Duodenum, Pancreas, and Abdominal Aorta LO5
5. Describe the autonomics of the abdominal portion of the gastrointestinal system.
Enteric Nervous System:
Embedded within the wall of the gut tube is the Enteric Nervous System (ENS), a complex series of nerve fibers and ganglia located either between the longitudinal and circular layers of the muscular wall (myenteric/Auerbach’s plexus), or within the submucosa (submucosal/Meissner’s plexus). The ENS is part of the autonomic nervous system (ANS), and it may also function independently.
Autonomic control of the gut tube function may be loosely divided into functions that are typically parasympathetic (stimulates gut motility, stimulates glandular secretion, vasodilation, & sphincter relaxation), and functions that are typically sympathetic (inhibits gut motility, inhibits glandular secretion, vasoconstriction, & sphincter contraction).
The fibers supplying the ENS typically follow arteries to the viscera. These fibers contribute to what are known as perivascular plexuses, networks of both sympathetic and parasympathetic fibers sometimes named for the vessels they travel along and/or originate near. Perivascular plexuses give input to the ENS and also control the smooth muscle of the tunica media of arterial vessels, especially at the level of the arterioles.
Perivascular plexuses of and about the aorta are contiguous elements of the ANS; fibers are shared between and among names plexuses, and there is not a clear, specific pattern of innervation from any particular thoracic splanchnic n. to any particular organ. Many named plexuses share a close proximity of similarly named ganglia (where presynaptic sympathetic fibers synapse, and where presynaptic parasympathetic fibers travel through). The existence and relative locations of individual ganglia may vary. Some major perivascular plexuses and the regions they innervate include:
- celiac plexus - all viscera served by the celiac trunk & branches (foregut)
- superior mesenteric plexus - all viscera served by the SMA & branches (midgut)
- renal plexus - kidneys and associated vasculature
- intermesenteric/aortic plexus - indistinct plexus of autonomic fibers between the celiac & superior mesenteric plexuses (superiorly) and the inferior mesenteric & superior hypogastric plexuses (inferiorly)
- inferior mesenteric plexus - all viscera served by the IMA (hindgut)
- superior hypogastric plexus - fibers communicate between aortic plexus (abdomen) and the inferior hypogastric plexuses (pelvis); communication to ureteric, gonadal, and iliac perivascular plexuses
The preganglionic sympathetic fibers that serve abdominal viscera and local perivascular plexuses come from the thoracic and lumbar splanchnic nn. These fibers synapse in pre-aortic (pre-vertebral) ganglia, which are often named for their associated perivascular plexus.
Thoracic splanchnic nn.
- Greater thoracic splanchnic nn. (T5-9) → celiac ganglia & suprarenal medullae
- Lesser thoracic splanchnic nn. (T9/10-11) → aorticorenal ganglion & celiac ganglia
- Least thoracic splanchnic nn. (T11/12) → renal plexuses (n.b. often the least t.s.n. is absent , and T12 fibers may be routed with the lesser t.s.n.).
Lumbar splanchnic nn.
- L1 → celiac & superior & inferior mesenteric plexuses, and renal plexus
- L2 → intermesenteric/aortic & inferior mesenteric plexuses
- L3 → superior hypogastric plexus
- L4 → superior hypogastric plexus & hypogastric nn.
The preganglionic parasympathetic fibers that serve abdominal viscera and local perivascular plexuses come from the vagal trunks (from vagus nn.) and the pelvic splanchnic nn. Recall that parasympathetic preganglionic fibers are relatively long and synapse in the walls of their target organs (i.e. either in the walls of arterioles, or in ganglia of the myenteric or submucosal plexuses).
Vagal trunks: The vagal trunks are formed by the L. & R. vagus nn. The anterior vagal trunk predominantly consists of fibers from L. vagus n. (with some contributions from the R. vagus n.), and the posterior vagal trunk predominantly consists of fibers from the R. vagus n. (and some fibers from the left vagus n.). The vagal trunks both innervate the thoracic & abdominal parts of the esophagus and the stomach. The ultimate targets of vagal trunks differ beyond the stomach.
- Anterior vagal trunk: enters the abdominopelvic cavity via the esophageal hiatus of the diaphragm (~T10), and divides shortly thereafter into hepatic, gastric, and pyloric brs. The hepatic br. serves the liver, gallbladder, and extrahepatic biliary tree. The gastric and pyloric brs. serve the anterior aspects of the stomach.
- Posterior vagal trunk: the posterior vagal trunk enters the abdominopelvic cavity via the esophageal hiatus of the diaphragm (~T10), with fibers contributing to the celiac plexus (and plexuses beyond). Many fibers will move through the celiac ganglia, but they do not synapse. Preganglionic parasympathetic fibers of the posterior vagal trunk serve the GIT and associated organs of the foregut and midgut and the perivascular plexuses associated with these regions.
Pelvic splanchnic nn.: The pelvic splanchnic nn. originate from the ventral primary rami of S2-4 as they exit the ventral foramina of the sacrum. The pelvic splanchnic nn. conduct preganglionic parasympathetic fibers to the perivascular plexuses and viscera of the pelvis and hindgut. The majority of the fibers will directly enter the inferior hypogastric plexuses (the major autonomic organizational plexuses of the pelvis), and fibers may either ascend from the IHP to the superior hypogastric plexus via hypogastric nn., of they make take an independent pathway over the pelvic brim to the inferior mesenteric plexus (to the hindgut). These fibers may bypass the IHP plexus altogether. The pelvic splanchnic nn. as well as the sacral splanchnic nn. (pelvic sympathetic outflow) will be more thoroughly discussed in the reproduction sessions.
General visceral afferent (GVA) fibers conduct sensory information from viscera and perivascular plexuses to the CNS. This information may relate to reflexes, distention, traction, or ischemia. GVA fibers that relate to smooth muscle reflexes are conducted with parasympathetics, whereas GVA fibers that relate to distention, traction, and ischemia (those loosely categorized as ‘pain’) travel with sympathetic fibers. While these are not true pain fibers, the brain will often interpret these signals as difficult to describe, poorly localized, and midline sensations of pain. Referred pain of abdominal viscera is localized to three regions:
- Epigastric - viscera and perivascular plexuses of the foregut
- Peri-umbilcal - viscera and perivascular plexuses of the midgut
- Suprapubic - viscera and perivascular plexuses of the hindgut
Below the levels of the peritoneum (typically, within the pelvis), GVA fibers that conduct ‘pain’ travel with parasympathetic fibers. Thus, the inferior-most extent of peritoneum roughly delineates the ‘pelvic pain line,’ above which ‘pain’ follows sympathetic fibers, and below which, ‘pain’ follows parasympathetic fibers. This will be discussed in greater detail in the reproduction sessions.