Pain "Gate" theory

Although the theory accounts for phenomena that are primarily mental
in nature - that is, pain itself as well as some of the psychological
factors influencing it - its scientific beauty is that it provides a
physiological basis for the complex phenomenon of pain. It does this
by investigating the complex structure of the nervous system, which is
comprised of the following two major divisions:

http://www.spine-health.com/conditions/chronic-pain/modern-ideas-gate-control-theory-chronic-pain

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