In order to make since of how Micro-Current is functioning in the body you first have to have some understanding of ATP. Let’s start with the definition of it.

Adenosine Triphosphate (ATP), is an organic compound serving as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of protein. 
Extracellularly, ATP has been found to act as a neurotransmitter . ATP receptors are widespread throughout the body. On its own it is known to have effects in the arteries, intestines, lungs, and bladder. It is also often released in tandem with other neurotransmitters, perhaps to add chemical stability.

ATP molecules are the storage and distribution vehicles for energy in the body. The breakdown of ATP into ADP (adenosine diphosphate) yields energy. It is the cleaving of the phosphate bond that yields the energy.

This energy is utilized in almost all energy related cellular reactions, in addition to being integral to the function of virtually every cell in our body.

In muscle contraction, the process occurs as such: each muscle spindle is composed of muscle fibers. Inside the muscle fibers are many muscle fibrils (wherein lies collagen formation). These muscle fibrils are suspended in a fluid matrix called sarcoplasm. Suspended in the sarcoplasm are thousands and thousands of mitochondria, which contain large amounts of ATP.
It should be noted that along with ATP, magnesium is very necessary in ATP energy releasing reactions. Before ATP can become "active ATP," magnesium must bind between the second and third phosphate. Clinically, magnesium deficiency may be related to such conditions as fibromyalgia and chronic fatigue syndrome.

The full explaination can be researched at:
----www.chiroweb.com/mpacms/dc/article.php?id=36227