Digestive System 1

• Digestive system is a continuous tubing from mouth to anus, also know as the digestive tract, gastrointestinal tract (GIT), gastroenteric tract, alimentary canal, or gut.
• Salivary glands, liver, and pancreas are organs that attach with ducts to digestive tubing.

Digestive System Functions

• Prehension: Grasping food with the lips or teeth, can be referred to as "horse prehending food"
• Mastication: Mechanical grinding and reducing food size with chewing.
• Chemical and physical digestion breaks food down.
• Absorption: Nutrients and water are absorbed.
• Elimination: Waste removal.

Oral Cavity

• Oral cavity also known as the buccal cavity.
• Lips: skeletal or smooth muscle?
• Philtrum: A cleft that divides the upper lip.
• Cheeks muscles lined with tissue.
• Palate includes the hard and soft palates

Palate

• Hard palate is made up of palatine, maxillary, incisive bones in the skull.
• Soft palate separates the pharynx into the oropharynx and nasopharynx.
• Elongated soft palate can cause brachycephalic dogs and cats difficulties

Oral Cavity Content: Teeth

• Teeth arrangement from rostral to caudal: incisor, canine, premolar, molar.

Oral Cavity Contents: Tongue

• Muscle type is not specified/
• Attached to the hyoid apparatus, sides of mandible.
• Papillae are specialized for grooming, moving food caudally.
• Taste buds are located on taste buds

Oral Cavity Innervation

• Sensory and motor nerves, Cranial Nerves VII and IX for sensory and XII for motor.
• Superficial blood vessels present in dogs, function is specified.

Salivary Glands - Four Pairs

• Salivary glands produce saliva; usually four pairs with ducts which moves saliva around the mouth.
• Parotid salivary glands are ventral to the ear canals (half).
• Mandibular salivary glands are ventral to the parotid glands at the caudal angle of the mandible.
• Sublingual salivary glands are medial to the shafts of the mandible just under the base of the tongue.
• Buccal salivary glands also called zygomatic salivary glands

Saliva

• Composed of water, proteins, electrolytes, antibodies and glycoproteins.
• Bicarbonate increases pH.
• Enzymes are lysozyme kills germs and in some species amylase which digest starch

Salivation

• Parasympathetic nervous system (CN X) creates a larger volume of more watery saliva.
• Sympathetic nervous system stimulation produces smaller volume more mucoid saliva.
• Conditioned responses exists.

Species Variation & Salivation

• Ruminants have more saliva and bicarbonate.
• Amylase is more important in humans, pigs and rats.
• Amylase is lacking in dogs, cats, ruminants.
• Horses have some Amylase.

Bolus & Deglutition

• Food that is swallowed by aniamls
• Pharynx shared with what other body system
• Cartilage flap preventing food from going down the trachea
• Cranial nerve is responsible for swallowing

Esophagus

• First and second sphincters encountered: upper esophageal and lower esophageal sphincter (aka cardiac sphincter).
• Inner muscle layer is circular, acts as sphincter when food reach stomach.
• Outer muscle layer is longitudinal.
• Peristalsis propels bolus.
• Mucus producing glands help the passage of bolus

Esophagus & Species

• Skeletal muscle found in dogs and ruminants for the entire esophagus.
• Skeletal muscle found in Horse at the cranial 2/3.
• Skeletal muscle found in Pigs cranial 1/3
• Primates and cats have smooth muscle on the distal esophagus.
• Birds and alligators smooth muscle throughout

Location: Stomach

• Located below the diaphragm

Stomach: Canine Anatomy

• Regions of stomach includes duodenum, pylorus, pyloric region, body, cardia and fundus

Stomach Functions

• Food Storage (fundus expands).
• Mechanical breakdown of food.
• Chemical breakdown of food.
• Generation of Chyme.
• Cardiac sphincter (aka LES).

Stomach Anatomy: species variation

• Cardia makes alkaline mucus.
• Canine, porcine stomach all glandular.
• Equine stomach fundus non-glandular (margo plicatus).
• Bovine stomach first 3 chambers are non-glandular.

Gastric Pits

• Structure found in cardia, fundus and pylorus of dogs.
• Parietal cells produce hydrochloric acid.
• Chief cells produce the enzyme precursor pepsinogen.
• Mucous (neck/progenitor) cells produce protective mucus.

Digestive Enzyme Precursors

• Pepsinogen converts to pepsin in presence of HCl.
• Pepsin digests proteins.

Pyloric Region Glands

• More branched than in body/fundus.
• Mucus secreting cells.
• G cells secretes gastrin (hormone) goes into bloodstream.

Actions of the Autonomic Nervous System

• Sympathetic stimulation: Peristalsis, gastric secretions, intestinal secretion all reduce
• liver increases glycogen conversion when stimulated by sympathetic stimulation
• Parasympathetic stimulation: Peristalsis, gastric secretions, intestinal secretion all increase
• Liver has no parasympathetic actions

Gastric Secretion

• Anticipate meal leads to parasympathetic NS stimulated.
• Ach release increases flow of saliva and release of HCI, pepsinogen and gastrin.
• Food entering to local reflexes and vagal reflexes increasing Ach release.
• This increases the process.

Stomach Action

• Stomach emptying: Balance between gastrin signaling peristalsis in pylorus and duodenum.
• Gastrin increases smooth muscle contraction at pylorus and relaxes pyloric sphincter.

Small Intestine Anatomy

• SI - duodenum, jejunum, ileum components of small intestine of dogs,

Duodenum Anatomy

• Runs down the right side of the abdomen.
• Spoons with pancreas.
• Contains the Mesoduodenum

Duodenum Functions

• Chemical Digestion.
• Sends signals to stomach depending on pH, osmolarity of chyme.
• Cholecystokinin (CCK) and secretin - hormones!
• Enteropeptidase enzyme for protein digestion.

Duodenum Secretions

• CCK is released from stomach inhibits contractions, tells pancreas to secrete pro-enzymes and bicarbonate, stimulates gall bladder contraction duodenum
• Secretin decreases acid production in stomach, increases bicarb from pancreas.

Jejunum

• Suspended from body wall by mesentery.
• Chemical nutrients is absorbed here

Ileum

• Peyer's patches here which are part of immune system .
• Absorption occurs here
• Ends at colon

Intestinal Tissue surface

• All increase area for absorption to occur

Movements in the Small Intestine

• Peristalsis
• Segmentation

Large Intestine

• Cecum, colon (ascending, transverse, descending), rectum, anus.
• Microbial digestion.
• Water and Electrolytes (ions) absorption.
• Variations of the LI
• Includes Segmentation, peristalsis, antiperistalsis, mass movement

Rectum/Anus

• When feces enters the anal canal.
• Defecation reflex internal anal sphincter relaxes.
• Brain sends message whether to proceed.
• External anal sphincter relaxes?

Pacemaker Cells

• Specialized smooth muscle cells are found in stomach, SI and colon..
• Between submucosa and inner circular muscle layer
• Also between circular and longitudinal muscle layers
• Slow fluctuations in resting potential - if stimulated to reach threshold then end up with pattern of contraction
• This has alternating periods of activity and rest

GIT Blood supply

• Celiac artery (a): Provides blood supply to stomach, pancreas, liver, spleen, proximal duodenum.
• Cranial mesenteric artery (a): Provides blood supply to rest of duodenum, jejunum, ileum, cecum, large intestine.
• Caudal mesenteric artery: Provide blood to some parts of the GIT