Digestive System

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Structures and Functions

lumen - internal hollow tube of digestive tract

secretions come from epithelial cells making up lining of digestive tract and exocrine accessory organs that have ducts emptying into the lumen

mucus - assists with movement of food from one part of the tract to the next

enzymes - assist with digestion of food within the lumen

absorption - material has to be moved from the lumen into the apical side (tube-side) of the epithelial cells lining the digestive tract and then out of the basal side of those cells into the body's interstitial fluid and into the blood or lymph capillaries for transport through the body; process can occur passively or with ATP

two main divisions of digestive system:

organs of the alimentary canal (also called the gastrointestinal (GI) tract or gut) - continuous muscular tube running from mouth to anus

mouth

ingestion

mechanical digestion - chewing

chemical digestion - salivary amylase breaks down starch

salivary mucus - helps dissolve food

motility - swallowing and propelling into pharynx

saliva - moistens food; cleans and lubricates teeth and oral cavity

pharynx

motility

propels bolus from oral cavity to esophagus

mucus lubricates food passageways

esophagus

motility

peristaltic waves - propel food bolus to stomach

mucus lubricates food passageways

stomach

motility/mechanical digestion

peristaltic waves - combine food with gastric juice and move it to duodenum

chemical digestion

hydrochloric acid - neutralizes ingested pathogens and stimulates protein-digesting enzymes

pepsin - begins protein digestion

absorption

intrinsic factor -allows B12 to be absorbed into intestines

some fat soluble substances absorbed

small intestine

chemical digestion

brush-border enzymes digest food

absorption

absorbs breakdown products of carbohydrates, protein, fat, and nucleic acid digestion, along with vitamins, water, and electrolytes

alkaline mucus - helps neutralize acidic chyme from stomach

large intestine

digestion

enteric bacteria digest some food residue and vitamins

absorption

absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria

motility

mucus smooths passage of feces through colon

propels feces toward rectum

defecation reflex eliminates feces

accessory digestive organs - contribute secretions to the GI tract, but food doesn't pass through these organs

liver - produces bile

gall bladder - stores and concentrates bile and then releases it

bile - emulsifies fat and stimulates digestion of fat and absorption of fatty acids, monoglycerides, cholesterol, phospholipids, and fat-soluble vitamins

pancreas - produces enzymes that digest food

pancreatic juice - helps neutralize acidic chyme (from stomach) and provide optimal environment for enzymatic activity

digestive functions:

ingestion

motility of food through GI tract lumen

mechanical and chemical digestion of food

absorption and breakdown of products

defecation to remove residues

Movement of Material

food material propelled through digestive system by voluntary skeletal muscles and involuntary smooth muscles

swallowing = deglution

tongue (voluntary)

pharynx skeletal muscles (involuntary)

peristalsis - wave-like propulsion of food

involves coordinated contraction and relaxation of two layers of smooth muscle in GI tract

esophagus - uses peristalsis to transport food from mouth to stomach

extra layer of smooth muscle in stomach creates a churning movement

propels food through small and large intestine

sphincters - control movement from one organ to the next

typically contracted and only relax to allow food to move to the next organ

most are involuntary

esophageal sphincter - separates esophagus from stomach to maintain linear movement and protect the esophagus from acidic chemicals of the stomach

pyloric sphincter - separates stomach from small intestine

ileocecal sphincter - controls movement into the large intestine (colon)

anal sphincter - skeletal muscle controlled consciously to relax during defecation

hepatopancreatic sphincter - controls release of accessory secretions from liver and pancreas into the beginning of the small intestine

Overview of Digestion

mechanical digestion - physically breaks down food into smaller pieces, increasing surface area for more efficient chemical digestion

chemical digestion - breaks down large food macromolecules into their chemical building blocks, which can then be absorbed through intestinal wall and into general circulation

Overview of Absorption

goal of mechanical and chemical digestive processes is to convert food into substances that can be absorbed by epithelial cells lining the digestive tract:

carbohydrates are broken down into:

monosaccharides

glucose

galactose

fructose

proteins broken down into:

amino acids

dipeptides

tripeptides

lipids broken down into:

monoglycerides

glycerol

fatty acids

most nutrients absorbed through active transport mechanisms

lipids, lipid-soluble vitamins, and most water-soluble vitamins are NOT absorbed through active transport

lipids are transported into intestinal cells in structures called micelles with the help of bile salts

absorbed fats then aggregate into chylomicrons for transport

absorption is enhanced by increasing surface area

anatomical structures at the organ, tissue, and cell level improve nutrient, water, and electrolyte absorption:

plicae circulares (organ level) - permanent ridges/folds forming successive rings along the length of the small intestine (increase surface area of the lining exposed to contents of the lumen)

villi (tissue level) - finger-like extensions of epithelial and connective tissue; increase surface area of epithelial tissue exposed to contents moving through lumen

connective tissue deep to the epithelial basement membrane supports the villi as it extends into the lumen

capillary networks extend from the deeper connective tissue into each villus to pick up and transport absorbed substances through the body

blood capillaries

lacteal capillaries - a central specialized lymph capillary for lipid absorption

microvilli (cellular level) - cell membrane extensions of apical surface epithelial cells

collectively known as brush border cells

cell membrane proteins on surface of apical epithelial cells

transporters - absorb nutrients by active transport

brush border enzymes - catalysts that complete the digestion of partially digested proteins and carbohydrates at the cell surface so they can pass through adjacent transporters

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Levels of Organization

chemical level - sodium, potassium, chloride, bicarbonate, and phosphate ions and hydrochloric acid

macromolecular level - enzymes of digestion, bile, and nutritive molecules such as carbohydrates, protein, lipids, and nucleic acids

cellular level - mucosa cells, secretory cells, immune cells

tissue level - epithelial, connective, and smooth muscle tissues

organ level - gastrointestinal tract and accessory organs

organ system level - integration of organs for nutrient and waste movement, digestion, and absorption

Chemical Digestion

chemical digestion - breaks down large food molecules into their respective chemical building blocks (called monomers) that can be absorbed

enzymes released by both intrinsic glands in GI tract and accessory glands

hydrolysis - breakdown of a chemical where a water molecule breaks a covalent bond and is added to each molecular bond that is broken; H+ attached to one component and OH- attached to the other

enzymatic hydrolysis - hydrolysis reaction catalyzed by an enzyme

Nutritive Molecules

Carbohydrates, proteins, and lipids are digested, absorbed, and then reassembled into macromolecules or used as fuel for metabolism

hydrolysis - breakdown of molecules using a water molecule

dehydration synthesis - reverse reaction that builds macromolecules from building blocks

enzymes speed up the rate of these reactions

Carbohydrates - primarily used for quick energy but also for signaling and structural molecules

monosaccharides - monomers of carbohydrates

glucose

galactose

fructose

disaccharides - two monosaccharides bound together

sucrose

lactose

maltose

polysaccharides - long chains of monomers, including polymerized glucose in different forms

glycogen - stored form of glucose in human bodies

starch - polysaccharide of glucose molecules from plant sources

cellulose - our bodies don't produce enzymes to break down these polysaccharides so they are indigestible but provide bulk (fiber) that helps propel food through digestive system

starch accounts for largest portion of digestible carbohydrates, with some glycogen, disaccharides, and monosaccharides

lactose - sugar found in milk

lack of enzyme lactase can lead to lactose intolerance

lactose doesn't get digested in small intestine, so it flows into the large intestine where bacteria metabolize lactose in a process called fermentation

fermentation produces gasses that cause abdominal bloating

sugar and fermentation products also cause large amounts of water to enter the large intestine - leading to loose stool

Proteins - usually found in the form of polypeptides and must be digested into its amino acid building blocks for absorption

numerous enzymes break large proteins into smaller peptides and then into amino acids

amino acids absorbed from digestive system into the circulatory system and delivered to the body

bonded together in the cells of the body to make proteins needed for cell function

can also be used as last resort for energy by converting them into glucose

Lipids - fats and oils

solid fats have more saturated and trans-fatty acids and are considered empty calories because they add calories but not any needed nutrients

most dietary lipids are in the form of triglycerides = one glycerol molecule + 3 fatty acids bound together

processed mainly by enzymes secreted by the pancreas (but also some enzymes from the stomach and saliva)

solubilized for absorption by salts secreted in bile (from the liver)

lipid absorption is required for absorption of some fat-soluble vitamins

Nucleic Acids - we digest the DNA or RNA nucleic acid that was in plant/animal foods

fundamental unit = nucleotide (phosphate group, pentose sugar, and a nucleic base)

pancreas releases two enzymes to break down DNA and RNA into nucleotides

brush border enzymes digest nucleotides into their molecular components for absorption

Saliva

intrinsic salivary glands - always excrete saliva (usually just enough to moisten mucous membranes and clean the mouth and teeth)

accessory saliva glands - exocrine organs under autonomic nervous system control with ducts leading into the mouth

secretion increases when their is food in the mouth or when food is smelled, seen, or thought about

saliva moistens and lubricates food and begins its chemical breakdown

components of saliva:

mainly water - dissolves chemicals in food

only dissolved chemicals activate the taste receptor cells on the tongue, palate and other parts of the mouth and pharynx

mucus

composed of glycoproteins

electrolytes

bicarbonate and phosphate ions help maintain pH of saliva as neutral or slightly basic (7.4)

protects teeth from acidic substances and plaque bacteria that grow best in an acidic environment

maintains optimal pH for enzyme amylase

digestive molecules

salivary amylase - starts breaking down complex carbohydrates like starch into sugars

this activity stops once the bolus (mixture of food and saliva)is swallowed and reaches the acidic environment of the stomach

salivary lipase - breaks down dietary lipids but is activated once it reaches the acidic gastric juice of the stomach

metabolic waste products

salivary glands play a role in some metabolic waste excretion so urea and ammonia are found in saliva

immune molecules - combat pathogens that might enter body with food or drink

lysozyme - antimicrobial enzymes

antibodies (Immunoglobulin A)

Gastric Juice

gastric glands produce:

HCl (hydrochloric acid) - responsible for high pH of stomach (1.5 - 3.5)

directly kills much of the bacteria ingested with food

helps denature proteins and substances found in plants

enzymes

pepsinogen (inactive enzyme) is modified to become pepsin (active protein enzyme) in low pH of gastric juice

protein enzymes of the digestive system are produced and secreted in an inactive form and are activated only in the lumen to prevent digestion of the cells themselves

intrinsic factor

necessary for absorption of vitamin B12 later in the small intestine

pH control:

inside stomach cell, the enzyme carbonic anhydrase converts one molecule of carbon dioxide and one molecule of water indirectly into a bicarbonate ion (HO3-) and a hydrogen ion (H+)

ion exchange is used to move H+ ions out of the cell and into the lumen of the stomach

bicarbonate ion (HCO3-) is exchanged for a chloride ion (Cl-) on the basal side

K+ and Cl- ions diffuse into the secretory region of the cell called the canaliculi

K+ is exchanged in this region for hydrogen ions via H+/K+ ATPase

H+ ions, which are millions of times more concentrated in this region of the cell than any other, are then pumped from the canaliculi into the lumen of the stomach

mucosal barrier - protects stomach cells and prevents the stomach from digesting itself

gastric mucus cells produce a thick mucus layer with lots of bicarbonate ions that will buffer the effect of the acid in the lumen

this pH neutral layer also protects the cells from the now activated pepsin enzymes in the lumen

surface gastric epithelial cells under the mucus layer have tight cell-to-cell junctions that prevent transcellular reabsorption of HCl from the lumen

gastric surface cells are replaced continually by gastric stem cells that are stimulated to divide more rapidly if there is tissue damage from gastric juice

prostaglandins - chemical messages involved in cell inflammation response to injury

prostaglandins in stomach stimulate production of gastric mucus and bicarbonate and promote tissue healing

peptic/gastric ulcer - when mucosal barrier breaks down and tissue destruction reaches into the deeper connective and muscular layers of the stomach

NSAIDs (non-steroid inflammatory drugs) and aspirin increase risk of gastric ulcers because they break down the mucosal layer and reduce secretion of bicarbonate ion

H. pylori - bacteria adapted to live in the stomach mucus layer

in some people, it can increase the risk of gastric ulcers

however, while the use of antibiotics has reduced incidence of gastric ulcers, it has increased the incidence of esophageal reflux and cancer

research shows inverse relationship between presence of H. pylori and GERD, esophageal cancer, childhood diarrhea, IBS, asthma, and tuberculosis

Intestinal Juice & Accessory Secretions

Pancreatic juice - clear, colorless liquid produced by pancreas

mostly water, along with some salts, sodium bicarbonate, and several digestive enzymes

sodium bicarbonate - responsible for slight alkalinity of pancreatic juice (7.1 to 8.2)

buffers the acidic gastric juice arriving from the stomach

inactivates pepsin from the stomach

creates optimal pH for activity of digestive enzymes in the small intestine

zymogens (inactive enzymes) produced in pancreas that are activated in the duodenum

trypsinogen -> trypsin (active enzyme)

procarboxypeptidase -> carboxypeptidase (active enzyme)

chymotrypsinogen -> chymotrypsin (active enzyme)

some pancreatic enzymes released in active form (but optimal activity requires interaction with ions or bile in intestinal lumen)

amylase

lipase

nuclease

Bile

made by hepatocytes of the liver but stored in, concentrated, and released from the gall bladder

produced in small quantities by the liver continuously and stored in gall bladder to release as needed

mixture of mainly water with other substances:

electrolytes

bile pigments - waste by-products from destruction of hemoglobin in aged or damaged red blood cells destroyed in the liver; contribute to color of feces

bilirubin

biliverden

bile salts - amphiphilic

hydrophobic/lipophilic portion is attracted to fat globules in the lumen

hydrophilic/lipophobic portion is attracted to water in the digestive juice

emulsification - chemical attraction and mechanical mixing of bile salts with fat globules and water breaks large fat globules into smaller droplets suspended in the fluid

bile salts prevent the droplets from rejoining and thus increase the surface area of the lipid exposed to lipase enzymes

don't actually digest lipids but make them easier to digest

once digested, the bile salts are still attracted to the lipid and form micelles

micelles come in contact with small intestine membranes, and then release and facilitate absorption of fatty acids, monoglycerides, and lipid-soluble vitamins

enterohepatic circulation - bile salts are reabsorbed by the large intestine and recycled through the hepatic portal vein back to the hepatocytes

soluble fiber interferes with the reabsorption of bile salts, so hepatocytes must metabolize new bile salts using cholesterol from the blood (which then lowers LDL cholesterol)

lipids

lecithin (phospholipid - has hydrophilic and hydrophobic portions)

cholesterol

cells lining the bile duct secrete biocarbonate ions into the bile

Intestinal Juice

intestinal glands (duodenal glands and goblet cells of the mucosa) at base of villi produce intestinal juice (mixture of water and mucus)

mucus produced by duodenal glands is alkaline and mucus produced by goblet cells is neutral

goblet cells become increasingly abundant in the jejunum and ileum

secretion is stimulated by distention of small intestine or the irritating effects of hypertonic or acidic chyme on the intestinal mucosa

mucus protects surface cells from abrasion as the food moves through the lumen

isn't directly involved in digestion but does serve as a substrate for digestion

enzymes extend into the mucus but remain embedded in the cell membrane of intestinal villi cells

cell membrane surface enzymes are important for final digestion of organic substances before absorption

Overview of Chemical Digestion

Carbohydrates

salivary amylase - breaks down starch polysaccharides into the disaccharide maltose, the trisaccharide maltotriose, and short chains of glucose called alpha-dextrins

pancreatic amylase - continues digestion of starch and other carbs in the small intestine that haven't been broken down by salivary amylase

brush-border enzyme (alpha-dextrinase) breaks alpha-dextrins down into one glucose unit at a time

disaccharides sucrose, lactose, and maltose aren't digested until they get to the small intestine

3 brush-border enzymes hydrolyze them into monosaccharides

sucrase splits sucrose into 1 molecule of fructose and 1 molecule of glucose

lactase breaks down lactose into 1 molecule of glucose and 1 molecule of galactose

maltase breaks down maltose and maltotriose into 2 and 3 molecules of glucose, respectively

digestive system then absorbs these monosaccharides

Proteins

digestion starts in the stomach where pepsin breaks down proteins into peptides

continues in small intestine

trypsin and chymotrypsin hydrolyze polypeptides into smaller peptides

elastase fragments whole proteins into peptides

small peptides are then broken down into their monomers

carboxypeptidase cleaves a single amino acid rom the carboxy terminus of a small peptide

aminopeptidase cleaves a single amino acid from the amino terminus

dipeptidase splits dipeptides in the middle, leaving two amino acids

Lipids

large lipid molecules must be separated into smaller lipid molecules through emulsification before breakdown can occur (because they do not dissolve in water)

bile salts aid in emulsification of large lipid globules into smaller ones and gives pancreatic lipase access to more lipid molecules

three lipases involved in digestion of triglycerides and phospholipids

lingual lipase

gastric lipase

pancreatic lipase - does most of the lipid digestion

breaks triglycerides into fatty acids and monoglycerides

Nucleic Acids

pancreatic juice contains two nucleases

deoxyribonuclease - digests DNA into nucleotides

ribonuclease - digests RNA into nucleotides

brush-border enzymes (nucleosidases and phosphatases) break down nucleotides into pentosesugars, phosphates, and nitrogenous bases, which can be absorbed through GI tract wall

Cells of the Stomach

mucosa - single layer of epithelial cells forming the innermost layer of stomach

gastric pits - depressions in the mucosa (still lined with epithelial cells)

gastric gland - located at bottom of the pit; contains multiple cell types:

mucous cells - secrete thick alkaline mucus to form protective mucosal barrier (much more alkaline than the mucus produced by goblet cells in other parts of the intestinal tract)

gastric neck cells - mucous cells found in the "neck" of the gastric pits

stem cells

parietal cells - secrete HCl and intrinsic factor

chief cells - secrete pepsinogen and gastric lipase

G cells - enteroendocrine cells of the stomach

enteroendocrine cells - found throughout organs of digestive system and release hormones and paracrine signals which regulate production of digestive chemicals and influence the nervous system for global regulation of digestion

G cells secrete signaling molecules, including gastrin, and other paracrines

gastrin signals parietal cells and chief cells to regulate production and alters stomach motility by altering the sphincter function on either side of the stomach

paracrines include serotonin, histamine, somatostatin, and other hormones involved in regulating digestion

Cells of Small & Large Intestine

small intestine composed primarily of simple columnar epithelial cells with microvilli facing the lumen

intestinal crypts (crypts of Lieberkuhn) formed by invaginations of the epithelium (much like gastric pits)

crypts contain:

paneth cells - secrete defensive proteins, including lysozyme definsins and phospholipase protect small intestine from pathogens which survived the stomach

goblet cells - secrete mucus for protection from acids of stomach and enzymes from accessory organs

enteroendocrine cells- secrete hormones for regulation of absorption of nutrients

S cells - produce secretin to buffer intestinal pH

I cells (CCK cells) - secrete cholecystokinin which stimulates release of digestive enzymes from pancreas and gall bladder into the intestine

K cells - secrete gastric inhibitory peptide (GIP; glucose-dependent insulinotropic peptide) which influences insulin levels

most of the mucosa of the large intestine is composed of simple columnar epithelial cells

distal anal canal - composed of nonkeratinized stratified squamous epithelial cells (more durable to abrasion that occurs when feces moves)

also has crypts containing epithelial and goblet cells

most digestion has already occurred so the only significant secretion is mucus to lubricate the passageway

Cells of the Liver

hepatocytes - liver's main functional cells that play a role in secretory, metabolic, and endocrine functions; make up 80% of the liver's volume

hepatic laminae - plates of hepatocytes radiate out from the center of hepatic lobules

continually secrete bile, but production and release accelerate when bile acids levels in the hepatic portal blood (blood coming from small intestine) increase (to secrete more bile as digestion and absorption are taking place)

Kupffer cells - found in blood filled spaces (sinusoids) between laminae

remove dead or damaged red and white blood cells, as well as bacteria and other foreign matter that potentially enter through the GI tract and travel to the liver in the hepatic portal vein

Cells of the Pancreas

acini cells - make up the majority of cells in the pancreas; named for the small clusters they form around a central space or lumen that connects to small ducts and looks like a cluster of grapes (acinus = grape)

each acinus cell has a wider basal side toward the basement membrane and a narrow apical side toward the lumen, which forms a ring of pie-shaped cells

these cells secrete the enzymes and bicarbonate ions that make up pancreatic juice into the central duct

endocrine cells

alpha cells - glucagon to increase blood glucose

beta cells - insulin and amylin to lower blood glucose

delta cells - somatostatin to restrict absorption

PP cells - pancreatic polypeptide to regulate pancreatic function

Tissue Layers of the GI Tract

Throughout the GI tract, walls are comprised of the same four fundamental layers:

mucosa

has 3 layers:

epithelium - closely packed cells without blood supply or nerves in direct contact with the lumen

mouth, pharynx, esophagus, and distal portion of anal canal are composed of primarily nonkeratinized stratified squamous epithelium

stomach and intestines are mainly simple columnar epithelium (and participate in secretion and absorption)

epithelial cells in the GI tract are sloughed off and replaced every 5-7 days

interspersed with exocrine cells (that secrete mucus and digestive fluid into the lumen) and enteroendocrine cells (that secrete hormones and paracrines)

lamina propria - layer of connective tissue supporting the epithelial cells

contains numerous blood and lymphatic vessels, which transports absorbed nutrients into the liver

contains most of the immune cells that make up the mucosa-associated lymphatic tissue (MALT)

muscularis mucosa - thin smooth muscle layer

isn't involved with movement of material through GI tract, but controls the exposed surface area

small alterations in the many small folds in the mucous membrane of the stomach and small intestine increases the surface area available for digestion and absorption

when this layer contracts, rugae (stomach) and plicae circulars (small intestine) are pushed together exposing less surface area to the lumen

when it relaxes, more of the mucosal surface is revealed

submucosa

binds mucosa to muscularis externa

composed of areolar connective tissue and includes blood and lymphatic vessels (which transport absorbed food molecules) and the submucosal plexus (which is part of nervous system control)

muscularis

in mouth, pharynx, superior and middle esophaguc, muscularis externa contains skeletal muscle for voluntary swallowing

external anal sphincter is also made of skeletal muscle for voluntary control of defecation

the rest of the GI tract is made of smooth muscle, which contracts involuntarily to break down food, mix it with digestive juices, and move it along the GI tract

complementary muscles in longitudinal and circular layers create peristalsis to move food from esophagus to anus

serosa - superficial layer of intestine that covers the parts of the GI tract exposed to the abdominal cavity

made up of areolar connective tissue and simple squamous epithelium (mesothelium)

esophagus has a single layer of tough connective tissue called the adventitia and does not have a serosa

Peritoneum & Peritoneal Folds

peritoneum - largest serous membrane in the body; lines the abdominal cavity

has several components:

mesothelium

underlying supporting areolar connective tissue layer, which has two layers:

parietal peritoneum - lines the abdominal wall

visceral peritoneum - covers some organs and serves as their serosae

peritoneal cavity - small space between parietal and visceral peritoneum that contains lubricating serous fluid

has large folds that bind organs to each other and to the abdominal walls to keep organs in place and suspend them when we are upright

folds contain blood vessels, lymphatic vessels, and nerves that innervate the abdominal organs

five major folds:

greater omentum - drapes over transverse colon and small intestine

falciform ligament - attaches the liver to the anterior abdominal wall and diaphragm

lesser omentum - suspends the stomach and duodenum from the liver

mesentery - attaches the jejunum and ileum of the small intestine to the posterior abdominal wall

mesocolon - attaches the transverse colon and sigmoid colon of the large intestine to the posterior abdominal wall

peritonitis - inflammation of the peritoneum; most common cause is a ruptured appendix

Mouth

mouth - also called the oral cavity or buccal cavity

framed by the cheeks, tongue, hard palate, and soft palate

mastication = chewing; tongue manipulates food, the teeth grind it, and saliva mixes with it

turns food into an easy-to-swallow bolus and breaks food into smaller pieces so there is more contact area for digestive enzymes

cheeks

outer side is made of skin

inner side is a mucous membrane made of nonkeratinized stratified squamous epithelium with the multiple layers being resistant to abrasion

between skin and mucous membrane are connective tissue, fat tissue, and buccinator muscles

lips

outer covering made of skin

inner lining is mucous membrane

orbicularis oris muscle in between

labial frenulum - midline fold of mucous membrane that attaches inner surface of each lip to the gum

oral vestibule - area between lips and teeth; puffing out cheeks increases area of oral vestibule

oral cavity - area between gums/teeth and entrance to the throat (oropharynx)

septum - a wall within a single organ or cavity that separates the space into distinct sides (for example, nasal septum divides the nostrils)

palate - the septum that separates the oral cavity from the nasal cavity; forms roof of mouth

allows us to breathe while chewing food

hard palate - anterior part of the palate

created by the maxillae and palatine bones and is covered by mucous membrane

soft palate - posterior part of the palate

also lined with mucous membrane

arch-shaped muscular structure that forms a dividing wall between the oropharynx and the nasopharynx

uvula - cone-shaped muscular process that hangs from the end of the soft palate

plays a role in speech and articulation of words

also plays a small role in preventing foods and liquids from entering the nasal cavity

2 muscular folds on each side of the base of the uvula extend down the lateral sides of the soft palate:

palatoglossal arch - anterior fold; terminates near base of tongue

palatopharyngeal arch - posterior fold

palatine tonsils - located between the two arches on the lateral wall

lingual tonsils - located at the base of the tongue

salivary glands - make secretions independent of the presence of food; these glands either open directly into the oral cavity or indirectly through short ducts

labial glands - lips

buccal glands - cheeks

palatal glands - in the palate

lingual glands - in the tongue

Tongue

composed of skeletal muscle covered with a surface of nonkeratinized stratified squamous epithelium and a mucous membrane covering

tongue and its associated muscles make up the floor of the oral cavity

a median septum extends the entire length of the tongue, dividing it into symmetrical halves

attached to the mandible, styloid process of the temporal bone, and hyoid bone

extrinsic muscles move tongue from side to side and in and out

intrinsic muscles change the size and shape of the tongue

lingual frenulum - attaches tongue to the floor of the mouth and limits the tongue's posterior movement

ankyloglossia - condition with having a too short or too rigid lingual frenulum that impairs speech

papillae - extensions of the lamina propria layer of mucosa (enshrouded in stratified squamous epithelium) that stud the top and sides of the tongue

most contain taste buds

those without taste buds have touch receptors to give a sense of the food's texture which increase the amount of friction between tongue and food and help the tongue move food around

lingual glands - found on the anterior, inferior surface of the tongue, in the lamina propria of the tongue

secrete mucus and a watery serous fluid that contains the enzyme lingual lipase, which begins the digestion of triglycerides

Teeth

teeth (dentes) - secured in sockets of the alveolar processes of the maxillae and mandible

gingivae (gums) cover the alveolar processes

periodontal ligament - lines the sockets; dense fibrous connective tissue that secures the teeth in place

enamel - covers the teeth; hardest substance in the body; prevents teeth from being worn down; and protects interior of tooth from acids

humans have two sets of teeth (dentitions):

20 deciduous teeth (baby teeth)

32 permanent teeth (replace the baby teeth)

types of teeth:

incisors - chisel-shaped teeth closest to the midline that tear into food

cuspids (canines) - next to the incisors

cusp (pointed edge of cuspid) tears and shreds food

2 bicuspids (premolars) on each side

up to 3 molars on each side

premolars and molars have broader, flatter surfaces to crush and grind food

Pharynx

pharynx - tube that runs from the choanae (internal nostrils) to the esophagus and the larynx

has 3 subdivisions:

nasopharynx - passageway for air during ventilation

oropharynx - both ventilation and digestion

laryngopharynx - both ventilation and digestion

mucosa includes an epithelium with mucous-producing glands

nasopharynx is lined with pseudostratified columnar epithelium

oropharynx and laryngopharynx are lined with stratified squamous

two skeletal muscle layers:

cells in the inner layer extend longitudinally

in the outer layer, the superior, middle, and inferior pharyngeal constrictor muscles are stacked one inside the other (like flower pots)

constricting these muscles propels food toward the esophagus

provoked by swallowed food entering the oropharyx and the laryngopharynx

during swallowing, the soft palate usually moves back to close off the nasopharynx and the trachea moves up under the epiglottis to cover the glottis

Esophagus

esophagus - collapsible tube located posterior to the trachea; about 10 inches long

upper esophageal sphincter - made of skeletal muscle

penetrates the diaphragm through an opening called the esophageal hiatus

cardiac orifice - where the esophagus joins the stomach

lower esophageal sphincter (gastroesophageal sphincter or cardiac sphincter) - surrounds the cardiac orifice; made of smooth muscle

relaxes to let food into stomach

contracts to prevent stomach contents from backing up into the esophagus

surrounded by the diaphragm which helps keep the sphincter closed when no food is being swallowed

heartburn and GERD (gastroesophageal reflux disease) caused by sphincter not closing completely

esophageal mucosa made up of:

nonkeratinized stratified squamous epithelium - protects against abrasion and erosion from food particles

lamina propria

muscularis mucosa

superior third of esophagus is skeletal muscle

middle third is both skeletal and smooth

inferior third is smooth muscle

mucosa near the stomach also has mucous glands

superficial layer of esophagus is called the adventitia, not the serosa, and is not covered by mesothelium

Deglutition

deglutition (swallowing) is movement of food from the mouth and into the stomach; takes about 4-8 seconds for solid or semisolid food and about 1 second for very soft foods and liquids

involves mouth, pharynx, and esophagus

facilitated by the secretion of mucus and saliva

3 phases of deglutition:

voluntary phase (oral phase, buccal phase)

tongue moves upward and backward against the palate, pushing the bolus to the back of the oral cavity and into the oropharynx

pharyngeal phase

stimulation of receptors in the oropharynx sends impulses to the deglutition center (collection of neurons that controls swallowing) in the medulla oblongata

impulses are then sent back to the uvula and soft palate, provoking them to move upward to close off the nasopharynx

sequential contractions of the pharyngeal constrictor muscles move the bolus through the oropharynx and laryngopharynx

relaxation of upper esophageal sphincter allows food to enter the esophagus

esophageal phase

peristalsis propels the bolus through the esophagus and toward the stomach

the circular muscle layer of the muscularis immediately superior to the bolus contracts, pinching the esophageal wall, forcing the bolus forward

at the same time, the longitudinal muscle layer of the muscularis inferior to the bolus also contracts, shortening this inferior area and pushing out its walls to receive the bolus

waves of contractions move the food toward the stomach

relaxation of the lower esophageal sphincter allows the bolus to pass into the stomach

during this phase, esophageal glands secrete mucus to lubricate the bolus and minimize friction

completely involuntary despite the presence of skeletal muscle in the first 2/3 of the esophagus

Stomach

position and size are always changing

the size of a large sausage when empty

stretches to hold 1 liter of food when full

total capacity is 2-3 liters

serves as a holding chamber and pushes out only a small amount of food into the small intestine at a time

semisolid bolus is converted into a liquid (chyme)

4 main regions:

cardiac - small area surrounding the cardia orifice where food enters

fundus - superior and left of the cardiac; dome shaped; functions as a temporary storage center for food

body - inferior to the fundus; the large central part of the stomach

pylorus - connects the stomach to the duodenum

pyloric antrum- wider, more superior portion of the pylorus that connects to the body of the stomach

pyloric canal - narrows from the pyloric antrum and leads into the duodenum

pyloric sphincter - controls food emptying into duodenum

greater curvature - convex lateral surface of the stomach

lesser curvature - concave medial border of the stomach

held in place by two fatty mesenteries (peritoneal folds):

lesser omentum - extends from liver to lesser curvature

greater omentum - extends from greater curvature to posterior abdominal wall

special adaptations of the stomach wall:

muscularis externa has a layer of inner oblique smooth muscle (in addition to the typical circular and longitudinal smooth muscle layers) that runs diagonally around the organ

this allows the stomach to more vigorously churn and mix food

gastric pits in the epithelium lead into gastric glands, which secrete gastric juice

glands in the cardia and pylorus are primarily composed of mucus-secreting cells

glands in the pyloric antrum secrete mucus and a number of hormones (including gastrin)

glands in the fundus and body produce most of the gastric secretions

Control of Gastric Secretion

secretion of gastric juice is controlled by nerves, hormones, and the presence of products of digestion

3 phases of gastric secretion (once gastric secretion begins, all phases can occur simultaneously):

cephalic phase

smell, taste, sight, or thought of food triggers this phase

lasts only a few minutes

it's a conditioned reflex (meaning it only occurs if we like or want a particular food)

gastric phase

set in motion by local neural and hormonal mechanisms triggered by entry of food into the stomach

lasts 3-4 hours

stomach distention activates stretch receptors that stimulate parasympathetic neurons to release acetylcholine, which then provokes increased secretion of gastric juice

partially digested proteins, caffeine, and rising pH stimulate the release of gastrin from enteroendocrine G cells, which in turn provokes parietal cells to increase their production of HCl and increase motility through the stomach by altering sphincter function

presence of proteins in the stomach raises pH

intestinal phase

Digestive Functions of Stomach

stomach participates in many digestive activities:

mechanical digestion

mixing waves - gentle type of peristalsis that softens and moistens food at intervals of 15-25 seconds; combine food with gastric juice to create chyme

followed by more intense waves to break food into smaller pieces and further mix with digestive juice, starting at the body of the stomach and increasing in force as they reach the pylorus

pylorus acts as a filter, permitting only liquids and small food particles to pass through the mostly closed pyloric sphincter

gastric emptying - process of periodic waves in the pylorus forcing chyme through the sphincter

the rest of the chyme is forced back into the body of the stomach where it continues mixing

this forward and backward movement accomplishes most of the mixing

this process is regulated by both the stomach and the duodenum

presence of chyme in the duodenum activates receptors that inhibit gastric emptying to prevent additional chyme from being released before the duodenum is ready to process it

contents are emptied into the duodenum within 2-4 hours after eating

carbohydrate-rich foods are the quickest to leave

high-protein foods exit more slowly

fatty meals with high triglyceride content remain in the stomach longest

chemical digestion

some foods may sit in fundus for an hour or more before being mixed with gastric juice; at this point, the digestive activity of salivary amylase can continue

mixing waves eventually combine chyme with acidic gastric juice which inactivates salivary amylase and activates lingual and gastric lipase

lingual lipase breaks down triglycerides into fatty acids and diglycerides

pepsin begins digesting protein

rennin is produced in infancy to help digest milk protein

parietal cells form HCl in the stomach, which denatures dietary proteins

absorption of some fat-soluble substances, including alcohol and aspirin

Small Intestine

the primary digestive organ in the body where most absorption occurs

circular folds, villi, and microvilli add extra surface area

3 sections:

duodenum

shortest piece

receives stomach contents, pancreatic juice, and bile

jejunum

responsible for the absorption of the majority of nutrients

ileum

vitamin B12 and bile salts are absorbed as well as any materials not absorbed by the jejunum

mucosa

distal portion of ileum have most of the solitary lymphatic nodules

blood supply running through the lamina propria and drains the villa is part of the hepatic portal system and not general circulation

lamina propria in ileum also has aggregated lymphatic follicles (groups of lymph nodules) called Peyer's patches that are more concentrated toward the end of the small intestine

submucosa

submucosa of duodenum is the only site of the complex mucus-secreting duodenal (Brunner's) glands, which produce a bicarbonate-rich alkaline mucus that helps buffer the acidic chyme coming in from the stomach

Absorption in Small Intestine

most absorption occurs in the jejunum

ileum mainly recycles bile salts

chyme entering large intestine is mostly indigestible food residue (plant fibers), some water, and millions of bacteria

most nutrients enter the capillaries in the villus and travel to the liver via the hepatic portal vein

certain lipids undergo passive absorption via diffusion and enter the modified lymph duct (lacteal) in the villus

carbohydrate absorption

all carbs are absorbed in the form of monosaccharides

indigestible fibers (like cellulose) are eliminated in the feces

glucose and galactose are transported via protein carriers through secondary active transport (which requires ATP) and leave the cells through facilitated diffusion where they enter capillaries through intercellular clefts

fructose is absorbed and transported by facilitated diffusion alone

monosaccharides combine with transport proteins immediately after disaccharides are broken down (by disaccharide splitting enzymes on microvilli)

protein absorption

active transport mechanisms in the duodenum and jejunum absorb amino acids

most carriers associated with amino acid transport are liked to the active transport of sodium

dipeptides and tripeptides are also transported actively but are broken down into amino acids before leaving the cell

lipid absorption

short-chain fatty acids are able to enter cells via simple diffusion due to their small size, and they take the same path as monosaccharides and amino acids

long-chain fatty acids and monoglycerides combine with bile salts and lecithin to make spheres called micelles

micelles have hydrophilic end facing outward and hydrophobic core

core of micelles also have cholesterol molecules and fat-soluble vitamins

micelles easily squeeze between microvilli and get close to cell surface

lipid substances exit the micelle and are absorbed via simple diffusion

free fatty acids and monoglycerides that enter the cells are reincorporated into triglycerides and mixed with phospholipids and cholesterol and surrounded with a protein coat to create chylomicrons

chylomicrons - milky white droplets of water-soluble lipoprotein processed by the Golgi apparatus for release from the cell

too big to enter blood capillaries in basement membrane so enter the much larger pores of lacteals

most fats don't enter venous blood until they reach the thoracic duct in the neck

lipoprotein lipase in the bloodstream breaks down triglycerides of chylomicrons into free fatty acids and glycerol

these breakdown products can pass through capillary walls and become energy for cells or stored in adipose tissues

liver cells combine remaining chylomicron material with proteins to form new lipoproteins that transport cholesterol in blood

nucleic acids - transported by special carriers via active transport

electrolyte absorption

sodium ions

sodium-potassium pump moves sodium ions

co-transport mechanisms link sodium ion absorption with glucose and amino acid absorption

chloride ions

active transport

potassium ions

facilitated diffusion (passive) via osmotic gradient

all nutrients absorbed, whether we need them or not (except for iron and calcium)

iron - active transport based on need

calcium - blood levels of Ca2+ determine absorption of calcium; vitamin D (under control of parathyroid hormone) regulates this process

vitamin absorption

mostly absorbed in small intestine

some vitamin K and B vitamins created by enteric bacteria are absorbed in the large intestine

fat soluble vitamins (A, D, E, and K) absorbed with lipids in micelles via simple diffusion

most water soluble vitamins absorbed by simple diffusion, except B12 (need intrinsic factor secreted in stomach)

water absorption

95% absorbed in small intestine via osmosis

active transport of solutes into mucosal cells creates concentration gradient -> water moves via osmosis into cells -> solutes left in lumen have higher concentration, which leads to absorption via passive diffusion

Large Intestine

primary function is to finish absorption, produce some vitamins, form feces, resorb water, and eliminate feces

four main regions:

cecum - small sac-like region inferior to the ileocecal valve

appendix - winding, coiled tube attached to cecum that contains lymphoid tissue and plays a role in immunity

mesoappendix - mesentery of appendix tethers it to the inferior part of the mesentery of the ileum

colon

ascending colon

right colic (hepatic) flexure - turn of the ascending colon into the transverse colon

transverse colon

left colic (splenic) flexure - turn of transverse colon into the descending colon

descending colon

sigmoid colon

rectum

rectal valves - trio of internal transverse folds within the rectum; allow passing of gas while preventing simultaneous passage of feces

anal canal

contains two sphincters:

internal anal sphincter (involuntary, made of smooth muscle)

external anal sphincter (voluntary, made of skeletal muscle)

ileocecal valve - controls flow of chyme from small to large intestine

mucosa

contains both absorptive and goblet cells like the small intestine

other than the anal canal, mucosa is formed of simple columnar epithelium

wall of large intestine doesn't have circular folds or villi and essentially no enzyme-secreting cells

large intestine has thicker mucosa and deeper (and more abundant) glands containing vast numbers of goblet cells

goblet cells secrete mucus to ease movement of feces and protect intestine from acids and gases produced by enteric bacteria

anal canal

stratified squamous epithelial mucosa in anal canal

anal columns - longitudinal folds of mucous membrane in anal canal; house a grid of veins

anal sinuses - depressions between anal columns; secrete mucus when they are crowded by feces to facilitate defecation

pectinate line - horizontal, jagged band running alongside the inferior margins of the anal sinuses

mucosa superior to this line is fairly insensitive to pain (innervated by visceral sensory fibers)

mucosa inferior to the line is very pain-sensitive (innervated by somatic sensory fibers)

teniae coli - 3 bands of smooth muscle that make up the longitudinal muscle layer of the muscularis externa of large intestine (except in the rectum)

tonic contractions of the teniae coli bunch up the colon into a succession of pouches

haustra - pouches responsible for wrinkled appearance of colon

epiploic appendages (omental appendages) - fatty pouches of peritoneum found in the serosa from the transverse colon through the sigmoid colon

Bacterial Flora in Large Intestine

bacterial flora - the many species of bacteria that populate the large intestine

functions

digest food products we don't have enzymes for (such as plant materials) and we absorb some of the nutrient products

produce some vitamins like vitamin K and B vitamins

gas (flatus) - byproduct of bacterial fermentation (digestion)

commensals - nonpathogenic; this makes up most of the enteric bacteria

dendritic cells - present antigens from commensal bacteria to T-cells in lymphoid follicles which triggers an IgA antibody-mediated response that blocks commensals from infiltrating the mucosa

Digestion in Large Intestine

3 types of movements in large intestine:

haustral contractions - pushes chyme from one haustra to the next

peristaltic movements

mass movements (mass peristalsis) - drawn-out, slow-moving, but strong peristaltic waves starting around the middle of the transverse colon that quickly force contents of colon into the rectum

distention in stomach and breakdown products of digestion in small intestine provoke the gastrocolic or duogenocolic reflex, which increases motility, including mass movements, in the colon

fiber softens the stool and increases the power of colonic contractions

Absorption/Excretion in Large Intestine

feces (stool) - water absorption from chyme changes it into a solid or semisolid substance

composed of undigested food residues, unabsorbed digested substances, millions of bacteria, sloughed-off epithelial cells from GI mucosa, inorganic salts, mucus, fat, and just enough water to let it pass smoothly from body

defecation reflex - provoked by stretching of rectal wall; eliminates feces from rectum

Valsalva maneuver - voluntary defecation in which we increase intra-abdominal pressure by contracting diaphragm and abdominal wall muscles and close the glottis

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Accessory Organs

Salivary Glands

3 pairs of major salivary glands:

parotid glands - lie between skin and masseter muscle near the ears and secrete saliva into mouth through parotid duct

secrete watery solution containing salivary amylase

submandibular glands - in floor of mouth and secrete saliva through the submandibular ducts

also secrete salivary amylase but in a liquid thickened by mucus

sublingual glands - lie below the tongue and use the lesser sublingual ducts to secrete saliva

contain mostly mucous cells and secrete the thickest saliva with least amount of salivary amylase

autonomic nervous system regulates salivation

parasympathetic stimulation keeps saliva flowing continuously in absence of food

most saliva is reabsorbed, preventing fluid loss

stress causes sympathetic simulation to reduce salivation and cause dry mouth

dehydration - salivation stops to conserve water, causing mouth dryness that stimulates feelings of thirst

salivation is stimulated by taste and sensation of food

food contains chemicals that stimulate taste bud receptors on the tongue

this sends impulses to the superior and inferior salivatory nuclei in the brain stem

nuclei send back parasympathetic impulses through fibers in facial and glossopharyngeal nerves to stimulate salivation

salivation increases even after food is swallowed to cleanse mouth and dilute and neutralize any irritating chemical remnants

salivation is also stimulated by thinking about, seeing, or smelling food

Pancreas

pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate

lies posterior to the greater curvature of the stomach

pancreatic regions:

head - next to duodenum

body - lies behind the stomach

tail - in contact with spleen

ducts

main pancreatic duct (duct of Wirsung) - fuses with common bile duct from liver and gall bladder before entering the duodenum through hepatopancreatic ampulla

hepatopancreatic ampulla (ampulla of Vater) - dilated common duct

hepatopancreatic sphincter - controls flow of pancreatic juice and bile into small intestine

accessory duct (duct of Santorini) - runs from pancreas directly into duodenum

pancreatic cells

acini - clusters of glandular epithelial cells surrounding the ducts (make up 99% of cells)

pancreatic islets - secrete pancreatic polypeptide, insulin, glucagon, and somatostatin (make up 1% of cells)

regulation of pancreatic secretions

secretin - release stimulated by presence of HCl in intestine; provokes cells in pancreas to release bicarbonate-rich pancreatic juice

cholecystokinin (CCK) - release stimulated by presence of fats (and some proteins) in the intestine; stimulates the release of enzyme-rich pancreatic juice and enhances the activity of secretin

parasympathetic regulation - cephalic and gastric phases of gastric secretion stimulate secretion of pancreatic juice

Liver

hepatic lobule - used to be considered functional unit of liver

each hexagonal (six-sided) hepatic lobule has a central vein

radiating from the vein are rows of hepatocytes and hepatic sinusoids

hepatic acinus - preferred model of functional unit of liver

each primarily oval hepatic acinus includes parts of two adjacent hepatic lobules

two primary lobes:

large right lobe

smaller left lobe

falciform ligament - mesentery that separates right and left lobes

visceral peritoneum enshrouds almost the entire liver

lesser omentum - tethers liver to lesser curvature of the stomach

porta hepatis - where hepatic artery and hepatic portal vein enter the liver on its inferior surface

hepatic artery - delivers oxygenated blood from heart to liver

hepatic portal vein delivers deoxygenated blood containing newly absorbed nutrients, drugs, and possibly bacteria from alimentary canal

branches of both vessels bring blood into liver sinusoids where hepatocytes take up most nutrients and some toxins

hepatocytes secrete substances, including nutrients needed by other cells, back into the blood, which drains into the central vein and then the hepatic vein

3 main components:

hepatocytes

bile canaliculi - small ducts between hepatocytes that accumulate bile produced by hepatocytes

bile then flows into bile ductules and then into bile ducts

bile ducts unite to form right and left hepatic ducts

these two ducts merge to form the common hepatic duct

this duct merges with cystic duct from gall bladder to form the common bile duct

hepatopancreatic sphincter prevents bile flow into duodenum between meals and bile backs up into the gall bladder where it is stored until needed

hepatic sinusoids - specialized blood vessels located between rows of hepatocytes

branches of hepatic artery deliver oxygenated blood to hepatic sinusoids

branches of hepatic portal vein deliver nutrient-laden blood

hepatic sinusoids combine and send blood into a central vein

blood then flows into the hepatic veins which drain into the inferior vena cava

contain immune cells called Kupffer cells

portal triad - arrangement in liver that includes a bile duct, a hepatic artery branch, and a hepatic portal vein branch

Gall Bladder

stores and concentrates bile produced by liver

divided into 3 regions:

fundus - wide portion

body - central portion

neck - tapered portion

simple columnar epithelium organized in rugae (like stomach)

muscle contractions release gall bladder's contents into the cystic duct

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Digestive System Homeostasis

Hormonal/Paracrine Control

cholecystokinin (CCK)

produced in duodenal mucosa

stimulated by fatty chyme and partially digested proteins

action

liver - potentiates activity of secretin

pancreas - increases production of enzyme-rich pancreatic juice

gall bladder - contracts gall bladder and releases bile

hepatopancreatic sphincter - relaxes hepatopancreatic sphincter, permitting bile and pancreatic juice to enter the duodenum

gastrin

produced in stomach mucosa

stimulated by presence of food in the stomach and release of acetylcholine

action

stomach - increased secretion by gastric glands & provokes gastric emptying

small intestine - provokes intestinal muscle contraction

ileocecal valve - relaxes ileocecal valve

large intestine - provokes mass movements

histamine

produced in stomach mucosa

stimulated by presence of food in stomach

action

stomach - stimulates parietal cells to release HCl

intestinal gastrin

produced in duodenal mucosa

stimulated by presence of acidic and partially digested foods in duodenum

action

stomach - activates gastric glands and stimulates motility

motilin

produced by duodenal mucosa

stimulated by fasting and neural stimuli

action

duodenum - activated migrating motility complex

secretin

produced by duodenal mucosa

stimulated primarily by acidic chyme

action

stomach - restricts gastric gland secretion and gastric motility in gastric phase of secretion

pancreas - increases production of pancreatic juice rich in bicarbonate ions

liver - enhances bile production

enhances activity of CCK

serotonin

produced by stomach mucosa

stimulated by presence of food in stomach

action

stomach - contracts stomach muscle

somatostatin

produced by stomach and duodenal mucosa and pancreas

stimulated by presence of food in stomach and sympathetic axon stimulation

action

stomach - restricts all gastric secretions & motility and emptying

pancreas - restricts secretion

small intestine - inhibits intestinal absorption by restricting GI blood flow

vasoactive intestinal peptide

produced by duodenal mucosa

stimulated by chyme with partially digested foods

action

duodenum - activates buffer secretion & dilates intestinal capillaries

stomach - restricts HCl production

small intestine - relaxes intestinal smooth muscle

Nerve Control of Digestion

two types of nerve innervation of GI tract:

enteric nervous system - contains motor neurons, sensory neurons, and interneurons

grouped in two plexuses:

myenteric plexus (plexus of Auerbach) - lies in muscularis externa layer; responsible for GI tract motility

submucosal plexus (plexus of Meissner) - lies in submucosal layer; regulates digestive secretions and reacts to presence of food

GI reflex pat

autonomic nervous system - parasympathetic and sympathetic innervation

parasympathetic nerves - increase GI secretion and motility by stimulating neurons of the enteric nervous system

sympathetic nerves - decrease GI secretion and motility by restricting activity of enteric nervous system