3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) Flashcards Preview

3.6 Organisms respond to changes in their internal and external environments (A-level only) > 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) > Flashcards

Flashcards in 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) Deck (17)
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Muscles work in ________?

- 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.


Give an example of how muscles work in antagonistic pairs?

=> 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


Describe the structure and composition of skeletal muscle.

1. Skeletal muscle is made up of large bundles of long cells - each cell called a muscle fibre.
2. Cell membrane of muscle fibre cells = sarcolemma.
3. Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (cytoplasm of muscle cells) = folds called transverse (T) tubules.
4. T-tubules help to spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
5. Network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm ---> sarcoplasmic reticulum stores and releases Ca2+ ions needed for muscle contraction.
6. Muscle fibres have many mitochondria to provide ATP needed for muscle contraction.
7. Muscle fibres are multinucleate (contain many nuclei) because they consist of several cells that
have fused together.
8. Muscle fibres have many long, cylindrical organelles called myofibrils. Made up of proteins and are highly specialised for contraction.


Describe the structure and composition of myofibrils.

1. 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.
2. 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).
3. Myofibril made up of many short units called sarcomeres.
4. Ends of a sarcomere are marked with a Z-line.
5. Middle of each sarcomere is an M-line (middle of myosin filaments.
6. Around the M-line is the H-zone - area containing only myosin filaments.


How does the sliding filament theory explain muscle contraction?

1. Myosin and actin filaments slide over each other to make the sarcomeres contract - myosin filaments don't contract.
2. Simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract.
3. 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.


Comment on structure of myosin heads and actin filaments? How do they interact?

1. Myosin filaments have globular heads that are hinged, so can move back and forth.
2. Each myosin head has a binding site for actin and a binding site for ATP.
3. Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
4. Another protein called tropomyosin is found between actin filaments ---> helps myofilaments move past each other.


How are binding sites in resting muscles blocked?

1. In a resting, unstimulated muscle the actin-myosin binding site is blocked by tropomyosin.
2. Myofilaments can't slide past each other because the myosin heads can't bind to the actin-myosin binding site on the actin filaments.


What triggers muscle contraction?

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.


How does an influx of Ca2+ ions trigger muscle contraction?

1. Action potential from a motor neurone stimulates a muscle cell, depolarising the sarcolemma. Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum.
2. This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.
3. 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.
4. This exposes the binding site, which allows the myosin head to bind.
5. The bond formed when a myosin head binds to an actin filament is called a actin-myosin cross bridge.
6. Calcium ions also activate the ATP-hydrolase enzyme to break down ATP to provide energy needed for muscle contraction.
7. Energy released from ATP causes myosin head to bend, which pulls the actin filament along (in a kind of rowing action).
8. 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.
9. 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.
10. Many cross bridges form and break very rapidly, pulling the actin filament along - shortening the sarcomere, causing the muscle to contract.
11. Cycle will continue as long as calcium ions are present.


Describe the processes taking place when stimulation of the muscle stops.

1. Muscle stops being stimulated, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (also requiring ATP).
2. This causes the tropomyosin molecules to move back, so they block the actin-myosin binding sites again.
3. Muscles aren't contracted because no myosin heads are attached to actin filaments (so there are no actin-myosin cross bridges).
4. The actin filaments slide back to their relaxed position, which lengthens the sarcomere.


Comment on the need for ATP.

- 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.


List the three ways in which ATP is continually generated to allow exercise to continue.

1. Aerobic respiration.
2. Anaerobic respiration.
3. ATP-Phosphocreatine (ATP-PCr) system.


Outline how aerobic respiration provides ATP for muscle contraction

1. Most ATP generated via oxidative phosphorylation in the cell's mitochondria.
2. Aerobic respiration only works when there's oxygen so it's good for long periods of low intensity exercise.


Outline how anaerobic respiration provides ATP for muscle contraction.

1. ATP is made rapidly by glycolysis.
2. End product of glycolysis is pyruvate, which is converted to lactate by lactate fermentation.
3. Lactate can quickly build up in the muscles and cause muscle fatigue,
4. Anaerobic respiration is good for short periods of hard exercise (400m sprint).


Outline how the ATP-Phosphocreatine (PCr) system provides ATP for muscle contraction.

1. ATP is made by phosphorylating ADP - adding a phosphate group taken from the PCr.
2. PCr stored inside cells and the ATP-PCr system generates ATP very quickly.
3. PCr runs out very quickly so used during short bursts of vigorous exercise (tennis serve).
4. 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.


Describe properties of slow twitch muscle fibres.

1. Muscle fibres that contract slowly.
2. Muscles used for posture - back muscles have many slow twitch muscle fibres.
3. Good for endurance activities.
4. Can work for a long time without tiring.
5. Energy released slowly through aerobic respiration ---> have many mitochondria and blood vessels supply the muscles with oxygen.
6. Reddish in colour because they're rich in myoglobin - protein (red-coloured) that stores oxygen.


Describe properties of fast twitch muscle fibres.

1. Muscle fibres that contract quickly.
2. Muscles used for fast movement ---> eyes and legs.
3. Good for short bursts of speed and power.
4. Tire very quickly.
5. Energy released quickly through anaerobic respiration using glycogen ---> few mitochondria / few blood vessels.
6. Whitish in colour as they have little myoglobin so store little oxygen.