lecture 9 - muscle tension/contraction

Question 1

What are the key components of the myofilament?

Answer 1

Actin and myosin

Question 2

What is actin in a myofibril?

Answer 2

A thin filament that runs along the myofilament and forms a structural scaffold.

Question 3

What is myosin in the myofibril?

Answer 3

Myosin is a thick filament that acts as a motor molecule, when attaching to actin, generating a force.

Question 4

What are the relative positions of Actin and Myosin at rest?

Answer 4

They are not in contact.

Question 5

What must be supplied to the myofilament for contraction to occur?

Answer 5

Calcium ions (Ca2+)

Question 6

What is a cross-bridge?

Answer 6

A bond between actin and myosin that allows the sarcomere to contract and shorten towards its centre.

Question 7

What are the 5 states of the cross bridge cycle?

Answer 7

Attached, Released, Cocked, Cross-bridge, Power-stroke.

Question 8

What is the attached state of the cross-bridge cycle?

Answer 8

Myofilament has just finished a power stroke (ADP and P has been released from myosin head), and the actin-myosin cross bridges are still present.

Question 9

What is the released state of the cross-bridge cycle?

Answer 9

A molecule of ATP binds to myosin, causing its head to detach from actin, removing the cross-bridge.

Question 10

What is the cocked state of the cross-bridge cycle?

Answer 10

The Myosin head burns ATP to make energy and a phosphate group binds to it. Energy is stored by changing the shape of Myosin, preparing it to attach to actin again.

Question 11

What is the cross-bridge state of the cross-bridge cycle?

Answer 11

If calcium is present, the myosin head binds to the actin myofilament forming a cross bridge.

Question 12

What is the power-stroke state?

Answer 12

The energised myosin head uses its stored energy to change conformation, releasing P and creating a power stroke that causes filaments to slide past each other to contract the sarcomere.

Question 13

What are the two factors that muscle tension is dependent on?

Answer 13

The number of muscle fibres recruited and the rate at which the muscle is stimulated.

Question 14

If a small number of neutrons are active, muscle force will be…?

Answer 14

Low

Question 15

What is the name of the process of activating more muscle fibres to generate more force?

Answer 15

Recruitment

Question 16

What will a single action potential generate in muscle fibres?

Answer 16

A pulse of Ca2+ will be released into the cytoplasm causing a brief period of tension - a twitch

Question 17

What is a twitch in terms of muscle contraction?

Answer 17

When a muscle if briefly stimulated when it is relaxed, causing a small amount of tension to be produced.

Question 18

What happens when many action potentials are fired to a muscle fibre in rapid sequence?

Answer 18

The muscle will be restimulated before Ca2+ has returned to the SR, meaning it will increase further, creating a longer actin-myosin interaction and a stronger, longer contraction.

Question 19

What is the state when maximum signalling and contraction of a muscle fibre is reached?

Answer 19

Tetanus

Question 20

What is the length-tension relationship?

Answer 20

Each muscle has an optimal length where it is strongest, where there is sufficient length for cross bridging AND contraction.

Question 21

Why are stretched muscles unable to contract effectively?

Answer 21

There is not enough overlap of the myosin and actin fibres, so cross-bridging is limited/does not occur.

Question 22

Why are slack muscles not effective for contraction?

Answer 22

The myosin and actin fibres overlap completely - there is no room for contraction to occur.

Question 23

Why do fast muscle fibres get fatigued rapidly?

Answer 23

They provide a lot of energy quickly, generating a short, powerful force.

Question 24

Which muscle fibre (slow/fast) has a larger fibre diameter?

Answer 24

Fast

Question 25

Which muscle fibre (slow/fast) has a greater capillary supply?

Answer 25

Slow (they need to produce a lot of energy when in use)

Question 26

Which muscle fibre (slow/fast) has more mitochondria?

Answer 26

Slow

Question 27

Why do fast fibres not have mitochondria?

Answer 27

Mitochondria provides energy too slowly, making it ineffective in providing the rapidly released high levels of energy required.

Question 28

What colour are fast muscle fibres?

Answer 28

White

Question 29

What colour are slow muscle fibres?

Answer 29

Red

Question 30

Which muscle fibre (slow/fast) has better fatigue resistance?

Answer 30

Slow

Question 31

Which muscle fibre (slow/fast) reaches peak tension more rapidly?

Answer 31

Fast

Question 32

How can muscle contraction be described in terms of the ‘lines’ of the sarcomere?

Answer 32

Myosin contracts, pulling the Z lines of the actin inwards the M line of the myosin, shortening the sarcomere on both sides of the M line.

Question 33

Is the M line found in the myosin or actin filaments?

Answer 33

Myosin (in the centre of the sarcomere)

Question 34

is the Z line found on actin or myosin filaments?

Answer 34

Actin

Question 35

What is the principle neurotransmitter at a neuromuscular junction?

Answer 35

Acetylcholine

Question 36

How does a synapse between a motor neuron and muscle fibre result in muscle contraction?

Answer 36

The action potential causes acetylcholine to be released from the neuron via the synaptic cleft to receptors in the muscle fibre. Ultimately, this leads to an action a potential which travels along the sarcolemma to the T-tubule, where it triggers the voltage gated receptor (DHPR) causing the release of calcium ions into the cytosol

Question 37

What is the position of the myosin heads in relation to the M line during the resting state of a muscle fibre?

Answer 37

Heads point away from the M line

Question 38

What part of the sarcoplasmic reticulum are Calcium ions stored in and released from?

Answer 38

Terminal cisternae

Question 39

What is the symbol for acetylcholine?

Answer 39

ACh