GI Motility & Regulation

Question 1

Segmentation

Answer 1

Mixing action
Type of motility
No net movement forward
Just back and forth

Question 2

Peristalsis

Answer 2

Propulsive movements

Moves food from mouth to the anus

Question 3

Smooth muscle contraction

Answer 3

Extracellular Ca enters cell or Ca is released from SR

Ca+ Calmodulin activates Myosin Light Chain Kinase which then phosphorylates Myosin which can then bind with Actin and contract

Myosin Light Chain Phosphatase removes the phosphate group from myosin, inactivating it

Question 4

Smooth Muscle in the GI Tract

Answer 4

Unitary (Single Unit) Cell Type

	- Held together by adherens junctions
	- Communicates electrically via gap junctions
	- Pacemaker cells with spontaneous activity
	- Intrinsically produces BER & muscle tone  without tension (myogenic properties)

Tension comes from neurotransmitters acting on muscle – Role of the ANS

Question 5

Basic Electrical Rhythm

Answer 5

The smooth muscle of the GI tract has a baseline rhythm of depolarization, but it’s not enough on it’s own to cause contraction. But it is pulsitile and highly regulated. When you get ACh on top of that, then you get enough for regular contractions and changes in tension.

Question 6

Which parts of the GI tract have what BERs?

Answer 6

Stomach has 3 cycles/min
Duodenum has 12 clycles/min

Want it to be slower at the top

Question 7

Phases of digestion

Answer 7

Cephalic- neural control
Gastric phase- Neural (early) and hormonal
Intestinal- Mostly hormonal but some neural

Question 8

Stages of swallowing (and which are voluntary and involuntary)

Answer 8

Stage 1. Voluntary- oral cavity then bolus pushed by tongue to oropharynx

Stage 2. Involuntary- glottis covers trachea, UES relaxes

Stage 3. Involuntary- esophageal peristalsis

Question 9

Esophagus

Answer 9

Transports and adjusts food temperature

Has upper & lower esophageal sphincters (UES, LES): basically just thickening of smooth muscle

Muscular walls transition from skeletal to smooth muscle

Thick muscular walls produce strong peristatic waves

Question 10

Release of substances to prepare intestine for food that’s coming

Answer 10

NO, VIP, and ATP are released downstream of the food in the GI tract, warning it to relax and prepare for food so get good peristalsis

Question 11

Lower esophageal sphincter (and what problems happen when it’s not working properly)

Answer 11

Gatekeeper between the esophagus and the stomach

Failure to relax due to damage/loss of the enteric nerves of the LES wall is called achalasia and can make swallowing difficult

Inappropriate LES relaxation can cause acid reflux and damage to the inner lining of the esophagus

Question 12

Stomach

Answer 12

Stores food (up to 3-4 L)

HCl disinfects food, denatures and digests proteins and produces intrinsic factor

Resulting chyme in released in peristatic spurts into the duodenum

Question 13

Pyloric sphincter

Answer 13

At end of the stomach

Regulates food going to duodenum

The pyloric sphincter serves as a sieve (prevents passage of particle >1-2 mm in size)

Question 14

Intrinsic factor

Answer 14

Important for Vit B12 absorption

Question 15

Gastric acid

Answer 15

Need low pH and pepsin to digest food

Question 16

Gastric motility patterns

Answer 16

Mixing and churning with retropulsion and receptive relaxation

A little bit is allowed to empty but most gets kicked back towards the fundus and gets mixed again

Question 17

Rate of emptying of different foods in the stomach

Answer 17

Carbs- fastest (few hours to leave stomach)
Protein- more slowly
Fat- slowest

Question 18

Vomiting

Answer 18

Centrally regulated by vomiting center in brain

Steps:

1. Salivation (HCO3-) and sensation of nausea (helps offset the acidic stomach contents)
2. Reverse peristalsis from upper small intestine to stomach
3. Ab muscles contract and UES and LES relax
4. Gastric contents are ejected

Question 19

Small Intestines

Answer 19

Liquification, pH adjustment, release of hormones and addition of enzymes in response to food entering the duodenum

Enzymatic digestion of food occurs in the lumen and the enterocyte surface

Absorption of nutrients, water, and ions occurs by cellular and paracellular pathways

Both peristaltic and segementation motility

Question 20

Key hormones released by small intestines

Answer 20

CCK
Secretin
GIP

Question 21

Gastroileal reflex

Answer 21

stomach activity stimulates movement of chyme through the ileocecal sphincter

Mediated by both ENS & external innervation

Question 22

Gastrocolic reflex

Answer 22

food in stomach stimulates mass movement in colon

Mediated by both ENS & external innervation

Question 23

Migrating Motor Complex (MMC)

Answer 23

Occurs in the absence of feeding (during fasting) – Housekeeping!

Occurs every 90-100 minutes with 3 phases starting from the stomach & propagating aborally to ileocecal valve. The hormone Motilin appears to initiate but appears to have a neural component as well.

Forces anything that wasn’t previously digested to get cleared out.

Question 24

Phases of MMC

Answer 24

Phase I
Quiescence occurs for 40-60% of the 90 min duration

Phase II
Motility increases but contractions are irregular
Fails to propel luminal content
Lasts 20-30% of MMC duration

Phase IIII
5-10 minutes of intense contractions
From body of stomach to pylorus to duodenum to ileocecal valve
Pylorus fully opens

Question 25

Ileocecal valve

Answer 25

normally closed (e.g. to prevent reflux of bacteria from colon into ileum)

Opened by distension of end of ileum (local reflex)

Closed by distension of proximal colon (local reflex)

Question 26

Large intestines

Answer 26

Serves as reservoir for undigested foodstuff

Main function is to reabsorb water and ions

Elimination of waste is controlled by two sphincters – internal (involuntary) and external (voluntary) anal sphincters

Two types of motility: haustration & mass movement

NO MMC

Question 27

Types of motility in large intestines and describe them

Answer 27

Haustration: Muscles of the colon wall are contracted intermittently to divide the colon into functional segments known as “haustra”

Mass movements: wave of contraction that usually follows a meal and that moves content over larger distance than with regular peristalsis; colon remains contracted for a while; strong peristaltic waves 1-3 X/day; slow

Question 28

Defecation

Answer 28

Filling of the rectum causes relaxation of the internal anal sphincter via release of VIP and NO from intrinsic nerves. At same time, the external anal sphincter contracts - rectoanal inhibitory reflex

Defecation (evacuation) occurs when the external anal sphincter is voluntarily relaxed and is enhanced by an increase in intra-abdominal pressure