Everything you didn’t know you wanted to know.

Microscopic Anatomy of Skeletal Muscle Sarcolema: Plalsma membrane of cell (fiber) Myofibril: Organelle composed of bundles of myofilaments & segmented into sarcomered Sarcomere: Single contractile unit (a segment of a myofibril) Light (I) Band- boundary zone of sarcomeres (walls) Dark (A) Band- the interior of sarcomeres (pullers) Z-Line- marks exact boundary of sarcomere H-Zone- center of sarcomere and the space into which the walls can contract

Microscopic Anatomy (continued) Myofilaments: Protein threads that perform the actual contraction. Thick Filaments- myosin- the grabbers Thin Filaments- actin- the hand holds Sarcoplasmic Reticulum- type of endoplasmic reticulum (smooth) that surrounds each myofibril. Stores calcium used to stimulate contraction.

Stimulation of Muscle Cells Skeletal muscle is controlled by voluntary (primarily) and involuntary stimulation by nerves. Here are the important features involved. Motor Unit: a neuron and the muscle cells its axon stimulates. Neuromuscular Junction: axon terminal “joins” sarcolema of a muscle fiber. Synaptic Cleft: gap between axon terminals and muscle cell. Neurotransmitter: a chemical released from axons that stimulates muscle cells (Acetylcholine- ACh is the neurotransmitter for skeletal muscle.

Stimulation of Muscle Cells Release of ACh makes Sarcolema permeable to Na+ and Na+ ions rush in. Action Potential: the electric current generated by the in rush of Na+ ions that provides the activation energy to cause the chemical reactions that lead to contraction. (more on this in Ch. 7)

Sliding Filament Theory The sliding filament theory is the explanation for how muscles produce force (or, usually, shorten). It explains that the thick and thin filaments within the sarcomere slide past one another, shortening the entire length of the sarcomere. In order to slide past one another, the myosin heads will interact with the actin filaments and, using ATP, bend to pull past the actin.

Six Steps of Cross Bridge Cycling 1. The influx of calcium, triggering the exposure of binding sites on actin. 2. The binding of myosin to actin 3. The power stroke of the cross bridge that causes the sliding of the thin filaments 4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin 5. The hydrolysis of ATP, which leads to the re- energizing and repositioning of the cross bridge 6. The transport of calcium ions back into the sarcoplasmic reticulum.

Step 1: A Closer Look The influx of calcium, triggering the exposure of binding sites on actin. This should be obvious… calcium has to expose actin in order for anything to start between actin and myosin

Step 2: A Closer Look The binding of myosin to actin. Actin and myosin must make contact with one another, or no sliding can occur. Important point: At the time myosin grabs onto actin, it is already “energized”. (in its “high energy state”) This means that in order to understand the steps you have to keep in mind that they start with an already “energized” myosin head.

Step 3: A Closer Look The Power Stroke: Since myosin is already energized, once it grabs onto actin it immediately begins to pull. Here you have to keep in mind that there is a strong attraction between actin and myosin. They will remain connected unless pulled apart, even after the power stroke is completed. One more thing: While the myosin head is bending in the power stroke, this conformational change in shap of the myosin head causes the ADP + P that remains attached to the myosin head to fall out. This falling out of the products of ATP hydolysis gives roomf or more ATP to bind to myosin.

Step 4: A Closer Look The binding of ATP The myosin head has already done its work. Now it is in its “low energy” state and needs to use more ATP in order to get ready for another power stroke. The ATP binding site is free, so anytime there is more ATP it will bind to the myosin head This time the conformational change prevents the myosin head from staying attached to actin causing them to separate.

Step 5: A Closer Look The hydrolysis of ATP Once the myosin head has ATP, it has to hydrolyze to get its energy The products of ATP hydrolysis are: ADP + P These products remain in the ATP binding site and myosin is back in its “high energy” state.

Step 6: A Closer Look The transport of Calcium ions back into the Sarcoplasmic Reticulum (SR) Assuming that the signal from the nervous system for contraction has ended, all the calcium will go back into the SR. Each action potential on the muscle fiber sarcolemma is extremely brief, so right after the calcium ions spill out, they have to be sucked back up again. (this requires an active transport mechanism)

Helpful Hints These steps are a part of a continuous cycle. Although we started on Step 1, you could imagine any step as the starting point. When studying this try to pick different starting points and move through the cycle in order from you chosen point.