. . . "it is released into the body as a result of injury, emotion, pain, infections, or other trauma.actin -- protein involved in muscle contraction.actin filaments -- one of the contractile elements in skeletal, cardiac, and smooth muscle fibers.actinium -- atomic number 89, symbol Ac; a silvery white highly radioactive element found in uranium ores, occurs as a decay product of uranium; half-life 21.5" . .