points

Answer 1

-the I band becomes smaller
-the Z lines become closer together
-the H band becomes narrower

Question 2

What additional structure do myosin filaments have?
What does this allow?

Answer 2

they have hinged globular heads

this allows them to move backwards and forwards

Question 3

What is present on the globular heads of mysoin?

Answer 3

binding sites for both actin and ATP

Question 4

Describe the first 3 steps that lead to muscle contraction at a neuro-muscular junction …

Answer 4

1)Action potential travels along axon of motor neuron
2)This causes VG calcium ion channels to open and calcium ions diffuse into the axon membrane
3)vesicles are pushed forward by calcium ions and fuse with the membrane of the motor neuron

Question 5

Describe the next 3 steps that lead to muscle contraction at a neuro-muscular junction …

Answer 5

4)ACh is released from vesicles by exocytosis and diffuses across the neuro-muscular junction
5)ACh binds to receptor proteins on the sarcolemma
6)binding of ACh opens VG sodium ion channels so sodium diffuses into the sarcoplasm

Question 6

Describe the final 3 steps that lead to muscle contraction at a neuro-muscular junction …

Answer 6

7)The action potential travels along the sarcolemma and along T-tubules
8)The action potential triggers the release of calcium ions from the sarcoplasmic reticulum
9)calcium ions bind to troponin on actin , which starts muscle contraction

Question 7

Describe the first 3 steps of muscle contraction , regarding the SFT …

start with release of calcium ions

Answer 7

1)calcium ions are released into the sarcoplasm
2)calcium ions bind to troponin on tropomyosin that is wrapped around actin filaments
3)binding causes tropomyosin to change shape which exposes the myosin binding sites

Question 8

Describe the next 3 steps of muscle contraction , regarding the SFT …

Answer 8

4)myosin heads(with ADP attached)bind to binding sites on actin and form cross bridges with actin
5)the angle at which myosin heads bind causes tension so heads change shape to achieve a lower energy state , meaning actin filaments are pushed along and ADP is released
6)mysoin heads then detach from binding sites because ATP attaches to them

Question 9

Describe the final 3 steps of muscle contraction , regarding the SFT …

Answer 9

7)ATP is hydrolysed by ATPase to form ADP and Pi , providing energy for mysoin heads to return to their original position
8)calcium ions are now released from troponin molecules and are reabsorbed into the T-tubule system
9)Tropomyosin returns to its normal state and blocks myosin binding sites , so contraction has finished

Question 10

Explain the role of ATP in muscle contraction

2 points

Answer 10

-ATP binding allows myosin heads to detach from actin
-ATP hydrolysis allows myosin heads to return to their normal position