The parasympathetic division of the ANS originates (bilaterally) from the brainstem and from sacral segments of the spinal cord. Preganglionic neurons synapse in discrete autonomic ganglia except in the case of the vagus nerve. Preganglionic axons in the vagus nerve synapse on terminal ganglia located within plexuses inside the visceral organ being innervated. A microscope is required to view terminal ganglia.

Sacral parasympathetic preganglionic neurons are located in intermediate gray matter of spinal cord segments (S1,2,3 in the carnivore). Bilaterally, preganglionic axons run through the lumbosacral plexus and pelvic nerve to synapse on pelvic ganglia within the pelvic plexus. Post ganglionic axons innervate pelvic viscera including the descending colon. Note: it is reported that 50% of the axons in the pelvic nerve are afferent (General Visceral Afferent).

Vagal parasympathetic preganglionic neurons arise from the parasympathetic nucleus of the vagus nerve (cardiac preganglionics reside in nucleus ambiguus). In the thorax, preganglionic axons leave the vagus nerves to innervate terminal ganglia in the heart and lungs. Bilaterally, preganglionic axons enter the abdomen via dorsal and ventral vagal nerve trunks. The axons reach terminal ganglia of abdominal viscera, including the transverse colon, by running in nerve plexuses on celiac and cranial mesenteric vessels. Note: it is reported that 80% of the axons in the vagus nerve are afferent (mostly, General Visceral Afferent).

Other brainstem parasympathetic nuclei give rise to preganglionic axons that run in the oculomotor nerve (to smooth muscle within the eye); facial nerve (to lacrimal glands, nasal glands, and ventral salivary glands); and the glossopharyngeal nerve ( to dorsal salivary glands). Brainstem nuclei are illustrated below.

Left. Parasympathetic pathways are illustrated for three cranial nerves that innervate viscera located in the head. The oculomotor nerve innervates smooth muscle within the eye. Facial and glossopharyngeal nerves innervate glands.

Right. Brainstem locations of parasympathetic nuclei are shown from three proints of view (dorsal, sagittal, and transverse).