Lesson 69: Stomach functions and controls

What are the three main control mechanisms for GI functions?

Enteric Nervous System, Autonomic Nervous System, and Endocrine System (and Immune System).

What is the purpose of controlling stomach functions (motility and secretions)?

To adjust stomach functions to the incoming quantity and quality of food.

What is another name for the Enteric Nervous System (ENS)?

Brain of the Gut.

Where is the Enteric Nervous System located?

In the smooth muscles between the esophagus and anus.

What are the two main plexuses that compose the ENS?

Myenteric plexus and Mucosal plexus.

What does the myenteric plexus primarily control?

GI motility.

What does the mucosal plexus primarily control?

GI secretions.

What type of reflexes do the ENS plexuses control?

Local (short) reflexes.

What stimulates GI motility via myenteric mechanoreceptors?

GI wall distension.

What stimulates GI secretion rates via mucosal chemoreceptors?

Mucosal chemoreceptors.

What is a mostly excitatory neurotransmitter used by the ENS?

Acetylcholine.

What is a mostly inhibitory neurotransmitter used by the ENS?

Norepinephrine.

What provides the parasympathetic supply to the gastrointestinal tract?

Vagus Nerve (CN X) and the Pelvic Nerves.

What type of fibers are contained within the Vagus Nerve supply to the GI tract?

Sensory fibers (from GI chemoreceptors & mechanoreceptors) and motor fibers.

What is the main neurotransmitter of the Parasympathetic System in the GI tract?

Acetylcholine (acting on nicotinic receptors).

What two main stimuli induce central (long) reflexes via the Parasympathetic System?

GI wall distension and nutrient density.

What three general effects does the Parasympathetic System have on GI function?

Increases GI motility (mostly), Increases GI secretion rates, and Relaxes GI sphincters.

What is the overall effect of the Parasympathetic System on digestion?

Facilitates digestion.

From where does the Sympathetic System originate?

The thoracolumbar chain (T5-L2).

What is the main neurotransmitter of the Sympathetic System in the GI tract?

Norepinephrine.

What three general effects does the Sympathetic System have on GI function?

Decreases GIT motility (beta-2, alpha-2), Decreases GIT secretion rates (alpha-2), and Decreases blood flow to the GIT (alpha-1).

What effect does the Sympathetic System have on GIT sphincters?

Stimulates GIT sphincters (alpha-1).

What is the overall effect of the Sympathetic System on digestion?

Slows down digestion.

What type of cells check incoming chyme via chemoreceptors on microvilli?

Endocrine cells (interspersed throughout the GI mucosa).

Name three examples of regulatory peptides/hormones released by gut endocrine cells.

Gastrin, Somatostatin, Secretin, Cholecystokinin, Gastric inhibitory peptide, Motilin, Enteroglucagon, or Peptide YY.

If endocrine cells release hormones into the blood stream, they are called what?

Endocrines.

If endocrine cells release hormones into the interstitium, they are called what?

Paracrines.

What are the components of the Enteric Immune System?

Mesenteric lymphnodes, gut-associated lymphoid tissue (GALT), Peyers patches, and mucosal WBCs.

What do immune cells release when challenged, acting as local paracrines?

Pro-inflammatory cytokines.

What effect do cytokines have on GI motility and secretions?

Greatly stimulates GI motility and secretions.

What is the protective mechanism called that flushes out pathogens?

Peristaltic rush.

What controls Mechanical Digestion in the stomach?

Motility Pattern, Storage of food (proximal), Grinding, mixing & release of chyme (distal), and Motility pattern during resting phase.

What are the self-excitatory pacemakers in GIT smooth muscles called?

Cells of Cajal.

What rhythmic undulating pattern of membrane potential changes do these pacemakers generate?

Slow waves.

What is the typical frequency of slow waves per minute?

3-12/min.

What triggers smooth muscle contractions (spike potential)?

When a slow wave reaches threshold potential (at -40/-35mV), opening Ca channels.

What are three factors that push slow waves towards the threshold potential (priming factors)?

Gut wall distention, parasympathetic stimulation, and some hormones.

What type of contraction travels long distances, pushing ingesta down the GI tube?

Peristaltic Contractions.

What type of contraction travels short distances, squeezing ingesta back and forth?

Mixing / Segmentation Contractions.

What reflex facilitates storage of food in the proximal stomach?

The Accommodation Reflex.

How does the Accommodation Reflex work (mechanistically)?

Sensory signals ascend via the vagus nerve to the medulla, motor signals descend via vagal motor fibers, releasing inhibitory NTs (nitric oxide), causing relaxation of the proximal stomach muscles.

What is the purpose of the Accommodation Reflex?

Enables expansion of the stomach without pressure increase.

Which feeding pattern is accommodated by the carnivore's ability to store up to 25% of their body weight in the proximal stomach?

“Feast-fasting” feeding pattern.

What initiates local and central reflexes in the distal stomach during feeding?

Stomach wall distension.

How is solid and semi-solid food processed in the distal stomach before release?

It requires grinding and mixing with gastric juices into fluid chyme.

What is the average stomach retention time of solid food for a dog?

4-12 hours.

What is the cyclical, recurring motility pattern during fasting called?

Migrating Motility Complex (MMC).

How often does the Migrating Motility Complex typically occur?

Ca every 2 hours.

What is the function of the Migrating Motility Complex?

Clears the stomach and small intestines (SI) of residual food and secretions, and avoids bacterial overgrowth in the SI.

What hormone largely stimulates the Migrating Motility Complex, and from where is it released?

Motilin, released from M-cells in the antrum and duodenum.

What is a potential disadvantage of the Migrating Motility Complex?

Time for foreign bodies to move into the small intestines, possibly causing ileus.

What cells produce hydrochloric acid (HCl)?

Parietal cells.

What is the typical pH range of pure HCl solution?

Less than 1 (<1).

What active pump is involved in HCl production in parietal cells?

The H+ / K+ pump (Proton Pump).

What is the strongest stimulator of HCl secretion?

Histamine.

What cells release Histamine in response to Gastrin?

Enterochromaffine-like (ECL) cells.

What substance, released from D cells, inhibits the proton pump, gastrin, and histamine release when stomach pH drops too low?

Somatostatin.

What are three functions of HCl?

Bactericidal, Denatures proteins, Activates gastric prepsinogen to pepsin, and Emulsifies fat.

What substance is co-secreted with HCl and binds to Vitamin B12 to protect it?

Intrinsic Factor.

In the stomach, what cells produce Pepsinogens?

Chief cells.

What condition is necessary for the activation of pepsinogens to active pepsins?

An acidic environment (HCl).

What type of enzymes are Pepsins, and what is their function?

Proteolytic enzymes

How much of carbohydrate digestion (polysaccharides into smaller units) may occur in the stomach by salivary amylase before it is destroyed by HCl?

30%.

How much of protein hydrolysis is typically initiated by pepsin in the stomach?

10-20%.

What process does HCl initiate on fat globules in the stomach?

Beginning emulsification (of larger into smaller fat globules).

What are the five components involved in creating the multifactorial Gastrointestinal Barrier?

Physical barrier, Chemical barrier, Immunological barrier, Microbiological barrier, and Healing capacity.

What physical barrier component is produced by Goblet cells and covers the entire stomach surface?

Mucus.

What substance, contained within the mucus, neutralizes back-diffusing H+?

Bicarbonate.

What cells form a continuous layer with tight junctions to seal the paracellular space as a physical barrier?

Epithelial Cells.

Name three functions of Prostaglandins (PGE2) in the chemical barrier.

Stimulates mucus and bicarbonate production, Improves blood supply (vasodilator), Reduces HCl production, or Supports cell proliferation & healing.

What are three barriers that are bactericidal?

HCl, Enzymes, and Bile Acids.

Name one factor that can disrupt the GI Barrier due to increased sympathetic tone.

Stress.

Name one common pharmaceutical cause of GI Barrier disruption.

NSAIDS.

What are three potential consequences of GI Barrier disruption?

Acute/chronic gastritis, autodigestion, erosions, or ulcerations.

What anatomical feature in horses prevents them from vomiting?

Strong cardiac sphincter & acute angle.

What is the definition of vomiting (emesis)?

The forceful active expulsion of GIT content via the mouth initiated by the vomiting reflex.

What is the definition of regurgitation?

The passive expulsion of ingesta from the esophagus or forestomach.