Sarcomere Physiology: Sliding Filament Theory This is pretty exciting!

1.The brain wants a certain muscle to contract. 2.A nerve impulse is sent down a nerve to the muscle from the brain in order to tell muscle to contract. 3.Ach (Acetylcholine) is released. 4.This triggers a muscle impulse. Myofibril of muscle fiber Nerve Acetylcholine

So What is the Sliding Filament Theory: Shortening of sarcomeres. When this happens, the thick and thin filaments DO NOT change length. Rather, they SLIDE past one another. Z lines move closer together, shortening the sarcomere. When this happens over and over again to a bunch of sarcomeres, the muscle eventually shortens.

Sarcoplasmic Reticulum: The sarcoplasmic reticulum has a high concentration of calcium ions. In response to a muscle impulse, the cisterna of the sarcoplasmic reticulum becomes permeable to calcium ions. Calcium diffuses OUT of cisterna and INTO the muscle fiber. Cisternae

When a muscle is at rest, the troponin-tropomyosin complexes block the binding sites on the actin molecules, preventing myosin to attach to actin. But the calcium that has entered the muscle cell binds to the troponin, which triggers the change of shape of the tropomyosin. –This exposes the binding sites of the actin. Now actin and myosin can bind, which in turn shortens a muscle.

Shortening of Sarcomere: The cross-bridge head then PULLS on the actin filament pulling the Z lines closer to one another. Then the head releases, straightens, and attaches to another actin binding site further down the thin filament and pulls again. This process happens over and over again, shortening the sarcomere.

ATP ATP = Energy!!! You use ATP whenever you do any muscle contractions. This breakdown of ATP to ADP releases energy into the body. This energy provides the force needed for a muscle contraction.

Role of ATP: The cross-bridge pulls on the actin filament. ATP comes in and attaches to the cross-bridge at the end of its “pull”. The cross-bridge is then released from the actin binding site. ATP is broken down to ADP + P (Phosphate). This releases the energy needed for the cross-bridge to return to the “cocked” position ready to pull on the actin again. So, ATP provides the energy needed for the cross-bridge to go from the final “pull” position back to its “cocked” position. Repeats over and over again.

Muscle Contraction at Zones: When the muscle contracts: –Z lines = come closer together –I band = shrinks –A band = stays the same –H zone = shrinks –M lines = come closer together as the entire muscle shortens

Muscle relaxation Active transport of Ca +2 into sarcoplasmic reticulum, which requires ATP, makes myosin binding sites unavailable. Ca +2 ADP + P Relaxed muscle 1 ATP Ca +2 ADP + P ATP splits, which provides power to “cock” the myosin cross-bridges New ATP binds to myosin, releasing linkages Contraction cycle Exposed binding sites on actin molecules allow the muscle contraction cycle to occur Cross-bridges bind actin to myosin ADP + P Cross-bridges pull thin filament (power stroke), ADP and P released from myosin PPADP Muscle contraction Release of Ca +2 from sarcoplasmic reticulum exposes binding sites on actin: Ca +2 binds to troponin Tropomyosin pulled aside Binding sites on actin exposed Tropomyosin Troponin Thick filament Thin filament

Sliding Filament Theory: Step-By-Step 1.Nerve impulse sent to muscle from brain in order to tell muscle to contract triggers => 2.Ach (Acetylcholine) to be released which triggers => 3.A muscle impulse which triggers => 4.The release of calcium from the sarcoplasmic reticulum. 5.Calcium enters the muscle fibers and attaches to the troponin. This triggers => 6.Tropomyosin to alter its shape, thus revealing its binding sites. 7.Myosin cross-bridges attach to the actin binding sites and PULL on thin (actin) filament. 8.ATP binds to myosin cross-bridge, which triggers release from actin. 9.ATP breaks down to ADP + Phosphate which provides the energy needed for myosin cross-bridge to get back into “cocked” position. 10.This happens over and over again until the muscle is completely shortened or the brain wants the muscle to relax. 11.Muscle relaxation: 1.Ach decomposes by enzyme (Acetylcholinesterase) 2.Calcium goes back to sarcoplasmic reticulum 3.Tropomyosin covers up binding sites

Rigor Mortis: Shortly after death (3-4 hours), Ca 2+ leaks out of sarcoplasmic reticulum. Causes sustained muscle contraction (sliding filament theory continuously happens). After a while, the calcium is degraded (decomposition of the body) and the sustained contraction stops. Lasts 3-4 days. Thus body is really, really stiff!