Motor unit recruitment, motor neurons increase with training?

When training your muscles for strength, does motor unit recruitment increase in the way that the neurons fire more efficiently and produce greater contractile strength, or does the amount of motor neurons increase in the muscle? Or both? Or neither?

Also, this is taken off medscape.com:

-"Increasing the number of active motor units (ie, spatial recruitment)"

Do they mean there are motor neurons in the vicinity of the muscle that are "dormant" or does the actual amount increase?

Also,

"As a general rule, motor units are recruited in order of their size.
When the muscle is activated initially, the first motor units to fire
are small in size and weak in the degree of tension they can generate."

Does this apply even if the weight being lifting initially is heavy for the lifter? Or would the larger motor units fire immediately if this was the case?

Edited March 24, 201312 yr by bodybuilder97