Physiology of Digestive System

sparse bundles of smooth m fibers that lie in deep layers of mucosa

mucosal muscle

outer layers of intestinal wall

serosa + smooth m layers (longitudinal and circular)

how does syncytial smooth muscle contract

syncytial interconnections among fibers - cells joined by many gap jxns (free flowing action potentials)

longitudinal vs circular muscle layers of gi tract

long - bundles extend down tract circ - extend around gut

Smooth muscle cells contain a large amount of a regulatory protein called _____________ which works by

calmodulin

initiates contraction by activating the myosin cross-bridge

role of calcium in calmodulin

influx Ca+ - binds w/ calmodulin - complex joins myosin light chain kinase - phosphorylated -> contraction

what must happen for smooth m to relax

Ca+ ions removed from intracell by pump (by ATP + myosin phosphatase enzyme)

where does enteric nervous system lie

in gut wall

Myenteric (Auerbach's) plexus

1 of 2 enteric nervous system plexuses - bw muscle layers, controls GI movements

Submucosal (Meissner's) Plexus

1 of 2 enteric nervous system plexuses in submucosa, controls GI secretion + blood flow

sensory n endings from GI send afferents to

enteric nervous system, prevertebral ganglia of symp, spinal cord and vagus n (to brainstem)

effects when myenteric plexus stimulated

increased tone, contractions, peristalsis

inhibitory actions of myenteric plexus

inhibition of sphincter ms (no food movement)

submucosal plexus controls what part of gi tract

inner wall of intestines

parasymp stimulation increases activity of

enteric nervous system (-> glandular secretion)

parasymp supply to gut is divided into

cranial and sacral divisions

cranial parasymp fibers of gut are almost all in ____________ while sacral parasymp fibers pass thru _______-

vagus nerves (beginning to first half of intestines) pelvic nerves (distal half of intestines down)

sigmoidal, rectal and anal regions are better supplied with which n fibers

parasympathetic

symp fibers to GI tract originate bw

T5 - L2

symp n endings secrete ___________

norepinephrine

norepinephrine secreted by symp nerves have an inhibitory effect to all except to

mucosal muscle

gut mucosa irritation, excess distention and chemicals in gut can stimulate

afferent sensory n fibers

reflexes that control secretions, movements and inhibitory effects are which reflexes?

reflexes integrated entirely within gut wall enteric nervous system

gastrocolic, entergastric and colonoileal reflexes are what reflexes?

reflexes from gut to prevertebral symp ganglia -> GIT

reflexes of the vagus n, inhibitory from pain and of defecation are what reflexes?

reflexes from gut to spinal cord -> GIT

endocrine vs paracrine secretions of hormonal GIT control

endocrine - peptidic (gastrin) paracrine - regulatory peptides (histamine)

functions of gastrin

increase acid production + trophic effect of mucosa + motor fxn of stomach + pyloric pump

products of protein digestion, stomach distentionn and vagal stimulation initiate what to secrete?

gastrin

how do somatostatin (D) cells inhibit gastrin secretion

low ph of gastric contents

Cholecystokinin (CCK) is secreted by ______ cells in ________

I cells mucosa of duodenum + jejunum

how does CKK digest fats

contracts gallbladder to eject bile, secretes pancreatic enzymes + HCO3-, inhibits gastric emptying

secretin is secreted by secretin cells of ________ in response to :

duodenum H+ + fatty acids in SI

secretin function

secretes HCO3- to neutralize acids for fat digestion, inhibits gastrin

___________ produces most of the actions of secretin and is the only GI hormone that is secreted in response to all nutrients

GIP

GIP is secreted by _______ + function:

K cells of duodenum + jejunum mucosa stimulates insulin secretion, inhibits gastrin

____________ has the only fxn of increasing gi motility by interdigestive myoelectric complexes

motilin

where is motilin secreted

stomach + duodenum

which hormones are released directly by neural stimulation

gastrin and motilin

CCK is stimulated by

fatty acids

2 types of movements in GIT

propulsive (forward peristalsis) + mixing (contents mixed)

peristaltic waves don't occur in the absence of

myenteric plexus

receptive relaxation of peristalsis

relaxation a bit before anus

muscles of chewing are innervated by and controlled by

5th cranial n, brainstem

digestive enzmes acts only on

surfaces of food particles

Swallowing is divided into three phases

1. voluntary phase

2. pharyngeal phase (epithelial swallowing receptors)

3. esophageal phase (peristalsis)

most sensitive areas of mouth for swallowing

pharyngeal opening (tonsillar pillars)

__________ impulses are transmitted thru trigeminal and glossopharyngeal n -> medulla oblongata

swallowing

________ stage of swallowing involves primary and secondary (distention) peristalsis

esophageal

secondary peristalsis of esophageal stage of swallowing are initiated by 2 mechanisms

intrinsic neural circuits of myenteric nervous system + pharyngeal reflexes (vagal afferents)

achalasia

failure of the lower esophagus sphincter muscle to relax

how is reflux prevented

contraction of lower esophageal sphincter + intraabd p

chyme

Partially digested, semiliquid food mixed with digestive enzymes and acids in the stomach.

stomach is divided into

orad (first 2/3) + caudad

vagovagal reflex when food stretches stomach

reduces muscular wall tone

retropulsion

The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach.

stomach emptying

migrating myoelectric complexes that cause contractions during fasting by motilin

pyloric pumping

when peristaltic wave reaches pylorus, tiny bit of chyme squeezes into small intestine

presence of breakdown products of chyme cause what effect

enterogastric inhibitory reflexes (inhibited stomach emptying)

which hormones slow stomach emptying

CCK, secretin, GIP

movements of _________ are influenced by chyme in order to increase absorption + lymph flow

muscularis mucosae and m fibers of villi

proximal vs distal halves of colon functions

absorption storage

Haustrations (mixing movements) in the large intestine

circular m + teniae coli contract -> large intestine bulges out

mass movement of colon

type of peristalsis w/ constrictive ring in response to distention/irritation -> contraction as a unit (stool moves down)

internal vs external anal sphincters

int - circular smooth m ext - striated m, controlled by pudendal n (concscious)

to be effective in defecation, the intrinsic myenteric defecation reflex must be helped by

parasymp defecation reflex (sacral n of spinal cord)

Peritoneointestinal reflex

results from irritation of peritoneum and causes intestinal paralysis

renointestinal + vesicointestinal reflexes inhibit

intestinal activity

types of alimentary tract glands

• single-cell mucous glands (mucous or goblet cells)

• pits (crypts of Lieberkuhn in SI)

• tubular glands (stomach + duo)

• complex glands (salivary, pancreas, liver)

tactile stimulus, chemical irritation and wall distention activate which system

enteric n system

symp stimulation effect on secretion

reduces (vasoconstriction)

_______________ are polypeptides that regulate secretions

GI hormones

2 major protein secretions in saliva

serous secretion - ptyalin for starches mucus secretion - mucin for lubrication

parotid glands secrete only _____________ secretion while submandibular+lingual glands secrete serous and mucus, but buccal glands secrete only _____________-

serous mucus

saliva has high amounts of

potassium + bicarb

primary secretion of saliva

Produces plasma-like isotonic saliva by acinar cells

secondary secretion of saliva

excretory and striated duct cells resorb Na+ and Cl-, creating hypotonic and alkaline saliva

in absence of salivation what can happen to the mouth

ulceration and infection, cavities

thiocyanate ions and proteolytic enzymes (lysozyme) are

factors in saliva that destroy bacteria

salivatory nuclei (at medulla x pons) are excited by

taste and tactile stimuli

how does parasymp (slightly symp too) n signals increase salivatory gland nutrition

increases saliva -> saliva dilates vessels (bradykinin)

agonists that release Ca+2 increase ___________ and those elevating cAMP lead to greater secretion of __________

acinar cell secretion enzyme + mucus

esophageal secretion is entirely _________ and is lined by

mucus simple mucous glands

stomach mucosa has 2 types of tubular glands

oxyntic (gastric, Hcl, pepsinogen, mucus) pyloric (secrete mucus + gastrin)

3 types of cells in the oxyntic glands

mucous neck cells (mucus) peptic chief cells (pepsinogen) parietal oxyntic cells (HCl and IF)

At the same time that HCl is secreted, bicarbonate ions diffuse into the blood so that gastric venous blood has ________ than arterial blood when the stomach is secreting acid.

higher ph

main driving force for HCl secretion by parietal cells is

H+-K+ ATPase pump

When does pepsinogen become pepsin?

in the presence of HCl

pyloric glands secrete

mucus (mucous neck cells of oxyntic gland) and gastrin

Ach by parasymp stimulation excites ___________ while gastrin and histamine stimulate __________

pepsinogen (peptic cells), HCl acid (parietal cells) + mucus (mucous cells)

HCl acid (parietal cells)

The parietal cells acitvity is directly related with another type of cell called ____________, the primary function of which is to secrete histamine.

enterochromaffin-like cells (ECL cells) of oxyntic glands

ECL cells are stimulated to secrete histamine by

gastrin

Stimulation of pepsinogen secretion by the peptic cells in the oxyntic glands occurs in response to two main types of signals:

Ach (vagus n)

stomach acid by enteric n reflexes

Gastric secretion occurs in three "phases":

1. cephalic (neurogenic vagus signals before + during eating)

2. gastric (vagovagal, enteric reflexes and gastrin)

3. intestinal (gastric juices from food in intestines)

when are gastric secretions inhibited

• presence of food in SI -> reverse enterogastric reflex

• presence breakdown products, fluids -> secretin, GIP, VIP, somatostatin released

interdigestive period secretes what

nonoxyntic mucous

pancreatic secretions into duodenum are made of

HCO3- (neutralizes acid from stomach) + enzymes for digestion

pancreatic digestive enzymes for each types of food

pancreatic amylase for carbs

pancreatic lipase, chol esterase + phospholipase for fat

trypsin + chymotrypsin for proteins

difference bw trypsin and chymotrypsin + carboxypeptidase (pancreatic digestive enzymes)

trypsin splits proteins into peptides, while carboxypolypeptidase splits peptides into amino acids (completed digestion)

When do the pancreatic enzymes become active?

once they are secreted into intestine

Although the enzymes of the pancreatic juice are secreted entirely by the acini of the pancreatic glands, the other two important components of pancreatic juice, bicarbonate ions and water, are secreted mainly by

the epithelial cells of the ductules and ducts that lead from the acini