1. How do muscle cells contract ?

2. What is the structure of a muscle fiber ? The sarcolemma, or plasma membrane contains invaginations called T (transverse) tubules. The sarcoplasm, or cytoplasm of the muscle cell contains calcium- storing sarcoplasmic reticulum. Muscle cells are multinucleate. Myofibrils are protein fibers. Two types of filaments are found here: - Thin filaments consist of two strands of the globular protein actin arranged in a double helix. Troponin and tropomysin cover special binding sites on the actin. - Thick filaments are myosin. Each has a protruding head at one end.

3. What is the sliding-filament model of muscle contraction ? 1) ATP binds to a myosin head and forms ADP + P. 2) Ca +2 exposes the binding sites on the actin filaments. 3) Cross bridges between myosin heads and actin filaments form. 4) ADP and P are released and sliding motion of actin results. 5) ATP causes the cross bridges to unbind. Without the addition of ATP, the cross bridges remain attached and no new muscle movement results.

4. How are neurons involved with muscle contraction ? 1) Action potential generates release of acetylcholine at the neuromuscular junction. 2) Action potential generated on the plasma membrane and along the T tubules. 3) Sarcoplasmic reticulum releases Ca +2 ions. 4) Myosin cross bridges form as Ca +2 ions attach to troponin and troponin pulls the tropomysin aside, exposing myosin head binding sites. 5) If ATP is available, muscle contraction begins.

5. What are the three kinds of muscles ? Skeletal muscle – striated, attached to bones, voluntary movements. Smooth muscle – lines the wall of blood vessels and the digestive tract. There are no T tubules in smooth muscle so contraction is slow and methodical. Cardiac muscle – rhythmic contractions of the heart. Cells are striated, branched and connected by gap junctions. They generated their own action potential.