Triad: this is the name of the structure at the end of the transversal tubulus. There, the cell membrane is close to the membranes of two sarcoplasmic reticula (pleural of reticulum). The proximity of three membranes together is called a Triad (= three).
Actin hots-spots: Ca2+ ions have a complicated effect on the actin molecule. There is actually an interplay between three molecules; actin, troponin and tropomyosin. For more (technical) information, go to a Physiology textbook.
Head rotation is a bit of an exaggeration. In reality, the head movement is more like a ‘tilting’ towards the middle of the sarcomere and, during the fourth step of the dance, the head tilts back to its starting position. It is more like the wipers on the windshield of your car that swipe back and forth.
During a typical contraction, this cross-bridge dance will happen millions of times, at all the thousands of cross bridges in the sarcomere, and in all the thousands of sarcomere stringed along a muscle fiber.
End of contraction: the cross-bridge dance continues as long as Ca2+ keeps the hot-spots on the actin molecule open. But, at the same time that the contraction takes place, Ca2+ is already being pumped back into the sarcoplamic reticulum (active transport). This will decrease the calcium-concentration in the neighborhood of the sarcomere. As soon as the Ca2+ concentration is low enough, the hot spots will be closed and no longer available for the cross bridges. This will stop the contraction.
Sliding Filament Theory: This process of the cross-bridge heads pulling the actin along the myosin molecules, makes the actin slide along the myosin molecule (= filaments). In the early days of this research, no one could actually see these cross bridges work and the idea of this mechanism was based on indirect evidence. But that evidence was enough to deduce the 'sliding' of the actin along the myosine molecule; hence the term ‘sliding filament theory’
D. The Stretch – Contraction relationship. Also called the Force-Length relation.
This is an important phenomenon in physiology.
If the muscle is stretched before the muscle is stimulated (by pulling at the tendons for example), then the contraction will be stronger.
The explanation of this phenomenon is shown in the diagram and in the following steps:
If the actin and myosin overlap a lot (as in the 'no stretch' situation in "a"), then there will be a small contraction. The contraction cannot be stronger because the myosin molecules are stopped against the Z-lines.
If the sarcomere (= the muscle) is stretched more (situation "b"), then the myofilaments can slide more before the Z-lines are reached and the contraction force will increase.
So, if you stretch the muscles more and more, then the filaments will slide more and more and the contraction force will increase.
But there is a limit to the amount of stretching. If you stretch too much (situation "c"), then the actin and myosin filaments are no longer in each others neighborhood, and the distance is too large to form cross-bridges. This will reduce the contraction force.
This effect of pre-stretch is also very important in the heart (there it is called the Frank-Starling mechanism) and in all other muscles.
In practice, the length of the skeletal muscles is determined by the position of the joints in the body. For example, the biceps muscles attach the lower arm to the upper arm. If the arm is fully flexed, then contraction will be small and when the arm is fully extended, it is more difficult to contract against a large force.
The optimal length of the biceps is at a angle of the elbow joint of about ninety degrees. This is the angle that most sportsman will use when having to lift heavy weights for examples (weight lifters).
E. Rigor Mortis ("stiffness in death"):
Question: When a person dies, the body, after a few hours will become very stiff. Why?
Contraction requires energy (ATP). Specifically, the ATP is required to de-tach the cross-bridge head from the actin molecule.
But, when a person dies, the normal repair mechanisms of the cells have stopped functioning. This means that the cells will start to deteriorate. One of the first signs of this deterioration is that membranes will start to fall apart.
Therefore, the membrane of the sarcoplasmic reticulum, which contains a lot of calcium ions, will break open and holes will appear in this membrane.
This will lead to a flow of Ca2+ ions from the sarcoplasmic reticulum into the sarcomere, and this will open the hotspots on the actin molecules.
Once the hotspots are open, the heads of the cross-bridges will automatically attach to the actin molecules.
But; because there is no ATP (the person is dead remember?), the heads will no longer be able to detach from the actin.
Therefore, the myosin and the actin molecules are now locked together forever.
Because this process happens in all skeletal muscles at more or less the same time, the corpse becomes very stiff.