The Sarcomere 1


The sarcomere is the fundamental unit in the skeletal muscle that makes the contraction happen.


A. Definitions and Structural components required:
  1. the sarcomere is the space from one Z-disc to the next Z-disc
  2. the sarcomere contains myosin and actin molecules, which are long and thin molecules
  3. (= filaments)
  4. the actin molecules are attached to the Z-disc
  5. the myosin molecules are arranged in the middle of the sarcomere and between the actin molecules
  6. from the myosin molecules, cross-bridges extend towards the actin  molecules
  7. there is also a sarcoplasmic reticulum that stores Ca2+
  8. and there are transverse tubuli (singular: tubulus). These are invaginations of the cell membrane (= sarcolemma) that come close to the sarcoplasmic reticulum.

structure of the sarcomere inside a muscle cell

diagram of a sarcomere


B. Functional Steps :


A muscle action potential propagates from the motor end-plate along the cell membrane, thereby activating (=exciting) the whole cell membrane.


Because the transversal tubuli are continuations of the cell membrane, the action potential also propagates into these tubuli.


The action potential at the end of the transversal tubuli have an effect on the neighboring sarcoplasmic reticulum


This will open the Ca2+ channels that are located in the membrane of the sarcoplasmic reticulum (= SR)


Because there is much more Ca2+ inside the SR then in the rest of the sarcomere, there will be Ca2+ diffusion into the cell (along the concentration gradient) and the Ca2+ concentration in the sarcomere will rapidly increase.


The Ca2+ ions will influence the actin molecules to open their hot spots.

hot spot small


Once these hot spots are available, the head of the nearest cross-bridge will attach to the hot spot.


It is important to note that the cross-bridges are a part of the myosin molecule. So, when the cross-bridges attach to the hot spots, then effectively the myosin is linked to the actin molecule.


Once the head is attached to the actin molecule, the head will rotate a little (see animation). The rotation is always towards the center of the sarcomere.


The rotation of the head will therefore pull the actin molecule a little towards the center of the sarcomere.


The head will then de-tach from the actin molecule and rotate back towards its orginal position. This step requires energy (one ATP molecule for every step).


Repeat previous steps; the head will attach again to the next hot spot, turn. detach and turn back.


In this manner, step-by-step, the actin molecule is pulled towards the middle of the sarcomere, thereby pulling the Z-discs closer to each other.


As the same thing is happening at the same time in all the other sarcomere along the muscle fiber, the whole fiber becomes shorter; this is the contraction.


The Cross-Bridge Dance:

In summary; once Ca2+ ions have opened the hot-spots on the surface of the actin molecules, the head of the cross bridges will start, what I call, the‘cross-bridge dance” (animation). This dance consists of four steps:


The Cross-Bridge Dance:

1. attach (myosin head to actin hot-spot)

2. turn (towards the middle of the sarcomere

3. detach (this requires energy in the form of ATP)

4. turn back



cross-bridge dance small size


animation of one cross-bridge

animation of one sarcomere


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