Gastrointestinal Lecture 2 Part 1 General Patterns of motility

Question 1

What is motility?

Answer 1

Motility refers to the movements of the gastrointestinal tract

Question 2

What are the two purposes of motility?

Answer 2

• Moves contents from the mouth to the anus
• Mixes contents to facilitate digestion and absorption

Question 3

What are the general patterns of motility?

Answer 3

• Peristalsis → Peristalsis propels food along the length of the intestine from the esophagus to the rectum
• Segmentation → Related to peristalsis, but does not propel luminal contents along the intestine
  
  • mixes contents in situ, facilitating digestion and absorption
  • primarily occurs in the small intestine

Question 4

peristalsis

Answer 4

Peristalsis is a series of wave-like muscle contractions that move food through the digestive tract. It starts in the esophagus where strong wave-like motions of the smooth muscle move balls of swallowed food to the stomach.

• Point of contraction in tube and then relaxation just upstream where food bolus resides and moves down in one direction

Question 5

reflex response of peristalsis

Answer 5

Outward pressure on wall and triggers reaction

• Reflex response, triggered by luminal contents stretching the intestinal wall and stretch initiates circular contraction behind the stimulant, and relaxation in front of it
• Reflex response is independent of extrinsic innervation, but can be modulated by autonomic input

Question 6

What is the peristalic reflec dependant on?

Answer 6

Reflex dependent on the integrated activity of the enteric nervous system

1. Local stretch responses cause release of serotonin → Serotonin (5HT) activates sensory neurons that activate the myenteric plexus
2. Neurons projecting “upstream” activated and release factors that cause smooth muscle contraction
3. Neurons projecting “downstream” activated and release factors that cause smooth muscle relaxation

Question 7

myenteric plexus

Answer 7

The myenteric plexus (also known as the Auerbach plexus) (plural: plexuses) refers to a network of nerves between the layers of the muscular propria in the gastrointestinal system. Among other things, the plexus helps regulate peristalsis in the gastrointestinal tract

Question 8

enteric nervous system

Answer 8

thought of as a digestive brain that helps to regulate motility, secretion, and growth. The enteric nervous system can act as a fast, internal response to digestive stimuli. When this occurs, it is called a short reflex.

Question 9

Segmentation

Answer 9

Segmentation separates chyme and then pushes it back together, mixing it and providing time for digestion and absorption. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary cana

• Ring of contraction does not travel in one direction occurs in different planes at the same time and pinching at different points to mix contents

Question 10

Where does segmentation occur?

Answer 10

primarily the small intestine

Question 11

Steps of segmentation

Answer 11

1. Local contractions separate intestine into pockets → multiple contractions
2. Subsequent contractions divide pockets centrally
3. Rhythmic contractions continue to subdivide pockets, mixing their contents

Question 12

BER

Answer 12

basic electrical rhythm

Question 13

basic electrical rhythm

Answer 13

Rhythmic contractions of segmentation have their basis in underlying oscillations in the membrane potential of smooth muscle cells = basic electrical rhythm (BER)

• Alternating rings of contraction occur at a set rhythm

Question 14

Where does BER originate?

Answer 14

Rhythmic activity originates from pacemaker cells → interstitial cells of Cajal within walls of intestine

Question 15

How is BER AP triggered?

Answer 15

If membrane potential reaches a threshold, an action potential will be triggered resulting in the contraction of smooth muscle cells

• Fluctuation. When its hits a certain threshold it releases AP which cause the contractions via tension, peaks are contractions

Question 16

What modulations (hormones) of the BER impact intestinal motility?

Answer 16

• Acetylcholine → increases activity, and stimulates contraction and tension
• Epinephrine → decreases activity, and depresses contraction causing relaxation

Question 17

How does the rate of the BER varies along the course of the intestinal tract?

Answer 17

• Stomach: 4/min
• Duodenum: 12/min
• Ileum: 8/min
• Cecum: 2/min
• Sigmoid colon: 6/min

Question 18

MMC

Answer 18

migrating myoelectric complex

Question 19

migrating myoelectric complex

Answer 19

After a meal has been absorbed, segmentation stops and is replaced by a sweeping wave of contraction known as a migrating myoelectric complex (MMC)

• last sweeping wave of contraction - getting rid of any undigested remnants

Question 20

MMC define by …?

Answer 20

Defined by a series of contractions starting from the stomach and progressing in ~2 foot sections along the small intestine (takes ~2 h to reach large intestine).

Question 21

MMC function

Answer 21

Function is to move undigested material into the large intestine

• serves the role of housekeeper of the small intestine by propelling undigested food residue and sloughed enterocytes

Question 22

MMC phases

Answer 22

Characterized by three phases → describes the pattern of stomach motility during the interdigestive period.

Phase 1 – quiescent period (no activity) → during digestion
Phase 2 – irregular electrical and mechanical activity → precursor to major wave
Phase 3 – burst of regular electrical and mechanical activity → burst of sweeping contraction

Phase 4 - Phase IV represents a short transition period back to the quiescence of phase I

Question 23

Why does ingestion of a meal completely inhibit MMC?

Answer 23

Do not want contents of SI to be swept away before it is absorbed

Question 24

MMC activity controlled by ….?

Answer 24

circulating levels of the intestinal hormone motilin

• increased plasma concentration of motilin triggers MMCs
• Ingestion of a meal inhibits motilin secretion, plasma levels drop, and MMCs stop
• After meal is absorbed motilin will increase again