Anatomy of the abdominal viscera: Innervation of the abdominal viscera

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Introduction0:00–0:34

Most of our abdominal organs, also called abdominal viscera, are a part of the digestive system. These include the stomach, the small and large intestine, the liver, gallbladder and pancreas.
The abdomen is also home to other organs, like the spleen, kidneys, suprarenal glands, and the ureters, which play important roles in other systems.
All of these organs work involuntarily, so they’re innervated by the autonomic nervous system. And luckily so - can you imagine trying to voluntarily control digesting each vegetable in a salad?!Ok, now remember that our nervous system is divided into the central nervous system and the peripheral nervous system.

Nervous system divisions0:34–1:29

Functionally, the peripheral nervous system is also divided into two parts. The somatic nervous system controlling the voluntary movement of skeletal muscles, and the autonomic nervous system that controls the involuntary movement of the smooth muscles and also the glands of our organs or viscera.
Now, the autonomic nervous system is also divided into two parts; the sympathetic, and parasympathetic nervous systems. The sympathetic nervous system is our fight or flight response and increases heart rate and breathing rate, while slowing down digestion through reducing peristalsis and constricting blood vessels to the gut to reduce blood flow.
The parasympathetic nervous system is our rest and digest response and it slows down heart rate, and increases digestion by promoting peristalsis, gland secretion, and dilation of gut vessels for increased blood flow.Alright, now, press pause, and see if you can tell whether the sympathetic or the parasympathetic nervous system promotes digestion!Let’s take a deep dive by first looking at the sympathetic nervous system, which consists of the abdominopelvic splanchnic nerves, prevertebral ganglia, the abdominal aortic plexuses and their extensions, the periarterial plexuses.

Quiz1:29–1:37

Sympathetic nerve supply1:37–1:53

Splanchnic nerves mean “nerves for the internal organs,” so they innervate the abdominal viscera. Abdominopelvic splanchnic nerves contain sympathetic fibers, which arise from cell bodies in the lateral horn of the gray matter of the spinal cord, from T5 to L2 or L3 spinal segments.

Abdominopelvic splanchnic nerves1:53–2:59

The fibers first pass through the anterior roots, the anterior rami, and the white rami communicantes of the thoracic and upper lumbar spinal nerves.
Then, they reach the chain of paravertebral ganglia, which are the sympathetic ganglia lying close to the spine, which are linked to form the sympathetic trunk.
Sympathetic fibers pass through the paravertebral ganglia without synapsing, then they enter the different abdominopelvic splanchnic nerves, which carry these fibers to the prevertebral ganglia in the abdomen.
There are two major groups of abdominopelvic splanchnic nerves based on which part of the sympathetic trunk they arise from.
These are the lower thoracic splanchnic nerves, which arise from the thoracic part of the sympathetic trunk, and the lumbar splanchnic nerves, which arise from the abdominal part of the sympathetic trunk.
Let's first look at the sympathetic pathway for the lower thoracic splanchnic nerves, which enter the abdominal cavity by piercing through the crus of the diaphragm.

Lower thoracic splanchnic nerves2:59–4:37

These nerves carry their preganglionic fibers to different ganglia located along the aortic plexuses, which are networks of nerve fibers overlying the abdominal aorta.
The greater splanchnic nerves, arising from T5 to T9, carry their fibers to the celiac ganglia which are located near the celiac arterial trunk.
The lesser splanchnic nerves, which arise from T10-T11, and the least splanchnic nerves, which arise from T12, synapse in the aorticorenal ganglia which are located on either side of the superior mesenteric artery, and the superior mesenteric ganglion located just superior to the superior mesenteric artery.
After synapsing, the postganglionic fibres travel along periarterial plexuses to their target organs. Postganglionic fibres from the celiac ganglia travel along the plexus on the arterial branches from the celiac trunk, and they mainly provide sympathetic innervation to the foregut, meaning the stomach, and the proximal part of the duodenum.
These fibres also innervate the liver, the gallbladder, pancreas, and spleen. Now the postganglionic fibers from the superior mesenteric ganglion provide sympathetic innervation to the midgut; that is the part of the gut from the distal parts of the duodenum through the proximal two thirds of the transverse colon, including the small intestine.
The postganglionic fibers from the aortico-renal ganglion provide the sympathetic innervation to the kidneys and some preganglionic fibres travel through this ganglion to the adrenal medulla where they synapse directly with cells within it.Next up, are the lumbar splanchnic nerves, which carry the sympathetic fibers from the abdominal part of the sympathetic trunk, which is from the L1 to L2 or L3 spinal segments.

Lumbar splanchnic nerves4:37–5:03

Preganglionic fibers of the lumbar splanchnic nerves synapse in the inferior mesenteric ganglion. Postganglionic fibers emerging from this ganglion provide sympathetic innervation to the hindgut which is the transverse colon, descending colon, sigmoid colon, and rectum.

Quiz5:03–5:09

Now, for a quick break. Can you remember where the greater splanchnic nerve synapses?Alright, now let’s get into the “rest and digest” mood, and talk about the parasympathetic innervation of the abdominal viscera which comes from the anterior and posterior vagal trunks, and the pelvic splanchnic nerves.The anterior and the posterior vagal trunks are the continuation of the left and the right vagus nerves that emerge from the esophageal plexus.

Parasympathetic nerve supply5:09–5:25

Vagus nerves5:25–6:05

They enter the abdominal cavity through the esophageal hiatus in the diaphragm, carrying preganglionic parasympathetic fibers through the abdominal aortic plexuses, the periarterial plexuses and finally synapse in enteric ganglia which are located on or within the organs.
The vagus nerves in the abdomen supply parasympathetic innervation to the lower esophagus, stomach, small intestine, ascending colon and most of the transverse colon.
The pelvic splanchnic nerves carry parasympathetic innervation to the final part of the gut, from the descending colon to the anus.

Pelvic splanchnic nerves6:05–6:33

Preganglionic fibers of the pelvic splanchnic nerves originate from the S2 to S4 spinal segments, and travel to the inferior hypogastric plexuses without synapsing.
From here, these fibres travel through the periarterial plexuses to synapse in the enteric ganglia of the descending colon, sigmoid colon, rectum and pelvic organs.
Now, let’s have a look at the sensory innervation of the abdominal viscera, which consists of afferent fibers, carrying visceral sensory information from the organs to the spinal cord.

Sensory supply6:33–7:36

Visceral afferent fibers for the abdominal viscera, up to the midpoint of the sigmoid colon, travel alongside sympathetic efferent fibers in the lower thoracic and lumbar splanchnic nerves, but in the opposite direction.
This means that an impulse starts from sensory receptors of a particular visceral organ, then passes along the splanchnic nerves to the sympathetic trunk, from where it moves through the white rami communicantes to the anterior rami of the spinal nerves.
At this level, afferent fibers diverge from the regular course of the sympathetic fibers, because instead of taking the anterior root pathway, they pass through the posterior root to get into the spinal cord.
Depending on the organ, pain stimuli are carried to different spinal cord segments. Pain stimuli from the foregut is primarily carried to the T6 to T9 spinal cord segments, the midgut travels to the levels of T8 to T12 and the hindgut to the T12 to L2 levels.

Reflex stimuli7:36–7:58

Finally, visceral afferent fibers from the midpoint of the sigmoid colon onwards run with the parasympathetic fibers of the pelvic splanchnic nerves, to relay to the S2 to S4 spinal cord segments.
Reflex stimuli from the abdominal viscera up to the transverse colon travel with the vagus nerves, and those from the transverse colon onwards travel with the pelvic splanchnic nerves.
Now, as sympathetic and parasympathetic fibers run in the abdominal cavity, they form networks of nerves clustered along the course of the abdominal aorta and its major branches called abdominal autonomic plexuses.

Autonomic plexuses7:58–9:16

Major abdominal autonomic plexuses include the celiac plexus, which surrounds the celiac arterial trunk. It has two celiac ganglia in it, and is composed of both the sympathetic fibers from the lower thoracic splanchnic nerves; and the parasympathetic fibers from the vagus nerves.
Next is the superior mesenteric plexus, which surrounds the superior mesenteric artery, and has one or more superior mesenteric ganglia in it.
Just inferior lies the intermesenteric plexus, which is located between the superior and inferior mesenteric arteries. Surrounding the inferior mesenteric artery is the inferior mesenteric plexus where an inferior mesenteric ganglion may also be found.
At the bifurcation of the aorta lies another plexus called the superior hypogastric plexus. It is followed by two more in the pelvis; the left and the right inferior hypogastric plexuses, which are connected to the superior hypogastric plexuses by the left and right hypogastric nerves.One last pause before we recap.

Quiz9:16–9:26

Can you try to remember which nerves provide parasympathetic innervation to the abdominal viscera?Alright, as a quick recap, abdominal viscera are innervated by the autonomic nervous system.

Review9:26–10:49

The sympathetic innervation is provided by the lower thoracic splanchnic nerves consisting of the greater splanchnic nerve, lesser splanchnic nerve, least splanchnic nerve, and lumbar splanchnic nerves.
The parasympathetic innervation is provided by the vagus nerve, and the pelvic splanchnic nerves. Nerve fibers from both the sympathetic and parasympathetic nervous systems create networks around the abdominal aorta called aortic plexuses or abdominal autonomic plexuses.
Major abdominal autonomic plexuses are the celiac plexus, the superior mesenteric plexus, the intermesenteric plexus, and the inferior mesenteric plexus.
There is also the superior hypogastric plexus, and two inferior hypogastric plexuses. Visceral afferent fibres from the abdominal organs up to the midpoint of the sigmoid colon run retrogradely with the sympathetic fibers in the lower thoracic and lumbar splanchnic nerves.
The visceral afferent fibres from the midpoint of the sigmoid colon onwards run with parasympathetic fibres to the S2-S4 sensory ganglia.
Lastly, the reflex fibres from the abdominal organs up to the transverse colon travel with the vagus nerves, and those from the transverse colon onwards travel with the