muscle contraction

 Step-by-step explanation

Actin and myosin are the main muscle proteins

Actin is the thin filament.

Myosin is the thick filament.

Muscle contraction happens only when myosin binds to actin.

Actin has myosin-binding sites

These are specific spots on actin where myosin heads attach.

If myosin cannot attach, contraction cannot happen.

Tropomyosin acts like a blocker

Tropomyosin is a long protein that lies along the actin filament.

In a relaxed muscle, tropomyosin covers the myosin-binding sites.

No binding = no contraction

Because the binding sites are covered:

Myosin cannot attach to actin

Cross-bridges cannot form

The muscle stays relaxed

What keeps tropomyosin in place?

Another protein called troponin holds tropomyosin in the blocking position.

This happens when calcium levels are low in the muscle cell.

What changes during contraction? (brief preview)

When calcium is released:

Calcium binds to troponin

Troponin moves tropomyosin

Myosin-binding sites are exposed

Myosin attaches → contraction begins

Simple exam-ready summary

“In a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin, preventing myosin from attaching and stopping muscle contraction.”

If you want, I can:

explain this using an analogy (like a lock and cover)

connect it to calcium and the sarcoplasmic reticulum

explain it in 1–2 sentences for TEAS

show the full contraction sequence step by step

Just tell me 😊