A&P Chapter 24 Regulation of digestion

mechanical and chemical stimuli

stretch, osmalality, and presence of substrate in the lumen stimulate receptors and autonomic reflexes

Intrinsic control

short reflex

• refers to the regulation starting with stimulation in the GI tract, which triggers receptors that then send information to the local enteric nerve plexus and then send directions to the effectors.

Extrinsic control

long reflex

• regulation of digestion by the central nervous system works through the local enteric nerve plexus.

• CNS sends instructions to smooth muscle or glands to act.

Phases of swallowing:

1. upper esophageal sphincter is contracted. During the buccal phase, the tongue presses against the hard palate forcing the bolus into the oropharynx where the involuntary phase begins.

2. The uvula and larynx rise to prevent food from entering the respiratory passageways. The upper esophageal sphincter relaxes allowing food to enter the esophagus.

3. The constrictor muscles of the pharynx contract forcing the food downward in the esophagus. The upper esophageal sphincter contracts (closes) after entry.

4. food is moved from the esophagus to the stomach by peristalsis.

5. the gastroesophageal sphincter opens and food enters the stomach

Characteristics of stomach mucosa?

• simple columnar epithelium

• mucus barrier with bicarbonate under it.

• cells need to be connected by super tight junctions.

Gastic Pits:

• extend deep into stomach mucosa

• contain gastric glands that secrete gastric juice which allows chemical digestion

enteroendocrine cells

gastrin, histamine, many others.

neck cells

mucus

parietal cells:

HCl and intrinsic factor (allow B12 uptake)

Chief cells

pepsinogen

pepsinogen activation

converted to pepsinogen by HCl (digests proteins)

Chyme

combination of mechanical and chemical digestion

Cephalic phase:

stimulatory events

1. sight and thought of food

2. stimulation of taste and smell receptors

Inhibitory events:

1. loss of appetite and depression

Body knows what you are feeding it.

Gastric Phase

stimulatory events:

• are a distension of the stomach.

• mechanical and stretch receptors activated.

• local stretch receptors say “HEY get ready!”

1. chemicals in the food stimulate G-cells for Gastrin secretion; gastrin is secreted into the blood and triggers stimulation in stomach as well as accessory glands to do their secretion.

Inhibitory events:

• excessive acidity and emotionally upset.

• this inhibits G-cells which means gastrin secretion is less.

• emotionally upset activates the sympathetic control and overrides parasympathetic control.

Intestinal phase:

Stimulatory events

—>The presence of low pH, partially digested food, fats, hypertonic solution in duodenum when the stomach begins to empty.

—> stimulates local nerve plexi to get stomach secretions aka gastrin secretion

Inhibitory events

• distension of duodenum; presence of fatty, acidic, hypertonic chyme and/or irritants in the duodenum

Gastric motility of the stomach

• two sphincters (esophageal gastric and pyloric end)

• “waves come up and hit wall then crash back”

• most vigorous mixing is done on the pyloric end.

• pyloric valve opens to let chyme enter the duodenum of the small intestine, then closes.

Rate of perstalic waves

3 per minute initiated by pacemaker cells

most vigorous peristalsis and mixing

happens at pylorus

Chyme movement

happens in small amounts at the duodenum (3 mL)

• forced back into the stomach for further mixing.

Peristalsis forces chyme into the duodenum. The presence of chyme in the duodenum stimulates:

the neural enterogastric reflex and hormonal enterogastrone mechanisms.

• the resulting inhibition is gastric secretion of duodenal filling.

carb rich chyme:

moves through duodenum quickly as opposed to fat rich.

what is olestra?

fat that humans cannot digest.

• challenges gut processing and causes the squirts.

plicae circularis

deep circular folds of mucosa and submucosa.

villi

finger like extensions of mucosa

microvilli

tiny projections of mucosal cells plasma membranes

mucosal epithelium

simple columnar absorptive cells, goblet cells, enteroendocrine cells, intraepithelial lymphocytes.

Intestinal juice produced in ….

intestinal crypts

characteristics: alkaline, mostly H2O, enzyme poor; mucus.

brunners glands

secrete alkaline mucous in duodenum

peyers patches

lymph tissue in ileum.

where is bile produced?

liver

list the composition of bile:

• yellow and green alkaline

• bile salts

• bile pigments

• cholestrol

• neutral fats

• phospholipids

  • electrolytes

bile salts:

cholesterol derivative

chief pigment of bile

bilirubin

functions of bile:

1. emulsify fat

2. facilitate fat and cholesterol

3. help solubilize cholesterol

path of bile

• produced in the liver

• leaves the liver through the hepatic ducts.

• travels up through the cystic duct into the gallbladder

• when it is needed, it is released back out the cystic duct into the common bile duct

  • then joins with the main pancreatic duct called the “hepatopancreatic ampulla”

cystic fibrosis

patients with this have too much mucus; if it gets to the small opening it can block the hepatopancreatic ampulla and bile and secretions will not make it to the pancreas.

• the pancreas makes enzymes for all food groups so if nutrients cannot get there then all nutrients are lost when they are turned into waste.

enzymes from the pancreas…

produced for all food groups

what stimulates the liver to make more bile?

bile salts and secretin

what starts the gallbladder to contract so bile squirts out?

vagus nerve

Cholecystokinin

stimulates the gallbladder to contract even more; the sphincter/ampulla needs to be open so the bile can actually get to the duodenum

secretin vs cholecystokinin

secretin = tells liver to make more bile

cholecystokinin = tells gallbladder to release the bile

function of the pancreas

secretes pancreatic juice

pancreatic acini

clusters of secretory cells contain zymogen granules w/ digestive enzymes

composition of pancreatic juice

water, inactive/active enzymes, electrolytes

• neutralize acid chyme

inactive enzymes

• trypsinogen activated to trypsin (protein)

• procarboxypeptidase activated to carboxypeptidase

active enzymes

amylase (carbs), lipase (fats), nuclease (RNA, DNA)

• however, they require ions or bile for optimal activity.

control of pancreatic secretion

1. parasympathetic stimulation from the vagus nerve causes the pancreas to release a secretion rich in digestive enzymes

2. secretin released from the duodenum stimulates the pancreas to release a watery secretion rich in bicarbonate ions.

3. cholecystokinin released from the duodenum causes the pancreas to release a secretion rich in digestive enzymes

Large intestine order starting with the colon

colon

ascending colon

hepatic flexure

transverse colon

splenic flexure

descending colon

sigmoid colon (joins the rectum)

large intestine mucosa

Simple columnar epithelium

crypts: lined with goblet cells

anal canal mucosa

stratified squamous epithelium

sinuses exude mucus and compress feces

superficial venous plexi

associated with anal canal

hemmroids

varicosities cause hemmrhoids

Bacteria flora of large intestine

1. those that survive the small intestine

2. those that enter via the anus

Bacteria in the large intestine do what?

1. colonize the colon

2. ferment digestible CHOs

3. Release irritating acids and gases

4. Synthesize B complex and K vitamins

Functions of Large Intestines

Digest bacteria only.

Reclaim vitamins; H2O and electrolytes.

Propels fecal matter to anus.

Embryonic development 3rd week

gut formation; mouth and anus nearly formed.

Embryonic development 8th week

accessory organ formation

Fetal development nutrition

via placenta

GI tract is stimulated by what?

swallowing embryonic fluid.

What is most important activity for babies?

feeding

Causes of malabsorption of nutrients

1. anything that interferes with the delivery of bile or pancreatic juice.

2. damaged intestinal mucosa.

Gluten sensitivity enteropathy

(celiac disease)

• gluten damages the intestinal villi and brush border

• treated by eliminating gluten from the diet (all grains but rice and corn)

Digestion small intestine

as chyme enters duodenum

• carbs and proteins are only partially digested.

• no fat digestion has taken place

• chyme is hypertonic and has a low pH

continues

• chyme is slowly released into the duodenum

• mixing is required for proper digestion

• substances required are produced by liver

• all nutrition absorption takes place in small intestine