Such a contraction is called a twitch. Thus, each motor unit is either relaxed or each presynaptic action potential causes an action potential in all of the muscle fibers of the motor unit, causing them all to contract at once.In the whole muscle, contractions of different strengths necessarily are created by activating different numbers of motor units.hus, each motor unit is either relaxed or each presynaptic action potential causes an action potential in all of the muscle fibers of the motor unit, causing them all to contract at once. In the whole muscle, contractions of different strengths necessarily are created by activating different numbers of motor units.
Johannes Gutenberg of Mainz, Germany is acknowledged as the first to invent a metal movable-type printing system in Europe, the printing press.
In 1377, the first movable metallic types were invented in Goryeo Dynasty in Korea, which were used to print Jikjishimcheyojeol or simply Jikji, which is the oldest extant movable metal print book. The printing press was introduced to the West in the Holy Roman Empire by Johannes Gutenberg, around 1440. Gutenberg, a goldsmith by profession, devised a hand mould to create metal movable type, and adapted screw presses and other existing technologies, to create a printing system.