A neurological explanation for the placebo effect?

It has been well documented in medical literature that when people believe they are receiving treatment, they will actually experience a reduction in symptoms—even if their “treatment” is an inactive placebo. This is particularly true when it comes to pain reduction, or analgesia; patients who believe they are being given powerful medication for pain will actually experience a drop in discomfort, even if no active pain medication has been administered. Yet what is it about how the brain is wired that causes this effect?