Role of the Nervous System in Muscle Contraction - Exercise Physiology

Role of the Nervous System in Muscular Contraction

An Alpha Motor Neuron and all of the muscle fibers that it innervates is called a motor unit. Some motor units may have as few as 15 muscle fibers associated with an alpha motor neuron, while others may have thousands of muscle fibers. The ratio of muscle fibers to alpha motor neurons is called the innervation ratio. The innervation ratio is dependent upon the amount of motor control necessary. When fine control is desired, I am going to have an alpha motor neuron and a small number of fibers. Large muscles, such as the muscles of the legs and the back, have thousands of muscle fibers per motor neuron.

All the fibers in a motor unit are of the same biochemical type, meaning that all of the fibers will be Type I, Type IIa, or Type IIb. However, that does not mean that all the fibers in a muscle must be of the same bichemical type. While the biochemical type of muscle fibers in a motor unit is the same, there are several motor units in a muscle and the biochemical type may vary among the motor units.

As mentioned in the article "Characteristics of Muscle Fibers: Type I and Type II," Type I and II fibers are innervated by alpha motor neurons with different characteristics.

The nerves going to a Type I fiber have a slower conduction velocity because the axons are not as thick. However, Type I alpha motorneurons also have a lower threshold for firing. In otherwords, if you have a series of alpha motor neurons next to each other and you stimulate each one of them at the same time with the same stimulus, the one that has the lowest threshold for firing will fire first.

According to the size principle, this makes sense. The size principle states that with a stimulus of increasing intensity, the smallest motor units will be recruited first, then the next largest, and so on. The motor units of Type I fibers are smaller than those of Type II fibers. Therefore, Type I fibers will be recruited first. If it is determined that we cannot perform the desired activity using only type I fibers, then the stimulus will increase in intensity until Type IIa fibers are recruited. If the activity still cannot be performed, the stimulus will continue to increase in intensity until the Type IIb fibers are recruited as well. As you can see, the type I fibers will be active the longest. That makes sense because Type I fibers have the most endurance and are the most energy efficient.

From a neural perspective, there are two ways that the strength of a muscular contraction can be varied.

Vary the frequency of motor unit firing.

An increase in frequency gives you an increase in force.

Vary the number of motor units recruited.

Recruit more motor units, recruit more fibers/ more crossbridges, therefore increase force.
The body will always recruit more fibers first, then increase the rate of firing.

Eventually, there are no more motor units to recruit. This point is called the one-repetition maximum, or 1RM, and represents the maximum amount of muscular force that may be produced in a single bout. This may also be referred to as a synchronous contraction, because all muscle fibers are active and therefore are working synchronously. An asynchronous contraction is one in which only some of the muscle fibers are active and therefore all the muscle fibers are not working synchronously.