Muscle Tissue- skeletal, cardiac, and smooth muscleSkeletal Muscle- attached to skeletal system, allow movement.Six Functions of Skeletal MusclesProduce skeletal movement- voluntaryMaintain posture and body positionSupport soft tissuesGuard entrances and exitsMaintain body temperatureStore nutrient reservesOrganization Includes-Muscle tissue- a muscle cells is also known as a muscle fiberConnective Tissue- three layersEpimysium- exterior collagen layer, connected to deep fascia, separates muscle from surroundingsPerimysium- surrounds bundles of muscle fibers (fascicles) provide blood vessels & nerves fascicleEndomysium- surrounds individual muscle fibers. Contain capillaries and nerve fibers contacting muscle cells. Contain myosatellite cells (stem cells) that repair damage (minimal repair ability)Muscle Attachments- endomysium, perimysium, and epimysium come together at ends of muscles to form connective tissue attachment to bone matrixTendon- bundleAponeurosis- sheetNerves 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 thatSupply large amounts of oxygen, supply nutrients, & carry away wastesSkeletal muscle cells develop through fusion of mesodermal cells (myoblasts). They contain hundreds of nuclei, become very large, and are very longCharacteristics of Skeletal Muscle FibersThe Sarcolemma and Transverse TubulesThe Sarcolemma- the cell membrane of a muscle fiber.Surrounds sarcoplasm (cytoplasm of muscle fiber)A change is transmembrane potential begins contractionsTransverse Tubules (T Tubules)- invagination of the sarcolemmaHave same properties as sarcolemmaTransmit action potential through cellAllow entire muscle fiber to contract simultaneouslyMyofibrils- lengthwise subdivisions within muscle fiberMade up of bundles of protein filaments (myofilaments)Myofilaments- responsible for muscle contractionThin filaments- made of the protein actinThick filaments- made of the protein myosinSarcoplasmic Reticulum (SR)- helps transmit action potential to myofibrilSimilar in structure to smooth ERA membranous structure surrounding each myofibrilForms chambers (terminal cisternae) attached to T Tubules.Triad- is formed by on T Tubule and two terminal cisternaeCisternae functions to concentrate Ca2+ (via ion pumps)- Ca2+ pumps use ATP to actively pump Ca2+ into the SR from the SarcoplasmRelease Ca2+ into sarcomeres when stimulated by an action potential to begin muscle contractionSarcomeres- The contractile unit of muscleStructural units of myofibrilsForm visible patters within myofibrilsA striped or striated pattern within myofibrilsAlternating dark, thick filaments (A bands) and light, thin filaments (I bands)StructuresA bandM Line- the center of the A Band, at midline of sarcomereH Band- the area around the M line, has thick filaments but no thin filamentsZone of overlap- the densest, darkest area on a light micrograph, where thick and thin filaments overlapI BandZ lines- the centers of the I bands, at two ends of sarcomereTitin- strands of protein. Reach from tips of thick filaments to the Z line. Stabilize the filamentsThin Filaments-F-actin (filamentous actin)- two twisted rows of globular G-actinActive sites on G-actin bind to myosinTropomyosin- double strand regulatory proteinPrevents actin-myosine interactionTroponin- a globular regulatory proteinBinds tropomyosin to G-actinControlled by Ca2+- binding of Ca2+ causes change in troponin.Initiating ContractionCa2+ binds to receptor on troponin moleculeTroponin- tropymyosin complex changesTropomyosin strand shifts in position so it no long covers the active site of the F-actin strandExposes active site of actinThick Filaments- contan about 300 twisted myosin subunits. Contain titin strands that recoil after stretchingThe mysin moleculeHead- made of two globular protein subunits, reaches the nearest thin filamentTail- binds to other myosin subunitsMuscle ContractionMysoin Action- during contraction, myosin heads interact with actin filaments, forming cross-bridges and pivot (produces motion)Sliding Filament Theory-Thin filaments of sarcomere slide toward M line, alongside thick filamentsThe width of A zone stays the sameZ lines move closer togetherSteps in muscle contractionNeural stimulation of sarcolemmaCauses excitation-contraction couplingMuscle fiber contraction involvesInteraction of thick and thin filamentsProduces muscle tensionThe Neuromuscular Junction (NMJ)Special intercellular connection between the nervous system and skeletal muscle fiberAction potential reaches the axon terminal of motor neuron causes release of ACh into synaptic cleftACh binds chemically-gated Na+ channels on muscle, opening themAction potentials generated in muscle fiber along inner surface of the sarcolemmaSkeletal Muscle Innervation1. 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 ACh2. 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 axon3. 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 sarcoplasm5. 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 sitesi. AChE breakdown of ACh prevents overstimulation of muscle by motor neuron.Excitation—Concentration CouplingAction potential reaches a triadReaching Ca2+ from the terminal cisternae of the SRTriggering contraction if:The
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