Somehow that signaling leads to the release of endorphins and monoamines, chemicals that block pain signals in the spinal cord and the brain. It looks like it's nerves to the brain. The acupoints that have been mapped over thousands of years are likely the spots where nerves are concentrated. There have been a number of clinical trials showing that acupuncture is extremely useful for the nausea and vomiting caused by chemotherapy and early pregnancy. That's not the endorphin system.
Nobody knows how that works.
Nor does the endorphin story explain what Cho found when exploring acupoints that are traditionally used to treat vision problems. Designated by Cho as VA1, VA2, VA3, and VA8, they are not found near the eyes but on the outside of the foot, running from the little toe to the ankle. Acupuncturists say that these points are on the urinary bladder meridian and refer to them as BL67, BL66, BL65, and BL60, respectively. And they hold that stimulation of these points with needles will affect the eyes via the system of meridians rather than through the central nervous system. While standard MRI provides static cross-sectional pictures of structures in the body, functional MRI goes further to reveal how those structures are working.