NPB 101-Exam 3

What is the major function of the digestive system?

To transfer nutrients from the food we eat into our body to be used as fuel and building blocks

General structure of the digestive tract wall

Four major tissue layers from outside in: 1. Serosa, 2. Muscularis externa, 3. Submuscosa, 4. Mucosa

Serosa

-secretes serous fluid-lubricates
-intestines are loosely suspended here
-supports digestive organs in proper place while allowing them freedom for mixing and propulsive movements

Muscularis Externa

-major smooth muscle coat for digestive tube
-contractile activity produces propulsive and mixing movements; propels the food along the GI tract
-two layers: inner circular layer & outer longitudinal layer

Inner circular layer

Contraction decreases diameter of lumen; inside muscle

Outer longitudinal layer

Contraction shortens the tube; outside muscle

What do the layers of the Muscularis externa do?

Allows for muscles to constrict and shorten to propel the foos along the GI tract

Myenteric plexus

-Part of the enteric nervous system
-in between muscle layer: Muscularis externa & mucosa

enteric nervous system

the nervous system of the digestive tract; doesn't require input of the brain

Submucosa

-thick layer of connective tissue; for distensibility & elasticity
-contains larger blood and lymph vessels
-contains submucosal plexus nerve network part of the enteric nervous system

Mucosa

-lines lumen: highly folded surface increases absorptive area
-contains 3 layers: 1. Epithelial, 2. Lamina propria, 3. Muscularis mucosa

Epithelial layer (mucous membrane)

-cells modified for secretion & absorption
-contains exocrine gland cells; secrete digestive juices, mucus, enzymes into lumen
-contains endocrine gland cells; secrete gastrointestinal hormones into capillaries

Lamina propria

-loose connective tissue
-small blood vessels, lymphatics, and enteric neurons
-contains gut-associated lymphoid tissue (GALT)

Muscularis mucosa

sparse layer of smooth muscle

The lumen of the gastrointestinal tract is..

Continuous w/ the external environment

4 basic basic digestive processes

1. Motility
2. Secretion
3. Digestion
4. Absorption

Voluntary control of digestive processes

Ingestion, chewing, swallowing, defecation; transfers food into the digestive tract via the mouth

Motility

-muscular contractions that mix and move forward the contents w/in the tract, facilitating later steps in the digestive process
-2 types: peristalsis & segmentation
-involuntary

Peristalsis (propulsive movements)

Move the contents forward through the digestive tract

Segmentation (mixing movements)

-mostly occurs in small intestines
-breaks down food into smaller absorbable units; then moved so that it has access to absorbing surfaces

3 functions of segmentation

1. Aid digestion by mixing food w/ digestive juices
2. Facilitate absorption by exposing food to absorbing surfaces
3. Forward movement (slow and non-linear)

Secretion (exocrine)

-digestive juices are secreted into the lumen by exocrine glands upon appropriate neuronal or hormonal stimulation

Secretion (exocrine) pt. 2

Secretions contain enzymes, acids, buffers, electrolytes, & water that promote digestion, adjust tonicity & provide lubrication for better movement throughout the tract

Secretion (endocrine)

Gut hormones are secreted into the blood by endocrine glands upon appropriate neuronal or nutritional stimulation

Secretion (endocrine) pt. 2

Gut hormones are chemical messengers released into circulation & act on receptors in distal locations to regulate motility, pancreatic secretions, & other digestive tract (& non-digestive tract) functions

Chemical digestion

Accomplishes the breakdown of structurally complex foodstuffs into smaller, & eventually absorbable units

Chemical digestion: Polysaccharides

-Starch & glycogen are broken down by amylase into maltose which is broken down by maltase into glucose
-Starch & glycogen—>maltose—>>glucose

Chemical digestion: Disaccharides

-sucrose broken down by sucrose into glucose/fructose
-lactose broken down by lactase into glucose/galactose

Chemical digestion: Proteins

-proteins are broken down by pepsin/trypsin/chymotrpsin/carboxypeptidase into peptide fragments which are broken down by aminopeptidase into amino acids

Chemical digestion: Fats

-triglycerides are broken down by lipase into monoglyceride/free fatty acids

Absorption

The transfer of small absorbable units along w/water, vitamins, & electrolytes from the lumen into the blood to lymph

Interstitial cells of Cajal (ICC)

-the membrane potential of these cells oscillate at 3-5 times per sec; this is the Basic Electrical Rhythm (BER) in the stomach
-moves on their own; rhythm created by Cajal cells don't lead to an action potential=leads to no or little contraction

Interstitial cells of Cajal (ICC) pt.2

-these depolarizations spread thru gap junctions to smooth muscle cells, then signal propagated through the tract by the enteric nervous system

migrating motility complex

-depolarizations excess spike threshold only 10-15 times per day; triggers contractions that are frequent enough to "Sweep" residual contents from the stomach & small intestine to the large intense
-triggered by motilin

Motilin

Hormone that increases the likelihood that on occasion action potential peak above the threshold cause infrequent contraction

Stretch and Gastrin

-activate neural circuits that increase amplitude & frequency of the basic electrical rhythm (BER) depolarizations
-increase digestive tract motility

When depolarizations excess spike threshold,

The smooth muscle spike & therefore contract

Intrinsic factors controlling digestive systems functions

1. Autonomous smooth muscle cells
2. Interstitial cells of Cajal (ICC)
3. Enteric nervous system

Autonomous smooth muscle cells

-connected by gap junctions, thereby forming a functional syncytium
-single-unit smooth muscle

Interstitial cells of Cajal (ICC)

-acts as pacesetter cells & generate slow-wave potentials (BER)
-if threshold is reached & action potentials are triggered, then the whole muscle sheet contracts as a unit

enteric nervous system (myenteric + submuscosal nerve plexuses)

-an interconnecting network of nerve cells localized w/in the digestive tract wall; coordinates local activity w/in the digestive tract

Extrinsic factor controlling digestive system functions

-extrinsic nerves
-gastrointestinal hormones

Extrinsic nerves

-coming from brain
-modifying activity of the enteric nervous system
-altering gastric hormone secretion
-acting directly on smooth muscle & glands

Gastrointestinal hormones

Long-range chemical messengers secreted into blood & act on receptors in distal locations to regulate digestive tract (& non-digestive tract) functions

Lips & tongue

-contain food in mouth
-guide food during chewing & swallowing

Teeth

Begin mechanical breakdown of chewing of food

Palate

-roof of the oral cavity
-separates oral cavity from nasal passage
-allows for chewing & breathing to occur simultaneously

Uvula

Soft tissue that hangs from the rear of the mouth & seals off nasal passage during swallowing

Salivary glands

1. Sublingual
2. Submandibular
3. Parotid

salivary glands function

-secrete saliva in response to autonomic stimulation

Saliva contains what?

-muscles to moisten food & lubricate
-lysozyme to lose bacteria (kills bacteria)
-bicarbonate buffers which neutralize acids
-amylase

What is the function of amylase in saliva?

Begins chemical digestion of carbohydrates by cleaving polysaccharides into maltose

Swallowing

-Refers to the entire process of moving food from the mouth, through pharynx & esophagus
-all or none reflex; initiated when bolus is voluntarily forced by tongue to rear of mouth into pharynx

Swallowing pt.2

-can be initiated voluntarily but cannot be stopped once it has begun
2 stages: oropharyngial & esophageal stage

Oropharyngial stage

-the pharyngoesophageal sphincter opens to allow for bolus to move in
-at the end of this stage, the sphincter closes & breathing resumes

Esophageal stage

-peristaltic (propulsive) waves move bolus into stomach
-no absorption happening here

Stomach

-A J shaped chamber located between the esophagus and the small intestine
-divided into 3 sections: fundus, body, & antrum

Fundus

Located above the gastroesophageal sphincter

Body

The middle portion

Antrum

-Bottom portion
-thick layer of smooth muscle
-connected to small intestine by the pyloric sphincter

Pyloric sphincter

-key regulator of gastric emptying
-acts kinda like a filter; small enough broken down particles get to pass through the half open/close opening to get into the duodenum
-if its too big, spends more time in stomach getting broken down

3 major functions of the stomach

1. Store ingested food
2. Gastric secretions
3. Converts pulverized food to chyme

Store ingested food

-store ingested food until it can be emptied into small intestine
-occurs in the body of the stomach

Gastric secretions

-create gastric secretion: including HCL & enzymes that begin chemical digestion of protein

Converts pulverized food to chyme

Gastric motility converts pulverized food to chyme: a thick liquid mixture of pulverized food & gastric secretions

Gastric filling

-gastric volume can expand~ 20-fold during a meal, by expansion/flattening of deep folds
-expansion of gastric volume is a vagally-mediated process receptive relaxation

Gastric secretions

Two distinct areas of secretory gastric mucosa:
1. Oxyntic mucosa (body & fundus)
2. Pyloric gland area (PGA & antrum)

3 types of gastric exocrine secretory cells in oxyntic mucosa, associated with gastric pits

1. Mucous cells
2. Chief cells
3. Parietal (oxyntic) cells

Mucous cell

-Secrete thin, watery mucus
-cells are lining the gastric pit

Chief cells

Secrete enzyme precursor, pepsinogen

Parietal (oxyntic) cells

-secrete HCL & intrinsic factor

What do chief & parietal cells do?

Contractions "push" out these secretions to mix w/ mucus & then mix w/ food

HCL & pepsinogen

-HCL turns pepsinogen into pepsin
-pepsinogen is released first to help combat cells being broken down

Functions of HCL

-activates pepsinogen to activate enzyme pepsin & provides acid medium for optimal pepsin activity
-denatures protein
-along w/ salivary lysozyme, kills most of the microorganism s ingested w/ food

ECL cells

-secrete histamine
-local amplifies stomach acid production as it occurs

Mucus coating

-much thicker
-has a bicarbonate buffer to prevent most mucosal cells from getting damaged from HCL; just in lumen or stomach

In pyloric gland area..

-endocrine secretory cells
-has G and D cells

G cells

-secrete the hormone gastrin into bloodstream

What are the functions of gastrin?

-stimulates parietal, cheif, & ECL cells
-increases gastric motility & promotes movement of leftover, undigested/unabsorbed material out of ileum into large intestine

D cells

Secrete the hormone somatostatin into bloodstream

What is the function of somastostatin?

Inhibits parietal & ECL cells

Gastric mixing and gastric emptying

Strong peristaltic contractions occur in the antrum that:
-mix food w/ gastric secretions to produce chyme
-propel chyme towards pyloric sphincter, where a small amount is pushed into the duodenum

Gastric mixing and gastric emptying pt.2

-in response to chyme, sphincter closes and remaining chyme is tumbled back into the antrum
-the food at the top of the stomach as its emptying isn't as mixed, amylase is still working by breaking carbohydrates

Factors arising in the stomach that control gastric mixing & gastric emptying

-volume of the chyme
-fluidity of the chyme

Volume of the chyme

Distention directly stimulates stretch receptors on the smooth muscle, stimulates enteric & parasympathetic nervous system as well as the stomach hormone gastric to increase motility

Fluidity of the chyme

Liquids do not require extensive mixing & churning; contents must be rendered fluid before they are evacuated

Factors arising in the duodenum that control gastric emptying

-fat
-acid
-hypertonicity
-distention

Fat

-only digested and absorbed w/in the small intestine
-when fat is present in the small intestine further emptying is inhibited

Acid

-highly acidic chyme from the stomach is neutralized by sodium bicarbonate (secreted from pancreas) in the duodenum
-unneutralized acid in the duodenum inhibits gastric emptying

Hypertonicity

Increased osmolarity in the duodenum indicates a back-up of nutrients & delays gastric emptying

Distention

Too much chyme in the duodenum inhibits gastric emptying

Enterogastric reflex

-neural responses are mediated through both intrinsic & automatic nerves
-in turn regulate gastric motility

Cholecystokinin (CKK)

-comes from I cells
-stimulated by fat in the duodenum
-inhibits natural contractions & induces contraction of the pyloric sphincter

Secretin

-stimulated by unneutralized acid in the duodenum
-released by C cells & slows gastric emptying

Control of gastric secretions has 3 phases

1. Cephalic (excitatory)
2. Gastric (excitatory)
3. Intestinal (inhibitory)

The accessory organs: pancreas & liver

-juice secreted by small intestine itself does not contain all the necessary digestive enzymes
-material emptying from stomach is acidic & partially digested
-need secretions of accessory organs to complete digestion & neutralize acid in chyme

Pancreas

-located dorsal & caudal to the stomach
-mixed gland that contains both endocrine & exocrine tissue

Exocrine pancreases includes..

1. Duct cells: release sodium bicarbonate into duodenum to neutralize acidic chyme
2. Acinar cells: release digestive enzymes into duodenum

What kind of enzymes for pancreatic acinar cells release?

-pancreatic amylase
-pancreatic lipase
-proteolytic enzymes

Types of proteolytic enzymes

1. Trypsinogen: gets converted by enteropeptidase to trypsin
2. Chymotrypsinogen: gets converted by trypsin to chymotrypsin
3. Procarboxypeptidase: gets converted by trypsin to carboxypeptidase