text: Mitochondria and other membranous organelles are normally enriched in the PNP region of peripheral myelinated axons, especially those large caliber axons. The actual physiological role of this accumulation and factors that regulate it are not understood; however, it is known that mitochondria are usually present in areas of the cell that expresses a high energy demand.  In these same regions, they are also understood to contain growth cones, synaptic terminals, and sites of action potential initiation and regeneration, such as the nodes of Ranvier.  In the synaptic terminals, mitochondria produce the ATP needed to mobilize vesicles for neurotransmission.  In the nodes of Ranvier, mitochondria serve as an important role in impulse conduction by producing the ATP that is essential to maintain the activity of energy-demanding ion pumps.  Supporting this fact, about five times more mitochondria are present in the PNP axoplasm of large peripheral axons than in the corresponding internodal regions of these fibers.