The Muscular System Microscopic Anatomy Lesson 20 Muscle Tissue skeletal cardiac and smooth muscle Skeletal Muscle attached to skeletal system allow movement Six Functions of Skeletal Muscles o Produce skeletal movement voluntary o Maintain posture and body position o Support soft tissues o Guard entrances and exits o Maintain body temperature o Store nutrient reserves Organization Includes o Muscle tissue a muscle cells is also known as a muscle fiber o Connective Tissue three layers Epimysium exterior collagen layer connected to deep fascia separates muscle from surroundings Perimysium surrounds bundles of muscle fibers fascicles provide blood vessels nerves fascicle Endomysium surrounds individual muscle fibers Contain capillaries and nerve fibers contacting muscle cells Contain myosatellite cells stem cells that repair damage minimal repair ability o Muscle Attachments endomysium perimysium and epimysium come together at ends of muscles to form connective tissue attachment to bone matrix Tendon bundle Aponeurosis sheet o Nerves and Blood Vessels skeletal muscles are voluntary muscles controlled by nerves of the central nervous system Brain and Spinal Cord Muscles have extensive vascular system that Supply large amounts of oxygen supply nutrients carry away wastes Skeletal muscle cells develop through fusion of mesodermal cells myoblasts They contain hundreds of nuclei become very large and are very long Characteristics of Skeletal Muscle Fibers o The Sarcolemma and Transverse Tubules The Sarcolemma the cell membrane of a muscle fiber Surrounds sarcoplasm cytoplasm of muscle fiber A change is transmembrane potential begins Transverse Tubules T Tubules invagination of the contractions sarcolemma Have same properties as sarcolemma Transmit action potential through cell Allow entire muscle fiber to contract simultaneously o Myofibrils lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments myofilaments Myofilaments responsible for muscle contraction Thin filaments made of the protein actin Thick filaments made of the protein myosin o Sarcoplasmic Reticulum SR helps transmit action potential to myofibril Similar in structure to smooth ER A membranous structure surrounding each myofibril Forms chambers terminal cisternae attached to T Triad is formed by on T Tubule and two terminal Tubules cisternae Cisternae functions to concentrate Ca2 via ion pumps Ca2 pumps use ATP to actively pump Ca2 into the SR from the Sarcoplasm Release Ca2 into sarcomeres when stimulated by an action potential to begin muscle contraction o Sarcomeres The contractile unit of muscle Structural units of myofibrils Form visible patters within myofibrils A striped or striated pattern within myofibrils Alternating dark thick filaments A bands and light thin filaments I bands Structures A band M Line the center of the A Band at midline of sarcomere H Band the area around the M line has thick filaments but no thin filaments Zone of overlap the densest darkest area on a light micrograph where thick and thin filaments overlap I Band Z lines the centers of the I bands at two ends of sarcomere Titin strands of protein Reach from tips of thick filaments to the Z line Stabilize the filaments Thin Filaments F actin filamentous actin two twisted rows of globular G actin o Active sites on G actin bind to myosin Tropomyosin double strand regulatory protein o Prevents actin myosine interaction Troponin a globular regulatory protein o Binds tropomyosin to G actin o Controlled by Ca2 binding of Ca2 causes change in troponin Initiating Contraction Ca2 binds to receptor on troponin molecule Troponin tropymyosin complex changes o Tropomyosin strand shifts in position so it no long covers the active site of the F actin strand Exposes active site of actin Thick Filaments contan about 300 twisted myosin subunits Contain titin strands that recoil after stretching The mysin molecule o Head made of two globular protein subunits reaches the nearest thin filament o Tail binds to other myosin subunits Muscle Contraction o Mysoin Action during contraction myosin heads interact with actin filaments forming cross bridges and pivot produces motion o Sliding Filament Theory Thin filaments of sarcomere slide toward M line alongside thick filaments The width of A zone stays the same Z lines move closer together o Steps in muscle contraction Neural stimulation of sarcolemma Causes excitation contraction coupling Muscle fiber contraction involves Interaction of thick and thin filaments Produces muscle tension Muscle Contraction Lesson 21 10 12 2011 21 24 00 The Neuromuscular Junction NMJ Special intercellular connection between the nervous system and skeletal muscle fiber o Action potential reaches the axon terminal of motor neuron causes release of ACh into synaptic cleft o ACh binds chemically gated Na channels on muscle opening them o Action potentials generated in muscle fiber along inner surface of the sarcolemma Skeletal Muscle Innervation 1 The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine is the neurotransmitter a chemical released by a neuron to change the permeability or other properties of other cell s plasma membrane The synaptic cleft and the moto end plate contain molecules of the enzyme acetylcholinesterase AChE which breaks down ACh 2 Stimulates for ACh release is the arrival of an electrical impulse or action potential at the synaptic terminal An action potential is a sudden change in the transmembrane potential that travels along the length of the axon 3 When the action potential reaches the neuron s synaptic terminal permeability changes in the membrane trigger the exocytosis of ACh into the synaptic cleft Exocytosis occurs as vesicles fuse with the neuron s plasma membrane 4 ACh molecules diffuse across the synaptic cleft and bind to ACh receptors on the surface of the motor end plate ACh binding alters the membrane s permeability to sodium ions Because the extracellular fluid contains a high concentration of sodium ions and sodium ion concentration inside the cell is very low sodium ions rush into the sarcoplasm 5 The sudden inrush of sodium ions results in the generation of an action potential in the sarcolemma AChE quickly breaks down the ACh on the motor end plate andin the synaptic cleft thus inactivating the ACh receptor sites i AChE breakdown of ACh prevents overstimulation of muscle by motor neuron Excitation Concentration Coupling Action
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