15.8 - Contraction Of Skeletal Muscle Flashcards
Why is the human skeleton described as incompressible?
The skeleton is made of bone, which is incompressible, meaning that muscles exert force via tendons to move the bone rather than changing shape themselves.
What is the function of joints in the skeleton?
Joints allow different parts of the skeleton to move relative to each other.
Why can muscles only pull and not push?
Muscles can only contract to pull on bones; to move in the opposite direction, an antagonistic muscle is required to pull the limb back.
What are antagonistic muscle pairs?
Antagonistic muscle pairs are pairs of muscles that pull in opposite directions, where one muscle contracts as the other relaxes.
What changes occur in a sarcomere during muscle contraction?
- The I-band becomes narrower.
- The Z-lines move closer together (sarcomere shortens).
- The H-zone becomes narrower.
- The A-band remains the same width.
Why does the A-band remain the same width during contraction?
The A-band width is determined by the length of the myosin filaments, which do not change length during contraction.
What is the sliding filament mechanism?
It is the process where actin and myosin filaments slide past one another to cause muscle contraction.
Describe the structure of myosin.
Myosin has:
- A fibrous tail, made of several hundred protein molecules.
- A globular head, with two bulbous structures at one end
Describe the structure of actin.
Actin is a globular protein with molecules arranged into long chains twisted into a helical strand.
What is tropomyosin?
Tropomyosin is a long, thin protein that winds around actin filaments, blocking the binding sites for myosin heads in a resting state.
How is a muscle stimulated?
- An action potential reaches neuromuscular junctions, opening calcium ion protein channels.
- Calcium ions diffuse into the synaptic knob, triggering the release of acetylcholine.
- Acetylcholine binds to receptors on the muscle cell membrane, causing depolarization.
What role do T-tubules play in muscle contraction?
T-tubules carry the action potential deep into the muscle fibre, reaching the sarcoplasmic reticulum.
How do calcium ions initiate muscle contraction?
- Calcium ions released from the sarcoplasmic reticulum bind to tropomyosin, causing it to move away from actin binding sites.
- This exposes the sites, allowing myosin heads to bind.
How does the myosin head interact with actin during contraction?
- Myosin heads bind to actin, forming cross-bridges.
- They change angle, pulling actin along and releasing ADP.
- ATP attaches to myosin heads, causing detachment.
- ATP hydrolysis provides energy for myosin heads to reset.
- The cycle repeats as long as calcium ions are present.
Why does the sarcomere shorten during contraction?
Myosin heads pull actin filaments toward each other, reducing the distance between adjacent Z-lines.
What happens during muscle relaxation?
- Nervous stimulation ceases, and calcium ions are actively transported back into the sarcoplasmic reticulum using ATP.
- Tropomyosin blocks actin binding sites again.
- Myosin heads can no longer attach, and the muscle relaxes.
How do antagonistic muscles contribute to muscle relaxation?
Antagonistic muscles pull actin filaments out from between myosin, lengthening the sarcomere.
What are the two main uses of ATP during muscle contraction?
- Moving myosin heads.
- Actively transporting calcium ions into the sarcoplasmic reticulum.
How is ATP regenerated in muscles during high activity?
- Aerobic respiration in mitochondria regenerates most ATP.
- Anaerobic processes, including glycolysis and phosphocreatine breakdown, provide additional ATP.
What is the role of phosphocreatine in muscle contraction?
Phosphocreatine acts as a phosphate reserve to rapidly regenerate ATP from ADP during intense activity.
Describe The Sliding filament mechanism of muscle contraction
- the action potential causes voltage-gated Ca+ ion channels to open, and Ca+ ions move from the sarcoplasmic reticulum into the sarcomere
- the actin filaments are associated with another protein called tropomyosin
- at intervals along the tropomyosin, molecules of a 4th protein called troponin are positioned, such that they prevent the myosin heads from binding
- when the calcium ions enter the sarcomere, they bind to the troponin molecules, causing them to deform which causes tropomyosin to sift position and expose the myosin binding site
- the myosin now binds, forming cross bridges
- the myosin carries inorganic phosphate and an ADP molecule: these are now released, causing a change in shape - the myosin relaxes
- this drags the filaments across each other, pulling the Z-line inwards. It’s called the “power stroke”
- A new ATP molecule binds to myosin, causing it to release actin (breaking down the cross bridge)
- Myosin then hydrolyses the ATP, releasing energy to extend the head once again.
How far does each skeletal muscle myosin head move each power stroke
10-12nm
