Muscle Physio Flashcards
Name the major proteins of the sarcomere.
- Myosin: thick filaments with ATPase activity in the myosin head; they dimerize and bind to actin, conduct the power stroke to shorten the sarcomere
- Actin: thin filaments that connect to the Z line; form fibrous helical chains
- Tropomyosin: covers myosin binding sites on actin
- Troponins: C - binds Ca; T - binds tropomyosin; I - inhibits actin by keeping troponin in its place that blocks the myosin binding sites on actin
Describe cross-bridge cycling starting with the entry of Calcium and how it uses ATP.
Ca enters the cytoplasmic space and binds to Trp-C, changing its conformation such that tropomyosin shifts and reveals the myosin binding site on the actin filament.
Myosin head uses its ATPase activity to cleave the ATP that’s bound to myosin; myosin bound to ADP has higher affinity for actin so it binds. The binding makes a shift in conformation which produces the hinge/stroke movement.
Myosin bound to actin forms a cross bridge.
When the stroke has occurred, the myosin now has higher affinity for ATP so the ADP is replaced, affinity for actin is reduced until the ATP is cleaved again.
Describe the return to the non-contracted state.
Ca is ACTIVELY pumped back into the sarcoplasmic reticulum. No more Ca in the cytoplasmic space means that trp-C returns to conformation that allows tropomyosin to cover the myosin binding site on actin.
*When rigor mortis sets in, the muscle stays contracted because there’s no more energy to actively pump away the Ca.” Well actually, as we saw, it is because there is no more ATP to release myosin from actin. This leaves the muscle contracted, hence: rigor mortis.
Describe the length-tension relationship, including the differences between passive vs. active tension.
The longer/more stretched the muscle fiber, the more tension within.
Passive tension increases with fiber length; there is an optimum length at which passive tension reaches approximately zero.
Active tension increases up to an optimum length; beyond this point there is a reduction in active tension because most myosin heads are now bound/contracting and steric hindrance is introduced. Below this length, there are many myosin heads able to bind so the tension can still increase.
What is the Sliding Filament theory?
a theory that explains muscle contraction based on muscle proteins that slide past each other to generate tension in the muscle fiber
Describe the events that couple excitation with contraction.
- AP from motor neuron is transmitted through NMJ via ACh; ACh binds to nicotinic ACh receptor.
- The nAChR is a ligand-gated ion channel which starts to depolarize the muscle cell membrane.
- When the cell membrane depolarizes past a threshold, the voltage-gated Na channels open and the membrane depolarizes rapidly. There are many more nAChRs than voltage-gated Na channels, so it takes a large quanta of released ACh to sufficiently depolarize the muscle.
- Transverse/T-tubules propagate the Na-depolarization. This depolarization activates the Dihydropyridine (DHP) receptor (a voltage-dependent Ca channel in the T-tubules).
- DHPR undergoes a conformation change. Because it is mechanically linked to the ryanodine receptor (RyR) in the SR, it causes allosteric activation of the RyR.
- RyR is a Ca channel which, when activated, releases Ca from SR into cytoplasmic space. Ca activates the sarcomere to contract.
Describe the muscle fiber types based on mechanical properties.
-Large motor units are faster to reach their isometric twitch tension, and their isometric twitch tension is greater.
-Small motor units are slower to reach their isometric twitch tension, and their isometric twitch tension is smaller.
The differences here are based on the size of the motor unit - the larger ones have a greater number of muscle fibers and a greater cross-sectional area of each. Smaller motor units are the opposite and therefore simply cannot produce the same magnitude of force.
Describe the muscle fiber types based on metabolic/fatigue properties.
- Small/Slow motor units have high mitochondrial density and lots of myoglobin. This means that ATP is produced via oxidative phosphorylation, and fatigue is not achieved for a while. Postural muscles.
- Large/fast motor units have lots of glycogen and mainly get energy from glycogenolysis. This means that they can achieve a high level of contraction for only a short period of time and fatigue quickly.
Describe the muscle fiber types based on recruitment properties.
-first to be recruited are the small motor units
-last to be recruited are the large motor units
Therefore, at first the generated force is small because only a few, small motor units are contracting. As the drive increases and more APs arrive, more fibers are depolarized and the larger motor units are recruited, thereby producing a larger contraction force and a greater velocity is achieved.
What is the Size Principle?
states that smaller motor units will be recruited first to fire action potentials, based on the fact that their cross-sectional area is smaller and the fiber is more easily depolarized
What is the force-velocity relationship in a muscle fiber?
- The relationship between the force exerted by the fiber (i.e., weight the muscle fiber must move) and the velocity with which it can move the weight (i.e., contract).
- It’s an inverse relationship: the greater the force, the slower the velocity of contraction.
- When force is so great that velocity is zero, this is the isometric twitch tension
- Theoretically, when force is zero, velocity of contraction is at maximum.
What structural features of a muscle fiber determine the velocity of contraction?
A: the frequency of the power strokes - more frequent can summate to a fast contraction velocity
Note - the frequency of the power strokes is dictated by the myosin head ATPase activity; the greater the activity, the faster ATP turnover and higher frequency of power stroke.
Describe the energy expenditure differences in performing resistance tasks vs. endurance tasks.
- Resistance tasks are those that need large forces for a short period of time; as such they favor larger muscle fiber diameter, more fast myosin expression, and increased glycogenolytic metabolism.
- Endurance tasks favor slow myosin expression and more oxidative metabolism.
How is ATP regenerated in the myofibrils?
by donation of high energy phosphate from creatine phosphate, the byproduct of which is creatinine (which is cleared by the kidney in a near 1:1 ratio with production)
