02-27 Enteric Nervous System and Motility Flashcards
At the end of this lecture, the learner should be able to: • Describe the embryologic development of the enteric nervous system • To outline the anatomic organization of the enteric nervous system • Describe the normal physiology of the enteric nervous system • Discuss how abnormalities of ENS function can produce disorders of GI motility and function —understand basics of gastric motility —understand the physiology of n/v —Review common etiologies of acute and chronic n/v —Define bas
T/F: DM is the most common etiology of IBS?
False, 50% of cases are idiopathic.
T/F: Hirschsprung’s disease is most commonly dx’ed in young women?
False, babies
T/F: Anx/depression cause IBS?
False!
—Though 80% of internists think so…
Def: functional bowel disorders
A group of disorders of sensation and motility classified by sx where testing reveals no organic cause.
—Classified into: myopathic, neuropathic, or mixed.
Where does enteric nervous system fall into the hierarchy of the NS?
PNS–> Autonomic–> enteric, para-symp, symp
OBECTIVE 1: Describe the embryologic development of the enteric nervous system
—Neurons and glial cells of the ENS are derived from the neural crest
—As neural crest cells migrate, they do not initially express any particular type of neuronal phenotype
—Foregut develops first, then midgut, then hindgut
OBECTIVE 2: outline the anatomic organization of the enteric nervous system
Similar to the CNS
—ENS consists of nerve cell bodies and their processes embedded in the wall of the gastrointestinal tract
—ganglia in two GIT wall areas forming plexi:
1. submucosal ganglia
2. myenteric ganglia/plexus
—Afferents back to CNS via CN X and IPANs (intrinsic primary afferent nn.)
—Can also send signals w/in gut
OBECTIVE 3: Describe the normal physiology of the enteric nervous system
—IPANs in mucosa sense stretch (other noxious stimuli) and communicate to myenteric plexus and CNS.
—
OBECTIVE 4: Discuss how abnormalities of ENS function can produce disorders of GI motility and function
blah
Submucosal (Meissner’s Complex)
—lies between inner circ muscle layer and mucosa
—Primarily in large and small intestine
—Doesn’t form extensive network in esophagus or stomach
—Nerve cells in the submucosal plexus innervate secretory cells, endocrine cells, and blood vessels in the mucosa and submucosa
—IPANs (intrinsic primary afferent neurons) project from the submucosal plexus to the myenteric plexus
Myenteric (Auerbach’s) Complex
All the way from esophagous to rectum (vs. submucosal)
—between inner circular and outer longitudinal layer
—motor innervation to the 2 muscle layers and
—also secretomotor innervation to mucosa
—Ganglia may contain up to 200 cells, and are connected to each other by small bands of nerve fibers called internodal strands
—Bundles of nerve fibers also connect the myenteric plexus to the submucosal and mucosal plexuses.
Which are major inhib neurotransmittiers in ENS?
—Excitatory?
—Role of serotonin?
EXCITATORY: ACh, Substance P, and the tachykinins
INHIBITORY: Nitric oxide and VIP are the major inhibitory NTs
SEROTONIN: (5-HT) plays a critical role in motor, sensory, and secretory function and in found in a large number of interneurons
—FACT: 95% of your 5-HT is in gut
The Law of the Intestine
“Local stimulation of the gut produces excitation above [i.e. contraction, orally] and inhibition below [aborally] the excited spot. These effects are dependent on the activity of the local nervous mechanism.”
What are the pacemaker cells of the GIT? Where are they located?
ICCs! (Intestinal Cells of Cajal)
—Located in patch along the greater curvature of stomach and q18” in small bowel
—Rate of firing slower in stomach, faster in intestines
OBECTIVE 5: understand basics of gastric motility (i.e. steps of motility in response to food ingestion)
- Food enters fundus (reservoir) which can which relaxes via NO release from vagus signaling to expand up to 7X its normal size to accommodate a large meal.
- Vagus signaling triggers contraction allowing trituration (or grinding) of food at pressures > 300mmHg!
- Food is filtered by pylorus such that only ground particles < 1mm leave the stomach for the duodenum.
- Peristaltic wave occurs when the constant firing from ICC occur in the correct hormonal milieu.
OBECTIVE 6: understand the physiology of n/v
—Vagal and sympathetic afferents (from the GI tract, the heart, the vestibular system) synapse in the vomiting center (medulla)
—Afferents from the chemoreceptor trigger zone (CTZ) synapse in the vomiting center, also
—CTZ is located in the area postrema of the floor of the 4th ventricle, outside of the blood-brain barrier, very sensitive to chemical stimuli, toxins, neurotransmitters, and medications. Surgical ablation prevents chemically induced vomiting (via apomorphine, a DA receptor agonist)
—Afferent stim from higher CNS (cortex, thal, hypothal) synapses in vomiting center, too. These pathways may be involved in vomiting that occurs with unpleasant tastes, offensive odors, or somatic pain. (memory vomiting)
The vomiting center in medulla is not a discrete area in the brain, but rather a collection of nuclei that are linked and which coordinate the complicated act of vomiting. When the vomiting center is appropriately activated by afferent stimuli from the CTZ, the ANS, somatic afferents, or higher brain centers, then vomiting occurs.
