Final answer:
The combining of a series of twitches to produce a more forceful contraction is called summation. Summation occurs when a motor neuron fires action potentials at an increased frequency, resulting in the release of more Ca++ ions and a stronger muscle contraction. Examples of summation include wave summation and incomplete tetanus.
Step-by-step explanation:
The combining of a series of twitches to produce a more forceful contraction is called summation.
When a motor neuron fires action potentials at an increased frequency, the tension produced in the muscle continues to rise until it reaches a peak point, resulting in a more forceful contraction. This is known as summation. Summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate additional sarcomeres while the muscle is still contracting from the first stimulus.
Examples of summation include wave summation, where successive stimuli are added together to produce a stronger muscle contraction, and incomplete tetanus, where the tension is three to four times greater than a single twitch. If the stimulus frequency is so high that the relaxation phase disappears completely, it is called complete tetanus.