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Skeletal Muscle Tissue Contraction Chapter 10 Anatomy and Physiology Mr. Knowles Liberty Senior High School A Brief Review of Skeletal Muscle Tissue Connective Tissue Structures • Epimysium Entire muscle from separates the… other tissue. • Perimysium divides Bundles of muscle the muscle into… fibers- fascicles. • Endomysium Individual skeletal surrounds the… muscle fibers. How does a skeletal muscle cell differ from most other eukaryotic cells? A Comparison Most Cells Skeletal Muscle • Small; < 100 μm long • Large; 12 inches long • One Nucleus • Multiple Nuclei • Normal metabolism- • High metabolism- needs normal enzymes; needs more enzymes; one copy of genes. more genes. • 100’s of mitochondria • 1,000’s of mitochond. • Endoplasmic Reticulum • Sarcoplasmic • No myofibrils Reticulum • Myofibrils Sarcolemma Sarcoplasm Nucleus Special Terms for Muscle Fibers • Sarcoplasm- cytoplasm of a muscle fiber. • Sarcolemma- unique cell membrane of a muscle fiber. • The sarcolemma forms tubes that travel into sarcoplasm at right angles- Transverse Tubules (T tubules). • Action Potentials (unequal charges) travel down these T tubules. Inside a Muscle Fiber… • Each T tubule encircles cylindrical structures- myofibrils. • Myofibrils- 1-2 μm in diameter and as long as entire cell. 100’s – 1,000’s of myofibrils/cell. • Myofibrils – are bundles of myofilaments – 2 kinds of protein filaments called actin (thin filaments) and myosin (thick filaments). Inside the muscle fiber… • Myofibrils can shorten and contract the muscle fiber. • Myofibrils are attached to the sarcolemma on its inner surface. • Collagen fibers are attached to the sarcolemma on its outer surface. These fibers extend into the tendon. • Myofibrils pull on sarcolemma pulls on tendon muscle contraction. Muscle Fiber Sarcolemma Tendon Myofibrils Collagen Intracellular Extracellular Triad Terminal Cisternae T Tubule Sarcoplasmic Reticulum • Pumps Ca+2 out of the sarcoplasm and stores it. • The resting cell has very little Ca+2 in the sarcoplasm. 1000X more Ca+2 in the SR than in the sarcoplasm. • The SR is made up of terminal cisternae that lie on above the junction of thin and thick filaments in the sarcomeres. The Organization of the Myofibril • Myofilaments are organized into repeating functional units- Sarcomere- smallest functional unit of the fiber. • Sarcomere = myosin + actin + other stabilizing proteins. Fig. 10-3. P. 282 Inside the Sarcomere… • Have two regions or Bands: 1. A Bands- are dArk bands and have three parts. a. M line- a protein that connects neighboring thick filaments, keeps their position. b. H zone- a region with only thick filaments. c. Zone of Overlap- thin and thick filaments overlap. Inside the Sarcomere… • Have two Bands: 2. I Bands- are lIght bands; only find thin filaments. a. Z lines- mark the boundaries between the adjacent sarcomeres on the myofibril; have connectin – a connecting protein that interconnects thin filaments. b. Titin- a protein that aligns thick and thin filaments; resist extreme stretching. Z Line A Band I Band Z lines Thin Filaments (Actin) Contains three proteins: • F actin- a twisted strand of 300-400 globular (G) actin molecules. – Each G actin has an active site- a region where thick filaments can bind. • Tropomyosin are strands of protein that wrap around F actin. They cover the active site. This prevents actin- myosin interaction. Thin Filaments (Actin) • Troponin- three globular subunits (parts). – One subunit binds to tropomyosin and locks the two together. – Another subunit binds to a G actin molecule, holding troponin and tropomyosin to the actin. – The third subunit binds to a Ca+2 ion. Resting cells have low Ca+2 and so this subunit is empty in the resting cell. Thick Filaments (Myosin) • Made of 500 myosin molecules. • Each myosin – two subunits twisted around each other. • Myosin has a long, attached tail bound to other myosin molecules in the thick filament. Thick Filaments (Myosin) • The free head projects outward, toward the nearest thin filament. Head can bind to the active site on the G actin. • Between the head and the tail there is a flexible hinge that lets the head swing back and forth. • All myosin molecules are arranged with their tails pointing toward the M line. Supporting the Myofibrils • Desmin- is a complex network of protein that twists around each Z line and connects adjacent myofibrils. • Vertical bands of desmin can be seen with a light microscope and give the fiber a banded appearance-striated muscle. The Sliding Filament Theory An explanation of how skeletal muscle cells contract.
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