Muscle contraction

A top-down view of skeletal muscle

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 muscles at 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 system to the motor neuron that 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. Reflexes are fast, unconscious muscular reactions that occur due to unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the brain.

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

Skeletal muscle contractions

Steps of a skeletal muscle contraction:

1. An action potential reaches the axon of the motor neuron.
2. The action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in.
3. The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber.
4. The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and 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 on the muscle fiber causes the sarcoplasmic reticulum to release calcium.
6. The calcium binds to the troponin present on the thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically obstructs binding sites for cross-bridge; once calcium binds to the troponin, the troponin forces the tropomyosin move out of the way, unblocking the binding sites.
7. The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding sites. It then delivers a power stroke.
8. ATP binds the cross-bridge, forcing it to change conformation in such a way as to break the actin-myosin bond. Another ATP is split to energize the cross bridge again.
9. Steps 7 and 8 repeat as long as calcium is present on thin filament.
10. All the time, the calcium is actively pumped back into the sarcoplasmic reticulum. When 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 cross-bridge ceases binding to the thin filament, and the contractions cease.

Smooth muscle contraction

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

The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sacoplasm. As the calcium ions are being actively pumped by the calcium pumps present in the membrane of the sarcoplasmic reticulum creating 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.