Darkthorn’s Blog

Muscle Structure and Contractile Properties

- Muscle fibre – cellular element, multinucleate, each innervated, sarcomere as a functional division

- Sarcolemma – cell membrane

- Sarcoplamic reticulum – source of Ca2+ that surrounds each myofibril

- Myofibril – bundles of actin and myosin

- T-tubules – ramifying tubular system that carried the depolarization of the sarcolemma internally to each myofibril

- Sarcomere – myosin – 6 polypeptide chains, a hinge region, and ATPase activity – 400 myosin to each filament, half in one direction, half in the other

- actin – anchored to the 2-disc by protein liten – tropomysoin (covers the active sites) is regulated by troponin (calcium reactions)

Excitation-Contraction Coupling

a) motor neuron releases Ach onto skeletal muscle fiber activating the nicotinic receptors and opening K+ and Na+ channels (depolarization)

b) action potential moves away and down t-tubule system

c) dihydropyridine receptors are activated by voltage change

d) ryanodine receptors (lateral sacs of sarcolemic reticulum) open to release Ca2+

e) Ca2+ diffuses across microfilamentsa, then binds to troponin

f) Myosin heads can now bind to g-actin molecules, beginning CBC

Cross-Bridge Cycling

- Continues while nerve depolarizes membrane and ATP is present

- Shortens sarcomere to relax, Ca2+ is returned to sarcoplasmic reticulum and actin filament returns to resting position (this process occurs several times)

- Contraction velocity – ATPase activity and length of power stroke

- Contraction frequency – greater contraction strength builds piggyback response

- Muscle length – isotonic – concentric (muscle shortens) vs eccentric (muscle lengthens)

- isometric – muscle does not shorten/lengthen

- ATP provided by phosphate, anaerobic and aerobic respiration