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What are the 2 basic GI processes?

One long tube.
- With circulatory system of blood (liver and heart)
- Things are absorbed and secreted to/from the blood.

Motility: muscular contractions that mix contents inside lumen and move forward the contents of the GI tract. Local areas

Secretion: glands along tract that secret contents into tract (sometimes cells themselves) i.e. salivary glands. Assist in motility, digestion and absorption.

Digestion: breakdown of large particles and molecules into small absorbable things that need to cross cell membranes

Absorption: into blood or lymphatic system


Describe motility?

Motility: smooth muscle cells
- Organized in different ways
- No alpha motor neurons.

- They maintain a constant contraction (tone) at their mid-length point.
○ Allows further contraction or relaxion (Could be longer or shorter if it was relaxed or contracted)
○ Maintains steady state pressure on contents.
- Keeps it from ballooning out.

A. Mixing
- Locally, redistribution of luminal contents
- Gives them to digestive enzymes
§ Need to get enzymes and fluids and secretions and enzymes into it. Helps with efficiency.
- Expose luminal contents to absorbing surfaces.

B. Propulsive: move it forwards
- From one area to other area (stomach to small intestine)
- Different rate of propulsion in different areas.
§ Esophagus: rapid
§ Small intestine: slow, need lots of mixing and absorbing by epithelial cells.


Describe secretion.

Secretion: digestive juices
- Exocrine glands (duct system)

i.e. salivary glands.
- Sacs with secretory cells
- Neck cells with mucus
- Contribute to digestive juice (enzyme, aqueous, mucus)
- All through an exocrien gland into the mouth

1. Water (but what about fats?? Uh oh!)
2. Electrolytes (ions, sodium, potassium, protons)
3. Organic substances
- Mucus (slippery), enzymes (digestion), bile salts. It VARIES.

All perform specific functions.

Production takes energy.
- Take raw materials, use ATP and secrete them to the lumen of exocrine gland itself.
- Assemble in ER certain things
- Neuronal control (enteric or autonomic)
- Hormonal (upregulating activity of epithelial cells of the gland)

Most secretions are reaborbed. Ls per day recycled.
- Very efficient.


Describe digestion

Breakdown into absorbable components.

Carbs: single sugar (monosaccarides) or linked sugar molecules (disaccharides, polysacchaides)
- Need to absorb the mono ones

Proteins: amino acids linked by peptide bonds and there are foldigns. All have to be broken down and digested by proteazes and peptidazes.

Fats: triglycerides (issue, hydrophobic aqueous solution)
- Fatty acids linked to a three carbon glycerol backbone.
- Charge + hydrophobic tail.

Hydrolysis: addition of water molecules to bonds that join ex: maltose.
- Maltase cleaves the two glucose molecules by adding water.
- Enzymes hydrolyze bonds

Digestion: hydrolysis.

You will have to know each of the enzyems in the blue boxes.
- Must understand which nutrients each enzyme is dealing with

Carbohydrate: amylase (breaks poly into disaassarides),
Sucrase, lactase and maltase
- All generate monosaccarides that ca be absorped.

Proteins: pepsin, trypsin, chymotrypsin, carboxypeptidase (pancrease), aminopeptidase
- Proteazes that leave proteins into amino fragments then amino acids (what is absorbed)

Fats: lipase. Leaves bonds between fatty acid chain and triglyceride molecule. So you have monoglycerides and fatty acids.
- Hydrophobic.


Describe absorption.

Membrane of epithelial cells across membrane in small intestine.

Nutrients stravel through epithelial cells into the blood.
- Breakdown products of carbs and proteins go into blood
Fats go into lymphatic system

Water and vitamin absorbs in large intestine (but that’s about it)


Describe gross anatomy of GI system?

Small intestine
Large intestine

Salivary glands


What are the layers of the GI tract wall (generally?)

4 primary layers (inside to outside)
- Mucosa
- Submucosa
- Muscular layer
- Serosa

What the lumen looks like changes, but each area has these 4 layers, but size and organization differs throughout the tract.

Shows duct going through the layers/wall to get through the lumen


Describe the mucosal layer

MUCOSA: very important
- Lines luminal surface of GI tract
1. Mucus membrane
- Epithelial cells linked by tight junctions (creates a barrier to the external-outside body technically)
- Exocrine cells: mucus or enzyems into lumen
- Endocrine cells: hormones to blood (paracrine)
- Epithelial cells for nutrient absorption

2. Lamina propria: supportive connective tissue
- Thin layer
- Connective tissue where epithelial cells sit
- Immune cells, capillaries, lymph ducts.

3. Muscularis mucosa
- Thin layer, 1st smooth muscle. Not propulsive.
For local mixing (villi that move things around)


Describe the submucosal layer

- Thick connective tissue
- Elasticity fibers
- Blood vessels
- Lympatic vessels
- Exocrine glands
- Background materials

- Within submucosa of small and large intestine, have interconnected neurons
Control GI motility and secretion


Describe the muscularis externa

Muscularis Externa: major muscle smooth layer
1. Inner circular layer: fibers run circularly around the lumen (connect to each other around the lumen, linked end to end)
- Contraction is constriction (partial or complete)
2. Outer longitudinal layer: fibers run length of GI tract
- Contraction is shortening of GI tract
- Change how long it is!

Myenteric plexus
- Lies between these two muscle layers.
- Coordinates muscularis externa contractions.
- Controls whats contracting and how much
i.e. peristalsis, constriction and shortening when you swallow.


Describe the Serosa

Serosa: connective tissue outer covering
- Anchors to other tissues
- Secretes a lubricating fluid to reduce friction (intestines don't get bound up with the liver for example)


Describe regulation of the GI system

Need to know regulation.

4 regulation factors that are involved in coordinating activity.
1. Intrinsic electrical properties of smooth muscle cells (mostly muscularis externa)
2. Enternic nervous system (GI tract has almost as many neurons as your brain) it can coordinate without your brain! Just not as efficient.
3. Autonomic nervous system (coordinates GI with actions and needs of the rest of the body)
4. GI hormones.


Describe intrinsic smooht muscle activity.

Drop out ang go to design school.

Specialized smooth muscle cells in GI tract that undergoes spantaneous transient epolarization
- Has RMP (difference in charge, negative)
- Pacemaker cells "interstitial cells of cajal" ICC cells. ICC is target for neuronal control.
- Non contractile
- Spontaneous depolarizations.
- ICC is connected to other smooth muscle cells. Propegate by itself.
- Potentials are in slow waves (speading to all connected other smooth cells)
§ Propegate from pacemaker to other smooth muscle cells through gap junctions.
- Flow of electric current between cells.
- Smooth muscles layers (circular and longtintidal) that form sheets connected by gap juncitons.

Slow wave, then a bigger one.
Slow wave reaches threshold for action potential, you'll get burst of AP

Number of Aps is the duration the slow wave remains above threshold.
- Larger wave, longer above threshold. Greater number of Aps In your burst.
- This elevates calcium.

Smooth muscle cells contract in response to a rise in intracellular calcium.
Elevated calcium causes contration and generation of force. As long as calcium is elevated.

More TENSION with more calcium. (greater slow wave, more tension, complete pinch off in innercucular muslce layer example)


Describe the ENS

10^8 neurons
Two nerve plexues within GI tract (submucosal and myenteric) wall comprise it.

Operate within GI wall ON ITS OWN without brain so its reflexive, but autonomic nervous system helps coordinate it with the body. Has its own sensors, integrations and motor outputs.

Functions via electrical communication and release of neurotransmitter between neurons.

Complete ability to sense what's going on in the lumen, process it in the wall and effect outputs (secretory or movements)
Don't need to memorize

Sensory components: sampling whats happening in wall and lumen (mechanoreceptors=pressure, chemoreceptors, and osmoreceptors)

- Excitatory and inhibitory

Secretomotor cells: smooth muscle, epithelial cells that secrete or absorb fluids/electrolytes
Enteric endocrine cells.
Movement or secretory.
- Smooth muscles or ICC cells or endocrine or epithelial cells can be targets.

Neurons part of enteric NS (black)
- Receive from sensors
- Communication across plexi
- Output structures.

Sense intercommunicate and effect what's happening


Describe the ANS

Two branches
- Sympathetic
- Between meals, or fight or flight
- Preganglionic: synapse outside synapse so postganglionic is going in.
- Parasympathetic
- Inputs to GI system 90% fibers in cranial nerve 10, major driver.
- Drive when you've eaten a meal
- Preganglionic fibers right into wall of GI tract
- Postganglionic: within nervous system.
Sympathetic and parasymathetic.

Sensory: conscious perception of whats going on.
- Only when something goes wrong. (choking or cramping)

Para: preganglionic fibers red, synapse with green postganglionic in green. ACH onto muscarinic
Synapse inside GI

Sympa: red and black, then postganglionic enters wall of GI tract.
Synapse outside GI

Different classes of sensory inputs and outputs. All communicate together to make this happen

When you're in parasympathetic drive (continuum of drives): increases motility and secretions
- During ingestion of a meal and processing of that content.
Opposite for sympatheitc drive time. (stress, exercise

If you increases sympathetic activity (add ach) get greater amplitude, get bursts of Aps.
Ach is a stimulatior of AP generation of smooth muscle cells and ICCs that can propegate.

NE (for sympathetic) the amplitude of slow waves is diminished, no action potentials. Decreased contraction of smooth muscles.


Describe GI hormones

GI hormones.
- endocrine cells that produce hormones
Among epithelial cells
Different between different organs.

Carried in the bloodstream NOT lumen (endocrine gland)
- Slower moving
- Effect downstream or distant areas through blood connections.

- Excitatory or inhibitory depending on which hormone and which receptor.

GI hormones: KNOW THESE.
- Gastrin: stomach
- Secretin: duodenum
- Cholecystokinin (CCK)
Gastric Inhibitory peptide (GIP) from duodenum and jejunum.


Summarize GI regulation pathway

Inputs (local or external changes)
Extrinsic autonomic nerves
GI hormones (in blood)
Enteric nervous system

Receptors in GI tract activate ENS, ANS or horomones
- Chemo
- Mechano
- Osmo

Smooth muscle, exograne gland, salivary gland and gastrin secretin CCK or GIP.

Stimulation of these receptors initiates neuronal reflexes (short loop or long loop, within or outside of wall neuronally) or hormoneal releases.
- Short: within wall of GI tract (pressing of bolus of food results in relaxation of wall of stomach in preparation)
- Long: activation of taste buds as you chew, changes motility in muslces of stomach to prep for food. (food in stomach soon) autonomic ns involved, left wall of GI tract and now you have a long loop.