Muscle Fibre Contraction Flashcards
Which part of the sarcomere is the A band, and which is the I band?
Which is the H band?
A band = the bit where myosin and actin overlap.
I band = the bit where there is actin but no myosin, including the z plate.
H band = bit where myosin is not overlapped by actin.
What does the tension developed by sarcomere shortening depend on?
The number of points of attachment between actin and myosin during the myosin power stroke.
What is the role of the protein actinin?
To attach actin to the z-lines.
What is the term for the myosin head ratcheting along the actin filament?
Cross-bridge cycling.
What is the role of the protein titin?
Titin is a structural protein that provides an elastic connection between the opposing ends of the actin and myosin filaments.
What are the roles of troponin T, troponin I, troponin C, and tropomyosin?
Troponin T binds the troponin complex to the tropomyosin. Troponin I inhibits myosin interacting with actin and tropomyosin. Troponin C has binding sites for Ca2+ to initiate contraction. Tropomyosin blocks the myosin binding sites on the actin molecule.
What is a motor unit?
The motor nerve and all the muscle fibres it supplies.
How does the number of muscle fibres in a motor unit influence the strength of contraction?
The more muscle fibres, the greater the strength of contraction.
In muscles with finer control of movement, are there more or fewer muscle fibres per motor unit?
Fewer.
What is muscle tone?
The degree of contraction a muscle exhibits.
What happens in the latent period of a muscle twitch?
The excitation-contraction coupling process is occurring (end plate potential propagates along sarcolemma and Ca2+ channels open) and tension is beginning to increase.
What happens in the contraction period of muscle twitch?
Cross bridge cycling, maximum tension is developed.
What happens in the relaxation period of muscle twitch?
Calcium is sequestered in the sarcoplasmic reticulum, tension decreases and the sarcomere returns to original length.
Why is the refractory period longer in cardiac muscle than skeletal muscle?
To allow complete contraction and emptying of the atria and ventricles.
What is a muscle twitch?
The response of a motor unit to a single action potential.
What is wave summation?
The stimuli have an increased frequency, so the muscle doesn’t have time to relax completely, meaning the contraction summates e.g temporal summation.
What is unfused tetanus?
Frequency of stimuli is increased so the relaxation between twitches becomes shorter and the concentration of calcium in the sarcoplasm becomes higher and higher. This leads to sustained (quivering) contraction. You can pick out individual twitches.
What is fused tetanus?
The frequency of stimulation is so great that the relaxations disappear and the contractions fuse.
When can maximum tension be developed?
When the muscle is at its optimum resting length, so the maximum number of cross-bridges can be formed.
What is the difference between isotonic and isometric muscle contraction?
In isotonic muscle contraction the force generated by the muscle is greater than the load so the muscle changes length.
In isometric muscle contraction the load is greater than the force generated by the muscle, so the muscle doesn’t shorten.
What is the difference between concentric and eccentric muscle contraction?
Concentric = muscle decreases in length Eccentric = muscle increases in length
What are thick and thin filaments composed of?
Thick filaments are composed of myosin. Think filaments are composed of actin, tropomyosin, and troponin complexes.
Why is skeletal muscle striated?
The A bands are lighter than the I bands.
What is a myofibril?
A thread like contractile element in the sarcolemma of muscle fibres, which is made up of end to end sarcomeres.
How many heavy and light chains form a myosin molecule?
Two heavy chains and two light chains/
What are the functions of troponin T, troponin I and troponin C?
Troponin T binds the troponin complex to the tropomyosin.
Troponin I blocks the myosin binding site on th actin.
Troponin C binds Ca2+ to produce a conformational change to free the myosin binding site from tropomyosin.
What is cross-bridge cycling?
A repeated process where the myosin heads ratchet along the actin filaments causing the thick and thin filaments to slide past each other.
What are the 6 stages of cross-bridge cycling?
1) ATP binds to ATP binding site on myosin head so it dissociates from actin
2) ATPase on myosin head hydrolyses ATP into ADP and iP, so myosin head is reorientated and energised and ready to bind to another binding site on the actin
3) A cross bridge forms as the myosin head binds to the actin
4) The iP is released from the myosin head, causing a conformational change which results in a power stroke causing the filaments to slide past each other (and the thin filament to slide towards the M line)
5) ADP is released from the myosin head
6) The cross bridge remains until another ATP molecule binds to the myosin head and it dissociates from the actin and starts the process again
What are the two conditions required for cross-bridge cycling to continue?
Ca2+ present and ATP still available.
What happens to the sarcomere if its length is decreased much below its optimal resting length?
Little tension will be developed because the thin filaments are crumpling and poking through the M line, and the myosin heads are bumping into the Z line.
What happens to the sarcomere if its length is increased much above its optimal resting length?
Zero tension will be developed because the A band is obliterated and no myosin heads are in contact with the thin filaments and able to form cross bridges.
What is the purpose of the junctional folds in the motor end plate?
To provide a large surface area for Ach receptors.
What is an End Plate Potential?
When acetylcholine causes ligand-gated Na+ channels to open in the sarcolemma, and the membrane is depolarised above -55mv to generate an End Plate Potential along the sarcolemma.
What are the triads in the sarcolemma comprised of?
One T tubule (invagination of sarcolemma running across the T tubule), and two terminal cisterns of the sarcoplasmic reticulum on either side.
What happens when the End Plate Potential propagates down the T tubule?
The depolarisation causes a conformational change in the L-type Ca2+ channels in the T tubule membrane (dihydropyridine receptors). The dihydropyridine receptors form a diad with the ryanodine receptors on the sarcoplasmic reticulum, so the conformational change causes the ryanodine receptors to open and release Ca2+ from the sarcoplasmic reticulum (allowing cross-bridge cycling to occur).
When there are no more action potentials, how is Ca2+ removed from the sarcoplasmic reticulum?
The ryanodine channels close, and the sarcoplasmic endoplasmic reticulum Ca2+/ATPase (SERCA) channels open to sequester Ca2+ back into the sarcoplasmic reticulum.
Also, the Na+/Ca2+ exchanger on the sarcolemma transports Ca2+ out of the muscle fibre.
This means the muscle relaxes.
Why is the tension developed in fused tetanus many many times higher than the tension developed in a single muscle twitch?
The temporal summation means each stimulus causes even more Ca2+ to be released from the sarcoplasmic reticulum, so the concentration if Ca2+ in the cytosol just continues to increase (positive inotropy).
How many neuromuscular junctions does each muscle fibre have and what does this mean?
Each muscle fibre has only one, near the middle, so the end plate potential propagates both ways down the fibre and the whole fibre contracts at once.
What is motor unit recruitment/motor unit summation?
A process by which the number of active motor units is increased.
Why is it that typically the weakest motor units are recruited first, and as the stimulus increases and more force is required progressively stronger motor units (larger diameter fibres) are recruited, and what is this called?
Size Principle - delays muscle fatigue and means the muscle can continue to contract for longer periods of time.
What two elements are muscles made up of?
Contractile elements and series elastic elements (e.g tendons), and the series elastic elements will stretch initially before transferring the tension developed by the contractile elements.
Velocity of muscle softening decreases as load increases, why is Vmax when load is zero?
Vmax is when load is zero because that is when the relaxed muscle is at the optimum length for tension development, lots of myosin heads can be in contact with the actin molecules.
Why is velocity of contraction zero at maximum load, and what is this called?
Not enough tension can be developed to slide the actin filaments together = isometric contraction.
Skeletal muscle fibres can be classified based on how fast the myosin heads hydrolyse ATP, what are Type 1 skeletal muscle fibres?
Slow oxidative fibres (slow twitch).
They appear dark red because they contain lots of myoglobin and blood vessels.
Contain large mitochondria and generate ATP by aerobic respiration.
Myosin heads hydrolyse ATP relatively slowly, so contraction occurs at a slower rate and takes longer to reach maximum tension.
Resistant to fatigue - contraction can be maintained for many hours.
Important for maintaining posture and aerobic endurance activities like running a marathon.
Skeletal muscle fibres can be classified based on how fast the myosin heads hydrolyse ATP, what are Type 2a skeletal muscle fibres?
Fast twitch glycolytic fibres.
White fibres due to low myoglobin content, and fewer blood capillaries.
Large glycogen stores to generate ATP mainly by glycolysis.
The myosin heads hydrolyse ATP very quickly, so the contraction is very quick and strong.
The fibres are susceptible to fatigue, so adapted for intense anaerobic movements of short duration.
Skeletal muscle fibres can be classified based on how fast the myosin heads hydrolyse ATP, what are Type 2b skeletal muscle fibres?
Fast oxidative-glycolytic (fast twitch fatigue resistant) fibres.
Pink or red in colour, because they contain an intermediate amount of myoglobin and a good blood supply.
They can generate considerable ATP by oxidative phosphorylation so are intermediately resistant to fatigue.
They have a high intracellular glycogen content so can also generate ATP by aerobic glycolysis.
They have fast contraction because the myosin head hydrolyses ATP faster than slow oxidative (type 1) fibres, and maximum tension is reached faster.
Used in activities like walking and sprinting.
In terms of the types of skeletal muscle fibre they contain, what is the difference between a muscle and a motor unit?
A muscle may contain a mixture of all three types of fibres (slow oxidative, fast twitch glycolytic, fast oxidative-glycolytic), but each motor unit will only ever contain one type of fibre.
In terms of recruitment, which of the three types of muscle fibre will be recruited first?
Slow oxidative 1, then fast oxidative-glycolytic 2b, then fast twitch glycolytic 2a
In what two aspects of muscle contraction is ATP used?
Cross bridge cycling
To pump Ca2+ back into the sarcoplasmic reticulum
What are the three ways muscles synthesise ATP?
1) Oxidative phosphorylation
2) Creatine phosphate
3) Glycolysis
How is creatine phosphate synthesised in muscle cells?
When the muscle is relaxed, more ATP is produced than is needed for metabolism, so the excess ATP is used to synthesise creatine phosphate from creatine.
The enzyme creatine kinase catalyses the transfer of a high energy phosphate group from ATP to creatine to form the energy rich molecule creatine phosphate.
How do muscle cells use creatine phosphate?
When muscle contraction begins, the level of ADP in the sarcoplasm rises. Creatine kinase catalyses the transfer of a phosphate group from creatine phosphate back to ADP to form ATP.
Why is creatine phosphate the first source of energy used when muscle contraction begins?
It involves direct phosphorylation, so the production of ATP from creatine phosphate is very rapid. But creatine phosphate only supplies enough energy for about 10 seconds.
Of the three ways muscle fibres produce energy, which is the fastest and which is the slowest?
Creatine phosphate is the fastest.
Anaerobic respiration is the next fastest.
Aerobic respiration is the slowest.
Of the three ways muscle fibres produce energy, which can sustain contraction for longest?
Aerobic respiration can sustain the contraction for hours.
Anaerobic respiration supplies enough ATP for 1 minute of contraction.
Creatine phosphate supplies enough ATP for 10 seconds of contraction.
What is myopathy?
A muscle disease.
What is myostitis?
Inflammatory muscle disease.
What is muscular dystrophy?
Inherited progressive weakening of the muscles.
(contraction force not transmitted to extracellular matrix due to abnormal dystrophin-glycoprotein complex (which normally binds actin to the sarcolemma)
What is myasthenia gravis?
Autoantibodies attacking nicotinic Ach receptors on the motor end plate causing weakness on exercise.
What is myotonia?
The inability to relax muscle after voluntary stimulation.
What are channelopathies?
Abnormal Na+, K+, Cl- channels.
