The motor unit is a functional system that consists of a motor nerve together with the muscle fibers that are innervated by that same nerve.
A motor nerve is a nerve that initiates a skeletal muscle contraction. Its soma (= cell body) is located in the ventral horn of the spinal cord and its axon projects from the soma, through the ventral root, out of the spinal cord, to several muscle fibers in the muscle.
A motor unit does NOT innervate ALL muscle fibers in a muscle. If that were the case, every action potential from this motor neuron would cause a massive and unregulated contraction of the whole muscle; i.e. a huge twitch.
A motor unit typically innervates 10-1000 muscle fibers. This depends on the type of muscles. Small muscles that are very delicate (like the small muscles in the fingers) innervate small size motor units (50-500 muscle fibers) whereas large muscles that do not need a lot of regulation (such as the large leg muscles) have motor units of 1,000 to 10,000 muscle fibers.
D. How does a motor unit work?
An example of several motor units is shown in the figure. In this case, there are three motor units (three motor neurons A, B and C and their respective muscle fibers; red, blue and brown).
If motor neuron A fires, then the red muscle fibers will contract. If motor neuron B or C fires, then the blue or brown fibers will contract. This is shown in the graph in situations 1, 2 and 3.
If two neurons fire simultaneously, (situations 4 and 5) then both groups of muscle fibers will contract, causing a stronger contraction. This effect is called summation. If all three neurons fire simultaneously (situation 6), then the contraction amplitude will further increase.
You must realize that the motor neuron is the boss. The muscle fibers have absolutely no say in this. If the motor neuron induces an action potential, then that action potential will always propagate to the muscle fibers that are innervated by that axon.
This is because transmission across the motor-end plate, the connection between the nerve axon and the muscle cell is alwayssuccessful. Therefore, if there is an action potential in the motor nerve, then there will always be action potentials in the connected muscle fibers and therefore contraction in those fibers. You could say that the muscle fibers are the slaves of the motor neurons.
This is of course the situation in a normal healty person. In diseases, this communication can easily be disturbed leading to all kinds of motor disturbances.