The first documentation the clearly describes the meridians was the The Yellow Emperor’s Classic of Internal Medicine, dating from about 100 BCE. There are 12 major meridians and 2 midline meridians. The 12 major meridians are further divided into 6 yang (downward flowing) and 6 yin (upward flowing) meridians that are paired based on whether the associated organs are considered 'hollow' or 'solid'. This pairing creates a looping, longitudinal circuit that is continuously flowing Chi up and down the body. These meridians have superficial aspects that allow them to be needled (acupoints) and deep pathways that run through the tissues and organs they’re named after.
In the early 1960s, a Korean researcher by the name of Bong-Han Kim proposed a system he called the Bonghan Ducts as the physical basis for the Chinese Meridian System. His work was largely passed over until in 2002 at the Seoul National University a team of researchers led by Professor Kwang-Sup Soh investigated further and conclusively confirmed that this system represented the physical construct of the acupuncture system. They renamed it the Primo Vascular system and have shown it exists in the same locations as the traditional meridian system acting as a third circulatory system after the cardiovascular and lymphatic systems. These "Primo vessels have bioelectrical activity, excitatory conductivity, and mechanical motility", Stefanov et el.
Here’s a great video from some researchers on how to identify the PVS using a staining dye injected into the body. In their example, they chose to look at a section of PVS inside of a Lymph vessel because the normally transparent tissue will clearly elucidate with the dye. The width of these tubes is 20-30 micrometers; contrast that with the thickness of human hair 60-120 micrometers. Thornton Streeter, a biofield scientist and recent guest on the podcast, has put together this lovely presentation on the PVS and its relations to both TCM and the Nadis of the Yogic/Ayurvedic traditions. https://www.slideshare.net/thorntonstreeter/primo-vascularmeridian-system-2016
Another previous guest, Rajan Narayanan, used this system as the basis for his study published in the International Journal of Yoga 2018. https://www.ncbi.nlm.nih.gov/pubmed/29755226 It seems the PVS ties together the endocrine, lymphatic, cardiovascular and nervous systems in an electromagnetic biomatrix that can be measured with Electro-Photonic Imaging. With this integrated perspective, Narayanan was able to use Pranayama (breath work), among other yogic techniques, as a successful and precise intervention for numerous patients.
Four meridians are known to travel to the brain including the Stomach/Spleen circuit which pass through the frontal lobes and the Liver/Gallbladder which flow through the brainstem and Limbic lobes (emotional centers). In particular, the liver meridian "connects to the hypothalamus, reticular structure, limbic system, eyes, visual pathways, proprioceptive pathways, vestibular system, cochlear and auditory pathways, and motor conduction pathways", Lui et al.
I recently came across some work at a joint Havard-MIT Bioemedical imaging center that used fMRI to see changes in the brain as a result of needling acupoints on various meridians in the body. Incredibly, they were able to show deactivation of the Limbic brain and Amygdala through needling the acupoints Large Intestine 4, Liver 3 and Stomach 36. In addition, this had an effect on deactivating the Default Mode Network, which is the same network affected in meditation.
An amazing correlation exists here in which LI4 and LV3 in TCM constitute the Four Gates. The Four Gates are reknown in acupuncture for creating physiological, homeostatic balance and a very relaxing, peaceful state.
And just in case you're interested in researching this topic further, here's another acupoint/fMRI study: The human brain response to acupuncture on same-meridian acupoints: evidence from an fMRI study.