7 - During the swivel, the MYOSIN HEAD is firmly attached to ACTIN.
So, when the HEAD swivels it pulls the ACTIN (and, therefore, the entire
thin myofilament) forward. (Obviously, one MYOSIN HEAD cannot pull the
entire thin myofilament. Many MYOSIN HEADS are swivelling simultaneously,
or nearly so, and their collective efforts are enough to pull the entire
8 - At the end of the swivel, ATP fits into the binding site on the cross-bridge & this breaks the bond between the cross-bridge (myosin) and actin. The MYOSIN HEAD then swivels back. As it swivels back, the ATP breaks down to ADP & P and the cross-bridge again binds to an actin molecule. 9 - As a result, the HEAD is once again bound firmly to ACTIN. However, because the HEAD was not attached to actin when it swivelled back, the HEAD will bind to a different ACTIN molecule (., one further back on the thin myofilament). Once the HEAD is attached to ACTIN, the cross-bridge again swivels, SO STEP 7 IS REPEATED. As long as calcium is present (attached to TROPONIN), steps 7 through 9 will continue. And, as they do, the thin myofilament is being "pulled" by the MYOSIN HEADS of the thick myofilament. Thus, the THICK & THIN myofilaments are actually sliding past each other. As this occurs, the distance between the Z-lines of the sarcomere decreases. As sarcomeres get shorter, the myofibril, of course, gets shorter. And, obviously, the muscle fibers (and entire muscle) get shorter. Skeletal muscle relaxes when the nervous impulse stops. No impulse means that the membrane of the SARCOPLASMIC RETICULUM is no longer permeable to calcium (., no impulse means that the CALCIUM GATES close). So, calcium no longer diffuses out. The CALCIUM PUMP in the membrane will now transport the calcium back into the SR. As this occurs, calcium ions leave the binding sites on the TOPONIN MOLECULES. Without calcium, TROPONIN returns to its original shape and position as does the attached TROPOMYOSIN. This means that TROPOMYOSIN is now back in position, in contact with the MYOSIN HEAD. So, the MYOSIN head is no longer in contact with ACTIN and, therefore, the muscle stops contracting (., relaxes).