Objective Previous studies indicated a close involvement of reflex activities of motoneurons in the spinal cord in the mechanism of meridian phenomena.The present study was designed to investigate the dendrite projections of meridianrelated motoneurons among the motoneurons and sympathetic preganglionic neurons in the spinal cord.Methods A total of 41 Sprague-Dawley rats were used in the present study.Cholera toxin B-subunit conjugated horseradish peroxidase(CB-HRP)containing 1.0% HRP was respectively injected to acupoint"Chengman"(ST 20)

"Liangmen"(ST 21)

"Guanmen"(ST 22)

"Taiyi"(ST 23)

"Huaroumen"(ST 24)

"Tianshu"(ST 25)and"Wailing"(ST 26)of the Stomach Meridian

and"Ganshu"(BL 18)

"Danshu"(BL 19)

"Pishu"(BL 20)

"Weishu"(BL 21)and"Sanjiaoshu"(BL 22)of the Bladder Meridian

and"Daimai"(GB 26)

"Wushu"(GB 27)

"Weidao"(GB 28)

"Juliao"(GB 29)

"Huantiao"(GB 30)

"Fengshi"(GB 31)

"Zhongdu"(GB 32)

"Xiyangguan"(GB 33)and"Yanglingquan"(GB 34)of the Gallbladder Meridian(for labeling preganglionic neurons)

and the celiac ganglion and superior mesenteric ganglion for labeling sympathetic preganglionic neurons.Three days after injection

the animals anesthetized were transcardia-cally perfused with 1.5%paraformaldehyde

the spinal cord was removed to be fixed routinely and then cut into sections for observing the labeled cells under microscope.Results In the ipsilateral ventral horn of the spinal cord

the motoneurons retrogradely labeled by CB-HRP formed dendritic projections oriented only to those motoneurons innervating the same meridian.In the longitudinal sections of spinal cord

the labeled motoneurons formed a bead-like column with a prominent network of longitudinal dendrites connecting the motoneurons innervating acupoints from the same meridian.In the transverse sections of spinal cord

two groups of dendrites from the labeled motoneurons projected to the identified sympathetic preganglionic regions:one group exten-ded dorsolateraly to the intermediolateral gray

another group extended intermediolateraly toward the central canal.In rats with injection of CB-HRP into both acupoint regions and ipsilateral celiac ganglion

the dendrites originated from the labeled motoneurons projected directly to the labeled sympathetic preganglionic neurons.Conclusion Each of the ST

BL and GB meridians is innervated by a specific group of motoneurons in the spinal cord.The motoneurons form a column with distinct border in the ventral horn of spinal cord

and the dendritic projections from the motoneurons oriented only to those innervating the same meridian.The dendrites from the meridian-related motoneurons can specifically project to the sympathetic preganglionic neurons at the thoracolumbar level.