A __________ consists of a group of muscle fibers that all get their signals from the same single motor neuron. As a result, all muscle fibers act together as a unit.

A(n) ____________________ is a single stimulus-contraction-relaxation sequence in a muscle fiber. It is a single, short contraction involving only a few motor units.

In a single muscle twitch, the ____________________ occurs when calcium (Ca2+) is being pumped back into the sarcoplasmic reticulum (SR) and actin and myosin stop the binding cycle.

In a single muscle twitch, the ____________________ occurs immediately after a muscle fiber has been stimulated by a nerve. Calcium (Ca2+) is released from the sarcoplasmic reticulum (SR). The stimulus has arrived, but NO force/tension is being produced. This is the SHORTEST phase of a single muscle twitch.

The image shown is an example of ____________________. This occurs if a second stimulus arrives IMMEDIATELY AFTER the RELAXATION phase has ENDED. The resulting contraction will have a slightly higher tension than the first contraction. Tension rises in stages (like a staircase, hence its name).

The image shown is an example of ____________________. This happens when another action potential (stimulus) occurs BEFORE the muscle is able to RELAX (before the relaxation phase). MORE calcium (Ca2+) is released resulting in MORE binding sites being revealed. As a result, MORE force is generated in that muscle fiber. In this case, twitches add to each other (get closer together in time).