muscles & skeleton locomotion chapter 50. muscle structure muscle fibers single cell with many...

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Muscles & Muscles & Skeleton Skeleton Locomotion Locomotion Chapter 50

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Muscles & Muscles & SkeletonSkeleton

LocomotionLocomotion

Chapter 50

Muscle structure

Muscle fibers Single cell with many nuclei Each fiber has a bundle of

myofibrils Each myofibril contains

myofilaments Thick(myosin) or thin (actin)

Muscle structure

Myosin (thick) proteins with a head region

Several actin (thin) form a double helix Cross-bridges Head region from myosin Extends to actin

Cross-bridgeCross-bridge

Cross-bridgeADP

P i

ATP

Actin

Myosin

Myosin binding sites

Muscle Muscle

Bundle ofmuscle fibers

Muscle

Single muscle fiber(cell)

Nuclei

Z lines

Plasma membrane

Myofibril

Sarcomere

Muscle structure

Sacromere Z-lines borders of the unit Thin filaments attached to Z-line I-bands thin filaments alone A-bands length of thick filaments H-zone center of the A-band only

thick filaments

Muscle

Muscle

Muscle

Muscle structure

Muscle contraction

Myosin head binds an ATP Spits ATP Forms cross-bridge Binds actin Pulls the actin Cross-bridge broken when binds new ATP Continues muscle is stimulated to contract Myofilaments move by sliding mechanism

Muscle structure

Control of contraction

Calcium Tropomyosin: Protein found on actin Troponin: Regulatory protein Transverse tubules (T tubules): Tube in the muscle fiber Sarcoplasmic reticulum (SR): Stores calcium in muscle

Mechanism of contraction

Relaxed muscle Myosin heads are ready (split ATP) Not attached to actin Cross-bridges cannot form Tropomyosin is on the actin Blocking binding site on actin

Mechanism of contraction

Contracting muscle Calcium binds the troponin Removes tropomysin off actin binding

sites Cross-bridges form Muscles contract

Contraction Contraction

Myosin-binding site

Tropomyosin

(a) Myosin-binding sites blocked

(b) Myosin-binding sites exposed

Ca2+

Ca2+-binding sites

Troponin complexActin

Mechanism of contraction

Relaxed muscle low calcium levels Contracting muscle high calcium levels Electric impulse (nerve) Stimulates calcium release from SR Travels down T tubules Binds troponin Contraction happens

Mechanism

Impulses stop Calcium is pumped back into SR Troponin no longer attached to

calcium Tropomysin returns to actin

Cardiac muscle

Shorter, branched cells Each with own nucleus Form a lattice Gap junctions electrical impulses

Smooth muscle

Internal organs Myosin/actin not organized into

sacromeres No sacroplasmic reticulum Need calcium to contract Capable of contracting when

stretched

Skeleton

Axial skeleton Supports Protects

Skeleton

Appendicular skeleton

Attach to axial skeleton

Skeleton

Articulations Joints, where movement happens

Lou-Gehrig’s disease