The organs illustrated in the model are the GI tract, urinary bladder and adrenal cortex. The autonomic innervation of most other organs in this region are illustrated elsewhere (see INDEX above).
"Splanchnic" means having to do with the viscera (i.e., internal organs). Sympathetic preganglionic axons (solid orange lines) emerge from the cord and pass through the sympathetic chain (line of yellow circles) without synapsing. They gather into three nerves called the greater, lesser & least splanchnic nerves. Though not obvious in the model the greater consists of bilateral outflow from T5-9, the lesser of outflow from T 10 & 11 and the least from T 12. Parasympathetic preganglionic axons (solid blue line) emerge from the sacral region of the cord to form the pelvic splanchnic nerve. These fibers penetrate pelvic organs to synapse with embedded terminal ganglia (light blue circle). From here, postganglionic axons (dashed blue line) target their receptors (not shown).
The prevertebral ganglia (large gold circles) are named the celiac, superior mesenteric and inferior mesenteric. There is much intertwining between the splanchnic nerves and these ganglia as represented by the contact of the lesser splanchnic with the celiac ganglion in the insert. These ganglia are where the postganglionic cell bodies are located; their axons (dashed orange lines) emerge to target various abdominal organs not shown in the insert.
The adrenal medulla is unique because it is the only organ innervated by preganglionic (solid orange line), instead of postganglionic, neurons. These neurons form a branch of the greater splanchnic nerve. Norepinephrine (not shown) is released onto stimulatory (color-code green) nicotinic 1 (N1) receptors on the cells of the medulla. Binding causes the release (arrows) of epinephrine (E) and norepinephrine (NE) into the capillaries. It is this release of epinephrine that is a hallmark of the "fight or flight response" resulting from sympathetic stimulation.
Abdominal blood vessels are represented in the lower portion of the insert above. They are innervated by postganglionic axons (orange dashed line) -- such neurons are called "vasomotor." They secrete norepinephrine (not shown) onto stimulatory (color-code green) alpha 1 receptors causing vasoconstriction and increased resistance to blood flow. The hormones epinephrine (E) and norepinephrine (NE) also activate this receptor. This explains why, during sympathetic stimulation, blood is diverted away from the abdominal organs.
This organ system contains a nervous system of its own called the enteric (i.e., intestinal) nervous system (ENS) ; some classify this as a third division of the autonomic nervous system. It is not discussed in this model but is mentioned here because the parasympathetic postganglionic neurons are a part of the enteric system.
The vagus nerve (X) contains parasympathetic preganglionic neurons (blue neuron) that penetrate the stomach wall to synapse within terminal ganglia (light blue circle). The postganglionic cells --also called 'ganglion cells'-- are a component of the enteric nervous system that secretes acetylcholine (not shown) onto the smooth muscle of the stomach wall that have stimulatory (color-code green) muscarinic 3 (M3) receptors responsible for increasing stomach motility. Gastric glands (small circles) contain stimulatory (color-code green ) muscarinic 1 (M1) receptors to increase the production of "gastric juice." It is the parasympathetic division that enhances digestive processes.
The celiac ganglion (large gold circle) -- see prevertebral ganglia above -- houses the nuclei of postganglionic neurons whose axons (dashed orange line) directly target the encircling smooth muscle of both gastric arteries and the pyloric sphincter. The norepinephrine (NE) --not shown-- released onto stimulatory (color-code green) alpha 1 receptors decreases blood flow to the stomach and decreases the passage of food from the stomach into the small intestine; blood-borne norepinephrine (NE) and epinephrine (E) have the same effect.
Postganglionics (dashed orange line) from the celiac ganglion also penetrate the stomach wall to synapse with inhibitory ( color-code red) alpha 2 receptors located on the prejunctional terminals of parasympathetic postganglionics of the enteric nervous system. Norepinephrine (NE) -- not shown-- binds with these receptors and decreases the amount of acetylcholine subsequently secreted by these terminals. The sympathetic division decreases gastric activities by decreasing the influence of the parasympathetic division. In addition, circulating epinephrine (E) binds with inhibitory (color-code red) beta 2 receptors on the smooth muscles further reducing gastric motility.
Like the stomach, the small intestine is innervated by branches of the vagus (X) nerve (solid blue line). Synapses occur within terminal ganglia (light blue circle) scattered between the muscle layers. Acetylcholine (not shown) released from enteric neurons binds with stimulatory (color-code green) muscarinic 3 (M3) receptors on the intestinal smooth muscle to increase motility.
Unlike the stomach, small intestine and upper portions of the large intestine, the decending colon is innervated by the pelvic splanchnic nerve that arises from the sacral region of the cord. Other that this, the enteric components, neurotransmitters, receptors and effects are the same. Parasympathetic stimulation increases intestinal activity.
The small intestine is innervated by postganglionic axons (dashed orange line) originating from the superior mesenteric ganglion; in the large intestine these neurons are from the inferior mesenteric ganglion. As in the stomach, norepinephrine (not shown) is released from these to bind with inhibitory (color-code red) alpha 2 receptors located on the postganglionic parasympathetic terminals (blue bars). This binding decreases subsequent acetylcholine release from the enteric neurons resulting in a reduction of motility. Blood-bourne epinephrine (E) binds with inhibitory (color-code red) beta 2 receptors on intestinal smooth muscle contributing to reduced motility. Sympathetic stimulation decreases intestinal activity.
The circular smooth muscle of the intestinal arteries have stimulatory (color-code green ) alpha 1 receptors. Vasomotor nerves (dashed orange line) release norepinephrine (not shown) onto these receptors causing vasoconstriction and decreased blood flow. Epinephrine (E) and norepineprine (E) have the same effect.
The internal anal sphincter (donut) is a region of circular smooth muscle at the outlet of the urinary bladder; its has stimulatory (color-code green) alpha 1 receptors. The primary regulation of this area is under the control of postganglionic neurons (dashed orange line) that release norepinephrine (not shown); binding causes constriction and prevents urine release.
Continence is the phase when the bladder is filling and the sphincters are contracted. Initial stretching of the bladder wall stimulates stretch receptors (not shown) to signal a sacral reflex center ---not a part of the autonomic nervous system. This results in contraction of the voluntary external urethral sphincter (skeletal muscle, not shown); it also activates the sympathetic outflow to the bladder. Postganglionics from the inferior mesenteric ganglion (dashed orange line) release norepinphrine (not shown) onto inhibitory (color-code red) beta 2 receptors on the bladder wall and onto stimulatory (color-code green) alpha 1 receptors of the internal urethral sphincter. The overall effect is that, while the bladder is filling, both sphincters are contracted and the bladder wall is relaxed and allowed to stretch.
Depending on the compliance (i.e., stretchability) of the bladder wall, a point will be reached when stretch receptors communicate with the brain as to the status of the bladder. A conscious decision can be made that results in simultaneous decreased sympathetic stimulation to the bladder wall and internal sphincter, decreased stimulation to the external sphincter and increased parasympathetic stimulation to the bladder wall via the pelvic splanchnic nerve. Postganglionics (dashed blue line) release acetylcholine (not shown) onto stimulatory (color-code green) muscarinic 3 (M3) receptors on the smooth muscle of the bladder wall. The effect is urination or voiding.
Last update: 11/10/2005