6.3 Muscle Contraction Flashcards
What attaches skeletal muscles to bones?
Tendons
What is a pair of muscles where one contracts and one relaxes called?
Antagonistic pair
What is the muscle that is relaxing called?
The antagonist
What is the muscle that is contracting called?
The agonist
Describe the structure of skeletal muscles
- Skeletal muscle consists of many bundles of muscle fibres
- Muscle fibres are long specialised cells
- The membrane of muscle fibres is called the sarcolemma.
- Muscle fibres also have many mitochondria and nuclei.
- The mitochondria provide lots of ATP to power muscle contraction.
- Myofibrils are cylindrical organelles that run along the length of muscle fibres.
- Myofibrils are the site of muscle contraction.
Describe the structure of myofibrils
- Myofibrils are made of multiple units that run end-to-end along the myofibril. These units are called sarcomeres.
- The end of a sarcomere is called the Z-line.
- Sarcomeres are made from two types of myofilaments.
- The two myofilaments slide past each other. This movement is what makes muscles contract.
The two types of myofilaments are: - Thick myofilaments - made of myosin protein.
- Thin myofilaments - made of actin protein.
The overlapping region is called the A-band.
The region with only myosin filament is called the H-zone.
Thin actin filaments only overlap with myosin filaments in the middle of the sarcomere. The middle is called the M-line.
The region with only actin filament is called the I-band.
Describe the structure of actin and myosin and how that structure allows contraction of the sarcomeres
Myosin filaments have globular heads that can move back and forth.
The movement of the globular heads is what allows actin and myosin filaments to slide past each other in muscle contraction.
There are two binding sites on every myosin head: One site can bind to actin. One site can bind to ATP which provides energy for the movement of the globular heads.
There is also a binding site for the myosin heads on actin filaments. This is called the actin-myosin binding site.
Tropomyosin is a protein that is located on actin filaments which plays an important role in muscle contraction because it blocks the actin-myosin binding site when muscle fibres are at rest.
When muscle fibres are stimulated, the tropomyosin protein is moved so that myosin heads can bind to the actin-myosin binding site.
When actin and myosin bind, they can slide past each other to cause muscle contraction.
Explain using the sliding filament theory how muscle contraction takes place
An action potential arrives at a neuromuscular junction from a motor neurone causing depolarisation of the sarcolemma.
Depolarisation spreads along the T tubules and into the sarcoplasm.
Depolarisation of the T tubules stimulates the sarcoplasmic reticulum (SR).
The SR releases Ca2+ ions into the sarcoplasm.
Ca2+ ions bind to a protein attached to tropomyosin. Tropomyosin is a protein that blocks the actin-myosin binding site.
Binding of Ca2+ ions causes the protein to change shape and move out of the way of the binding site.
The myosin head can now bind to the actin filament forming actin-myosin cross bridges.
Ca2+ ions also activate ATP hydrolase which hydrolyses ATP to ADP and inorganic phosphate.
The energy released from this reaction causes the myosin head to bend.
The actin filament is pulled by the myosin head.
ATP allows the myosin head to detach from the actin. It then binds to the next binding site pulling the actin along.
Describe how phosphocreatine can supply ATP for muscle contraction
During intense muscular effort, phosphocreatine donates phosphate to ADP to produce ATP. The ATP produced is used to sustain muscle contraction.
During low periods of muscle activity, ATP can be used to phosphorylate creatine back to phosphocreatine.
This process is anaerobic and produces no lactate but phosphocreatine is in short supply.
Explain how muscle contraction is halted / how muscles relax
If action potentials are no longer stimulating the muscle cells, the release of Ca2+ ions by the sarcoplasmic reticulum (SR) will stop. The Ca2+ ions are transported back into the SR by active transport.
Removal of Ca2+ ions means that the protein attached to tropomyosin undergoes a conformational change.
This causes tropomyosin to shift so that it is blocking the actin-myosin binding sites.
Myosin heads can no longer bind to actin filaments.
The actin filaments return to their resting position.
The sarcomere lengthens again. The muscle is no longer contracting.
What are the two types of muscle fibres?
- Fast twitch fibres
- Slow twitch fibres
Where are slow twitch fibres found and what is their function?
Found in muscles used for posture such as the back and neck.
Adapted for endurance and slow movement over long periods of time.
Used for maintaining posture.
Muscle fibres are long and thin.
Where are fast twitch fibres found and what is their function?
Found mainly in muscles such as the arms and legs.
Adapted for fast or strong movement over short periods of time.
Muscle fibres are short and wide.
Do slow twitch fibres fatigue quickly or slowly?
Slowly
Do fast twitch muscle fibres contract slowly or quickly?
Quickly
What types of respiration do do slow twitch fibres rely on? What about fast twitch fibres?
Slow - aerobic respiration
Fast - anaerobic respiration
What are the structural differences between slow and fast twitch muscle fibres?
Slow:
- 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 that stores oxygen), so appear reddish.
- Less sarcoplasmic reticulum (contains calcium ions).
Fast:
- Fewer mitochondria.
- Fewer capillaries.
- High levels of glycogen.
- High levels of phosphocreatine.
- Small stores of myoglobin.
- More sarcoplasmic reticulum.
