Muscular System

The 3 Types of Muscles

SKELETAL MUSCLE STRUCTURE

– Epimysium (epi = upon) – Perimysium(peri = around) – Endomysium(endo = within) Myo = Muscle Um= Membrane Skeletal Muscle Structure: Connective Tissue Coverings

Skeletal Muscle Structure: Structure Hierarchy 1.Whole Muscle 2.Muscle Facicle 3.Muscle Fiber (cell) 4.Myofibril 5.Filament Largest Smallest

Skeletal muscle fiber (cell) Muscle Fascicle Surrounded by perimysium Surrounded by endomysium Endomysium Perimysium Skeletal muscle Surrounded by epimysium Epimysium Tendon Skeletal Muscle Structure

Microanatomy of a Muscle Fiber (Cell) sarcolemma transverse (T) tubules sarcoplasmic reticulum terminal cisternae myofibril thin myofilament thick myofilament triad mitochondria nuclei myoglobin

I-Band A-Band H- Zone Z-Line M-Line

SARCOMERE

Sarcomere Z line A band H zone I band Zone of overlap M line Zone of overlap Thin myofilaments Thick myofilaments

Muscle fiber myofibril Thin filamentsThick filaments Thin myofilament Myosin molecule of thick myofilament sarcomere Z-line

Sarcomere Structure Striations form a repeating pattern along the muscle fiber called sarcomeres. Z Line – separates sarcomeres. I bands (light bands) are composed of actin filaments attached to Z lines. A bands (dark bands) are composed of myosin overlapping actin attached to Z lines. A central region (H zone) consists of myosin only with a thick line, the M line.

Organization of these protein filaments leads to light and dark striations seen in skeletal muscle under the light microscope which denote each sarcomere.

Z line Thin myofilaments M line Thick myofilaments Z line Myofilaments of Sarcomere

Protein Filaments Myosin: Thick filament of twisted protein strands with globular ends called cross-bridges. Actin: Thin filament protein which can be found in a complex with two other muscle proteins, tropomyosin and troponin.

Thick myofilament ( has ATP & actin binding site)

Thin Myofilament (myosin binding site)

Actin filaments – Has an active sites where it interacts with myosin – Resting – active site covered by tropomyosin which is held in place by troponin Myosin – Head attaches to actin during contraction – Can only happen if troponin changes position, moving tropomyosin to expose active site

Activation of muscles releases calcium which causes troponin to move tropomyosin out of the binding site.

Sliding Filament Theory Myosin heads attach to actin molecules (at binding (active) site) Myosin “pulls” on actin, causing thin myofilaments to slide across thick myofilaments, towards the center of the sarcomere Sarcomere shortens, I bands get smaller, H zone gets smaller, & zone of overlap increases

43 Contraction Cycle Repeating sequence of events that cause the thick & thin filaments to move past each other. 4 steps to contraction cycle Cycle keeps repeating as long as there is ATP available & high Ca+2 level near thin filament

STEP 1 – ATP binds to head of myosin causing it to detachment from actin

STEP 2 – Hydrolysis of ATP (breaks ATP to use energy) – Energy used to “spring load” the myosin head.

Step 2

STEP 3 – attachment of myosin to actin to form crossbridges

STEP 4 – Power stroke!

Myosin and Actin will remain in “expended” state until a new ATP binds the myosin

51 Steps in the Contraction Cycle Notice how the myosin head attaches and pulls on the thin filament with the energy released from ATP

LETS SEE IT IN ACTION