serosa

longitudinal muscle layer

circular muscle layer

submucosa

mucosa

Smooth muscle in the GI ahve muscle bundles that fuse at many points so that it functions as a ______.

Why is this imporatant?

Functions as a synctium - a large single cell-like structure with many nuclei.

Allows an AP initiated anywhere within the muscle mass will travel in all directions in the muscle.

slow waves (resting)

spike (stimulated by stretch, ACh)

stomach = 3

duodenum = 12

terminal ileum = 8-9

There is a circular layer that runs around the intestine whose contraction causes segmentation

The longitudinal layer which runs along length of intestine, causes wave like contractions.

Slow waves DON'T cause muscle contraction by themselves.

Mainly control appearance of intermittent spike potentials.

The spike potentials excite most of the muscle contraction.

The normal resting potential in smooth muscle fibers of GI tract is ______.

A spike potential will occur when the resting potential reaches what threshold level?

normally:  -50 to -60mV

Contraction at more positive than -40mV

GI spike potentials can last as long as ____.

How long does this compare with large nerve fibers?

spike potentias last as logn as 10-20 milliseconds.

This is 10-40X longer than large nerve fibers.

Explain how an action potential of GI smooth muscle differs from normal nerve fiber AP.

Why is this important?

In GI smooth muscle, Ca2+ enters with smaller number of Na+ (lower depolarization threshhold).

Na+ channels much slower to open and to close than normal rapid Na+ channels. This results in a longer duration of an AP.

This leads to long slow muscular contractions.

What are the two plexuses of the enteric nervous system that are importnat to GI function.

How do the broadly effect the GI tract?

Myenteric plexus- contribute mostly to GI movements

Submucosal plexus - contribute mostly to GI secretion and local blood flow.

Increased tone of the gut wall

increased intensity of rhythmic contractions

Increased velocity of excitatory wave conduction along gut wall - more rapid movement of peristalic waves

Local intestinal secretion

Local absorption

Local cotraction of submucosal muscle

ACh most often excites GI activity

Norepi and epi mostly inhibits GI activity

what secretes cholecystokinin?

What causes its secretion?

cholecystikinin is screted by I cells in mucosa of duodenum and jejunum.

Secreted in response to presence fo breakdown products of fat, fatty acids, and monoglycerides in intestine.

CCK increases gallbladder contractility to expel bile into the small intestine. (bile helps emulsify fatty substances to be digested/absorbed.)

Moderatly inhibits stomach motility (to allow fat digestion in upper intestial tract)

What secretes secretin?

What is secretin secreted in response to?

by what and where is gastric inhibitory peptide secreted?

what is its main function?

secreted by mucosa of the upper intestinal tract mainly in repsonse to fatty acids and amino acids.

Mildly decreases motor activity of stomach to slow emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food.

What are the two major movements of the GI tract?

What are each of their main functions?

Peristalisis - propels food through the GI tract. This s a unidirectional movement (until you vomit).

Mixing - circular movement (mostly for mechanical breakdown and chemical mixing)

Stimulus is gut distension which signals the enteric nervous system to contract gut wall.

Chemical or physical irritation of the epithelial linign of the gut is also a stimulus.

What is the "Law of the Gut"?

blood flowing from the gut, spleen, and pancreas flows into liver via portal vein.

Decontaminated blood then enter the hepatic veins to vena cava.

Villi (in small intestine) experience countercurrent blood flow.

Blood oxygen diffuses out of arterioles into adjacent venules without reaching the tips of villi.

This means that the tips of villi are very susceptible to circulatory shock etc.

Parasympathetic stimulation to stomach and lower colon causes incread blood flow and glanduar secretion.

Sympathetic stimulation leads to vasoconstrction of arterioles temporarily (also constricts veins - adding between 200-400 mL extra blood to the rest of cirulation)

What are the three stages of swallowing?

What happens during each? Is it voluntary?

Voluntary phase: food at pharynx, intiates swallowing process

Pharyngeal phase: Involuntary, reflex. Food passes from pharynx into esophagus

Esophageal: Involuntary. Food passes from pharynx to stomach.

What brain area controls swallowing?

How does it complete its job?

The brainstem controls successive stage of swallowing.

Medulla and lower pons (aka the swallowing center)

Inhibits respiratory center of medulla at time fo swallowing for swalling to procede.

Primary peristalsis: continuation of peristaltic wave that begins in the pharynx and spreads to esophagus.

Secondary Peristalsis: initiated if primary peristalss fails ,secondary peristalsis takes over due to distension of esophagus.

Increased food volume in stomach causes increased emptying of stomach.

Food entering duodenum can slow stomach emptying: if pH of chyme is low (3.5-4) it will signal for slowed gastric emptying until it ca neutralize the chyme it has.

Also fatty substances lead to CCK release from jejunum to block stomach motility from gastrin.

What is one stimulus for SI to illicite localized contractions?

How often will contraction will contractions occur to mix chyme with SI secretions?

When portion of SI is distened with chyme, stretching of intestinal wall elicits localized contraction (segmentation).

Chop chyme about 2-3X per minute to mix stuff up.

Which hormones enchance intestinal motility?

Proximal 1/2 colon: absorption of water and electrolytes from chyme to form solid stool

Distal 1/2 colon: storage of fecal matter until it can be expelled

The basic functional unit of the liver is the ____.

A liver usually contains about how many of these units?

liver lobule

Contains 50,000-100,000 individual lobules

Hepatic cells

Endothelial cells

Kupffer cells (reticuloendothelail cells)

Explain what occurs to the liver with Cirrhosis.

what are some possible causes of liver cirrhosis.

Fibrous tissue replaces normal parenchyma

Fibrous tissue contracts around blood vessels, imdeding portal blood flow.

Causes: Alcohol, toxins, viral disease

Oxidizes fatty acids to supply energy for other body functions.

synthesis for large quantity of cholesteral

Deamination f AAs

Formation of urea for removal of ammonia from body fluids

Formation of plasma proteins (e.g. albumin)

Interconversions of some AAs

The liver is a storage site for Viatmins ______

(particulary what general type of vitamin?)

Vitamins A, D, B12

particularly good at storing fat soluble vitamins