text: This may be more than you wanted, but here goes:\n\nSkeletal muscles contract according to the sliding-filament model:\n\nAn action potential originating in the CNS, reaches the axon of the motor neuron. \nThe action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in. \nThe calcium causes the neurotransmitter, acetylcholine, within vesicles in the axon to fuse with the plasma membrane, releasing the acetylcholine into the synapse between the axon and the motor end plate specialization in the sarcolemma of the muscle. \nThe acetylcholine diffuses across the synapse and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and potassium. Sodium rushes in, while potassium trickles out through the sodium-potassium (Na/K) pump located in the sarcolemma. However, because sodium is more permeable, the muscle fiber membrane becomes more positively charged, triggering an action potential. \nThe action potential spreads through the muscle fibre's network of T tubules, depolarizing the inner portion of the muscle fibre. \nThe depolarization activates voltage-gated calcium channels in the T tubule membrane, which are in close proximity to calcium-release channels in the adjacent sarcoplasmic reticulum. \nActivated voltage-gated calcium channels physically interact with calcium-release channels to activate them, causing the sarcoplasmic reticulum to release calcium. \nThe calcium binds to the troponin C present on the 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. \nMyosin (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. Myosin is now bound to actin in the strong binding state. The release of ADP and inorganic phosphate causes the myosin head to turn, causing a ratchet movement (Actin acts as a cofactor in the release of inorganic phosphate, expediting the release). This will pull the Z-bands towards each other. It also shortens the sarcomere and the I-band. \nATP binds 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 energy to move into the "cocked back" state while releasing ADP and inorganic phosphate. In general, evidence indicates (predicted and in vivo evidence) 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. \nSteps 7 and 8 repeat as long as ATP is available and calcium is present on thin filament. \nWhile 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. \nThe 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.\n\nThere are two types of voluntary muscular contractions, eccentric and concentric. In the case of eccentric contraction, the force generated is insufficient to overcome the resistance placed on the muscle and