Figure 3  A myelinated nerve fiber

Muscle 

One anterior or ventral horn cell, and thus one axon, innervates a few hundred or even a few thousand muscle fibers.  The muscle fibers innervated by a single anterior horn cell are collectively known as a motor unit.  The "territory" of such a motor unit spans 10 - 15 mm in a muscle, however it is rare that directly adjacent muscle fibers are innervated by the same anterior horn cell / axon.  The figure below shows the seemingly random pattern of innervation of adjacent muscle fibers by individual anterior horn cells.  The clear fibers below are innervated by a single anterior horn cell and comprise a motor unit.  The vertically oriented fibers are innervated by a different anterior horn cell constituting a second motor unit and the horizontally oriented yet another. 

We also need to distinguish between type 1 (slow contracting) muscle fibers and type 2 (fast contracting) muscle fibers.  The type of muscle fiber is dependent on the type of anterior horn cell that innervates it.  Thus if a muscle fiber is innervated by a type 1 anterior horn cell, it will contract slowly.  Certain histochemical reactions, amongst others myosin ATPase, distinguish between type 1 and type 2 fibers.  Thus muscle reacted with myosin ATPase will normally exhibit a checkerboard pattern as it is likely that the adjacent muscle fibers are innervated by another anterior horn cell of a different fiber type (figure 4). 

see the speed of nerve fibers

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