Objective: To investigate the morphological relationship between cardiogenic referred pain and Heart Meridian of Traditional Chinese Medicine. Methods: A total of 11 SD rats anesthetized with 10% urethane (1 g/kg) were used in the present study. The fluorescent traces

fast blue (2～5%)

propidium iodide (2～5%) and bizbenzimide (2～5%) were injected into heart and the acupoints “Shaohai” (HT 3) and “Qingling” (HT 2) of the Heart Meridian and “Xiabai” (LU 4) and “Chize” (LU 5) of the Lung Meridian respectively

with 5 μL in each acupoint. The labeled cells were observed in dorsal root ganglia (DRG) from C 6 to T 5 segments. Results: After injection of the abovementioned three fluorescent traces into the heart and the acupoints of the Heart Meridian and Lung Meridian of the left forelimb the ratios of the single-labeled cells over the total-labeled neurons in DRG of the C 6～T 5 segments were 29.07%

24.03% and 24.42% respectively; the ratios of the double-labeled neurons over the total-labeled neurons were 12.14% and 7.5% following injection of the traces into the heart and the acupoints on the left Heart Meridian separately; and the ratio of triple-labeled cells over the total-labeled neurons was 1.16% after injection of the traces into the heart and the acupoints on the left Heart Meridian and Lung Meridian respectively. After injection of the three fluorescent traces into the acupoints with the same names as those mentioned above on the right forelimb the ratios of single-labeled over the total-labeled neurons were 26.98%

26.89% and 21.97% respectively; the ratios of the double-labeled/total-labeled neurons after injection of the traces into the heart and right Heart Meridian

and the heart and right Lung Meridian were 10.81% and 9.58% respectively; and the ratio of the triple-labeled neurons to the total-labeled neurons following injection of the traces into the heart and the acupoints of the right Heart Meridian and the right Lung Meridian was 2.02%. Conclusion: The present study provides direct evidence showing the existence of dichotomized peripheral afferents that supply both visceral and somatic structures. The relationship between the Heart Meridian and the heart organ is closer than that between the Lung Meridian and the heart.