The Digestive System and Tract

• The digestive system processes food, extracts nutrients, and eliminates residue
• Digestion involves five stages: ingestion, digestion, absorption, compaction, and defecation
• Ingestion is the selective intake of food
• Digestion is the mechanical and chemical breakdown of food to be usable by the body
• Absorption involves uptake of nutrients into epithelial cells and then into blood and lymph
• Compaction involves absorbing water and consolidating indigestible residue into feces
• Defecation is the elimination of feces
• Mechanical digestion is the physical breakdown of food
• Cutting and grinding via the teeth and churning in the stomach and small intestines contribute to mechanical digestion
• Chemical digestion breaks down dietary macromolecules into monomers

The Digestive System and Tract

• Consists of the digestive tract and accessory organs
• The digestive tract is about 30 ft long
• It extends from the mouth to the anus
• Includes the mouth, pharynx, esophagus, stomach, small intestine, & large intestine
• The Gastrointestinal (GI) tract includes just the stomach and the intestines

Enteric Nervous System

• A nervous network exists in the esophagus, stomach and intestines
• This regulates digestive tract motility, secretion, and blood flow
• It is thought to have over 200 million neurons, and functions independently of the central nervous system
• Is often considered part of the autonomic nervous system
• The submucosal plexus controls glandular secretions of mucosa
• Controls muscularis mucosae movements
• The myenteric plexus controls peristalsis and other contractions of muscularis externa

Intestinofugal Neurons

• Provide sensory inputs for gut sympathetic reflexes that bypass the CNS
• All prevertebral ganglion neurons receive synapses from preganglionic neurons that have cell bodies in the spinal cord.
• Postganglionic neurons elicit vasoconstriction of arterioles in the gut
• Postganglionic neurons inhibit secretomotor or motility neurons in the enteric nervous system.
• Intestinofugal neurons send fibers to prevertebral ganglia (e.g. inferior mesenteric ganglion)
• Activate sympathetic efferents that inhibit gut function, and do not enter the CNS
• Secretomotor inhibitory and motility inhibitory neurons receive connections from intestinofugal neurons
• Intrinsic neurons of the gut send inputs to the intestinofugal neurons

Mastication

• Mastication/chewing breaks food into small swallowed pieces and exposes a larger surface to digestive enzymes
• It's the first step in mechanical digestion
• Food stimulates oral receptors, triggering an involuntary chewing reflex
• The tongue, buccinator (cheeks) and orbicularis oris (lips) manipulate food
• The masseter and temporalis elevate the lower teeth to crush food
• Medial and lateral pterygoids and masseters swing teeth in side-to-side grinding

Salivation

• Saliva moistens mouth, begins starch and fat digestion, cleanses teeth, dissolves molecules to stimulate the taste buds
• It binds food together into bolus to aid in swallowing
• Intrinsic salivary glands are small minor glands dispersed amid oral tissues
• Lingual glands are in the tongue and produce lingual lipase
• Labial glands are inside the lips
• Palatine glands are on the roof of the mouth
• Buccal glands are inside of the cheek
• All secrete saliva at a fairly constant rate

Salivary Glands

• Major extrinsic include the parotid, sublingual, and submandibular glands
• Mucous cells secrete mucus
• Serous cells secrete thin fluids rich in enzymes and electrolytes
• Autonomic nerves control salivation
• The sympathetic nervous system regulates salivation, activating alpha adrenoreceptors for fluid-rich secretion
• Activating beta adrenoreceptors for protein-rich secretion
• The parasympathetic nervous system releases acetylcholine, stimulating fluid-rich secretion

Salivation Medulla Oblongata and Pons

• Salivatory nuclei respond to signals from the presence of food
• Tactile, pressure, and taste receptors excite salivation
• Salivatory nuclei receive input from higher brain centers
• Odor, sight, and thought of food, stimulates salivation

Swallowing

• Swallowing (deglutition) involves 22+ muscles
• The swallowing center (pair of nuclei in medulla oblongata) coordinates swallowing
• Communicates with pharynx and esophagus muscles via trigeminal, facial, glossopharyngeal, and hypoglossal nerves

The Stomach

• J-shaped elastic organ
• Food enters from the esophagus, passing through the cardiac sphincter
• The cardiac sphincter is valve that stops back flow of stomach contents
• Food leaves the stomach through the pyloric sphincter into the duodenum
• Churns food with gastric juices
• Lubricates food with mucus and kills bacteria with hydrochloric acid

Gastric Motility Regulation

• Medulla oblongata's swallowing center signals the stomach to relax
• The vagus nerve relays this signal, which activates receptive-relaxation response
• The stomach resists stretching briefly but relaxes to hold more food
• Pacemaker cells in the longitudinal layer of the muscularis externa trigger peristaltic contractions
• The ring of constriction occurs every 20 seconds
• Contractions strengthen at the pyloric region
• These contractions become strong after 30 minutes, churning the food

Gastric Regualtion Phases

• Cephalic phase: The stomach responds to sight, smell, taste, or thought of food, input converges on the hypothalamus

• Hypothalamus relays signals to the medulla oblongata

• Vagus nerve stimulates the enteric nervous system, stimulating gastric secretion

• 40% of stomach acid secretion occurs in the cephalic phase

• Gastric phase: Swallowed/semi-digested food activates gastric activity and ⅔ of gastric secretion in this phase

• Ingested food activates activity in two ways: by stretching the stomach, and by increasing the pH of its contents

• Intestinal phase: Duodenum responds to arriving chyme and moderates gastric activity with hormones and reflexes

• Duodenum enhances secretion then inhibits it

• Acids/fats trigger enterogastric reflex; duodenum sends inhibitory signals to the stomach via the enteric nervous system

• Duodenum signals medulla to inhibit vagal nuclei (reduced vagal stimulation); stimulates sympathetic neurons

The Small Intestine

• Chemical digestion & nutrient absorption mainly occurs
• The longest part of the digestive system at 5m (living) or 8m(cadaver)
• "Small" refers to diameter, not length as the diameter is about 2.5 cm (1 in.)
• Divided into 3 parts: duodenum, jejunum, and ileum
• The duodenmum receives stomach contents, pancreatic juice & bile
• The stomach acid is neutralized here and fats are physically broken up via bile acids
• Enzymes conduct chemical digestion
• Forms the first 40% of the small intestine beyond the duodenum
• The wall has large tall folds and muscularis so blood supply rich in color
• Forms the last 60%, thinner less vascular. Paller pink color
• Ileocecal junction

Intestinal Motility

• Segmentation = stationary ring-like constrictions appear along the intestine
• They relax and new constrictions form elsewhere
• Most common contraction, set by pacemaker cells
• Contractions occur at 12x per minute
• 8x per minute ileum
• Mixes and churns, but does not move material along as in peristalsis
• When most nutrients are absorbed, segmentation declines and peristalsis begins
• Peristalsis moves contents toward the colon with a wave starting in the duodenum
• Followed by another wave starting down the tract
• Move chyme toward colon in 2 hours

The Large Intestine

• The large intestine is 5 ft long, and 2.5 inches in diameter
• This organ receives 500 mL of indigestible residue per day through 48 hours
• Large Intestine then eliminates the leftover volume to 150mL
• Does this through water and salt absorption
• Mass Peristalis Movements 1-3 times per day/ triggered by filling of the stomach
• Rectum/ Portion ending at the anal canal
• Anal Canal/ final 3cm of intestine and terminates at the anus
• Anal Column then exude mucus and lubricant into the anal canal for Defecation
• Haustral Contraction every 30 min or "churning"
• Large Hemorrhoidal Veins for a superficial plexus in anal columns and around the orfice
• When Water gets reabsorbed and makes you slow this is Constipation
• Diarrhea large intestine absorbs water too fast

Defecation

• Involves filling of the rectum
• Resulting in the reflex contraction of rectum
• Causes relaxation of Internal and external analy Sphincters