Muscle Innervation
Muscle Innervation
The innervation of each type of muscle is controlled by the nervous system responsible for its
function. Involuntary muscles are innervated by the autonomic nervous system; voluntary
muscles by the peripheral
In skeletal muscle the point of innervation is called the motor point
The motor unit consists of a motor nerve fiber and all the muscle fibers it innervates. A motor
unit functions together (as a whole). The strength of the contraction needed determines how many
motor units are activated. When lifting a paperback book from a table, few units would be
recruited; lifting a large hardback book would require more units to be activated in order to
accomplish the same motion.
Large muscles have large motor units. There are a greater number of muscle fibers per motor
nerve fiber. Muscles like the large gluteus maximus may have one motor unit responsible for
activating 20 muscle fibers. Smaller muscles are needed for coordination and fine motor activities
that require greater control. Small muscles have smaller motor units. Some have a ratio of 1:1.
Consider the differences in the activities produced by a large versus a small muscle. The larger
muscle is needed for force and power; a motor fiber needs to fire a group of muscle fibers
together. Smaller muscles need to be finer tuned for controlled motions. Imagine a group of fibers
all firing at once as you contract the fingers to thread a needle. It would be impossible to control
the motion enough to guide the thread through the small hole without more individual control of
the muscle fibers.
As the name motor unit implies, all the muscle fibers within a given motor unit contract or relax
nearly simultaneously; that is, it is not possible for some of the muscle fibers of a motor unit to
contract while others in the same motor unit relax. Also, if the muscle fibers of a motor unit are
activated by the nerve sufficiently to contract, those fibers will contract maximally. This is
known as the all-or-none law. The law applies only to individual motor units. Physiologic
mechanisms exist for the fine gradation of the force of contraction of the muscle as a whole. This
means that the trapezius muscle can contract only the upper fibers to activate motions associated
with that portion or activate both upper and lower fibers to neutralize movement in that plane.
The strength of the contractions will depend on the number of motor units recruited.
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