According to traditional Chinese medicine, meridians constitute an invisible network that connects body surface and viscera. Life-supporting substances, Qi and blood, circulate in meridians; acupuncture regulates meridian function. Since meridian channels have no dedicated anatomical structure and the concepts underlying acupuncture are difficult to rigorously test, acupuncture has remained controversial. Nevertheless, many published observations support the existence of meridians. We propose a mechanism for the objective sensations acupuncture elicits and present a hypothesis that, for the first time, is amenable to theoretical description and experimental testing. We postulate that meridians consist of interstitial spaces rich in nerve endings, muscles, and vessels within a connective tissue matrix. We propose that needle puncture elicits propagated sensations (DeQi) along the channels. Based on an analysis of published data, we posit that DeQi is a relay of action potential that is transmitted from one excitable cell to another, via two modes of ion transport: migration driven by gradients of extracellular, negative potential impulses; and ion diffusion originating from potential-associated charge density fluctuation in the interstitial fluid of meridian channels. According to the hypothesis, the initial negative potential derives from skin battery potentials, pierced cells, and the collagen piezoelectric/converse-piezoelectric effect. We further propose that acupuncture effects are due to the depolarizing action of the traveling negative potential that accompanies DeQi. This hypothesis explains a variety of observations and provides readily testable predictions of acupuncture action.