Gi #13: Gastrointestinal Motor Function

Front 1

What are the two main forms of neural control in the GI tact?

Name the layers of the gut going from the lumen outward.

Back 1

CNS (via Autonomic nervous system) and ENS (via myenteric and submucosal plexus)

Lumen - mucosa (epithelium,LP, and MM)- Submucosal plexus- IC- myenteric plexus- OL- adventitia/serosa

Front 2

Longitudinal muscle layer vs. Circular muscle

- intestinal length
- radius of intestine
- how is excitation-contraction triggered?

Back 2

Longitudinal = shorten's intestine, increases radius
Circular = elongates the intestine, decreases radius

Longitudinal = Excitation-contraction triggered by influx of extracellular Ca2+ influx
Circular = triggered by intracellular Ca2+ release from stores

Front 3

What are two mechanisms for triggering contraction in GI muscles?

Back 3

1. Pharmaco-mechanical coupling (binding of ligand, neurocrine, pararine, endocrine, etc.) that activates Ca2+ release extracellularly and intracellularly.

2. Membrane depolarization (from action potential) causes opening voltage-gated Ca2+ channel, elevating cystolic Ca2+.

Front 4

Describe the steps involved in smooth muscle contraction once Ca2+ is released into the cytoplasm.

Back 4

Ca2+ binds calmodulin --> activates Myosin LC kinase --> Myosin is P* and binds actin --> forms crossbridge leading to contraction

When Ca2+ levels return to low level --> Myosin LC Kinase phosphatase cleaves Pi --> relaxation of myosin+ actin crossbridge

Front 5

What is active vs. passive tension as it applies to smooth muscles?

Back 5

Active = with stimulus (ex: Ach) due to cross bridge formation and sliding filaments
Passive = passive stretch alone. Remember, Total tension = Active + Passive.

There is a length-tension relationship for smooth muscle, maximum force is developed when there is optimal overlap of thick and thin filaments.

Front 6

What are slow waves (BER or basic electrical rhythm)?

What triggers spike potentials? How does BER vary within the gut?

Back 6

Slow waves= Rhythmic fluctuation of membrane, thought to be myogenic. They are responsible for timing of contractions (but require action potential discharges in conjunction).

Spike potentials triggered by stretch or neuronal/hormonal input. BER frequency changes in gut (faster in stomach, slower in duodenum).

Front 7

Describe the basic four phases of slow waves.

Back 7

Phase 0 = RMP
Phase 1= depolarization (Ca2+ channel activated)
Phase 2= transient repolarization (due to K+ channel activation)
Phase 3= Ca2+ IN and K+ out battle (plateau phase)
Phase 4= Repolarization (K+ wins, and calcium channels are inactivated)

Front 8

Segmental contraction is involved in what action of the gut?

What about peristaltic wave? What about Tonic contraction?

Back 8

Segmental = bolus mixing (simultaneous contraction of various lengths of bowel)

Peristaltic wave = esophagus (wave of pressure moves down to propel bolus)

Tonic contraction= sphincter muscles (always contracted at rest).

Front 9

During the resting phase of sphincters, how is pressure distributed between adjacent segments?

What about during distal distension (stretch on the wall beyond the sphincter)?

Back 9

Resting = Pressure > Adjacent segment

Distal distension = Pressure >> Adjacent segment (to prevent retrograde flow)

Front 10

A normal swallow consists of what three integrated phases?

Which phase is involuntary?

Back 10

1. Oral phase (prepare the bolus, chew)
2. Pharyngeal phase (tongue thrusts bolus into the pharynx, when tongue touches palate swallowing is triggered. Pharynx contracts and UES opens to let food out)
3. Esophageal (wave of peristalsis brings the food down, INVOLUNTARY).

Front 11

What is the difference between Primary and Secondary Peristalsis as it applies to the esophagus?

What CN are responsible for bringing the sensory signal for swallowing?

Back 11

Primary = activated by swallowing
Secondary = activated by distension of esophageal wall (can't be controlled)

CN 5, 9, 10 bring sensory signal to the swallowing center.

Front 12

What two neurotransmitters regulate esophageal peristalsis? How do they work in tandem?

Back 12

1. Ach (causes wave of contraction to propel food)
2. NO (released from NANC nerve causing wave of relaxation to provide room for bolus in lumen)

Front 13

What produces the resting LES tone?

Back 13

Myogenic (intrinsic to muscle) as well as some contraction of diaphragm.

Front 14

What are the three anatomic regions of the Stomach?

What are the two functional regions and what is their role?

Back 14

Fundus, Body (corpus), and Antrum (contains pylorus)

Reservoir (tonic contraction and relaxation to accommodate bolus)
Antral pump (phasic contractions, retropulsion of food to break it down)

Front 15

Where are the pacemaker cells of the stomach found and what is their role?

Back 15

Pacemaker cells in Mid-Corpus create basal rhythm of contraction. Antrum clams down and causes retropulsion of food that breaks it down to 1-2 mm.

Front 16

What are the three types of relaxation that occur in the Gastric reservoir?

Back 16

1. Receptive relaxation (allows bolus to move into stomach after swallowing)
2. Adaptive (stretch receptors in stomach allow accommodation of food)
3. Feedback (chemoreceptors sense nutrients entering duodenum and send signals to stomach to slow down, allow for appropriate time to digest).

Front 17

What is the effect of high caloric content or high lipid content foods on gastric emptying? What about a solid vs. liquid meal?

Back 17

High caloric/ lipid content = slow down gastric emptying

Solid meal = long lag time and emptying phase (large particles), Liquid meal = fast emptying phase (small particles)

Front 18

Describe whether the circular muscle and longitudinal muscle are Contracted or Relaxes in the-

Propulsive segment of SI
Receiving segment of SI

Back 18

Propulsive segment-
Circular is contracted (radius decreases), Receiving is Relaxed

Receiving segment-
Circular is Relaxed, Receiving is contracted (radius expands)

Front 19

What are the two types of motility of the Small Intestine controlled by the ENS? Describe when the are active (during fed or fasting)?

What is the stimulus for each?

Back 19

1. Segmental contraction (fed)- mixing of chyme, allows for absorption. Stimulus= CCK, gastrin, and vagal activation.

2. Migrating Motor Complex (fasting, interdigestive)- allows cleaning of large particles in stomach and emptying of SI. Also propels bile back to liver. stimulated by Motilin every 90 min.

Front 20

What is the function of the right colon? the left?

What three sphincters make up the Large intestine? Which one is under voluntary control?

Back 20

Right = absorption of water & electrolytes, Left = storage of waste

Ileo-cecal. Internal anal sphincter. External anal sphincter (voluntary).

Front 21

What gives the large intestine the lumpy bumpy look?

Back 21

Longitudinal m. is not continuous in colon (3 long bands - tenae coli). Where there is no longitudinal muscle, the circular muscle bunches up.

Front 22

What three movements does the large intestine exhibit?

How does the motility of the large intestine differ from that of the stomach and SI?

Back 22

1. Mixing movements (segmental)
2. Haustral migrations (short spike bursts)
3. Mass movements, peristaltic propulsion.

Large intestine is ALWAYS ACTIVE (unlike stomach and SI which have distinctive fasting and fed motor patterns).

Front 23

What maintains anorectal continence?

Back 23

1. internal anal sphincter (contiguous with circular muscle of colon)
2. External anal sphincter (striated muscle)
3. Puborectalis sling (pulls on anal canal obstructing flow)

Front 24

Describe how a bowel movement (defecation) occurs.

Back 24

1. waste enters rectum causing a stretch on wall
2. Internal anal sphincter reflexivelyrelaxes and a tiny bit of stool is released which is sensed by anal mucosa (discriminates texture of stool)
3. afferents are sent to cortex to determine if time is appropriate
4. If timing is appropriate, peristalsis occurs, and puborectalis and EAS relax.

Front 25

What are some changes that occur with age in the-

1. Oropharynx
2. Esophagus
3. Stomach
4. Small intestine
5. Colon, Anorectal

Back 25

1. OROPHARYNX: xerostomia, difficulty swallowing
2. ESOPHAGUS: ↑ GERD, ↓ LES tone, reduced myenteric ganglion cell
3. STOMACH: delayed gastric emptying (dyspepsia, early satiety)
4. SI: ↓ contraction and MMC, ↑bacterial overgrowth, cramping
5. COLON/ANORECTAL: delayed colonic transit, ↓ rectal sensation- fecal incontinence or impaction.