Blood Supply Muscles are richly supplied with blood vessels. Arteries and veins enter the muscle along with the connective tissues and are oriented parallel to the individual muscle fibers. They branch repeatedly into numerous capillaries and venules, forming vast networks in and around the endomysium. In this manner each fiber is assured of an adequate supply of freshly oxygenated blood from the arterial system and of the removal of waste products such as carbon dioxide via the venous system. The amount of blood required by skeletal muscle depends, of course, on its state of activity. During maximal exercise the muscles may require as much as 100 times more blood than when resting. Besides the large number of vessels which supply each muscle there are other ways in which this blood flow requirement can be met. For example, the alternating contraction and relaxation of active muscle causes periodic squeezing of the blood vessels. This pumping or milking action speeds up the flow of blood to the heart, thus increasing the amount of fresh blood which can be returned to the muscles. Constriction of the arteries supplying blood to the inactive areas of the body (such as the gut, kidney, and skin) and dilation of those to the active skeletal muscles also aids in regulating muscle blood flow. Nerve Supply The nerves supplying a muscle contain both motor and sensory fibers. The motor nerves, which when stimulated cause the muscle to contract, originate in the central nervous system (spinal cord and brain). They constitute about 60 per cent of the nerves which enter the muscle. The sensory nerves, which make up the remaining 40 per cent, convey information concerning pain and orientation of body parts from the muscle sense organs to the central nervous system. The Motor Unit. If we were to count the number of motor nerves entering a muscle and calculate the number of muscle fibers within the muscle, we would find that a great difference exists between the two. There are about a quarter of a billion separate muscle fibers which make up the skeletal musculature in man, but there are only about 420 thousand motor nerves. Inasmuch as the number of muscle fibers greatly exceeds the number of nerve fibers and keeping in mind the fact that every muscle fiber is innervated, we see that the nerve fibers must necessarily branch repeatedly. In other words, a single motor nerve fiber innervates anywhere from 5 to 150 or more muscle fibers. All the muscle fibers served by the same motor nerve contract and relax at the same time, working as a unit. For this reason, the single motor nerve and the muscle fibers it supplies are called the motor unit. The ratio of muscle fibers innervated by a single motor nerve is not determined by the size of the muscle, but rather by the precision, accuracy, and coordination of its movement. Muscles that are called on to perform fine and delicate work, such as the eye muscles, may have as few as 5 to 10 muscle fibers in a motor unit; muscles used for rather heavy work, such as the quadriceps, may have as high as 150 or more muscle fibers per motor unit. A stimulated muscle or nerve fiber contracts or propagates a nerve impulse either completely or not at all. In other words, a minimal stimulus causes the individual muscle fiber to contract to the same extent that a stronger stimulus does. This phenomenon is known as the all-or-none law. Because a single neuron supplies many muscle fibers in the formation of the motor unit, it naturally follows that the motor unit will also function according to the all-ornone law. While this law of physiology holds true for the individual muscle fibers and motor units, it does not apply to the muscle as a whole. It is possible for the muscle to exert forces of graded strengths, ranging from a barely perceptible contraction to the most vigorous type of contraction, depending on the number of motor units stimulated.		Home Components of Physical Fitness Clinical Examination Differences Between Fit and Unfit Men Physiological Elements of Fitness The Step Test The Effects of Training on the Physiological Systems Work Output Physical Condition of Muscle Application of Muscular Force Maintaining Optimum Strength Tests of Muscular Strength Strength and appearance Strength as a measure of physical fitness Strength as affected by organic problems Strength as prophylaxis Role of inactivity in production of disease Lack of Sociability in Work Lactic Acid System Muscular Form Capillary Growth Heart Size Heart Rate Training produces physicochemical changes Muscular Strength Nervous Stimulation and Muscular Strength Quality of Champions Factors Which Limit Skill Blood Supply Kinesthesis Balance Reaction Time Skill Muscular Tension Visual Aim Skill Center in the Brain Endurance for Exhaustive and Moderate Work Distribution of Energy