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THE SARCOMERE

Contraction of the Skeletal Muscle Cell

I.  Events at the Neuromuscular Junction and Sarcolemma

     A.  Depolarization – Repolarization of the nerve cell membrane

  1. Vesicles containing neurotransmitter fuse with the membrane of the nerve cell button.
  2. Vesicles release their contents of acetylcholine (Ach) into the synapse.
  3. Ach molecules bind with receptors on the surface of the motor end plate.

     B.  Depolarization – Repolarization of the Motor End Plate

  1. The motor end plate depolarizes.
  2. Cholinesterase breaks down Ach.
  3. The depolarization-repolarization spreads through the entire sarcolemma.
  4. The wave of depolarization passes down through the T channels past the adjacent cisternae of the sarcoplasmic reticulum (SR).

II.  Events within the sarcoplasm and sarcomere

     A.  Release of calcium from the SR.

  1. Calcium binds to troponin molecules.
  2. This binding causes a shape change in the troponin-tropomyosin complex.
  3. The tropomyosin strands move away from actin molecules exposing binding sites on the actin for myosin.

     B.  Interactions of myofilaments

  1. ATP molecules bind to myosin cross bridges energizing them.
  2. Energized myosin cross bridges flex at their hinge and bind to actin molecules.
  3. The cross bridge pulls on the actin molecule in a power stroke. This causes the actin molecule to slide on the myosin.
  4. Another ATP molecule binds to the myosin cross bridge disconnecting the cross bridge from the actin molecule.
  5. The myosin returns to its unflexed position

     C.  Calcium ions are pumped back into the cisternae of the SR

  1. ATP molecules breakdown releasing the energy necessary to operate the calcium channels in the membranes of the SR.
  2. The free troponin-tropomyosin molecules return to their original positions covering the actin binding sites for myosin cross bridges.

     In an actual contraction, some cross bridges are bound to actin and some are not. The resulting shortening of the sarcomere is a rachet-like movement involving the successive binding and detachment of cross bridges and actin molecules.