Functional magnetic resonance imaging studies of acupuncture at ST36: a coordinate-based meta-analysis

Acupuncture, the ancient Chinese practice that involves the insertion of needles at specific points on the body, has been gaining growing scientific recognition for its therapeutic benefits. Among the most commonly used points is Zusanli (ST-36), located three centimeters below the knee joint on the anterior aspect of the leg. This point is traditionally used in the treatment of various conditions, ranging from gastrointestinal problems to cognitive disorders and pain. With the development of modern neuroscience, functional magnetic resonance imaging (fMRI) has allowed researchers to observe how acupuncture affects the human brain in real time.

However, individual studies on the ST-36 point presented inconsistent results, making it difficult to understand the neural mechanisms involved in this therapy.

To clarify these issues, researchers conducted a comprehensive meta-analysis, combining data from multiple functional MRI studies on acupuncture at the ST-36 point. The objective was to create a reliable "brain map" of this acupuncture point, identifying which areas of the brain are consistently activated when ST-36 is stimulated. The researchers followed a rigorous protocol, previously registered in scientific databases, to ensure transparency and methodological quality. They performed extensive searches in multiple databases through August 2021, including studies in different languages.

The inclusion criteria were rigorous: only studies that used fMRI to investigate the effects of acupuncture on the human brain, involving exclusively healthy participants, with true (non-sham) acupuncture at the ST-36 point, and that reported precise three-dimensional brain coordinates. For analysis, they employed an advanced statistical technique called SDM-PSI (Seed-based d mapping with permutation of subject images), considered state-of-the-art in neuroimaging meta-analyses.

The analysis included 27 studies involving 542 healthy participants. The results revealed a consistent pattern of brain activation when the ST-36 point was stimulated. The main areas activated included the bilateral cerebellum (an important structure for motor coordination and cognitive functions), the bilateral Rolandic operculum (region involved in processing bodily sensations and body awareness), the right supramarginal gyrus (area related to temporal perception, attention, and memory), and specific lobes of the cerebellum. Interestingly, the analysis showed low heterogeneity between studies, indicating that these results are consistent and reliable.

There was no evidence of publication bias, strengthening the validity of the findings.

To better understand the significance of these brain activations, the researchers investigated the functional characteristics of the identified areas. They discovered that stimulation of ST-36 is mainly associated with functions related to action and perception, including motor control, cognition, learning, memory, and body awareness. The cerebellum, for example, not only coordinates movements but also participates in cognitive and emotional processing, in addition to playing an important role in visceral pain, which explains why ST-36 is traditionally used for digestive problems. The Rolandic operculum processes signals that integrate external and internal sensations, contributing to body awareness and self-perception.

The right supramarginal gyrus is involved in complex cognitive functions such as temporal perception, episodic memory, and emotion recognition.

A particularly important discovery was the distinction between brain activations related to the painful sensation of the needle versus specific therapeutic effects of acupuncture. The researchers identified that some activated areas (such as the cerebellum, insula, and superior temporal cortex) are part of the pain neural network, reflecting the physical stimulation of the needles. However, other regions (including the right inferior frontal gyrus, left superior temporal gyrus, and specific areas of the cerebellum) represent networks unrelated to pain, suggesting specific therapeutic mechanisms of acupuncture. This differentiation is crucial for understanding how acupuncture produces therapeutic benefits beyond simple sensory stimulation.

The clinical implications of these findings are promising for patients and health care professionals. The brain mapping of ST-36 provides a solid scientific basis for its therapeutic use, especially in conditions involving motor function, cognitive processes, learning and memory problems, as well as issues related to body awareness. For patients, this means greater confidence in the efficacy of acupuncture based on concrete neurobiological evidence. For professionals, these results can guide more precise and informed therapeutic decisions.

The identification of specific patterns of brain activation also paves the way for more personalized therapies, where the choice of acupuncture points can be based on specific neurological goals.

The study has important limitations that should be considered. The included studies varied in experimental design, analytical methods, and software used, although the low heterogeneity suggests that this did not significantly compromise the results. The analysis focused exclusively on healthy participants, limiting generalization to populations with specific diseases. In addition, the relatively small number of studies included requires caution when interpreting the results.

The lack of rigorous control over sham acupuncture designs also reduces the reliability of conclusions about the specificity of the effects.

This research represents a significant advance in the scientific understanding of acupuncture, providing the first comprehensive and reliable brain map of the ST-36 point. The results demonstrate that acupuncture produces specific and consistent patterns of brain activation, validating its neurobiological mechanisms. Looking ahead, these findings lay the foundation for the development of more precise and personalized acupuncture therapies. The possibility of mapping other acupuncture points using similar methodologies may eventually create a complete brain atlas of acupuncture, revolutionizing clinical practice by allowing treatments based on specific neurological targets.

Future studies should include larger samples, more rigorous designs, and investigation of clinical populations to expand and validate these promising findings.