Muscle contraction

Image:Skeletal muscle.jpg

A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) shortens. Locomotion is possible only through the repeated contraction of many musclesat the correct times.

For most muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous systemto the motor neuronthat innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to originate from the brain. Some reflexesare fast, unconscious muscular reactions that occur due to unexpected physical stimuli. Other reflexes such as locomotion, breathing, chewing have a reflex aspect to them; the brain will start the contractions, but continuation of the movements can become reflexive. The action potentials for reflexes to unexpected stimuli originate in the spinal cordinstead of the brain.

There are three general types of muscle contractions: skeletal musclecontractions, heart musclecontractions, and smooth musclecontractions.

Skeletal muscle contractions

Skeletal muscles contract according to the sliding-filament model:

1. An action potential reaches the axonof the motor neuron.
2. The action potential activates voltagegated calciumion channelson the axon, and calcium rushes in.
3. The calcium causes acetylcholinevesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plateof the muscle fiber.
4. The acetylcholine diffuses across the cleft and binds to nicotinic receptorson the motor end plate, opening channels in the membrane for sodiumand potassium. Sodium rushes in, and potassium rushes out. However, because sodium is more permeable, 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, depolarizingthe inner portion of the muscle fiber.
6. The depolarization activates voltage-gated calcium channelsin the T tubule membrane, which are in close proximity to calcium-release channelsin 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 Cpresent on the thin filamentsof the myofibrils. The troponin then allosterically modulatesthe 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 is bound to ADPand is in a ready state) binds to the newly uncovered binding sites on the thin filament. It then releases ADP and an inorganic phosphate and delivers a power stroke. Myosin is now bound to actin in the strong binding state.
10. ATPbinds myosin, allowing it to release actin and be in the weak binding state. (A lack of ATP makes this step impossible, resulting in rigor mortis.) The myosin then hydrolyzes the ATP and uses the enery to move into the "cocked back" state while releasing ADP and inorganic phosphate.
11. Steps 7 and 8 repeat as long as ATP is available and calcium is present on thin filament.
12. All the while, the calcium is actively pumpedback 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 fiaments and contraction ceases.

Smooth muscle contraction

1. Contractions are initiated by an influx of calciumwhich binds to calmodulin.
2. The calcium-calmodulin complex binds to and activates myosin light-chain kinase.
3. Myosin light-chain kinase phosphorylatesmyosinlight-chains, causing them to interact with actinfilaments. This causes contraction.

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Muscle+contraction Wikipedia article Muscle contraction.