GI Motility

Question 1

4 basic GI processes

Answer 1

Motility
Secretion
Digestion
Absorption

Question 2

2 broad categories of motility

Answer 2

Mixing movements

Propulsive movements

Question 3

Mixing movements

Answer 3

Redistribute luminal contents locally
Enhancing the exposure to digestive secretions
Expose luminal contents to GI tract absorbing surfaces (epithelium)

Question 4

Propulsive movements

Answer 4

Move the luminal contents forward

Rate of propulsion varies with specific function of region (esophagus = fast, SI = slow)

Question 5

Myenteric plexus

Answer 5

Also called Auerbach’s plexus
Lies between circular and longitudinal muscle layers
Coordinated muscularis-externa contractions

Question 6

What type of muscle is in the GI tract?

Answer 6

Smooth muscle

Have gap junctions between them for electrical continuity

Question 7

How do the muscle cells contract?

Answer 7

Contraction is initiated by increased cytoplasmic Ca
Sarcoplasmic reticulum associated with PM at indentations (calvaeoli)
Contractile acto-myosin filaments are obliquely arranged
Cytoskeletal filaments are anchored at dense body junctions
Cells are physically and electrically coupled at gap junctions

Question 8

Thick Filament regulation

Answer 8

When intracellular Ca concentration is increased, Ca binds to calmodulin
Ca-calmodulin complex activates Myosin Light Cain Kinase
MLCK phosphorylates myosin so it can bind to actin (begins the shortening process)
The phosphate is removed by Myosin Light Chain Phosphatase
De-phosphorylated myosin can no longer bind to the actin filament and the contraction is terminated

Question 9

What are the 2 ways that Ca is released?

Answer 9

Depolarization of the membrane

Production of IP3 at the PM (stimulated by hormones and NTs)

Question 10

How does depolarization lead to muscle contraction?

Answer 10

Cell is depolarized via synaptic transmission
Voltage gated Ca channel is in the base of the caveolae, allows some Ca to enter
Ca binds ryanodine receptor on SER
Huge efflux of Ca into the cytoplasm

Question 11

How does the production of IP3 lead to muscle contraction?

Answer 11

Hormones and NTs bind at the sarcolemma
IP3 is produced, and diffuses into the cell
Binds to the IP3 receptor on the SER
Ca diffuses out of the SER into the cytosol

Question 12

How is the Ca removed from the cytoplasm?

Answer 12

Ca ATPases
On the sarcoplasmic membrane
On the SER

Question 13

4 factors that regulate the GI system

Answer 13

Intrinsic electrical properties of smooth muscle cells
Enteric NS
Autonomic NS
Other systems (brain, immune, hormones)

Question 14

Interstitial cells of Cajal

Answer 14

Non-contractile pacemaker cells
Specialized smooth muscle cells that undergo spontaneous, transient membrane depolarizations
The depolarizing potentials are termed slow waves
Propagate through gap junctions (smooth muscle network)

Question 15

How are the duration and amplitude of slow waves modulated?

Answer 15

By neurotransmitters/agonists that are released by enteric motor neurons in the wall of the GI tract

Question 16

How do slow waves cause APs?

Answer 16

If slow wave depolarizations reach AP threshold, a burst of APs result
The number of APs is proportional to the duration the slow wave remains above the threshold
Greater number of APs = greater elevation of intracellular Ca = greater strength of contraction of muscle cell

Question 17

2 ways the ENS functions

Answer 17

Via electrical communication and the release of NTs
Can operate without external input (brain), so is considered reflexive
Made up of the submucosal and myenteric plexus

Question 18

3 components of the ENS

Answer 18

Sensory neurons (including mechanoreceptors, chemoreceptors, and osmoreceptors)
Interneurons (excitatory and inhibitory)
Secretomotor cells (influence smooth muscle, epithelial cells that secrete/absorb fluid, enteric endocrine cells)

Question 19

3 functions of the ANS in the gut

Answer 19

Influences ongoing ENS activity
Directly affects smooth muscle and glands
Alters GI hormone levels

Question 20

Pre and post ganglionic fiber locations with the

1. Sympathetic NS
2. Parasympathetic NS

Answer 20

1. Preganglionic fibers synapse in ganglia, post ganglionic (NE) synapse in the ENS
2. Preganglionic fibers synapse with a postganglionic fiber (ACh) within the ENS

Question 21

Smooth muscle En Passant innervation

Answer 21

Motor axons have multiple varicosities (swellings containing packets of NT)
Release the transmitter in the vicinity of smooth muscle
NT diffuses to its targets

Question 22

Parasympathetic vs sympathetic function in GI

Answer 22

Parasymp: increases motility and secretions, relaxes sphincters (occurs during and immediately after a meal)
Symp: decreased motility and volume of secretions, contracts sphincters (during stress response)

Question 23

Where is the ‘swallowing center’?

Answer 23

Medulla oblongata

Question 24

3 phases of swallowing

Answer 24

Oral
Pharyngeal
Esophageal

Question 25

Oral phase of swallowing

Answer 25

Pushing a food bolus towards the back of the oral cavity and up against the palate
Requires the tongue
Initiation of swallowing is voluntary, but once initiated it becomes reflexive (next stage)

Question 26

Pharyngeal phase of swallowing

Answer 26

Pharynx is located at the posterior of the oral cavity, at its junction with the nasal passages
Touch and pressure receptors in the pharyngeal palate are activated by the food bolus
Info is sent to the medulla via the trigeminal nerve
This initiates the reflexive component of swallowing

Question 27

Once the trigeminal nerve is activated, the swallowing reflex begins. What does this entail?

Answer 27

Contraction of the pharyngeal wall behind the bolus pushes food toward the esophagus
Tongue position prevents the bolus from travelling back into the mouth
Uvula elevates to seal the nasal passages
Vocal cords contract and the epiglottis closes over the trachea
Swallowing center inhibits respiratory center in brain

Question 28

Pharyngoesophageal sphincter

Answer 28

Top of the esophagus

Remains closed except during swallowing to prevent air entering the stomach

Question 29

Esophageal phase of swallowing

Answer 29

Swallowing center relaxes the pharyngoesophageal sphincter

Swallowing center then initiates primary peristaltic waves by interacting with the EN

Question 30

Primary peristalsis

Answer 30

Inner circular layer contracts, pinching a ring
Outer longitudinal layer contracts in front, reducing the tube length
Primary peristaltic wave takes 5-9 seconds to travel from the beginning to the end of the esophagus

Question 31

Secondary peristalsis

Answer 31

Reflexive
Does not involve the swallowing center
Luminal contents become lodged, the distension of the Gi wall activates stretch receptors, they stimulate the ENS, the ENS coordinates a strong peristaltic wave to dislodge the luminal contents
Sweeper of the esophagus

Question 32

Gastroesophageal sphincter

Answer 32

Opens when a peristaltic wave pushes a food bolus against this region
Reflexive relaxation
Mediated by the vagus nerve
Contracts after to prevent reflux

Question 33

2 parts of motility in the small intestine

Answer 33

Segmentation (mixing during meals)
Migrating mobility complex (moves contents in between meals)
ENS controls both

Question 34

Segmentation

Answer 34

Alternating contractions and relaxations of adjacent sections of the small intestine
Initiated by distension of lumen, presence of gastrin, parasympathetic input
Helps move chyme toward LI

Question 35

Migrating motility complex

Answer 35

Begins at the duodenal/gastric junction
Consists of weak peristaltic contractions that travel for a short distance
Second wave begins slightly more distally than the initiation site of the first wave, and travels slightly further
Takes about 2 hours to get from stomach to LI