To make a muscle contract in a sensible and useful manner.
A. Definitions and Structural components required:
A skeletal muscle; this can be either a small muscle like those that contract our eyelids or a large muscle such as in the upper leg (the rectus femoris for example).
These muscles consist of thousands of fibers and millions of sarcomeres.
Nerve endings that innervate all these muscle fibers.
B. The Twitch :
If there is only one action potential in the nerve that excites the muscle, then the muscle will contract only once. This is called a twitch. It is the minimum contraction for the whole muscle.
Notice that the duration of the contraction, the twitch, is much longer than the duration of the action potential.
In fact the twitch has three phases:
the delay. This is the time between the arrival of the action potential at the motor end-plate and the beginning of the contraction. This time includes diffusion of the acetylcholine, the propagation of the muscle action potential, release of calcium ions from the sarcoplasmic reticulum etc.
the contraction phase: this is when the actin molecules are bound to the myosin fibers and the head of the cross-bridges turn in cycles.
The relaxation phase; the contraction is over, calcium-ions are pumped back into the sarcoplasmic reticulum, the cross-bridges are no longer formed etc.
C. From Twitch to Tetanus :
If we only had twitches, we would have difficulty in moving around. We need many twitches (and therefore many action potentials) to make a proper movement.
Remember that the duration of the twitch, is much longer than the duration of the action potential.
If, after the first twitch, a second action potential excites the muscle, this will then generate a second twitch.
If the time between the first and the second action potential is made shorter and shorter then the second contraction will start while the first contraction is still going on.
Therefore, the second contraction will start at a higher level and reaches a higher peak. This phenomenon in which one contraction gets a "lift" from a previous contraction is called summation.
In the figure above, a third action potential was also generated earlier, exiting a third contraction, which induced additionalsummation. After this third action potential, the nerve became quiet and contraction force returned to the original level.
If the time between the successive action potentials is made even shorter, then the individual twitches will summate on top of each other even more and the total force of contraction of the whole muscle will further increase (figure: roughtetanus)
With a further increase in action potentials, the individual twitches will “blur” into each other, creating a smoothtetanus, a smooth contraction.
D Additional Notes :
In daily and normal life, all contractions of our skeletal muscles are smooth tetani (=plural for tetanus) . Examples of these are walking, writing, turning your head etc. Possibly the only example of a twitch is the blinking of an eyelid (lasts about 0.1 second).
In pathological situations however, when there is a problem with the coordination between nerves and muscles, it is possible for roughtetanus to become visible. Examples of such abnormal contractions are shaking, tremors and spasms.
The whole principle of summation depends on the fact that the action potential duration (and its refractory period) is much shorter than the duration of the ensuing contraction. A typical action potential in a skeletal muscle lasts 2-5 msec while a contraction may last from 100 to 1000 milliseconds.
If the action potential duration is increased, this will decrease the possibility of summation. This would be abnormal in the skeletal muscle and visible as tremors.