| 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. |
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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.
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