1. Microscopic Anatomy of Skeletal Muscle Fibers
Long cylindrical cells
Multiple nuclei just below sarcolemma
Glycosomes- granules of stored glycogen
Myoglobin- red pigment stores O2
Myofibrils- rodlike structures that run parallel to length of cell, densely packed
Made of myofilaments- contractile elements of skeletal muscles

2. Myofilaments Thick filaments- myosin Thin filaments- actin
rodlike tail, ends in flexible hinge with 2 globular heads
Form cross bridges with thin filaments
Contain actin binding sites and ATPase enzymes
Thin filaments- actin
subunits are globular= G actin
Tropomyosin- strands run along actin, help stiffen actin molecule, in a relaxed muscle cell they block myosin binding sites
Troponin- 3 polypeptide complex, 1= binds to actin to inhibit contractions, 2= binds to tropomyosin and helps position actin, 3= Ca2+ binding site

3. Striations and Sarcomeres
Striations- repeating series of dark A band & light I bands
H zone- lighter stripe in midsection of A band
M line- dark line that bisects H zone vertically
Z disc- dark midline interruption of I bands
Sarcomere- region of myofibril between 2 sucessive Z discs, smallest functional unit of muscle fiber

4. Microscopic Anatomy Cont’d
Sarcoplasmic reticulum- elaborate SER, interconnecting tubules surround each myofibril
most run longitudinal but some larger channels cross perpendicular at A band- I band junctions= terminal cisternae
T tubules- at each A/I band junction, sarcolemma of muscle cell penetrates into cell interior= elongated tube
can/do conduct impulses to deep regions of muscle cell and to every sacromere, ensure that every myofibril contracts at same time

5. Sliding Filament Theory
“during contraction, thin filaments slide past thick ones so that the actin and myosin filaments overlap to a greater degree”
Shortening occurs when tension generated by cross bridges exceeds forces opposing shortening, contraction ends when cross bridges become inactive, tension declines
Sarcomere Shortening (499.0K)
Actin Myosin Bridge