Describe the mechanism of muscle contraction?

Question

Describe the mechanism of muscle contraction?

Answer 1

The person before me gave you the correct answer. However, I will try to summarize to make it easier.

You nerves send a signal for muscle contraction to occur. This causes a release of calcium from your SR (sarcoplasmic reticulum, your muscles specialized ER). The calcium release will cause contraction by binding myosin to actin (crossbridge interacts by removal of tropomyosin troponin complex sites from actin by addition of calicum allowing the binding of myosin to actin) giving powerstroke AKA muscle contraction. To release the contraction, ATP (our energy source) is needed. To use the ATP we must break of one of the P to release the energy, leaving ADP and Pi.

ATP has three phosphates    ADP has 2 phosphates

Pi means inorganic phosphate

1. nerve signal

2. calcium released

3.myosin binds actin

4. Powerstroke occurs

5. ATP used to release contraction

6. Crossbridge waiting for calcium to be released so it can attach to actin.

*Myosin and actin are your filaments. Your thick filament is myosin, actin your thin.

I hope this helped. I try to simplify to students so they actually understand what is going on. If you understand the basics, than you can add more details.

Answer 2

Skeletal muscle?  Cardiac or smooth?

Anyway, I would assume it is skeletal:

(copy paste from wiki: http://en.wikipedia.org/wiki/Muscle_cont...

Skeletal muscles contract according to the sliding filament model:

   1. An action potential originating in the CNS reaches an alpha motor neuron, which then transmits an action potential down its own axon.

   2. The action potential activates voltage-dependent calcium channels on the axon, and calcium rushes in.

   3. Calcium causes vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine into the synaptic cleft between the motor neuron terminal and the motor end plate of the skeletal muscle fiber.

   4. The acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptor on the motor end plate. Activation of the nicotinic receptor opens its intrinsic sodium/potassium channel, causing sodium to rush in and potassium to trickle out. Because the channel is more permeable to sodium, the muscle fiber membrane becomes more positively charged, triggering an action potential.

   5. The action potential spreads through the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber.

   6. The depolarization activates L-type voltage-dependent calcium channels (dihydropyridine receptors) in the T tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

   7. Activated voltage-gated calcium channels physically interact with calcium-release channels to activate them, causing the sarcoplasmic reticulum to release calcium.

   8. The calcium binds to the troponin C present on the actin-containing thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin sterically obstructs binding sites for myosin on the thin filament; once calcium binds to the troponin C and causes an allosteric change in the troponin protein, troponin T allows tropomyosin to move, unblocking the binding sites.

   9. Myosin (which has ADP and inorganic phosphate bound to its nucleotide binding pocket and is in a ready state) binds to the newly uncovered binding sites on the thin filament (binding to the thin filament is very tightly coupled to the release of inorganic phosphate). Myosin is now bound to actin in the strong binding state. The release of ADP and inorganic phosphate are tightly coupled to the power stroke (actin acts as a cofactor in the release of inorganic phosphate, expediting the release). This will pull the Z-bands towards each other, thus shortening the sarcomere and the I-band.

  10. ATP binds myosin, allowing it to release actin and be in the weak binding state (a lack of ATP makes this step impossible, resulting in the rigor state characteristic of rigor mortis). The myosin then hydrolyzes the ATP and uses the energy to move into the "****** back" conformation. In general, evidence (predicted and in vivo) indicates that each skeletal muscle myosin head moves 10-12 nm each power stroke, however there is also evidence (in vitro) of variations (smaller and larger) that appear specific to the myosin isoform.

  11. Steps 9 and 10 repeat as long as ATP is available and calcium is present on thin filament.

  12. While the above steps are occurring, calcium is actively pumped back into the sarcoplasmic reticulum. When calcium is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the contractions cease.

The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sarcoplasm. The active pumping of calcium ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of calcium ions from the troponin. Thus the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases.