6 Responding to Change- Muscle Contraction

Question 1

What do antagonistic pairs consist of and what do they allow?

Answer 1

-consist of an agonist & an antagonist
- are how skeletal muscles contract and relax to allow body movement

Question 2

What attaches skeletal muscles to bones?

Answer 2

Tendons

Question 3

What is the muscle that’s relaxing called?

Answer 3

The antagonist
Which muscle in a pair this is can vary depending on the movement

Question 4

What is the muscle that’s contracting called?

Answer 4

Agonist
Which muscle in pair this varies depending on movement

Question 5

What do skeletal muscles consist of?

Answer 5

Many bundles of muscle fibres - long, specialised cells

Question 6

What is the sarcolemma and what does it fold inwards to?

Answer 6

-The membrane of muscle fibres
-Folds inwards to sarcoplasm (muscle fibre cytoplasm) at certain points

Question 7

What are the inwards folds in muscle fibres called and what are they important for?

Answer 7

• Transverse (t) tubules
• very important in initiating muscle contraction

Question 8

What is the sarcoplasmic reticulum and its importance?

Answer 8

• An organelle in the sarcoplasm
• store for calcium (Ca²+ ions); important in muscle contraction

Question 9

What other organelles do muscle fibres contain and why?

Answer 9

-Mitochondria & nuclei
-Mitochondria provide lots of ATP to power muscle contraction

Question 10

What are myofibrils and their function?

Answer 10

• Cylindrical organelles, run along length of muscle fibres
• site of muscle contraction

Question 11

What are sarcomeres?

Answer 11

-Ask up myofibril; multiple units running end-to-end along myofibril
-End of sarcomere = Z line

Question 12

What are the 2 types of myofilaments and their function?

Answer 12

-Sarcomeres → made from 2 types of myofilaments; thick (made of myosin protein) & thin (made of actin protein)
-The two slide past each other → movement makes muscles contract

Question 13

How are myosin and actin filaments arranged with each other?

Answer 13

-Myosin & actin filaments → arranged in alternating pattern in sarcomeres
-Thick myosin filaments overlap w/ thin actin filaments at each end

Question 14

What is the A-band and the H-zone?

Answer 14

• A-band = overlapping region between myosin and actin filaments
• H-zone = region w/ only myosin filament

Question 15

What is the M-line and the I-band?

Answer 15

M-line = middle of the sarcomere, where myosin and actin filaments overlap
I-band = region with only actin filament

Question 16

What does the sliding filament theory explain?

Answer 16

How muscle contraction is coordinated in myofibrils

Question 17

Sliding filament; how is the sarcolemma depolarised?

Answer 17

-Muscle contraction → initiated when an action potential arrives at muscle cells
-Action potential depolarises sarcolemma

Question 18

Sliding filament; what causes the sarcomeres to contract?

Answer 18

-Depolarisation of sarcolemma causes myosin & actin filaments to slide over each other
-Sliding movement causes sarcomeres to contact

Question 19

Sliding filament; what causes the whole muscle to contract?

Answer 19

-There’s multiple sarcomeres along myofibril length
-As many sarcomeres contract simultaneously, muscle fibres contract
-Contraction of fibres causes whole muscle to contract

Question 20

Sliding filament; what causes the muscle to relax?

Answer 20

-After muscle contracts, sarcomeres relax
-Filaments slide back over each other & muscle relaxes

Question 21

What feature allows the myosin and actin filaments to slide past each other in muscle contraction?

Answer 21

-Myosin filaments—> have globular heads; move back and forth
-Movement of globular heads—> allows actin & myosin filaments to slide past each other in muscle contraction

Question 22

What binding sites are present on the myosin head/actin filaments?

Answer 22

-2 binding sites on each myosin head; one binds to actin, another to ATP
-Also binding site for myosin heads on actin filaments; actin-myosin binding site

Question 23

What is tropomyosin and its function?

Answer 23

-Protein located on actin filaments
-Plays important role in muscle contraction as blocks actin-myosin binding site when muscle fibres—> at rest
-When muscle fibres= stimulated, tropomyosin protein is moved so myosin heads can bind to actin-myosin binding site
-When actin + myosin bind, can slide past each other—> muscle contracts

Question 24

Why does ATP need to be made rapidly in muscle contraction?

Answer 24

It’s a very energetically demanding process

Question 25

How does aerobic respiration make ATP for muscle contraction?

Answer 25

-Makes ATP via oxidative phosphorylation
-Requires oxygen
-Mainly used for extended periods of low-intensity muscle use

Question 26

How does anaerobic respiration make ATP for muscle contraction?

Answer 26

-Makes ATP by glycolysis & lactate fermentation; makes lactate, build up can cause fatigue
-Mainly used in short periods of high-intensity muscle use

Question 27

What is phosphocreatine and how does it make ATP for muscle contraction?

Answer 27

-Molecule that can supply ATP for muscle contraction
-In intense muscular effort, phosphocreatine donates phosphate to ADP—> makes ATP, sustains muscle contraction
-In low periods of muscle activity, ATP can phosphorylate creatine—> phosphocreatine; short supply made, anaerobic process, makes no lactate

Question 28

Contraction; How is depolarisation of the sarcolemma and then the sarcoplasm caused?

Answer 28

• Muscle contraction initiated when action potential arrives at neuromuscular junction from motor neurone
  -Action potential causes depolarisation of sarcolemma and spreads along T-tubules—> sarcoplasm

Question 29

Contraction; What causes the influx of calcium ions into the sarcoplasm?

Answer 29

-Depolarisation of T-tubules stimulates sarcoplasmic reticulum (SR)
-SR releases Ca²+ ions—> sarcoplasm

Question 30

What is the role of tropomyosin and what occurs due to it in the muscle contraction process?

Answer 30

-Ca²+ ions bind → protein attached to tropomyosin (protein blocking actin-myosin binding site)
-binding of Ca²+ ions causes protein to change shape
-Altering protein causes tropomyosin to be moved. Actin-myosin binding site no longer blocked by tropomyosin

Question 31

How is the actin-myosin cross bridge formed?

Answer 31

-Actin-myosin binding site no longer blocked by tropomyosin so myosin head can now bind to actin filament
-bond between the 2 is called the actin-myosin cross bridge

Question 32

How is ATP hydrolase activated and what is its role in muscle contraction?

Answer 32

-Ca²+ ions also activate ATP hydrolase (enzyme hydrolysing ATP → ADP + inorganic phosphate)
-Process releases energy that can power muscle contraction

Question 33

How does the bending of the myosin heads occur and what does this lead to and why?

Answer 33

-Ca²+ ions activate ATP hydrolase—>ATP hydrolysed, energy released.
-energy released causes myosin head to bend.
-movement of myosin head causes actin filament to slide past myosin filament.
-actin filament pulled by myosin head due to actin-myosin cross bridge.

Question 34

How is the actin-myosin bridge broken and what does this result in?

Answer 34

-After actin filament has slid past myosin filament, actin-myosin cross bridge= broken (driven by energy from ATP)
-myosin head no longer attached to actin filament

Question 35

How is the actin-myosin cross bridge reformed?

Answer 35

-myosin head bends back to OG position after it’s released from actin binding site.
-myosin forms new cross bridge w/ binding site further along actin filament.

Question 36

How does the forming/breaking cycle of the actin-myosin bridges lead to muscle contraction?

Answer 36

-cycle of forming + breaking actin-myosin cross bridges occurs quickly & continuously.
-As actin filaments are pulled past myosin filaments, overall result= shortening of sarcomere; causes muscle contraction

Question 37

Halting contraction; how are calcium ions removed?

Answer 37

-If action potentials—> no longer stimulating muscle cells, release of Ca2+ ions by sarcoplasmic reticulum (SR) will stop.
-Ca2+ ions= transported back into SR by active transport

Question 38

Halting contraction; why does tropomyosin move and what does this cause?

Answer 38

-Removal of Ca²+ ions= protein attached to tropomyosin undergoes conformational change.
-protein changes shape—> causes tropomyosin to shift so it’s blocking actin-myosin binding sites.
-Myosin heads can no longer bind to actin filaments

Question 39

Halting contraction; what causes the sarcomere to lengthen and what does this mean?

Answer 39

-Myosin heads can no longer bind to actin filaments.
-actin filaments return to resting position.
-sarcomere lengthens again; muscle no longer contracting.

Question 40

What are the 2 types of skeletal muscle fibres?

Answer 40

Slow & fast twitch fibres

Question 41

How do slow and fast twitch muscle fibres differ in location?

Answer 41

-Slow twitch fibres; found in muscles used for posture, eg back & neck
- fast twitch fibres; found mainly in muscles like arms & legs

Question 42

How do slow and fast twitch muscle fibres differ in adaptation to function?

Answer 42

-slow twitch fibres; adapted for endurance & slow movements over long time periods. Are long & thin. Fatigue slowly, contract slowly
- fast twitch fibres; adapted for fast/strong movement over short time periods. Are short & wide. Fatigue quickly, contract quickly

Question 43

How do slow and fast twitch fibres differ in energy source?

Answer 43

• slow twitch fibres; rely on energy released via aerobic respiration
• fast twitch fibres; rely on energy released via anaerobic respiration

Question 44

What is the cell structure of slow twitch fibres?

Answer 44

• Lots of mitochondria to maintain aerobic respiration
• lots of capillaries to supply muscle fibres with oxygen
  -Low levels of glycogen
  -Low levels of phosphocreatine
• large stores of myoglobin (pigment storing oxygen) so appear reddish
• less sarcoplasmic reticulum (contains calcium ions)

Question 45

What is the cell structure of fast twitch fibres?

Answer 45

-fewer mitochondria
-fewer capillaries
-high levels of glycogen
-high levels of phosphocreatine
-small stores of myoglobin
-more sarcoplasmic reticulum