5-muscle physiology - Lecture: Muscle Physiology I. Anatomy...

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Lecture: Muscle Physiology I. Anatomy of Skeletal Muscle CELL (Muscle Fiber) A.General Features 1. multinucleated cells (syncytium: from fusion) 2. sarcolemma - special name for plasma membrane 3. very long compared to other cells (1 - 300 mm) 4. not unusually wide diameter (10 - 100 microns) 5. sarcoplasm - rich in glycogen and myoglobin 6. myoglobin - stores oxygen; similar to hemoglobin 7. special structures: myofibrils and sarcoplasmic reticulum B. Ultrastructure of Myofibrils ¼ 1. muscle cell contains many parallel myofibrils 2. myofibrils have DARK bands (A bands) and LIGHT bands (I bands) that cause "striated" appearance of muscle 3. A band and I band result from the arrangement of overlapping and non- overlapping regions of two types of myofilaments a. thick filaments (myosin) b. thin filaments (actin) 4. sarcomere - smallest contractile unit of muscle cell a. Z-line - connection of actin filaments; dividing line between two adjacent sarcomeres b. M-line - connection of myosin filaments c. H-zone - non-overlapping region of the myosin filaments around the M-line d. A-band - length of myosin filaments e. I-band - length of non-overlapping actin filaments Each muscle cell (fiber) is composed of many myofibrils . Each myofibril contains hundred of accordion-like sarcomeres laid end-to-end. Muscle contraction occurs when the sarcomeres contract by the sliding motion of actin and myosin filaments. 1. thick filaments (myosin filaments) 12-16 nm a. composed of about 200 myosin proteins i. myosin has a golf club like shape ii.2 heads (cross bridges) - can bind to the actin filaments and use ATP iii.tail - shaft of the thick filament
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2. thin filaments (actin filaments) 5-7 nm a. 2 helical chains of F actin (G actin subunits) I. G actin can bind with myosin heads ii.tropomyosin - rod-like protein that helps to stiffen F actin structure iii.troponin - globular protein that can bind Ca ++ to regulate actin/myosin binding D. Sarcoplasmic Reticulum and T Tubules 1. sarcoplasmic reticulum - smooth ER that houses Ca ++ a. surrounds each myofibril b. fused to each other at H zones and A/I bands c. terminal cisternae - around A/I bands 2. T (transverse) Tubules - passageways from extracellular space to the terminal cisternae of SR a. passage of nerve message directly to SR b. passage of glucose, oxygen, salts to fiber II. Contraction of Skeletal Muscle Cell A. Sliding Filament Model (Actin/Myosin Sliding Mechanism) 1. Ca ++ released from sarcoplasmic reticulum 2. Ca ++ binds to TnC region of Troponin 3. Troponin changes shape, moving Tropomyosin, exposing binding site on actin filament 4. Attachment - myosin head with ADP + P i binds actin 5. Power Stroke - myosin head bends, pulling along the actin filament, ADP + P i are released 6. Detachment - ATP binds to the myosin head, causing detachment from Actin 7. Re-cocking the Head - hydrolysis of ATP ADP + P releases energy to re-cock the myosin 8. some myosin heads are in contact with actin at all times, allowing "walking
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This note was uploaded on 10/19/2011 for the course PCB 3703C taught by Professor Asd during the Spring '09 term at University of Central Florida.

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5-muscle physiology - Lecture: Muscle Physiology I. Anatomy...

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