Excitation stops

• 1)What happens when the muscle stops being stimulated?

  calcium ions leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum (this requires ATP)\
• 2) What does this cause?

  The tropomyosin molecules to move back, so they block the actin-myosin binding sites again
• 3- Why are muscles not contracted?

  because no myosin heads are attached to actin filaments

  • (so there are no actin-myosin cross bridges)
• 4) what happens?

  the actin filaments slide back to their relaxed position, which lengthens the sarcomere