The Motor Nerve

The Motor Nerve


Every single motor nerve entering a muscle can stimulate anywhere from one to several thousand muscle fibers. All of the muscle fibers activated by an individual motor nerve react together to its impulses, contracting and relaxing in unison. A single motor nerve together with the muscle fibers it activates creates a motor unit.

When a motor nerve is stimulated, the impulse sent to the muscle fibers within the motor unit either spreads completely or does not spread at all. This is the all-or-none law, which means that a weak impulse creates the same tension within the motor unit as a strong impulse.

This all-or-none law does not apply to the muscle as a whole. While all muscle fibers within a single motor unit respond to the stimulation of the motor nerve, not all motor units are activated during a muscular contraction. The number of motor units involved in a contraction depends on the load imposed upon the muscle, and this has a direct correlation to the force produced. For example, if the load is light, only a small number of motor units will be recruited and the strength of contraction will be low. On the other hand, if the load imposed upon the muscle is extremely heavy, then all or almost all of the motor units will be recruited resulting in a maximal force output. Since a muscle's motor units are recruited in sequential order, the only way to train the entire muscle is to expose it to maximum loads, so every motor unit is used for muscle contraction.

While the force exerted by a muscle depends upon the number of motor units recruited during a contraction, it also depends upon the number of muscle fibers within a motor unit. The number of fibers can vary between 20-500, with the average number being around 200. The more fibers in a motor unit, the higher the force output. The genetic factor that determines the number of fibers helps to explain why some people can increase muscle size and strength quite easily, while others have to fight for every small gain.

When a nerve impulse stimulates a motor unit, it responds by giving a twitch or a very quick contraction, followed by relaxation. If another impulses reaches the motor unit before it has time to relax, the two twitches summate (join forces) and produce greater tension than that produced by a single twitch.

The summation of motor units depends upon the load imposed on the muscle. During maximum loads, all of the muscle's fibers summate in synchronization leading to maximum force output; during medium loads, some motor units are twitching while others are relaxing, leading to medium force output. This is one of the main reasons why heavy loads lead to higher gains in maximum strength.

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