Ganglion impar

The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. It consists of four or five small sacral ganglia, connected together by interganglionic cords, and continuous above with the abdominal portion. Below, the two pelvic sympathetic trunks converge, and end on the front of the coccyx in a small ganglion, the ganglion impar (or ganglion of Walther).

Clinicial significance
Physicians at New Jersey Medical School specializing in Physical Medicine and Rehabilitation have published that sometimes even just a single local nerve block injection at the ganglion impar can give 100% relief of coccydynia (tailbone pain, also called coccyx pain), when performed under fluoroscopic guidance.