Lecture 6 (GI): organs, basic processes, structure of wall, enteric nervous system, phases of GI contorl, mouth, pharynx, esophagus (swallowing), stomach

primary organs

where food actually enters and passes (e.g., mouth, pharynx, esophagus, stomach, small intestine, large intestine)

accessory organs

add things to the GI tract that is useful for digestion and absorption (e.g., salivary glands, liver, gallbladder, pancreas)

is regulation of GI digestion and absorption homeostatic

NO. regulation is designed to maximize digestion and absorption, regardless of whether nutrients are required for function at the moment

motility

contraction of the tract. occurs along entire length from esophagus to anus

secretion

movement of nutrients from ECF to GI tract. occurs throughout the entirety of the GI tract, but mostly in small intestine.

digestion

breakdown of food into smaller units. mostly occurs in small intestine, but also occurs in mouth (salivary glands) and tiny bit in stomach

absorption

movement from GI tract lumen to blood. mostly occurs in small intestine

short reflexes

isolated to lumen and wall. does not involve CNS input.

long reflexes

involve CNS

ICCs and what do they determine

generate slow wave depolarizations and are connected to adjacent smooth muscle cells. determine FREQUENCY of GI smooth muscle contractions

how much fluid in the tract is from ingestion vs secretion

• ingestion ~1.2L

• secretion ~7 L

• total fluid = 8.2 L/day

cephalic phase

initiated when receptors in the head are stimulated by chewing and/or thinking, sight, smell, and/or taste of food

gastric phase

initiated by distension, acidity change, proteins and peptides in the stomach to regulate stomach functions

intestinal phase

initiated by distension, acidity, osmolarity, and digestive products in the small intestine to regulate both intestinal (activated) and stomach (inhibited) functions

amylase

enzyme in saliva that digests starch (carbohydrates)

lipase

enzyme in saliva that digests fat

initial saliva vs final saliva osmolarity

• initial saliva = isotonic solution, plus mucin and proteins (e.g., amylase) are secreted from acinar cells

• final saliva = hypotonic. NaCl reabsorbed, K+ and HCo3- secreted. water not reabsorbed (impermeable to water)

autonomic innervation of salivary gland arterioles

• PS neurons release VIP (vasoactive intestinal peptide), and acinar cells release paracrine → dilation of vessels → increased blood flow (this is one exception to the “blood vessels get only sympathetic innervation” rule)

• symp neurons release NE → blood vessels (α2 receptors) → vasoconstriction

• dual innervation to blood vessels w/ opposite effects

autonomic innervation of salivary gland acinary cells

• PS neurons release ACh → acinar cells (M-AChR’s) → increased isotonic fluid secretion

• symp neurons release NE → acinar cells (α and β receptors) → increase mucin (mucous) and proteins (e.g., amylase secretion)

• dual innervation with similar effects… both stimulate an aspect of salivary secretion)

swallowing reflex

1. tongue pushes food to back of pharynx → reflex activated: afferent information sent to

2. swallowing center in medulla oblongata which sends efferent impulses to: inhibit breathing, raise larynx, close glottis, and relax upper esophageal sphincter (skeletal muscle)

peristalsis

progressive wave of contractions that moves food from esophagus to stomach

lower esophageal sphincter (LES) opens vs closes during

• opens/relaxes during swallowing (PS activity)

• closes after food (symp activity)

secondary peristalsis

repeated peristaltic waves not initiated by swallowing if food doesn’t reach stomach or is there is distension at esophagus

chyme

mixture of broken up food particles and gastric juice

mucous cells

mucus, bicarbonate secretion

parietal cells

acid, intrinsic factor secretion

enterochromaffin-like cells (ECL cell)

histamine secretion

chief cells

pepsingogen secretion

G cells

enteroendocrine cells that secrete gastrin (a hormone)

D cells

secrete somatostatin (paracrine factor)

what is the only essential function of stomach

secretes intrinsic factor, which is essential for absoprtion of vitamin B12 by the ileum (B12 is required for production of RBC)

pepsinogen

inactive enzyme that requires acidic environment to be activated (HCl) and converted to pepsin to breakdown proteins

motility of the stomach steps

1. stomach produces peristaltic waves in response to arrival of food

2. wave begins in body and progresses to the antrum

3. antrum has more smooth muscle, so stronger contraction. peristaltic wave closes the pyloric spincter — only a small amount of chyme enters the small intestine

4. most chyme is forced back toward the body, which mixes the chyme