B4. Muscle Contraction Flashcards
Myosin filaments
Myosin filaments have ________heads that are _______, so they can move back and forth. Each myosin head has a _________site for ______and a binding site for _____
Myosin filaments have globular heads that are hinged, so they can move back and forth. Each myosin head has a binding site for actin and a binding site for ATP
Actin filaments
Actin filaments have binding sites for ________ ______, called ______-_______ __________ _______. Another protein called ________________is found between ______filaments. It helps _______________move past each other
Actin filaments have binding sites for myosin heads, called actin-myosin binding sites. Another protein called tropomyosin is found between actin filaments. It helps myofilaments move past each other
Figure 1: The structure of myosin and actin filaments.
Binding sites in resting muscles
For myosin and actin filaments to slide past each other, the myosin head needs to _____to the ______-_______ ________ _____on the ________filament. In a resting (unstimulated) muscle the _____-_______ __________ _____ is ___________ by _______________-see Figure 2. This means _________________can’t slide past each other because the _________ ______can’t bind to the _____filaments.
Figure 2: Actin and myosin filaments in resting muscle.
For myosin and actin filaments to slide past each other, the myosin head needs to bind to the actin-myosin binding site on the actin filament. In a resting (unstimulated) muscle the actin-myosin binding site is blocked by tropomyosin-see Figure 2. This means myofilaments can’t slide past each other because the myosin heads can’t bind to the actin filaments.
The process of muscle contraction - 1. Arrival of an action potential
When an action potential from a _______neurone ____________a _________cell, it ____________the ___________. Depolarisation spreads down the _-________to the ______________ ___________. This causes the sarcoplasmic reticulum to release stored __________ ____into the _____________. This __________of _________ ____into the ___________triggers ________ ______________.
___________ions bind to a __________ ____________to ________________, causing the __________to ________ ________. This pulls the __________ __________out of the ______-________ ___________ _____on the _____ filament. This exposes the __________site, which allows the _________ _____to _____. The bond formed when a __________ _____ _______to an ______filament is called an _______-________ _______ _______- see Figure 3
Figure 3: Formation of an actin-myosin cross bridge.
When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum. This causes the sarcoplasmic reticulum to release stored calcium ions (Ca) into the sarcoplasm. This influx of calcium ions into the sarcoplasm triggers muscle contraction.
Calcium ions bind to a protein attached to tropomyosin, causing the protein to change shape. This 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 an actin-myosin cross bridge- see Figure 3
The process of muscle contraction - 2. Movement of the actin filament
_____________ions also activate the enzyme ____ ___________, which _____________(breaks down) ATP (into ADP+P) to provide the energy needed for ________ ____________. The energy released from ATP causes the ________ ______to _____, which ____the ______ ___________ _______in a kind of rowing action (see Figure 4).
Figure 4: Movement of the myosin head.
The process of muscle contraction - 2. Movement of the actin filament
Calcium ions also activate the enzyme ATP hydrolase, which hydrolyses (breaks down) ATP (into ADP+P) to provide the energy needed for muscle contraction. The energy released from ATP causes the myosin head to bend, which pulls the actin filament along in a kind of rowing action (see Figure 4).
Tip: The movement of the myosin head to the side is called a ‘_________ _______.
Tip: The movement of the myosin head to the side is called a ‘power stroke.
The process of muscle contraction - 3. Breaking of the cross bridge
Another ____molecule provides the energy to _______the _______-_______ ______ _______, so the ________ _______ _________from the ______ ___________after it’s ______. The _________ _____then returns to its ___________position, and ___________to a different __________ _____further along the _____filament-see Figure 5. A new _____-_________ _______ ______is formed and the cycle is ___________(attach, move, detach, reattach to new binding site…).
Many actin-myosin cross bridges form and ______ _____ ________, pulling the ______filament along-which shortens the __________, causing the muscle to _________. The cycle will continue as long as _________ _____are present.
Figure 5: Myosin head forms a new actin-myosin cross bridge.
The process of muscle contraction - 3. Breaking of the cross bridge
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. The myosin head then returns to its starting position, and reattaches to a different binding site further along the actin filament-see Figure 5. A new actin-myosin cross bridge is formed and the cycle is repeated (attach, move, detach, reattach to new binding site…).
Many actin-myosin cross bridges form and break very rapidly, pulling the actin filament along-which shortens the sarcomere, causing the muscle to contract. The cycle will continue as long as calcium ions are present.
Tip: As the actin filaments are being moved along, the _-bands are getting shorter and the _-lines are moving closer together.
Tip: As the actin filaments are being moved along, the I-bands are getting shorter and the Z-lines are moving closer together.
The process of muscle contraction - 4. Return to resting state
When the muscle stops being stimulated, __________ _____leave their __________sites and are moved by ________ _________back into the _____________ __________(this needs ____too).
This causes the ____________molecules to move _____, so they block the ______-_______ _________ ______again-see Figure 6. Muscles aren’t contracted because no ________heads are attached to ________filaments (so there are no ______-_______ _______ _______). The _______filaments slide back to their ___________position, which __________the _____________.
Figure 6: Blocking of the actin-myosin binding sites as the muscle returns to its resting state.
The process of muscle contraction - 4. Return to resting state
When the muscle stops being stimulated, calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (this needs ATP too).
This causes the tropomyosin molecules to move back, so they block the actin-myosin binding sites again-see Figure 6. 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.
Energy for muscle contraction
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 this happens in three main ways:
- Aerobic respiration
- Anaerobic respiration
- ATP-phosphocreatine (PCr) system
- Aerobic respiration
Most ATP is generated via ___________ ____________in the cell’s _______________. Aerobic respiration only works when there’s _________so it’s good for ______ periods of low-intensity exercise, e.g. a long walk.
Most ATP is 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, e.g. a long walk.
- Anaerobic respiration
ATP is made rapidly by ___________. The end product of glycolysis is ________, which is converted to _________by _________ _____________. Lactate can quickly build up in the muscles and cause ________ __________. Anaerobic respiration is good for ______periods of hard exercise, e.g. a 400 m sprint.
ATP is made rapidly by glycolysis. The 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, e.g. a 400 m sprint.
- ATP-phosphocreatine (PCr) system
ATP is made by phosphorylating ADP-adding a phosphate group taken from PCR. The equation for this is shown in Figure 7. PCr is stored inside cells and the ATP-PCr system generates ATP very quickly. PCr runs out after a few seconds so it’s used during short bursts of vigorous exercise, e.g. a tennis serve. The ATP-PCR system is anaerobic (it doesn’t need oxygen) and it’s alactic (it doesn’t form any lactate).
_____+_____→ _____+ ___(_________)
Some of the creatine (Cr) gets broken down into ___________, which is removed from the body via the kidneys. Creatinine levels can be higher in people who exercise regularly and those with a high muscle mass. High creatinine levels may also indicate kidney damage.
ATP is made by phosphorylating ADP-adding a phosphate group taken from PCR. The equation for this is shown in Figure 7. PCr is stored inside cells and the ATP-PCr system generates ATP very quickly. PCr runs out after a few seconds so it’s used during short bursts of vigorous exercise, e.g. a tennis serve. The ATP-PCR system is anaerobic (it doesn’t need oxygen) and it’s alactic (it doesn’t form any lactate).
ADP+PCr→ ATP + Cr (creatine)
Some of the creatine (Cr) gets broken down into creatinine, which is removed from the body via the kidneys. Creatinine levels can be higher in people who exercise regularly and those with a high muscle mass. High creatinine levels may also indicate kidney damage.
Slow twitch muscle fibres
Slow twitch muscle fibres contract _________and can work for a _____time without getting tired. This makes them good for ____________activities, e.g. long-distance running and maintaining posture. High proportions of slow twitch muscle fibres are found in the muscles you use for posture, such as the muscles in the back and in the calves.
Energy is released _________through ________respiration (see above) in slow twitch muscle fibres. They have lots of _____________and _________ _______to supply the muscles with _______. The mitochondria are mainly found near to the ______of muscle fibres, so that there’s a _______ ____________ _________for ___________from the blood vessels to the _______________. Slow twitch muscle fibres are also rich in ___________, a red-coloured protein that stores oxygen, so they’re reddish in colour.
Slow twitch muscle fibres contract slowly and can work for a long time without getting tired. This makes them good for endurance activities, e.g. long-distance running and maintaining posture. High proportions of slow twitch muscle fibres are found in the muscles you use for posture, such as the muscles in the back and in the calves.
Energy is released slowly through aerobic respiration (see above) in slow twitch muscle fibres. They have lots of mitochondria and blood vessels to supply the muscles with oxygen. The mitochondria are mainly found near to the edge of muscle fibres, so that there’s a short diffusion pathway for oxygen from the blood vessels to the mitochondria. Slow twitch muscle fibres are also rich in myoglobin, a red-coloured protein that stores oxygen, so they’re reddish in colour.
Figure 9: A slow twitch muscle fibre.
Figure 10: A fast twitch muscle fibre, with few blood vessels, myoglobin or mitochondria.
