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

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

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

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

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