The sacral component of the parasympathetic nervous system includes the cell bodies of presynaptic parasympathetic nerves in the lateral horns of spinal cord segments S2, S3 and S4. These fibers exit the spinal cord via the ventral roots of segments S2, S3 and S4, pass through the spinal nerve, and then enter the ventral primary rami. Pelvic splanchnic nerves, containing presynaptic parasympathetic fibers, branch from the ventral primary rami shortly after dorsal and ventral primary rami branch. (Here is another type of splanchnic nerve, so we have now encountered splanchnic nerves that are presynaptic, postsynaptic, sympathetic and parasympathetic. So, don't learn these nerves as simply splanchnic!) The pelvic splanchnic nerves provide parasympathetic innervation to the descending and sigmoid colon, the rectum, and pelvic viscera via the inferior hypogastric plexus. Parasympathetic innervation to the colon and rectum increases peristalsis, and inhibits the internal anal sphincter to allow for defecation. Parasympathetic innervation to the bladder inhibits contraction of the internal sphincter of the bladder and contracts the detrusor muscle of the bladder wall allowing for urination. The postganglionic cell bodies for the pelvic splanchnics are in microscopic terminal ganglia that sit on or in the wall of the target organ.