Structure Of Skeletal Muscle

Dr. Niranjan Murthy H L Asst. Prof., Dept. of Physiology Sree Siddhartha Medical College & Hospital, Tumkur

STRUCTURE OF SKELETAL MUSCLE • • • •

Forms 40% of body weight Locomotion Striated or voluntary Multinucleated

Sarcolemma• Consists of an inner plasma membrane and outer collagenous layer • Invaginates at numerous points to form Ttubules • Carries action potential Sarcoplasm• Consists of myofibrils • Numerous mitochondriae lying parallel to myofibrils Sarcoplasmic reticulum• L-tubules • Storage of calcium ions

• • • • •

Skeletal muscle Fasciculus Muscle fibers Myofibrils Myofilaments

• Muscle fiber- 10 to 80μ in diameter each is composed of 1000s of myofibrils • Each myofibril is in turn made up of myofilaments • Myofilaments (i) contractile- myosin II, actin (ii) modulatory- tropomyosin, troponin

LIGHT MICROSCOPIC STRUCTURE • • • • • • •

Cross striations due to alternate dark and light bands Light band- Isotropic band- I band- thin filaments Dark band- Anisotropic band- A band- birefringent H zone- lighter zone in A band Z line- in the center of I band M line- in the center of A band Sarcomere- between two z lines- basic unit of muscle

ULTRASTRUCTURE • •

Myofilaments- thick & thin filaments Myosin-10 to 14nm wide & 1.6μm long 1500 in each myofibril Myosin filament made of around 200molecules of myosin • Myosin molecule- composed of 6 polypeptide chains- 2 heavy chains and 4 light chains A tail, an arm and 2 globular heads Arm and head form cross-bridge Two hinges Head has sites for ATP and actin binding

• Actin filament- 7nm wide and 1.0μm long Extend on both sides of z-lines F-actin forms a double helix Made of 300 G-actin molecules (MW 42,000) G-actin has active sites for interaction with myosin heads

• Tropomyosin- 40nm length and MW of 70,000. Wrapped around actin helix & covers active sites in resting phase • Troponin- made of 3 protein subunits (Tn I, Tn C & Tn T)

OTHER PROTEINS 3) Actinin- binds actin to Z lines 2) Desmin- binds Z lines to the plasma membrane 3) Titin- connects Z lines to M lines and provide scaffolding for sarcomere. Provides elasticity 4) Dystrophin, dystroglycans and sarcoglycans