What are the surfaces of the GI wall?
the mucosal surface (consisting of epithelial cells, a lamina propria, and a muscularis mucosae) faces the lumen and the serosal surface faces the blood
Describe the muscle layers of the GI wall
The longitudinal muscle layer is thin and contains few nerve fibers, whereas the circular muscle layer is thick and more densely innervated
Note that neurons do not make synapses on the GI smooth muscles, rather they release transmitters from varicosities (swellings) along their axon length
Where is Meissner's Plexus located?
aka the submucosal plexus lies between the submucosa and the circular muscle
Where is Myenteric Plexus located?
between the circular and longtiduinal muscle layers
What is the parasympathetic innervation of the GI?
the vagus nerve (CN X) in the upper Gi tract including the striated muscle of the upper 1/3rd of the esophagus, the wall of the stomach, the small intestine, and ascending colon, and
the pelvic nerve in the lower GI tract including the striated muscle of the external anal canal and the walls of the transverse, descending, and sigmoid colons
Postganglionic neurons of the parasympathetic nervous system are classified as either cholinergic or peptidergic. What are the transmitters of each in the GI?
Peptidergic- Substance P, VIP, others
T or F. 75% of Vagus nerve fibers are afferent
T. Delivering sensory info from the periphery to the CNS.
NOTE that both the afferent and efferent transmission of GI impulses/functioning are mediated via the vagus nerve, a phenomenon called a vagovagal reflex
What are the main sympathetic ganglia of the GI?
celiac, superior mesenteric, inferior mesenteric, and hypogastric
What transmitter do post-ganglionic sympathetic fibers release in the GI?
T or F. The intrinsic or enteric nervous system can direct ALL functions of the GI tract, even in the absence of extrinsic (ANS) innervation
T. Although typically the enteric plexus receives input/gives output to the ANS in the GI wall, as well as mechanoreceptors/chemoreceptors
How does Ach (parasympathetic) affect the GI?
-Contraction of smooth muscle in GI wall and relaxation of sphincters
-increased salivary, pancreatic, and gastric secretion
How does VIP affect the GI?
Smooth muscle relaxation
increased intestinal and pancreatic secretion
How does gastrin-releasing peptide (GRP)/Bombesin affect the GI?
increases gastrin secretion