It acts by several mechanisms:
1. Gate control theory- stimulation of larger diameter nerve fibers inhibits pain carrying small diameter fibers.
2. Secretion of putative neurotransmitters that inhibits pain signals.
3. It produces supraspinal inhibition of pain either by inhibiting pain transmission or modulation of pain impulses.
4. Sympathetic inhibition by some mechanisms.
In Spinal Cord Stimulation, implanted electrodes stimulate it. In TENS electrodes applied over skin. Former is better as several other mechanisms acts to relieve pain.
Simulation of nerves electrically with implanted leads close to the involved segment or nerve with/ without implanted power generator to inhibit or block the sensation of pain is called Neuro-stimulation.
When the lead is implanted in the epidural space to stimulate spinal cord itself is called Spinal Cord Stimulation.
TENS is Trans-cutaneous Electrical Nerve Stimulation. Here nerve or spinal segment is stimulated with superficial current applied over the skin of same dermatome.