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1.Functions of the muscular system: -movement, -posture & body position -body temperature -protection. 2. The structure of skeletal muscle: a. Muscle cells are called muscle fibers. they are long, tubular cells. b. Each fiber is surrounded by connective tissue. c. The fibers are striated and voluntary. d. A fascicle is a group of muscle fibers. Each fascicle is surrounded by connective tissue. The fascicles in a given area comprise the muscle and are surrounded by connective tissue. This connective tissue = fascia. Fascicle Tendons (connect muscle to bone) are formed from fascia that continues past the muscle. 3. The structure of the muscle fiber: a. Sarcolemma – cell membrane around each muscle fiber (or myofiber) b. Myofilaments – actin and myosin that make up the myofibril. Actin (thin) and myosin (thick) are made of protein. The actin and myosin overlap slightly to give the muscle fiber the striped (striated) appearance -They are organized in repeating units called sarcomeres. c. T Tubules (transverse tubules) – extend from the cell membrane into the interior of the muscle fiber. d. Sarcoplasmic reticulum – store calcium ions which are necessary for contraction. myosin actin 4. The Neuromuscular Junction a. Skeletal muscles are controlled by motor neurons. Where they connect is called a synapse or a neuromuscular junction. b. Presynaptic terminal – located at the end of the neuron’s axon. c. Postsynaptic terminal – synaptic area of the muscle fiber membrane. d. Synaptic cleft – the space between the pre and postsynaptic terminals. e. Neurotransmitter–chemical (acetylcholine) located in the synaptic bulb. http://www.blackwellscience.com/matthews/nmj.html 5. So how exactly does a muscle fiber contract? a. A nerve impulse (action potential) travels down the axon to the presynaptic terminal. Ca++ influx causes vesicles (filled with Ach) to fuse with the membrane of the presynaptic bulb. b. The neurotransmitter (Ach) is released from the presynaptic terminal (bulb) and travels across the cleft. c. The neurotransmitter binds with receptors on the postsynaptic terminal (motor end plate or post synaptic terminal) of the muscle fiber. d. The binding of the neurotransmitter and the receptors triggers a nerve impulse to travel across the muscle fiber membrane (sarcolemma). Synaptic terminal of motor neuron Synaptic cleft PLASMA MEMBRANE T TUBULE SR ACh Ca2+ CYTOSOL Ca2+ e. This nerve impulse (action potential) travels along the cell membrane (sarcolemma) and down through the T tubules to the interior of the cell. f. Calcium is released from the sarcoplasmic reticulum in response to the nerve impulse. g. The calcium acts on one of the two regulatory proteins: -troponin= attached to actin. When it binds with calcium it causes a conformational change to tropomyosin. -tropomyosin = filament that covers the myosin binding site on the actin. Tropomyosin Ca2+-binding sites Actin Troponin complex Myosin-binding sites blocked. Ca2+ Myosinbinding site Myosin-binding sites exposed. h. The change in the shape of the tropomysin causes it to move and allows a cross-bridge to form between the actin and the myosin.. i. So, how does a cross-bridge form? ATP bound to myosin is hydrolized = now high energy (ADP + Pi complex). and rapidly binds to myosin head. Then the Pi is released causing a conformational change in the myosin. When the myosin changes shape, it pulls on the actin it is attached to. This is called a power stroke. j. Lastly, the ADP is released. A new ATP binds to the myosin head allowing it to release from actin. This ATP is hydrolized and the whole cycle can begin again. ATPase Cycle: Website #1 http://www.nismat.org/physcor/muscle.html Animation #1 http://www.blackwellscience.com/matthews/myosin.html Animation #2 http://www.sci.sdsu.edu/movies/actin_myosin.html 6. A contraction ends when there is no more stimulus (no AP). Ca++ pumps pump out the calcium. This causes the tropomyosin to move over the myosin binding site breaking the cross bridges. Synaptic terminal of motor neuron Synaptic cleft PLASMA MEMBRANE T TUBULE SR ACh Ca2+ CYTOSOL Ca2+ 7. If the stimulus is large enough, the muscle fiber contracts fully. All or nothing principal 8. Motor Unit – Motor neuron and all of the muscle fibers controlled by it. Most control hundreds of muscle fibers. All or nothing contraction. Motor Unit 9. Whole muscles respond in a graded fashion. Stronger stimulus = stronger contraction. When contractions of greater strength are needed, larger and larger motor units are activated = recruitment. 10. Types of Muscle Fibers a. Slow Twitch – good for endurance, have many capillaries and many mitochondria. Distance runners b. Fast Twitch – higher glucogen content and faster ATP production. Sprint runners. “Color” Contraction Metabolism Blood Supply Fiber Diameter Myosin ATPase Mitochondria Myoglobin Performance Fast Twitch, Glycolytic (white muscles) White Fast Glycolytic / anaerobic Low Large High Few Low Rapid, powerful contractions (jumping) Slow Twitch,Oxidative (red muscles) Red Slow Oxidative / Aerobic High Small Low Many High Prolonged, continued muscle activity (marathon races)