npb101 digestive system

t/f: the digestive lumen is continuous with the outside of the body

t

what are the 4 major tissue layers of the digestive tract wall (from the outside working in)?

1. serosa
2. muscularis externa
3. submucosa
4. mucosa

This layer of the digestive tract wall:
- secretes serous fluid
- supports digestive organs in proper place (suspension)
- allows organs to perform mixing/propulsive movements

serosa

This layer of the digestive tract wall:
- is the major smooth muscle coat of the digestive tube
- has 2 layers (inner circular, outer longitudinal)
- mixes and propels food via contractile activity
- contains the myenteric plexus

muscularis externa

what are the 2 layers of the muscularis externa?

inner circular and outer longitudinal

what is the purpose of the inner circular layer of the muscularis externa?

contraction decreases lumen diameter

what is the purpose of the outer longitudinal layer of the muscularis externa?

contraction shortens tube

describe the myenteric plexus

- between inner and outer layers of muscularis externa
- part of enteric nervous system
- collection of nerve bodies

This layer of the digestive tract wall:
- is a thick layer of connective tissue
- has distensibility and elasticity
- contains larger blood and lymph vessels
- contains submucosal plexus nerve network
- is part of the enteric nervous system

submucosa

This layer of the digestive tract wall:
- separates outer and inner lumens
- has a highly folded surface that increases absorptive area
- contains an epithelial layer, lamina propria, muscularis mucosa

mucosa

what is the purpose of the highly folded surface of the mucosa?

to increase absorptive area

another name for the epithelial layer of the mucosa

mucous membrane

describe the epithelial layer of the mucosa

- has cells modified for secretion, absorption
- has exocrine and endocrine gland cells

describe the lamina propria of the mucosa

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

describe the muscularis mucosa of the mucosa

- sparse layer of smooth muscle
- small, local movements

purpose of exocrine gland cells

secrete digestive juices, mucus, enzymes into lumen

purpose of endocrine gland cells

secrete GI hormones into capillaries

why is it important that the lumen of GI tract is continuous with the external environment? (3 factors)

- pH of GI is very acidic, not compatible with normal body pH range
- harsh GI enzymes can destroy the body's tissues
- millions of microorganisms inhabit GI tract, could be lethal if they entered other parts of body, so the GI tract makes a mucosal barrier

t/f: some GI enzymes are synthesized in its inactive form and are only activated when they reach the GI lumen

t

what are the 4 basic digestive processes in order?

motility, secretion, digestion, absorption

ingestion, chewing, swallowing, and defecation are all (voluntary/involuntary) actions

voluntary

main purpose of motility

muscular contractions mix and move contents forward within the GI tract

what are the 2 types of movements involved in motility?

propulsive (peristalsis), mixing (segmentation)

purpose of peristalsis

move contents forward through tract

purposes of mixing/segmentation (3)

- aid digestion by mixing food with digestive juices
- facilitate absorption by exposing food to absorbing surfaces
- facilitate slow, non-linear forward movement

what is exocrine secretion?

secretion into compartment continuous with outside of body

purposes of exocrine secretion

- secrete digestive juices into lumen
- secrete enzymes, acids, buffers, electrolytes, water
- promote digestion
- adjust tonicity
- provide lubrication for better movement

what is endocrine secretion

secretion of hormones (chemical messengers) into the bloodstream

purposes of endocrine secretion

regulate motility, pancreatic secretions, and other digestive tract/non-digestive tract functions

purpose of digestion

breakdown of structurally complex foodstuffs into smaller, eventually absorbable units

what are the 3 macronutrients involved in digestion?

carbohydrates, protein, fat

what are the 2 sources of amylase

salivary and pancreatic

name the 4 enzymes that convert proteins into peptide fragments and where they come from

- pepsin from stomach
- trypsin, chymotrypsin, carboxypeptidase from pancreas

what is the main enzyme involved in fat digestion, and where does it come from?

lipase from pancreas

why is the pancreas especially important for fat digestion?

the pancreas is the only organ in the body that produces enzymes that can digest fats

purpose of absorption

transfer of small absorbable units from lumen into blood

specialized cells in GI tract that generate electrical signals to control GI contractions

interstitial cells of Cajal (ICC)

what is the basic electrical rhythm (BER) in the stomach?

3-5 Hz

t/f: ICCs in the small intestine depolarize more frequently

t

depolarizations in the GI tract spread through ____ _____ to smooth muscle cells

gap junctions

GI signals are propagated by the _____ nervous system

enteric

t/f: GI depolarizations exceed spike threshold only 10-15x/day

t

this triggers contractions that are frequent enough to "sweep" residual contents from the stomach/small intestine to the large intestine. results in "stomach growls"

migrating motility complex

what hormone triggers the migrating motility complex?

motilin

_____ and ______ activate neural circuits to increase amplitude + frequency of BER depolarizations

stretch, gastrin

where does gastrin come from

induced by protein in stomach

t/f: stretch + gastrin increase digestive tract motility

t

what 3 factors move ICCs below threshold to inhibit the motility signal?

hormones, intrinsic nerve reflexes, sympathetic nerve output

intrinsic factors controlling digestive system functions (3)

- autonomous smooth muscle cells
- ICCs
- enteric ns

do autonomous smooth muscle cells form a functional sanctum? if so, what allows this?

yes, gap junctions

extrinsic factors controlling digestive system functions

extrinsic nerves, GI hormones

how do extrinsic nerves influence motility and secretion? (3 ways)

- modifying activity of enteric ns
- altering gastric hormone secretion
- acting directly on smooth muscle and glands

what 2 factors stimulate the salivary center in medulla?

- pressure receptors/chemoreceptors in mouth
- cerebral cortex (thinking of, seeing, and smelling food)

purpose of lips + tongue in digestive system

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

purpose of teeth in digestive system

begin mechanical breakdown by chewing food

purpose of palate in digestive system

- separates oral cavity from nasal passage
- allows chewing and breathing to occur simultaneously

purpose of uvula in digestive system

- seals off nasal passage during swallowing

purpose of salivary glands in digestive system

secrete saliva via autonomic stimulation

contents of saliva

- mucus
- lysozyme
- bicarbonate buffers
- amylase

purpose of mucus in saliva

moistens food, lubricates

purpose of lysozyme in saliva

lyse bacteria

purpose of bicarbonate buffers in saliva

neutralize acids

purpose of amylase in asliva

begin chemical digestion of carbohydrates by cleaving polysaccharides into maltose

does the digestion of carbohydrates start in the mouth?

yes

entire process of moving food from mouth -> pharynx and esophagus -> stomach

swallowing

t/f: swallowing is an all-or-none reflex

t, it's initiated voluntarily, but it can't be stopped once started

what are the 2 stages of swallowing

oropharyngeal and esophageal

describe the oropharyngeal stage

at the end, the pharyngoesophgeal sphincter closes -> breathing resumes

describe the esophageal stages of swallowing

peristaltic (propulsive) waves move bolus into stomach

what are the 3 sections of the stomach? describe their relative locations within the stomach

- fundus: above gastroesophageal sphincter
- body: middle
- antrum: bottom

functions of pyloric sphincter

- connects stomach to small intestine
- key regulator of gastric emptying

how does the pyloric sphincter regulate gastric emptying?

it only allows small enough particles through (if not small enough, substance stays in stomach to be further broken down)

3 major functions of stomach

- store ingested food
- create gastric secretions that begin chemical digestion of protein
- gastric motility

t/f: gastric filling/expansion is vagally mediated

t

what 2 locations do gastric secretions come from, and in what parts of the stomach are they found?

- oxyntic mucosa (body, fundus)
- pyloric gland area (antrum)

name the 3 types of gastric exocrine secretory cells from the oxyntic mucosa (associated with gastric pits)

mucous, chief, parietal/oxyntic

mucous cells

secrete thin, watery mucus

chief cells

secrete pepsinogen, protect stomach from itself

pepsinogen is the enzyme precursor for

pepsin

parietal (oxyntic) cells

secrete HCl and intrinsic factor

functions of HCl in stomach (3)

- activates pepsinogen to active pepsin, provides acid medium for optimal pepsin activity
- denatures protein
- kills most microorganisms ingested with food

name the 2 types of endocrine secretory cells

g and d cells

g cells purposes

- secrete gastrin, which:
- makes space for new food
- stimulates parietal, chief, ECL cells
- increases gastric motility
- promotes movement of leftover material out of ileum -> large intestine

d cells purposes

- secrete somatostatin hormone, which:
- inhibits parietal, ECL cells

purpose of gastric motility

convert pulverized food into chyme

thick liquid mixture of pulverized food and gastric secretions

chyme

describe gastric mixing and emptying

- strong peristaltic contractions in antrum
- mixes food with gastric secretions to produce chyme
- propels chyme towards pyloric sphincter

describe the 2 factors arising in the stomach that control gastric mixing and emptying

- chyme volume increases gastric motility
- chyme fluidity increases gastric motility

t/f: GI contents must be fluid before being evacuated from the stomach

t

describe the 4 factors arising in the duodenum that control gastric emptying

- fat inhibits emptying
- acid inhibits emptying if un-neutralized
- hypertonicity: increased osmolarity indicates nutrient back-up -> delays emptying
- distention (too much chyme) inhibits emptying

what neutralizes stomach acid?

sodium bicarbonate, secreted from pancreas

describe the neural responses regulating gastric motility

- intrinsic (short) and autonomic (long) nerves
- enterogastric reflex slows down motility/emptying

neural response that slows gastric motility/emptying

enterogastric reflex

what are the 2 hormonal responses regulating gastric motility?

cholecystokinin (CCK) and secretin

what cells secrete CCK?

i cells

what cells secrete secretin?

s cells

cck is stimulated by

fat in duodenum

secretin is stimulated by

unneutralized acid in duodenum

cck and secretin both (speed up/slow down) gastric emptying

slow down

describe the excitatory phases of gastric secretion

- cephalic: stimuli in head (seeing, smelling, tasting, chewing, swallowing food)
- gastric: stimuli in stomach