3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) Flashcards
1
Q
Muscles work in ________?
A
- Muscles work in antagonistic pairs.
- Skeletal muscles attached to bones by tendons.
- Ligaments attach bones to other bones to hold them together.
- Pairs of skeletal muscles contract and relax to move bones at a joint —> bones of the skeleton and incompressible so they act as levers.
=> Give the muscles something to pull against. - Muscles that work together to move a bone are called antagonistic pairs.
—> contracting muscle = agonist.
—> relaxing muscle = antagonist.
=> Example - Biceps and Triceps:
-> Bending arm upwards, biceps contracts (agonist - for this movement) and triceps relaxes (antagonist - for this movement). - other way around for bending arm downwards.
2
Q
Give an example of how muscles work in antagonistic pairs?
A
=> Example - Biceps and Triceps:
- > Bending arm upwards, biceps contracts (agonist - for this movement) and triceps relaxes (antagonist - for this movement).
- other way around for bending arm downwards
3
Q
Describe the structure and composition of skeletal muscle.
A
- Skeletal muscle is made up of large bundles of long cells - each cell called a muscle fibre.
- Cell membrane of muscle fibre cells = sarcolemma.
- Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (cytoplasm of muscle cells) = folds called transverse (T) tubules.
- T-tubules help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
- Network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm —> sarcoplasmic reticulum stores and releases Ca2+ ions needed for muscle contraction.
- Muscle fibres have many mitochondria to provide ATP needed for muscle contraction.
- Muscle fibres are multinucleate (contain many nuclei) because they consist of several cells that
have fused together. - Muscle fibres have many long, cylindrical organelles called myofibrils. Made up of proteins and are highly specialised for contraction.
4
Q
Describe the structure and composition of myofibrils.
A
- Myofibrils contain bundles of thick and thin myofilaments than move past each other to make muscles contract.
- > thick myofilaments are made of the protein myosin.
- > thin myofilaments made of protein actin. - Looking at myofibriI sample under an electron microscope, we can see pattern of alternating dark and light bands.
- > Dark bands contain the thick myosin filaments and some overlapping thin actin filaments - A bands (blue and red overlap).
- > Light bands contain thin actin filaments only - I bands (blue). - Myofibril made up of many short units called sarcomeres.
- Ends of a sarcomere are marked with a Z-line.
- Middle of each sarcomere is an M-line (middle of myosin filaments.
- Around the M-line is the H-zone - area containing only myosin filaments.
5
Q
How does the sliding filament theory explain muscle contraction?
A
- Myosin and actin filaments slide over each other to make the sarcomeres contract - myosin filaments don’t contract.
- Simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract.
- Sarcomeres return to their original length as the muscle relaxes.
- > A-bands stay same length.
- > I-band gets shorter.
- > H-zones get shorter.
- > Sarcomeres get shorter.
6
Q
Comment on structure of myosin heads and actin filaments? How do they interact?
A
- Myosin filaments have globular heads that are hinged, so can move back and forth.
- Each myosin head has a binding site for actin and a binding site for ATP.
- Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
- Another protein called tropomyosin is found between actin filaments —> helps myofilaments move past each other.
7
Q
How are binding sites in resting muscles blocked?
A
- In a resting, unstimulated muscle the actin-myosin binding site is blocked by tropomyosin.
- Myofilaments can’t slide past each other because the myosin heads can’t bind to the actin-myosin binding site on the actin filaments.
8
Q
What triggers muscle contraction?
A
An influx of calcium ions, caused by depolarisation of the sarcolemma spreading down T-tubules to sarcoplasmic reticulum, after action potential from a motor neurone stimulates a myocyte.
9
Q
How does an influx of Ca2+ ions trigger muscle contraction?
A
- Action potential from a motor neurone stimulates a muscle cell, depolarising the sarcolemma. Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum.
- This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.
- Calcium ions bind to a protein attached to tropomyosin, causing protein to change shape —> pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.
- This exposes the binding site, which allows the myosin head to bind.
- The bond formed when a myosin head binds to an actin filament is called a actin-myosin cross bridge.
- Calcium ions also activate the ATP-hydrolase enzyme to break down ATP to provide energy needed for muscle contraction.
- Energy released from ATP causes myosin head to bend, which pulls the actin filament along (in a kind of rowing action).
- Another ATP molecule provides the energy to break the actin-myosin cross bridge, so the myosin head detaches from the actin filament after it’s moved.
- Myosin head then reattaches to a different binding site further along the actin filament —> new actin-myosin cross bridge is formed and the cycle is repeated.
- Many cross bridges form and break very rapidly, pulling the actin filament along - shortening the sarcomere, causing the muscle to contract.
- Cycle will continue as long as calcium ions are present.
10
Q
Describe the processes taking place when stimulation of the muscle stops.
A
- Muscle stops being stimulated, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (also requiring ATP).
- This causes the tropomyosin molecules to move back, so they block the actin-myosin binding sites again.
- Muscles aren’t contracted because no myosin heads are attached to actin filaments (so there are no actin-myosin cross bridges).
- The actin filaments slide back to their relaxed position, which lengthens the sarcomere.
11
Q
Comment on the need for ATP.
A
- So much energy is needed when muscles contract that ATP gets used up very quickly.
- ATP has to be continually generated so exercise can continue.
12
Q
List the three ways in which ATP is continually generated to allow exercise to continue.
A
- Aerobic respiration.
- Anaerobic respiration.
- ATP-Phosphocreatine (ATP-PCr) system.
13
Q
Outline how aerobic respiration provides ATP for muscle contraction
A
- Most ATP generated via oxidative phosphorylation in the cell’s mitochondria.
- Aerobic respiration only works when there’s oxygen so it’s good for long periods of low intensity exercise.
14
Q
Outline how anaerobic respiration provides ATP for muscle contraction.
A
- ATP is made rapidly by glycolysis.
- End product of glycolysis is pyruvate, which is converted to lactate by lactate fermentation.
- Lactate can quickly build up in the muscles and cause muscle fatigue,
- Anaerobic respiration is good for short periods of hard exercise (400m sprint).
15
Q
Outline how the ATP-Phosphocreatine (PCr) system provides ATP for muscle contraction.
A
- ATP is made by phosphorylating ADP - adding a phosphate group taken from the PCr.
- PCr stored inside cells and the ATP-PCr system generates ATP very quickly.
- PCr runs out very quickly so used during short bursts of vigorous exercise (tennis serve).
- ATP-PCr system is anaerobic and it’s alactic (doesn’t form any lactate.
- –> some of the creatine gets broken down into creatinine which is removed from the body by the kidneys.
- –> creatinine levels can be higher in people who exercise regularly and those with a higher muscle mass. High creatinine levels may indicate kidney damage.
