Sarcomeres and Calcium regulation in skeletal vs. smooth muscle.

In a sarcomere, where are the (+) ends of actin filaments anchored?

They are anchored at the Z-discs.

What is the function of Titin in the sarcomere?

It acts as an elastic spring connecting the Z-disc to the myosin thick filament, preventing overstretching and providing elastic recoil.

What is the function of Nebulin in the sarcomere?

It acts as a molecular ruler that dictates the precise length of the actin thin filaments.

How does Tropomyosin regulate skeletal muscle contraction?

It physically covers the myosin-binding sites on the actin filament, preventing the myosin heads from attaching.

In skeletal muscle, what does Calcium specifically bind to in order to trigger contraction?

Calcium binds to Troponin C (part of the Troponin complex).

What happens immediately after Calcium binds to Troponin in skeletal muscle?

Troponin undergoes a conformational change that shifts Tropomyosin away from the myosin-binding sites on actin, allowing cross-bridge cycling to begin.

Does smooth muscle use Troponin and Tropomyosin to regulate contraction?

No, smooth muscle lacks Troponin. It relies on a myosin-linked regulatory mechanism.

In smooth muscle, what protein does Calcium initially bind to?

Calmodulin.

What is the role of Myosin Light Chain Kinase (MLCK) in smooth muscle contraction?

Activated by Ca2+-Calmodulin, MLCK phosphorylates the myosin regulatory light chain, unfolding and activating the myosin motor so it can bind actin.

How is smooth muscle contraction turned off?

Myosin Light Chain Phosphatase (MLCP) removes the phosphate from the myosin light chain, returning the motor to its inactive, folded state.