While a bit outdated and lacking up-to-date evidence of the anatomical basis for acupuncture, this wonderful summary of the physiological basis for acupuncture still holds a solid review of the understanding of the impacts and mechanisms of acupuncture from a biomedical perspective:
"Several conclusions can be made based on the above neurophysiologic studies. First, afferent nociceptive pathways are essential for acupuncture analgesia. Second, acupuncture analgesia is mediated by way of various endogenous neurotransmitters, systemic release of enkephalin and dynorphin, and probably by decreasing the local inflammatory response via N-methyl-d-aspartate receptors. Third, the acupuncture-induced increase in pain threshold is gradual, with a peak effect at 20–40 min, followed by an exponential decay with a half-life of approximately 16 min. Fourth, a prolonged period of acupuncture stimulation results in tolerance that is mediated via release of cholecystokinin octapeptide. Lastly, immunocytochemistry studies indicate that both pain and acupuncture activate the hypothalamic-pituitary-adrenocortical axis."
If you notice, there is a reference to the research that corroborates the classic understanding of acupuncture's benefits peaking at around 25-35 minutes.
We also see the early summary of functional imaging studies of acupuncture and its impacts and effects:
Even in 2008, when this paper was published in Anesthesia and Analgesia, there were sufficient imaging studies that we could understand the basic effects and impacts of acupuncture on the nervous system:
"Several conclusions can be made based on the above CNS imaging studies. First, the hypothalamus may play a central role in acupuncture analgesia. Second, the significant overlap between acupuncture and pain CNS pathways suggests that acupuncture stimulation may affect pain signals processed in the CNS. Third, superficial needling and traditional acupuncture needling activate two different central pathways and yet both provide clinical analgesia."
Much research has progressed these areas of understanding since then, allowing us to better understand, explain, and apply acupuncture to help our patients. This kind of acupuncture research is endlessly fascinating as it unifies classic empirical evidence and clinical tradition with modern biomedicine in ways that inspire us and remind us of how amazing and how elegant the body and acupuncture are.
Shaw, V., Diogo, R., & Winder, I. C. (2020). Hiding in Plain Sight ‐ ancient Chinese anatomy. The Anatomical Record, ar.24503. https://doi.org/10.1002/ar.24503
Wang, S.-M., Kain, Z. N., & White, P. (2008). Acupuncture Analgesia: I. The Scientific Basis. Anesthesia & Analgesia, 106(2), 602–610. https://doi.org/10.1213/01.ane.0000277493.42335.7b