Craniosacral Outflow: Cranial Nerve X and the Pelvic Splanchnic Nerves of the Parasympathetic Division of the ANS.

The vagus nerve (CN X) and three pelvic splanchnic nerves (S1, S2, S3) carry parasympathetic fibers to the infracranial portion of the body, where their preganglionic fibers synapse in diffuse ganglia present in the substance of the target viscus. Although its nucleus resides in the caudal brainstem, the vagus nerve does not supply structures in the head. Rather, it is a vagabond, supplying a host of tissues and organs in the thorax and abdomen. In the thorax, the vagus supplies the upper GI tract (for secretion and smooth muscle control), the heart, and respiratory airways. In the abdomen, the nerve innervates the gastrointestinal tract approximately to the left colic (splenic) flexure, other viscera derived from the gut tube, and kidneys. The remainder of the GI tract (descending colon, sigmoid colon, and rectum) and the urogenital structures (bladder and erectile bodies) are provided preganglionic parasympathetic innervation via the three pelvic splanchnic nerves. These arise from the lateral horn of spinal cord levels S2, S3 (largest contributor), and S4. A number of important clinical and surgical considerations derive from this anatomy.