MP - Vallano - 2/10 - GI Introduction

Question 1

What are the major functional processes of the GI system?

Answer 1

1. motility
2. secretion
3. digestion
4. absorption
5. excretion

Question 2

List the muscular sphincters that regulate unidirectional flow between different sections of the GI via inherent myogenic mechanisms

Answer 2

1. upper esophageal sphincter (UES)
2. lower esophageal sphincter (LES)
3. pyloric sphincter
4. ileocecal sphincter
5. internal anal sphincter
6. external anal sphincter

Question 3

Where does the majority of the absorption of fluid occur?

Answer 3

Small intestine

Question 4

What are the primary plexuses in the Enteric Nervous System?

Answer 4

1. Myenteric (Auerbach’s)
   - between the longitudinal and circular muscle layers of GI tract
   - contraction
2. Submucosal (Meissner’s)
   - between the circular muscle and submucosa of small and large intestines
   - secretion

Question 5

T or F: The Enteric Nervous System (ENS) is able to function independently responding to input from the local environment even in the absence of extrinsic innervation

Answer 5

True

This is possible through mechanoreceptors, chemoreceptors and osmoreceptors in the epithelial lumen

Question 6

T or F: ENS activity is modulated by both sympathetic and parasympathetic input from the ANS

Answer 6

True

In general, stimulation from the parasympathetic nerves enhances motility and secretions through the ENS, whereas sympathetic stimulation reduces motility and secretions

Question 7

A 35-year-old woman with a recent breast cancer diagnosis reports gastrointestinal (GI) distress following chemotherapy treatments targeting fast-replicating cells. Which GI layer or signaling molecule is most likely affected by the treatment?

a. Longitudinal muscle
b. Submucosal plexus
c. Epithelium
d. Motilin
e. Vasoactive intestinal peptide

Answer 7

C. Epithelium

Gastrointestinal (GI) epithelium has a very high turnover rate and, thus, is most affected by chemotherapy

Question 8

Action potentials in the GI smooth muscle:

a. Occur only in the upper GI tract
b. Are stimulated when the slow waves are depolarized above -40 mV
c. Occur only in the lower GI tract
d. Result primarily from influx of sodium ions into the smooth muscle cells
e. Are only stimulated by extrinsic nerves

Answer 8

b. Are stimulated when the slow waves are depolarized above -40 mV

Action (or spike) potentials in the GI tract are caused primarily by the influx of calcium into the smooth muscle when the slow waves are depolarized above -40 mV. This mechanism is active throughout the tract, and depolarization can result from local stretch (mechanoreceptors acting on enteric nerves), extrinsic nerves, and peptides

Question 9

In the absence of extrinsic innervation to the gastrointestinal tract:

a. Propulsive movements would not occur in the small intestine
b. Propulsive movements would occur in the small intestine, but secretion into the small intestine would be abolished
c. Secretion would be abolished in the entire GI tract
d. Motility and secretion throughout the tract would be stimulated by local mechanoreceptors, chemoreceptors, and osmoreceptors, as well as hormones
e. Motility and secretion would be affected only in the large intestine

Answer 9

d. Motility and secretion throughout the tract would be stimulated by local mechanoreceptors, chemoreceptors, and osmoreceptors, as well as hormones

The GI tract has an intrinsic nervous system (enteric nerves), which responds to signals from lumenal receptors and hormones, as well as extrinsic nerves. When the extrinsic nerve input is severed, motility and secretion continues, but not as efficiently

Question 10

In the small intestine, peristalsis:

a. Occurs primarily in response to the chemical composition in the chyme
b. Proceeds in both forward and backward directions from the site of contraction
c. Involves receptive relaxation of the circular muscle proximal (toward the mouth) to the bolus of chyme
d. involves contraction of the circular muscle proximal to the bolus of the chyme
e. Is dependent on parasympathetic innervation

Answer 10

d. involves contraction of the circular muscle proximal to the bolus of the chyme

Peristalsis follows the law of the intestines, where circular muscle is contracted proximal to the bolus and relaxed distal to the bolus, producing aboral movement of the chyme

Question 11

Select the FALSE answer about the migrating myoelectric complex (MMC):

a. The MMC is a series of contractions that “sweep” undigested fibers and bacteria into the colon
b. In each cycle, the main contractions of the MMC occur during phase II
c. The hormone motilin stimulates phase III contractions
d. The MMC occurs during fasting, and stops upon feeding

Answer 11

b. In each cycle, the main contractions of the MMC occur during phase II

The main/strongest contractions occur during phase III, not phase II

Question 12

Which of the following are functions of the gastrointestinal tract?

a. Secretion of endocrine hormones.
b. Secretion of digestive enzymes.
c. Absorption of nutrients from chyme.
d. Storage of chyme
e. All of the above

Answer 12

e. All of the above

Question 13

What is achalasia?

Answer 13

A condition in which the lower esophageal sphincter (LES) fails to relax during swallowing -as a result, food swallowed into the esophagus then fails to pass from the esophagus into the stomach

Question 14

Migrating motility complexes (MMC) occur about every 90 min between meals and are thought to be stimulated by the gastrointestinal hormone, motilin. An absence of MMCs causes an increase in which of the following?

a. Duodenal motility
b. Gastric emptying
c. Intestinal bacteria
d. Mass movements
e. Swallowing

Answer 14

c. Intestinal bacteria

Migrating motility complexes (sometimes called interdigestive myoelectric complexes) are peristaltic waves of contraction that begin in the stomach and slowly migrate in an aboral direction along the entire small intestine to the colon. By sweeping undigested food residue from the stomach, through the small intestine, and into the colon, MMCs function to maintain low bacterial counts in the upper intestine. Bacterial overgrowth syndrome can occur when the normally low bacterial colonization in the upper gastrointestinal tract increases significantly. It should be clear that an absence of MMCs would decrease duodenal motility and gastric emptying

Question 15

Parasympathetic stimulation increases gastrointestinal motility and sympathetic stimulation decreases motility. The autonomic nervous system controls gut motility by changing which of the following?

a. Gastrin secretion
b. Pacemaker discharge frequency
c. Secretin secretion
d. Slow wave frequency
e. Spike potential frequency

Answer 15

e. Spike potential frequency

Gastrointestinal smooth muscle undergoes rhythmical changes in membrane potential called slow waves. The slow waves are thought to be caused by variations in the calcium conductance of specialized pacemaker cells, called interstitial cells of Cajal. The discharge frequency of the pacemaker cells and hence the frequency of slow waves is fixed (i.e., does not change) in various parts of the gut. Slow-wave frequency averages about 3 per minute in the stomach, 12 per minute in the duodenum, 10 per minute in the jejunum, and 8 per minute in the ileum during active digestion. When a slow wave depolarizes sufficiently, it elicits spike potentials which are true action potentials. In the small intestine, slow waves cannot initiate smooth muscle contraction in the absence of spike potentials; however, slow waves themselves can initiate the contraction of smooth muscle in the stomach. The number of spike potentials associated with a given slow wave is increased by parasympathetic stimulation and decreased by sympathetic stimulation, therefore, the autonomic nervous system controls gut motility by changing the frequency of spike potentials

Question 16

Which of the following structures undergoes receptive relaxation when a bolus of food is swallowed?

a. Orad stomach
b. Palatopharyngeal folds
c. Pharynx
d. Thoracic esophagus
e. Upper esophageal sphincter

Answer 16

a. Orad stomach

During a swallow, the orad portion of the stomach and lower esophageal sphincter relax at about the same time. Intraluminal pressures in both regions decrease before the arrival of the swallowed bolus. This phenomenon is called receptive relaxation. Because the orad stomach relaxes with each swallow, the stomach can accept a large volume of food with only a few mm Hg rise in intragastric pressure. Receptive relaxation is mediated by afferent and efferent pathways in the vagus. Receptive relaxation and gastric distensibility are impaired following vagotomy. The palatopharyngeal folds are important for determining whether a bolus of food is small enough to be swallowed. The pharynx and thoracic esophagus undergo peristaltic contractions during swallowing, but they do not undergo receptive relaxation. The upper esophageal sphincter opens during a swallow, but this is not considered to be receptive relaxation.