Control of motility and secretion in the upper GI

Question 1

Components of the ENS

Answer 1

myenteric plexus (between two external muscle coats)
submucosal plexus (submucosa, small and large intestine only)

Question 2

Outputs of ENS & ANS

Answer 2

muscularis externa
muscularis mucosae
exocrine/endocrine cells

Question 3

Muscularis externa

Answer 3

By myenteric plexus

Controls motility: tonic contraction, rhythmic segmentation, oscillatory movements, peristalsis

Question 4

Muscularis mucosae

Answer 4

By submucosal plexus
further mixing (movement of the villi)

Question 5

Role of hormones in GI motility

Answer 5

Modulate responses to ICCs and their impact on the intracellular calcium concentration during initiation of a slow wave
Influence resting contractility of muscle in the upper GI

Question 6

Secretory components of the stomach

Answer 6

gastrin (endocrine)
histamine (endocrine)
ACh (ENS)

Question 7

Enterochromaffin-like cell

Answer 7

Modulated by gastrin and ACh of ENS to produce histamine to trigger the release of acid, in addition to their own effects on parietal cells

Question 8

Enteric intrinsic interneurons

Answer 8

influence acid secretion by parietal cells directly
secretion of gastrin through GRP (would also increase acid secretion by parietal cells)
in the cepahlic, gastric, and intestinal phases of stimulation of gastric secretion

Question 9

Enterogastric reflex

Answer 9

certain substances are sensed in the duodenum, then enterogastrones (hormones) are released as a reflex that travels through the celiac ganglions to the effectors in the stomach

Question 10

Myenteric (Auerbach’s) plexus

Answer 10

within muscularis externa, between two layers of muscle
Sensory: receptors in gut wall that senses mechanical, chemical or thermal stimuli, then relates signal to interneurons within ganglia

Interneurons receive from sensory then transmits to motor neurons, which innervate the smooth muscle, exocrine and endocrine/parietal cells

Question 11

Submucosal (Meissner’s) plexus

Answer 11

located in the submucosa
Similar reflex as myenteric (sensory –> motor), except only a small number of innervates the smooth muscle layer (muscularis mucosae)

Question 12

NTs in longitudinal layers of the muscularis externa

Answer 12

Excitatory: ACh
Inhibitory: NO, vasoactive intestinal peptide, purines (ATP, beta-NAD)

Question 13

NTs in circular layers of the muscularis externa

Answer 13

Excitatory: ACh, substance P, serotoninin (5-HT)
Inhibitory: NO, vasoactive intestinal peptide, purines (ATP, beta-NAD)

Question 14

Submucosal plexus NT

Answer 14

Stimulatory: ACh (motility), VIP (glandular/exocrine)
Regulates secretion
Regulates motility of the muscularis mucosae

Question 15

Co-localization

Answer 15

neurons that contain more than one type of NT

Question 16

Secretomotor neuron

Answer 16

neuron that is able to induce a gland to secrete a substance

Question 17

Slow wave

Answer 17

Spontaneous rhythmic fluctuations in the RMP intrinsic to the GI tract
1-5 s/oscillation
frequency (region-specific): distal stomach - 3 cycles/min, duodenum - 12 cycles/min
Determine the fq of phasic smooth muscle contractile events

Question 18

Tonic contraction (tone)

Answer 18

muscularis externa circular muscle contractions that functionally separate different areas of the GI tract
allows for unidirectional movement

Question 19

Sphincter

Answer 19

site of prolonged tonic contraction

ensures unidirectional movement

Question 20

RMP in the GI tract

Answer 20

-40 to -80 mV

Determined by: Na and K channels, Na/K ATPase

Question 21

Phases of a slow wave

Answer 21

Rising phase: entry of Ca through CaV channels and others, and release from intracellular storage

Repolarization: activation of Ca-dependent K channels, concominant reduction in cytosolic Ca

Plateau: due to inward Ca current and outward K fluxes

Question 22

Spike potentials

Answer 22

contraction begins when electrical threshold is reached for opening of CaV channels

In some regions: spike action potentials are also generated, results in larger Ca influxes through CaV –> greater contractions

Question 23

Interstitial Cells of Cajal (ICC)

Answer 23

Pacemaker cells
concentrated in myenteric plexus (ICCMY) and muscle (ICCMI)
ICCMY responsible for generating and propagating slow waves (Ca oscillations)

Gap junctions allow for electrical coupling

Question 24

Factors that contribute to Ca influx in GI tract (6)

Answer 24

1. CaV (L-type) - predominant
2. Store-operated Ca channels (SOCs) - open in response to depletion of intracellular Ca
3. Stretch-activated Ca channels
4. NTs and hormones acting on receptors coupled to activation of phospholipase C and release of IP3, resulting in release of intracellular Ca (ACh, substance P)
5. Receptor-operated Ca channels (ROCC)
6. Ca ATPases, Na/Ca exchangers: extrude Ca following contractions, also reuptake into intracellular storage

Question 25

Peristaltic reflex arc

Answer 25

``` Intrinsic/local reflex involving aborally propagated circular muscle contractions behind the bolus, initiated by intraluminal distention coupled to a wave of relaxation (reflex relaxation) in the distal part of the GI tract NT: 5HT/serotonin secreted by epithelial enteroendocrine cells to stimulate sensory neurons Rhythmic segmentation (circular) Oscillatory movements (Longitudinal) ```

Question 26

Functions of the esophagus

Answer 26

Deglutition (movement of food from pharynx to stomach) prevention of air entry into the stomach/reflux of gastric contents Eructation (belching) Vomiting

Question 27

Resting pressure of the pharynx

Answer 27

0

Question 28

Resting pressure of the esophagus

Answer 28

UES: 25-60 Body: approximates intrathoracic pressure (-5 to -15 inspiration, -2 to +5 expiration), flaccid LES: 10-40

Question 29

Pressure during swallowing

Answer 29

Pharynx: >100 (bolus moves from pharynx --> body) UES: 0-60 reflex relaxation in response to food in pharynx, then closes Body: 90-100 - high pressure peristaltic movements moves food in 9-10 seconds LES: 0-40 - bolus moves from high-P body into lower-P stomach (5-10), then closes

Question 30

Pathway in pharyngeal peristalsis

Answer 30

1. Bolus of food --> tactile receptors --> sensory info sent to swallowing centre (brainstem), received by afferent reception neurons. Reflex relaxation of UES occurs. 2. Afferent info sent to cortex and central pattern generator of the swallowing centre 3. Efferent info --> coordinated contraction of pharyngeal muscles, peristalsis to propel food into UE 4. Reflex relaxation of the LES occurs with swallowing or distention of esophagus/stomach. Myenteric cholinergic neurons activate VIP, NO, purine (ATP, beta-NAD)-containing inhibitory neurons to mediate relaxation

Question 31

Pathway in primary peristalsis

Answer 31

Continuation of the pharyngeal peristalsis, transverses esophagus in 9-10 seconds Does not require sensory input from the esophagus 1. Swallowing centre coordinates the action of efferent vagal somatic nerves firing from the Nucleus Ambiguus --> sequential activation of striated muscles in the esophagus 2. Vagal PS input from the dorsal motor nucleus of the vagus --> initiates peristaltic wave in smooth muscle; control of the wave progression by myenteric plexus (vagal initiation, intrinsic regulation)

Question 32

Pathway in secondary peristalsis

Answer 32

If primary peristalsis is insufficient in clearing the food from the esophagus, distention initiates secondary peristalsis Similar to primary wave except that it can occur in the absence of pharyngeal swallowing 1. Local distention in the esophagus sends sensory info to the swallowing centre 2. Involves extrinsic innervations via the dorsal motor nucleus of vagus for initiation in UE 3. involves both the vagus and myenteric plexus in the smooth muscle 4. Clears food remnants and refluexed gastric juice following primary wave

Question 33

Reflex relaxation of the ES's

Answer 33

UES: neurally mediated in response to food bolus in the pharynx LES: neurally mediated in response to swallowing or distention of the esophagus/stomach, both vagal and efferent fibres and intrinsic neurons are involved. Myenteric cholinergic neurons activate VIP, NO, and/or purine-containing inhibitory neurons

Question 34

Spontaneous transient LES relaxation (TLESR)

Answer 34

Occurs 20-30x/day, lasting 10-60 seconds Pathological conditions: more frequent and prolonged - excessive reflux/extended contact time of gastric contents with the mucosa, can result in heartburn and damage

Question 35

Extrinsic modification of ENS

Answer 35

1. ANS - PS motor from brainstem via vagus to the pharynx to first 2/3 of colon - sacral spinal cord/pelvic nerves to distal - sym motor from thoracic/lumbar SC - sensory neurons from ENS --> ANS 2. Higher CNS brain centres - hormonal factors in GI and external senses (smell, sight) - influence ANS 3. immune system - locally acting paracrine factors that can stimulate immune cells (e.g. mast cells) to produce their own factors (i.e. histamine) - can directly influence GI motility and secretion

Question 36

Myenteric plexus innervation

Answer 36

esophagus to rectum

Question 37

Submucosal plexus innervation

Answer 37

small and large intestines only