pelvic splanchnic nerves Pelvic splanchnic nerves carry parasympathetic fibers from the lateral horn of the spinal cord at the S2, 3, and 4 levels. They can be seen coming off of the ventral primary rami of S2, 3, and 4 and going to the inferior hypogastric plexus. These nerves provide parasympathetic innervation to the pelvic viscera and the GI tract distal to the left colic flexure. (Remember, the vagus gives parasympathetic innervation to the rest of the gut.) Sacral splanchnic nerves come off the sacral sympathetic chain ganglia, carrying sympathetic fibers that will go to the inferior hypogastric plexus. To remember the difference between the pelvic and sacral splanchnics, just remember that the sacral splanchnics are named after a spinal cord segment, just like the thoracic, lumbar, and cervical splanchnics that you know and love. These all carry sympathetic fibers. Pelvic splanchnics are not named after a spinal cord segment and they're different--they carry parasympathetic fibers.
The grey rami communicantes are structures that postganglionic sympathetic neurons travel on to get out of the sympathetic trunk and rejoin a spinal nerve. There are gray rami in the pelvis. The white rami communicantes are structures that preganglionic sympathetic fibers use to get out of a spinal nerve to enter the sympathetic trunk. White rami are seen between the T1 to L2 levels, but not in the pelvis. Finally, the hypogastric nerves carry postganglionic sympathetic nerves from the superior hypogastric plexus to the inferior hypogastric plexus.