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.
OBECTIVE 8: Define basics for eval of pt w n/v
.
Define: retching
“Commonly referred to as “dry heaves”. Very similar to vomiting, but this typically occurs without expelling gastric or small bowel contents.”
Define: nausea
A vague, unpleasant or uneasy feeling in the epigastric area; usually accompanied by the sensation that vomiting might occur. Typically preceded by anorexia.
Objectively, nausea is associated with a reduction in gastric tone and gastric peristalsis; small bowel tone is increased
Define: Vomiting
Typically preceded by anorexia and nausea. Autonomic symptoms are usually present (hypersalivation, tachycardia, pallor, diaphoresis).
Define: gagging
Non-specific; may involve nothing more than a hyper-sensitive pharyngeal reflex, or may be the initial motor movements involved in vomiting that are voluntarily suppressed.
Define: Regurgitation
The sudden, effortless, involuntary movement of small amounts of liquids and/or solids into the esophagus or mouth.
Define: Rumination
Food is chewed, swallowed, and then voluntarily regurgitated.
—Note difference from vomiting and retching.
—Textbooks often say this is only in developmentally handicapped individuals: NOT TRUE: Many high functioning people do this for some reason. Psych? Cultural?
What are the 3 functions of the stomach?
- accommodation
- trituration (grinding/crushing)
- emptying
MMC
Migrating Motor Complex
—waves of activity that sweep through the intestines in a regular cycle during a fasting state
Phase I - period of quiescence
Phase II - irregular phasic activity
Phase III - short period of intense contractions
Occurs at least once a day to clean things out
OBECTIVE 7a: Review common etiologies of ACUTE n/v
Infections, Infections, Infections!
Toxins
Metabolic d/o
CNS d/o
Oculovestibular d/o
Pregnancy
Meds
Intra-abdominal disorders
- GI Tract
- Viscera
Others
- MI - CHF
- Excessive vitamin intake
- Excessive fasting or starvation
- Severe somatic pain
OBECTIVE 7b: Define chronic nausea
—Review common etiologies of CHRONIC n/v
DEFINITION
—nausea for > 6-8 wks
DDX
~~Common Causes~~
Gastroparesis
Intestinal Pseudo-obstruction (CIP)
Psychogenic Vomiting
Others (functional nausea and vomiting, kidney disease, medications, diabetic gastropathy, etc)
~~Gastric Motility Causes~~
Gastroparesis
Functional dyspepsia
Dumping Syndrome
Rumination Syndrome
Scleroderma
A connective tissue disorder
Smooth muscle of the GI tract is lost
Aperistalsis may result
—Causes dysphagia and/or reflux
Esophagus most commonly affected, then the small intestine, and then the anorectal area
Stomach is least likely to be affected
—Because has three layers of muscle
Hollow Visceral Myopathy
An uncommon disorder
May be hereditary or sporadic
Smooth muscle of the GI tract (and other hollow organs including eyes which would be symptomatic) wastes away
Aperistalsis occurs
Colonic Anatomy
—Length?
—Sphincters?
—Layers?
—Musculature?
~4-5 feet long
—internal anal sphincter (smm) is resp for 70% of continence mechanism
—external anal sphincter (skm) is a true sphincter
Four layers
1. mucosa
2. submucosa
3. muscularis
4. serosa
MUSCULATURE
Inner circular muscle layer and outer longitudinal muscle layer
Colonic Innervation?
Intrinsic - ENS
Extrinsic - via the symp/parasymp
—Symp = INHIBITORY; through LUMBAR s.c. neurons synapsing on prevertebral ganglia.
—Parasymp = STIMULATORY; via vagus and pelvic splanchnic nerve
Small Bowel Anatomy
—Length?
—Sphincters?
—Layers?
—Musculature?
Approximately 21-25 feet long
The ileocecal valve acts as a braking mechanism, (not a true sphincter)
Four layers
1. mucosa
2. submucosa
3. muscularis
4. serosa
Villi (finger-like projections of epithel w/ lamina propria core), dramatically increase surface area of the small intestine
MUSCULATURE
Inner circular muscle layer and outer longitudinal muscle layer
Small Bowel Innnervation
Intrinsic - ENS
Extrinsic - via the symp/parasymp
—Symp = INHIB; thru THORACIC s.c. neurons synapsing on prevertebral ganglia
Parasymp = STIMULATORY; via the vagus (only vs. colon which has also pelvic splanchnic parasymp innvervation)
Function of colon
Absorption
—fluid and electrolytes
—nutrients
—Bacterial fermentation products
Formation of residue
—mixing, retention, formation
Storage of material
—reservoir function
Transport of material
—conduit
Normal frequency of bowel movts?
3/day to 3/week!
What sorts of contraction occur in the colon?
A) Segmenting contractions
—Promote mixing to aid absorption and bacterial breakdown
B) Propagating contractions
- Low Amplitude Contractions
Propel colonic contents over short distances in either direction
- High-amp propagating contractions
Propagate caudally over long distances
Often assoc’d w/ defecation
Definition of constipation
—Change from patient’s normal
—Figure out patient’s definition: straining, hard stool, incomplete evacuation, sitting on the toilet for hours
Motility Disorders of the Colon and Anorectum
Constipation
Diarrhea
Hirschsprung’s disease
Pelvic floor dyssynergia
Proctalgia fugax
Hirschprung’s Disease
Uncommon - @ 1:5000 births
Caused by failure of the neural crest cells to migrate to the colon
Aganglionosis results
Colonic dilation occurs above the aganglionic segment
Symptoms include: failure to thrive, distention, bloating, constipation
Diagnosed by anorectal manometry and/or full thickness rectal biopsy
Treatment: surgery
RAIR
RectoAnal Inhibitory Reflex
—Test with rectal manometry: blow up balloon which should
Colonic inertia
Typically a disorder of young women
Infrequent stools – every 1-4 weeks
Generally a neuropathic process
Frequently due to loss of ICC
Review Slide decks #s 1-3
That we reviewed the night before
