A muscle fiber generates tension when properly stimulated, for instance by the nervous system or by electrical impulses. This physiological process is called muscle contraction. While under tension, the muscle may lengthen, shorten, or remain the same length. Although in current English the term contraction implies a reduction in length or size, when referring to the muscular system it simply means muscle fibers generating tension (traction) which may or may not be intense enough to produce shortening. A contraction will not produce shortening when the ends of the muscle are pulled apart from each other by external loads which are larger than or equal to the pulling force applied, in the opposite direction, by the muscle. Voluntary muscle contraction is controlled by the central nervous system. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates several muscle fibers. In the case of some reflexes, the signal to contract can originate in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli proceeding in the body to the muscle itself.