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Skeletal muscle tissue and the Muscular SystemThree types of muscle• Skeletal – attached to bone• Cardiac – found in the heart

• Smooth – lines hollow organs Skeletal muscle functions• Produce skeletal movement• Maintain posture and body position• Support soft tissues• Guard entrances and exits• Maintain body temperature

Anatomy of Skeletal MuscleOrganization of connective tissues• Epimysium surrounds muscle• Perimysium sheathes bundles of muscle fibers

• Epimysium and perimysium contain blood vessels and nerves• Endomysium covers individual muscle fibers• endons or aponeuroses attach muscle to bone or muscle Skeletal muscle fibers• Sarcolemma !cell membrane"• Sarcoplasm !muscle cell cytoplasm"• Sarcoplasmic reticulum !modified E#"• $tubules and myofibrils aid in contraction• Sarcomeres – regular arrangement of myofibrils Myofibrils• hick and thin filaments• %rgani&ed regularly Muscle FiberThin filaments• '$actin• (ebulin• ropomyosin

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• Covers active sites on G$actin• roponin

• )inds to G$actin and holds tropomyosin in place

Thick filaments• )undles of myosin fibers around titan core• Myosin molecules have elongate tail* globular head• +eads form cross$bridges during contraction• ,nteractions between G$actin and myosin prevented by tropomyosin

during rest Sliding filament theory• Explains the relationship between thick and thin filaments as

contraction proceeds• Cyclic process beginning with calcium release from S# • Calcium binds to troponin• rponin moves* moving tropomyosin and exposing actin active site• Myosin head forms cross bridge and bends toward + &one• - P allows release of cross bridge

The Contraction of Skeletal Muscle

Tension• Created when muscles contract• Series of steps that begin with excitation at the neuromuscular

.unction• Calcium release• hick/thin filament interaction• Muscle fiber contraction• ension

Control of skeletal muscle activity occurs at the neuromuscular junction• -ction potential arrives at synaptic terminal• -Ch released into synaptic cleft• -Ch binds to receptors on post$synaptic neuron

• -ction potential in sarcolemma

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!citation"contraction coupling• -ction potential along $tubule causes release of calcium from

cisternae of S# • ,nitiates contraction cycle

• -ttachment• Pivot• 0etachment• #eturn

#ela!ation• -cetylcholinesterase breaks down -Ch• 1imits the duration of contraction

Tension $roductionTension production by muscle fibers• -ll or none principle• -mount of tension depends on number of cross bridges formed• Skeletal muscle contracts most forcefully over a narrow ranges of

resting lengths• Twitch

• Cycle of contraction, relaxation produced by a single stimulus• Treppe

• Repeated stimulation after relaxation phase has been completed

Summation• Repeated stimulation before relaxation phase has been completed

• Wave summation = one twitch is added to another • Incomplete tetanus = muscle never relaxes completely• Complete tetanus = relaxation phase is eleminated

Tension production by skeletal muscles• ,nternal tension generated inside contracting muscle fibers• External tension generated in extracellular fibers• Motor units

• -ll the muscle fibers innervated by one neuron• Precise control of movement determined by number and si&e of

motor unit• Muscle tone

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• Stabili&es bones and .oints

Tension production by skeletal muscles

• ,nternal tension generated inside contracting muscle fibers• External tension generated in extracellular fibers• Motor units

• -ll the muscle fibers innervated by one neuron• Precise control of movement determined by number and si&e of

motor unit• Muscle tone

• Stabili&es bones and .oints

Contractions• ,sometric• ension rises* length of muscle remains constant

• ,sotonic• ension rises* length of muscle changes

• #esistance and speed of contraction inversely related• #eturn to resting lengths due to elastic components* contraction of

opposing muscle groups* gravity

nergy %se and Muscle ContractionMuscle Contraction re&uires large amounts of energy• Creatine phosphate releases stored energy to convert -0P to - P• -erobic metabolism provides most - P needed for contraction• -t peak activity* anaerobic glycolysis needed to generate - P

nergy use and level of muscular activity• Energy production and use patterns mirror muscle activity • 'atigued muscle no longer contracts

• )uild up of lactic acid• Exhaustion of energy resources

#ecovery period• )egins immediately after activity ends• %xygen debt !excess post$exercise oxygen consumption"

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• -mount of oxygen re2uired during resting period to restore muscleto normal conditions

Muscle $erformance

Types of skeletal muscle fibers• 'ast fibers• Slow fibers• ,ntermediate fibers Fast fibers• 1arge in diameter• Contain densely packed myofibrils• 1arge glycogen reserves• #elatively few mitochondria• Produce rapid* powerful contractions of short duration

Slo' fibers• +alf the diameter of fast fibers• ake three times as long to contract after stimulation• -bundant mitochondria• Extensive capillary supply • +igh concentrations of myoglobin

• Can contract for long periods of time (ntermediate fibers• Similar to fast fibers• Greater resistance to fatigue

Muscle performance and the distribution of muscle fibers• Pale muscles dominated by fast fibers are called white muscles• 0ark muscles dominated by slow fibers and myoglobin are called red

muscles• raining can lead to hypertrophy of stimulated muscle

$hysical conditioning• -naerobic endurance

• ime over which muscular contractions are sustained by glycolysisand - P/CP reserves

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• -erobic endurance• ime over which muscle can continue to contract while supported

by mitochondrial activities

Cardiac Muscle TissueStructural characteristics of cardiac muscle• 1ocated only in heart• Cardiac muscle cells are small

• %ne centrally located nucleus• Short broad $tubules• 0ependent on aerobic metabolism

• ,ntercalated discs where membranes contact one another

Functional characteristics of cardiac muscle tissue• -utomaticity • Contractions last longer than skeletal muscle• 0o not exhibit wave summation

• (o tetanic contractions possible

Smooth Muscle TissueStructural characteristics of smooth muscle• (onstriated

• 1ack sarcomeres• hin filaments anchored to dense bodies

• ,nvoluntary Functional characteristics of smooth muscle• Contract when calcium ions interact with calmodulin

• -ctivates myosin light chain kinase• 'unctions over a wide range of lengths

• Plasticity • Multi$unit smooth muscle cells are innervated by more than one

motor neuron• 3isceral smooth muscle cells are not always innervated by motor

neurons• (eurons that innervate smooth muscle are not under voluntary

control

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