SLIDING-FILAMENT THEORY (Explanation of how muscle contracts)

Proposes that a muscle shortens or lengthens because the thick and thin myofilaments sliding past each other without filament lengths being effected. Thus, muscle, fiber, and myofibrils change lengths, but not myofilaments!!!

Process of Muscle Contraction

1. Nerve impulse moves along the sarcolemma (fiber membrane) passing T-tubules and depolarizing them.

2. Ca++ is released from the lateral sacks (terminal cisternae) of the Sarcoplasmic Reticulum into the Sarcomere among the Myofilaments.

3. Ca++ binds with Troponin located on Tropomyosin causing active attachment sites to be exposed.

4. Energized Myosin heads attach to exposed sites on Actin filaments (those in proper alignment), attachment causes head deflection resulting in tension production (pulling action).

5. Available ATP re-energizes Myosin head, thus breaking Myosin head attachment to Actin filament (breaking the cross-bridge).

• As attached heads release other heads are attaching (that are properly aligned with active sites) and deflecting causing muscle tension through its range of motion via a racheting process.

• If impulse is removed Ca++ is actively pumped back into Lateral Sacks uncoupling with Troponin allowing Tropomyosin to recover active sites on the Actin filament.

• CONTRACTION WILL ONLY CONTINUE if there is available Ca++ in the Sarcomere and ATP is available for use.

Animation illustrating and describing the sliding filament theory.