On to microscopic muscle structure and order, and the theory behind contraction. Oh joy! I remember this from Physiology class--thank GOD because I'd be reeling if this was all totally new to me.
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Myofibrils: elements of skeletal muscle that allows muscles to contract; consist of two primary proteins:
actin and
myosin, which are referred to as
myofilaments. Actin is the thinner protein, myosin the thicker, and under a microscope the arrangement of the myofilaments create light and dark striations. They exist throughout the muscle fiber and thus skeletal muscle is also called striated muscle.
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A Band: term for the darker zone of the striation
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I Band: term for lighter zone
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Z Line or Z Disk: bisects the I band, attaches to the sacrolemma to bring
stability to the ultrastructure
Ok this is getting confusing here for me!
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Sarcomere: Um....the repeating sequence from Z line o Z line that delineates the functional unit of skeletal muscle--the functional and contractile unit of skeletal muscle. Also, the basic functional unit of the myofibril and the specialized order of actin and myosin fliaments
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H Zone: region in the center of the A band, a gap where only myosin is present and within its center is a darker area known as the
M line, the center of the sarcomere. Produced by proteins that link adjacent myosin filaments.
Actin-Myosin Alignment:
Actin is a double helix with two protein constituents,
tropomyosin and
troponin (
I am so glad this book has pictures!!) which help with actin's function in muscle contraction. Tropomyosin is a long, rope-like protein that spirals around the actin double helix, lies in a groove formed by the actin strands, and blocks the binding sites for actin-myosin interaction or coupling. Troponin is a globular molecule that attaches at regular intervals to the tropomyosin. It has a strong affinity to calcium ions and is crucial to muscle action and fatigue. Myosin is a thicker filament with distinct components--a molecule with two globular heads,
S1 units, or cross bridges, that are attached to protein strands (heavy chains). The protein stands intertwine to form long shafts (light chains, or tails). In muscle contraction, the myosin globular heads extend as cross bridges and bind to specific sites on the actin filament, forming the structural and functional link between the two filaments.
OK I'm going to call it a night before I move to the Sliding-Filament Theory. 
