GI Tract Function and Feeding Mechanisms

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Questions and Answers

Which function of the GI tract is primarily concerned with the immune system?

Transportation of food

Regulation of water and electrolyte balance

Thermoregulation

Immunologic barrier (correct)

In which species is the snout primarily used for prehension during feeding?

Pigs (correct)

Horses

Cattle

Goats

What is the primary difference in mastication between carnivores and herbivores?

Carnivores spend more time masticating than herbivores

Herbivores have large chewing surfaces on their teeth (correct)

Herbivores have primarily vertical jaw movements

Carnivores use primarily horizontal jaw movements

What is one function of the motility movements in the GI tract?

To retain ingested feed for digestion Signup and view all the answers

What is the first motility pattern in the GI tract after feeding?

Deglutition Signup and view all the answers

What initiates the involuntary phase of deglutition?

Food enters the pharynx Signup and view all the answers

Which structure prevents food from entering the trachea during swallowing?

Epiglottis Signup and view all the answers

Which type of dysphagia is associated with malfunction of the upper esophageal sphincter?

Oropharyngeal dysphagia Signup and view all the answers

Which molecule is known to inhibit hunger and increase energy consumption?

Melanocyte-stimulating hormone Signup and view all the answers

What is the role of Cholecystokinin (CCK) in the regulation of food intake?

Inhibits hunger Signup and view all the answers

During which stage of deglutition does breathing temporarily stop?

Pharyngeal phase Signup and view all the answers

Which area of the hypothalamus is primarily known as the hunger center?

Lateral hypothalamus Signup and view all the answers

What effect does Leptin have on Neuropeptide Y (NPY) release?

Decreases NPY release Signup and view all the answers

Which mechanism is responsible for the stimulation of gastric acid secretion during the cephalic phase?

Neural mediation via acetylcholine Signup and view all the answers

What is the role of histamine in gastric acid secretion?

It acts as a paracrine mediator enhancing acid secretion Signup and view all the answers

Which species produces chymosine to aid in the digestion of milk proteins?

Calves Signup and view all the answers

During gastric motility, which phase is primarily induced by the presence of food in the duodenum?

Intestinal phase Signup and view all the answers

What hormone is released primarily in response to fasting and is known to increase appetite?

Ghrelin Signup and view all the answers

Which of the following substances is primarily responsible for stimulating mucus secretion in the gastric mucosa?

Acetylcholine and prostaglandins Signup and view all the answers

Which of the following glands is primarily responsible for producing serous saliva?

Parotid gland Signup and view all the answers

Which type of secretion is characterized by a mixture of both mucous and serous fluids?

Seromucous glands Signup and view all the answers

Which function of saliva is directly related to the initiation of carbohydrate digestion?

Enzymatic activity Signup and view all the answers

Which component is added to saliva in the ducts, enhancing its regulatory function?

HCO3¯ Signup and view all the answers

In which species is saliva involved in thermoregulation primarily through behavior like panting?

Dogs Signup and view all the answers

What is a common consequence of disturbances in saliva production?

Bacterial proliferation and dental caries Signup and view all the answers

What is the primary role of bicarbonate in salivary gland secretions?

Neutralizing acidity and pH regulation Signup and view all the answers

What process is primarily involved in the production of carbonic acid within parietal cells?

Chemical reaction of water and CO2 Signup and view all the answers

Which component is actively transported into the canaliculus during HCl production?

H+ ions Signup and view all the answers

Intrinsic factor is necessary for the absorption of which vitamin?

Vitamin B12 Signup and view all the answers

What is the primary role of enteroendocrine cells in the gastrointestinal tract?

Production of hormones Signup and view all the answers

What happens to H+/K+ ATPase when parietal cells are not stimulated?

It is stored in vesicles within the cytoplasm. Signup and view all the answers

Which hormone is secreted by enteroendocrine cells?

Gastrin Signup and view all the answers

What is the function of mucous neck cells in the gastric epithelium?

Producing mucin for lubrication Signup and view all the answers

Which of the following correctly describes the role of carbonic anhydrase?

Catalyzes the formation of carbonic acid Signup and view all the answers

When enteroendocrine cells release their secretions, where do they primarily release them?

Into the blood capillaries in the lamina propria Signup and view all the answers

Which type of channel is involved in the transport of Cl- into the canaliculus?

Conductance channels Signup and view all the answers

Which statement accurately describes the role of gastric pits in the stomach?

They contain cells that secrete mucus and lead to deep gastric glands. Signup and view all the answers

What is the primary stimulus for the secretion of CCK (Cholecystokinin)?

Presence of fats and amino acids Signup and view all the answers

In terms of gastric acid production, which animal has the highest daily production?

Horses Signup and view all the answers

Which hormone is responsible for stimulating bicarbonate secretion and inhibiting acid secretion in the stomach?

Secretin Signup and view all the answers

What is the primary action of parasympathetic stimulation on salivary gland secretion?

Results in secretion of more dilute saliva. Signup and view all the answers

Which type of secretion involves substances that act locally and diffuse through the interstitial space?

Paracrine substances Signup and view all the answers

Which of the following cell types is responsible for producing hydrochloric acid (HCl) in the gastric glands?

Parietal cells Signup and view all the answers

The stimulation of saliva secretion purely through contact with the oral mucosa is categorized as which type of reflex?

Innate reflex Signup and view all the answers

Which hormone acts primarily to inhibit gastric secretion and stimulate insulin release in response to fats and glucose?

GIP Signup and view all the answers

In the context of GI tract motility, what role does acetylcholine play during fasting?

Induces motility through MMC Signup and view all the answers

What is the primary mechanism by which parietal cells produce gastric acid?

Secretion of hydrochloric acid through an ATP-dependent process Signup and view all the answers

Which hormone plays a crucial role in stimulating the secretion of gastric acid in the stomach?

Gastrin Signup and view all the answers

How does the digestive system of herbivores typically differ from that of carnivores?

Herbivores have longer intestines to aid in fiber digestion Signup and view all the answers

Which of the following best describes the phases of motility movements in the GI tract during digestion?

Both peristalsis and segmental contractions facilitate movement Signup and view all the answers

What defines the role of enteroendocrine cells in the gastric mucosa?

They secrete hormones affecting gastric secretions and motility Signup and view all the answers

Which factor primarily influences the composition of saliva produced by salivary glands?

The type of food being ingested Signup and view all the answers

Which aspect of gastric secretion is primarily regulated by the presence of food in the stomach?

Intrinsic and extrinsic neural signals Signup and view all the answers

What is the primary function of mucus secreted in the stomach?

To protect the gastric lining from acidity and mechanical damage Signup and view all the answers

In terms of energy source specialization, how do ruminants adapt their digestive systems?

They have a complex stomach structure that allows for fermentation of plant materials Signup and view all the answers

What is primarily responsible for the reflection of satiety in the digestive system?

Hormonal signals released from the gut and pancreas Signup and view all the answers

Study Notes

GI Tract Function

The GI tract is composed of concentric muscle cylinders lined with an epithelium, and functions in the transport, digestion, absorption of food.
The GI tract also plays a role in water and electrolyte balance, provides an immunologic barrier through the Gut Associated Lymphoid Tissue (GALT), and regulates thermoregulation.

Prehension (Food Intake)

Prehension involves the process of taking food into the mouth. Different species have different methods of feeding.
Horses use their lips when eating from a manger or their incisors when grazing.
Cattle use their tongue to wrap around forage and their incisors.
Goats and sheep use their tongue and lips.
Pigs use their snout and mandible.
Carnivores utilize their canines, incisors, and forelimbs.

Mastication (Chewing)

Mastication is the first step in digestion. It involves the teeth, jaws, tongue, and cheeks.
Carnivores chew sparsely, with vertical movements of the mandible.
Herbivores spend a significant amount of time masticating, with large upper and lower jaws that accommodate teeth with wide chewing surfaces.

GI Tract Motility

After mastication, motility of different GI tract segments is activated.
Functions of GI tract movements:
- Propelling ingested feed from one location to the next.
- Retaining ingested feed at a specific site for digestion, absorption, or storage.
- Physically breaking up food material and mixing it with digestive secretions.
- Circulating ingested feed to ensure all portions come in contact with the absorptive surfaces.

Deglutition (Swallowing)

Deglutition is the first motility pattern in the GI tract.
Voluntary phase (oral phase): Food is molded into a bolus in the oral cavity and pushed back into the pharynx.
Involuntary phase (swallow reflex): The pharynx and esophagus direct food into the digestive system, away from the upper airways.

Deglutition Mechanisms

During deglutition, breathing momentarily stops.
The soft palate elevates, blocking the pharyngeal opening of the nasopharynx.
The tongue is pressed against the hard palate, closing off the oral opening.
The epiglottis moves backwards, covering the entrance to the trachea.
The upper esophageal sphincter opens, and food is transported through the esophagus by peristaltic contractions.

Deglutition Disorders

Difficulty in swallowing (dysphagia) can result from neuromuscular disorders or mechanical obstruction.
Oropharyngeal dysphagia arises from malfunctions in the pharynx and upper esophageal sphincter.
Esophageal dysphagia is caused by malfunctions in the esophagus.

Aspiration

Aspiration occurs when food particles, fluids, or stomach contents reach the upper airways.
This can be a result of dysphagia.

Regulation of Food Intake

The hypothalamus regulates energy homeostasis.
Hunger center: Located in the paraventricular nucleus, lateral hypothalamus fields, and perifornical region.
Satiety center: Located in the ventromedial nucleus.

Neuropeptides in the Hypothalamus

Stimulatory: Neuropeptide Y (NPY) and Orexin.
Inhibitory: Melanocyte-stimulating hormone (MSH) inhibits hunger and increases energy consumption.

Non-Hypothalamic Hormones

Stimulatory: Ghrelin.
Inhibitory: Cholecystokinin (CCK), Peptide YY (PYY), Leptin (secreted by fat cells, inhibits NPY release and activates MSH activity), and Insulin (secreted by the pancreas, regulated by glucose availability).

Ghrelin

Produced in the stomach by endocrine cells in the gastric fundus
Increases during fasting and decreases after eating (postprandially)
Helps increase appetite and food intake
Capromorelin is a ghrelin receptor agonist used to stimulate appetite in dogs

Salivary Glands

Major salivary glands: Parotid, Mandibular, Sublingual
Small salivary glands: Ventral jaw, Palate, Pharyngeal, Lip, Zygomatic
Classified based on secretion type: Serous, Mucous, Seromucous

Saliva Functions

Primary (Digestive)
- Protects oral mucosa and teeth
- Facilitates swallowing (deglutition)
- Initiates carbohydrate digestion (via amylase)
- Regulates pH through bicarbonate (HCO3¯) secretion
Secondary
- Immunological (Lysozyme, Immunoglobulins)
- Thermoregulation (panting in dogs)
- Defense mechanism (llamas, alpacas, guanacos)

Saliva Composition

Water (99%)
Electrolytes (Na+, K+, Cl-, HCO3¯)

Salivary Gland Components

Acini: Basic secretory unit of salivary glands; produces primary saliva (H2O, Cl-, Na+)
Ducts: Add K+ and HCO3¯ to form secondary saliva

Saliva Production Disturbances

Dry mouth (xerostomia)
Buccal ulcers
Dysphagia (difficulty swallowing)
Increased bacterial growth, leading to dental caries

Saliva Secretion Regulation

Parasympathetic System:
- Nerves: Facial and Glossopharyngeal
- Act through M3 receptors
  - Contracts myoepithelial cells
  - Increases dilute saliva secretion
Sympathetic System:
- First three thoracic segments
- Act through alpha-1 receptors
  - Secretes small volumes of viscous (mucous) saliva

Saliva Secretion Stimulation

Innate (Reflex): Contact with oral mucosa (mechanoreceptors)
Conditioned: Sight, smell, or imagination of food

Gastric Secretion Types

Endocrine: Secretions enter blood and act on distant targets
Paracrine: Local action through interstitial space, affecting nearby cells
Autocrine: Acts on the same cell that produced it
Neurocrine: Secretions from enteric neurons affecting muscle, glands, and blood cells

Digestive Hormones

Secreted by one cell and affect another
Transported in blood
Stimulated by food and mimicable by synthetic analogs
The "Big Five" GI Hormones:

Secretin
- Produced in duodenum (primarily) and jejunum
- Stimulates bicarbonate secretion
- Inhibits acid secretion
- Stimuli: Acids, fats, proteins
Gastrin
- Produced in stomach (antrum) and duodenum
- Stimulates acid secretion
- Stimuli: Protein, high pH
CCK (Cholecystokinin)
- Produced in duodenum, jejunum, and ileum
- Stimulates pancreatic enzyme secretion
- Stimulates gallbladder contraction
GIP (Glucose-dependent Insulinotropic Peptide)
- Produced in duodenum and jejunum
- Inhibits gastric secretion
- Stimulates insulin secretion
- Stimuli: Fats, glucose
Motilin
- Produced in duodenum and jejunum
- Induces intestinal motility during fasting (migrating motor complex - MMC)
- Stimuli: Acetylcholine

Monogastric Stomach

Regions: Esophageal part, cardia, fundus, corpus, pylorus
Glandular Zones:
- Cardia: Mucus production dominant
- Fundus: HCl and enzyme production dominant
- Pylorus: Mucus production dominant
Gastric Acid Secretion: Varies by species
- Dogs: 0.2-0.5 liters/day
- Humans: 1.0-2.0 liters/day
- Pigs: 2.0-3.0 liters/day
- Horses: 6.0-8.0 liters/day

Monogastric Stomach Cells

Surface Mucous Cells: Produce thick mucus to protect stomach from acid
Gastric Pits: Lined with mucus-secreting cells, leads to gastric glands
Gastric Glands:
- Parietal cells: Produce HCl and intrinsic factor (IF)
- Chief cells: Produce pepsinogen (inactive), prochymosine (in calves and labs)
- Mucous Neck Cells: Produce thin mucus and serve as progenitor cells for gastric mucosa
- Enteroendocrine Cells: Produce hormones (gastrin, histamine, somatostatin)
- Stem Cells: Located in the neck of the gland, differentiate into various gastric pit cells

Gastric Acid Production in Parietal Cells

Carbonic acid dissociates to produce protons (H+)
H+/K+ ATPase pumps on the apical membrane actively pump 2H+ and Cl- into the lumen of the canaliculus
Intrinsic factor (IF) is secreted by parietal cells and is essential for vitamin B12 absorption

Gastric Enzyme Secretion

Stimulated by:
- Neural (acetylcholine, noradrenalin)
- Hormonal (secretin, CCK)
Release of CCK and secretin from the small intestine is triggered by food particles and pH
Amino acids and fatty acids stimulate CCK release
Low pH stimulates secretin release

Gastric Secretion Regulation

Three Phases:
- Cephalic Phase: Before food enters the stomach (sight, smell, thought, taste)
- Gastric Phase: Induced by vagovagal reflexes and food in the stomach
- Intestinal Phase: Induced by food in the duodenum (feedback mechanism)

Gastric Ulcer

Mucus secretion is less affected by food compared to acid and enzyme secretion
Acetylcholine (Ach) and prostaglandin E (PGE) stimulate mucus secretion
Non-steroidal anti-inflammatory drugs (NSAIDs) block prostaglandin synthesis, compromising mucus production and increasing ulcer risk

Learning Objectives

Understand the functions of the GI tract
Differentiate food and water intake between species
Describe deglutition (swallowing) and its phases
Explain the control of hunger and satiety
Describe salivary gland structure and function
Understand the composition and regulation of saliva production
Compare endocrine, paracrine, neurocrine, and autocrine actions
Describe the synthesis, function, and regulation of key digestive hormones (gastrin, GIP, secretin, CCK, motilin)
Analyze the different parts of the monogastric stomach
Identify cell types in the stomach and their functions
Explain the production of gastric acid, enzymes, hormones, and mucus
Understand the regulation of gastric secretion and enzyme production
Explain the effects of NSAIDs on gastric mucosa
Recognize causes of gastric ulcer in animals

Digestive System

GI tract + Liver, pancreas, gallbladder (accessory organs)
GI tract consists of a concentric muscle cylinder lined with epithelium
Species have specialized digestive systems based on their diet

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Description

Explore the key functions of the GI tract, including digestion and absorption, as well as its role in maintaining homeostasis. Learn about various prehension methods used by different animal species during food intake and the importance of mastication in the digestion process.