Autonomic Nervous System
The Autonomic Nervous System consists of parasympathetic and sympathetic fibers. The vagus, along with the pelvic splanchnic plexus, forms the main parasympathetic supply to the foregut, midgut and hindgut. It stimulates peristalsis and secretion of the glands in the gut, thereby assisting in the digestion of ingested food. The thoracic splanchnic nerves, along with the greater and the lesser, and a few fibers from the upper lumbar splanchnic nerves, form the sympathetic innervation which prevents digestion by diverting blood flow to the limbs and trunk for the “flight or fight response.”
The parasympathetic nerve stimulation results in increased intestinal peristalsis with the opening of the valves, while sympathetic stimulation causes closure of the valves with inhibition of motility. The sight, smell and taste of food stimulate the vagus. This effect can last for up to thirty minutes and leads to the direct stimulation of the chief cells in the stomach to secrete pepsinogen in anticipation of digesting proteins, and indirectly stimulates the gastric parietal cells to secrete hydrochloric acid.
The tenth cranial nerve, the vagus, forms the anterior and posterior trunks and carries the parasympathetic fibers to the esophagus and the abdomen. The vagal trunks carry visceral afferent fibers to the periarterial and aortic plexuses, as well as presynaptic parasympathetic fibers.
The anterior rami of the 2nd, 3rd, and 4th sacral nerves help to form the pelvic splanchnic nerves which carry presynaptic parasympathetic fibers to the pelvic plexus. The walls of the abdominal organs also have parasympathetic ganglia.
The sympathetic fibers arise from the cells of the lateral horn of the seventh thoracic to the second lumbar vertebrae. The presynaptic fibers travel through the anterior nerve roots, then the anterior rami and the communicating branches of the spinal nerves towards the sympathetic trunks. The presynaptic fibers pass through the paravertebral ganglia without synapsing and then form the splanchnic nerves, but, as they pass through the prevertebral ganglia and autonomic plexuses located along the abdominal aorta and its branches, they synapse with the postsynaptic neurons. Sympathetic fibers are sensory, as well as motor. The sensory fibers respond to the distension of the viscera. The sympathetic chain terminates at the ganglion impar in the pelvis.
There are two main splanchnic nerves:
- Thoracic splanchnic nerves: carry the majority of the presynaptic sympathetic fibers through the greater, lesser and the lowest or least splanchnic nerves. They accompany the aorta as it passes through its diaphragmatic hiatus into the abdomen. Their fibers contribute to the celiac, aorticorenal and superior mesenteric ganglia and plexuses.
- Lumbar splanchnic nerves: they originate within the abdominal sympathetic trunk and form about three to four lumbar splanchnic nerves which enter the intermesenteric, inferior mesenteric and hypogastric plexuses.
Autonomic Plexuses in the Abdomen
The Autonomic plexuses in the Abdominopelvic cavity carry both sympathetic and parasympathetic fibers. They are located in the abdominal aorta and its various branches. The different plexuses are the celiac, aortic, hepatic, superior mesenteric, inferior mesenteric, intermesenteric, renal, superior hypogastric and the right and left inferior hypogastric plexuses.
Celiac plexus: is situated at the origin of the celiac trunk at the level of the 12th thoracic and 1st lumbar vertebra. It contains the paired celiac ganglia and receives parasympathetic fibers from the vagus nerve and sympathetic fibers from the lesser and greater splanchnic nerves.
Left gastric plexus: innervates the lesser curvature of the stomach.
Hepatic plexus: derives fibers from the celiac plexus through the right and left vagus nerves, as well as the phrenic nerves. It is situated around the hepatic artery, portal vein and their branches. It provides fibers to the cystic plexus.
Splenic plexus: receives fibers from the right vagus nerve, the celiac plexus and the left celiac ganglion. It innervates the blood vessels, the splenic capsular smooth muscle and trabeculae.
Suprarenal plexus: is connected to the celiac plexus and sends fibers to the suprarenal medulla.
Superior mesenteric plexus is located on the superior mesenteric artery. It is formed by fibers from the celiac plexus and the lesser and least splanchnic nerves. It sends fibers to the pancreas, the duodenum, jejunum, ileum and the proximal two-thirds of the transverse colon (midgut derivatives).
Inferior mesenteric plexus is located on the inferior mesenteric artery and is formed by the fibers from the intermesenteric plexus and the second and third lumbar splanchnic nerves. It innervates the distal one-third of the colon and the rectum.
Aortic plexus: is situated around the abdominal aorta and its branches. It supplies the inferior vena cava and connects the superior and inferior mesenteric plexuses.
Intermesenteric plexus is located between the superior and inferior mesenteric plexus and provides fibers to the renal, testicular and uterine plexuses.
Renal plexus: is formed by fibers from the celiac ganglion and plexus, the aortic plexus, first lumbar splanchnic and the least splanchnic nerve. It is situated on the renal arteries. It gives off fibers to the ureteric and gonadal plexuses.
Superior hypogastric plexus is a continuation of the intermesenteric plexus and lies anterior to the abdominal aortic bifurcation, fifth lumbar vertebra and the sacral promontory. It receives fibers from the third and fourth lumbar splanchnic nerves and the aortic plexus. It bifurcates into the right and left hypogastric nerves which join the inferior hypogastric plexus inferiorly.
Inferior hypogastric plexus receives fibers from the superior hypogastric plexus and from the pelvic splanchnic nerves (S 2- 4) which provide the parasympathetic fibers. It is located lateral to the base of the urinary bladder, vagina and the rectum against the posterolateral pelvic wall. It supplies nerve fibers to the hindgut derivates, namely the descending colon, the sigmoid colon, the rectum, as well as the pelvic organs up to the anorectal junction. It gives rise to several plexuses: the middle rectal plexus, the vesical plexus, the prostatic plexus, the uterovaginal plexus and the deferential plexus (plexus of the ductus deferens).
The portion of the anal canal and external anal sphincter below the pectinate line are not derived from the hindgut and are innervated by the somatic pudendal nerve; hence, the anal canal below the pectinate line is very sensitive to pain.
Enteric Nervous System
It is formed with parasympathetic fibers from the vagus and sympathetic fibers from the prevertebral ganglia and is located within the lining of the gastrointestinal tract. The enteric motor neurons act on the intestinal smooth muscles to control peristalsis, while the other neurons control enzymatic secretion. The enteric neurons are aggregated in two types of ganglia /plexuses:
Myenteric (Auerbach’s) plexus located between the layers of the muscularis externa.
Meissner’s submucosal plexus located within the submucosa of the small intestine.
Cerebrospinal nerves: The parietal peritoneum from the sixth thoracic vertebra to the twelfth thoracic vertebra is innervated by cerebrospinal nerves with the same segmental supply as the lower thoracic dermatomes. The visceral peritoneum has no innervation.
Abdominal pain is transmitted via splanchnic and cerebrospinal afferents nerve fibers. The splanchnic afferent nerve receptors, the free nerve endings, and the Pacinian corpuscles, are located within the walls of the viscera and are stimulated only by spasm or distension (stretch). On the other hand, the cutaneous receptors of the cerebrospinal nerves are stimulated by temperature, pressure, friction or cuts. The ganglia of the splanchnic nerves and the cerebrospinal nerves lie next to each other within the dorsal root ganglia. This close proximity may account for the spilling over of visceral pain arising from rapid distension into the cerebrospinal nerve ganglia, resulting in “referred pain” to the skin overlying the affected viscus.
As the gastrointestinal organs are derived from the primitive embryonic gut as midline structures, their innervation from the splanchnic nerves is bilateral and hence pain originating from the viscera is first noted in the midline; hence, pain arising from the foregut viscera is referred to the region below the costal intersection and epigastrium. The pain from the midgut viscera is referred to the umbilical region and the pain from the hindgut viscera is referred to the suprapubic region.
- Understanding the innervation of the gastrointestinal tract is essential to accurately diagnose the cause of abdominal pain. This is best explained in acute appendicitis when the pain starts initially in the midline, periumbilical region, as splanchnic nerves are only stimulated by stretch and distension within the hollow, inflamed appendix. As the condition progresses and the parietal peritoneum overlying the appendix becomes inflamed, the pain localizes to the right iliac fossa overlying the appendix.
- Painful external hemorrhoids versus painless internal hemorrhoids: Here to the knowledge of the embryonic origin of the gastrointestinal tract and its innervations is important. As the internal hemorrhoids originate in the region of the hindgut supplied by the splanchnic plexuses, there is no sensation of pain. However, external hemorrhoids arise in the region inferior to the pectinate line supplied by the pudendal (somatic nerve) which is sensitive to pain.