The Muscular System

Muscular System Functions MOVEMENT MOVEMENT Maintain Posture Maintain Posture Stabilize Joints Stabilize Joints Generate HEAT Generate HEAT –40% body mass –80% body heat –Endothermy!

Muscle Tissue Tissue Review: Tissue Review: –Cardiac  Cardiovascular System  Involuntary, Striated –Smooth  Cardiovascular, Digestive, Reproductive, etc.  Involuntary, non-striated –Skeletal*  MUSCULAR SYSTEM  Voluntary, striated

Muscles are… Excitable (irritable) Excitable (irritable) Contractile Contractile Extensible Extensible Elastic Elastic Myo-, Mys, Sarco- (muscle prefixes) Myo-, Mys, Sarco- (muscle prefixes)

Skeletal Muscle Tissue MUSCLE CELL STRUCTURE MUSCLE CELL STRUCTURE –Arrangement: large, long FIBERS  Fiber = muscle cell –Two major protein filaments present:  Actin  Myosin myofilaments

Skeletal Muscle Structure Striated Striated –Due to actin/myosin Elongated Elongated –Varied lengths Multinucleated Multinucleated

TEXT: Structures / Regions to know: Structures / Regions to know: –Muscle –Tendon –Epimysium –Facsicle –Muscle Fiber –Perimysium –Myofibril –Endomysium

Muscle Anatomy

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Skeletal Muscle Connections Direct connection to Bone Direct connection to Bone Indirect connection via TENDON Indirect connection via TENDON ORIGIN: bone that does NOT move when muscle contracts ORIGIN: bone that does NOT move when muscle contracts INSERTION: bone that MOVES when muscle contracts INSERTION: bone that MOVES when muscle contracts

Microscopic Structure of Muscle Fiber Cell membrane = Sarcolemma Cell membrane = Sarcolemma Cytoplasm = Sarcoplasm Cytoplasm = Sarcoplasm Multiple Mitochondria = High E output Multiple Mitochondria = High E output Fiber is filled with long myofibrils Fiber is filled with long myofibrils Myofibrils filled with filaments arranged in contractile units called SARCOMERES. Myofibrils filled with filaments arranged in contractile units called SARCOMERES. –Myosin (thick filament) –Actin (thin filament)

Sarcoplasmic Reticulum Specialized ER connected to cell surface by T-tubules Specialized ER connected to cell surface by T-tubules Surrounds each myofibril Surrounds each myofibril At rest, filled with Ca++ maintained by a calcium “pump”, uses ATP At rest, filled with Ca++ maintained by a calcium “pump”, uses ATP When activated, pores open and release calcium, initiating contraction When activated, pores open and release calcium, initiating contraction

Actin (thin) filament Composition Long chains of actin globules in double spiral arrangement Long chains of actin globules in double spiral arrangement Each actin contains binding site for myosin Each actin contains binding site for myosin Tropomyosin spiral around chain Tropomyosin spiral around chain –blocks active site on actin Troponin clustered along spiral Troponin clustered along spiral –Binding site for calcium!

Actin Filament

Myosin (thick) filament Composition Contains 2 tails each with globular heads Contains 2 tails each with globular heads Heads have ATP binding sites and ATPase for splitting ATP Heads have ATP binding sites and ATPase for splitting ATP Heads attracted to active sites on actin molecules Heads attracted to active sites on actin molecules Heads form cross-bridges with actin Heads form cross-bridges with actin

Myosin Filament

Multiple Myosin Filaments

Sarcomere structure Alternating dark and light bands Alternating dark and light bands Central H-zone  contains MYOSIN only Central H-zone  contains MYOSIN only Lateral A-bands  contain both Lateral A-bands  contain both ACTIN and MYOSIN filaments ACTIN and MYOSIN filaments End in I-bands  contain ACTIN only End in I-bands  contain ACTIN only (with Z-line in center) (with Z-line in center) Z LINE TO Z LINE = 1 SARCOMERE Z LINE TO Z LINE = 1 SARCOMERE

SARCOMERE STRUCTURE

Sarcomere

Nerve supply to Muscle Fiber Each muscle fiber served by a motor neuron Each muscle fiber served by a motor neuron Motor neuron ends in a pad filled with vacuoles packed with neurotransmitter Motor neuron ends in a pad filled with vacuoles packed with neurotransmitter Pad sits above specialized piece of sarcolemma called motor end plate Pad sits above specialized piece of sarcolemma called motor end plate

Neuron pad + motor end plate = Neuromuscular junction (Space between called synaptic cleft )

Sliding Filament Theory of Muscle Contraction Sequence of Steps: 1. Neuron releases neurotransmitter, acetylcholine (ACh) into synaptic cleft 2. ACh diffuses to motor end plate 3. ACh binds to receptor on motor end plate 4. Gated channel protein opens, Na+ rushes into cell interior, upsets RMP!

Generation of Action Potential RMP = -70 Mv RMP = -70 Mv Sudden influx of Na+ generates Sudden influx of Na+ generates Action Potential Action Potential RMP later restored to normal by RMP later restored to normal by sodium-potassium pump sodium-potassium pump

5. Action potential carried along the sarcolemma to transverse (“T”) tubules connected to Sarcoplasmic Reticulum 6. SR membrane becomes permeable to calcium 7. Sarcoplasm is flooded with calcium ions

8. Ca++ binds to troponin 9. Ca/Troponin pulls tropomyosin out of the way, unmasks active site on actin molecules 10. Myosin heads attach to actin 11. Heads rotate, pull actin in to H-zone 12. Z lines get closer…

13. Myosin splits ATP to recharge 14. Continues until Action Potential is restored and Ca++ is pumped back into SR 15.All sarcomeres shorten, shortening muscle cell

16. Shortening cell pulls on tendons attached to bones, moving bone at articulation 17. Contraction of opposite muscle required to fully elongate shortened muscle

Reminders: Refractory Period Refractory Period All or None Effect All or None Effect Breakdown of ACh by ACh-esterase Breakdown of ACh by ACh-esterase

Disorders/Conditions of the Muscular System Duchenne’s Muscular Dystrophy Duchenne’s Muscular Dystrophy –Sex linked inheritance –Dystrophin protein deficiency –Tearing of sarcolemma –Accumulation of CT/fat –Muscular ATROPHY

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