- transport, grinding, and mixing of food

- enzymatic and chemical procesing of food

- absorption of nutrients, water, electrolytes, etc.

- evacuation of waste and vomiting

- protection of body from toxins and pathogens

Auerbach's plexus (aka myenteric plexus)

Meissners plexus (aka submucosal plexus)

They are a network of nerce cells and fibers embedded in the wall of the bowel

Mouth

Esophagus

Stomach

Duodenum

Small Intestine

Large Intestine (colon)

Rectum

Anus

Digestion and elimination of food.

Approx. 8.3 meters (30 feet) long

Chewing

Salivating (CNS -> SNS/PNS)

Swallowing: coordinated by brainstem, tope 1/3 of esophagus

Store and digest food: secretory cells produce acidic environment

Kneads large clumps of food into small particles in the antrum

Propulsion and mixing via longitudinal and circular smooth muscles (individual smooth cells are electrically coupled for coordinated actions)

Mucous secretion

Pancreas and liver produce/secrete digestive enzymes.

Enterocytes in SI produce enzymes for digestion.

Water and electrolyte absorption.

Immune system

- Control of gut motility

- Regulation of fluid exchange and local blood flow

- Regulation of gastric and pancreatic secretion

- Regulation of GI endocrine cells

- Defense against pathogens

Within the walls of the digestive organs: esophagus, stomach, intestines, pancreas, gall bladder, and pancreato-biliary ducts

Approx. 200-600 million nerve cells in ENS

Lies between longitudinal and circulat muscles in the esophagus, stomach, SI, and colon

Effector cells are smooth muscle cells of longitudnal and circular musculature

Main function is to regulate motility of GI tract

Lies bewteen circular muscle and the submucosa in SI and colon

Effector cells are: mucosal muscle cells (incl. secretory glands); hormone secreting cells; mucosal and submucosal arterioles

Main function is regulation of GI secretion

Sensory neurons

Interneurons

Motor neurons

Histochemical

Electrophysiological

Functional

IPAN: intrinsic primary afferent neurons - sensory

Located in myenteric and submucosal plexi

Located within secretory cells of the GI tract

Mechanical pressure on the apical end of EC cells stimulate 5-HT release into the gut wall activating IPANs and ENS reflexes

Ascending

Descending

Intestinofugal

Orally projecting

Involved in excitatory motor reflexes

Release Ach that bind to nAch receptors on excitatory motor neurons

Aborally projecting.

Involved in secretory reflexes and inhiitory motor reflexes

Release Ach with ATP or 5-HT as a co-trasmitter onto secretomotor and inhibitory motor neurons

Involved in extrapsinal intestino-intestinal reflexes: when part of intestine is overly distended or mucosa become irritated, activity in other parts of intestine inhibited

Use Ach to signal to sympathetic prevertebral ganglia

Located in subucosal and myenteric plexi

Act directly on smooth muscle cells and secretory cells (chief, parietal, mucosal, enterocyte, and pancreatic exocrine cells)

Motor neurons not synaptically connected with each other

Secretomotor and vasodilator neurons

Elicit glandular seceretion and vasodilation via NT ACh and VIP

Enterochromaffin cells: repsonse to mechanical pressure to activate IPANs

Interstitial cells of Cahal (ICC): pacemake of the ENS

Located at submucosal and myenteric border of musle layers

Form elecrticlally couples networks contact GI smooth muscle cells (via gap junctions)

ICC are the pacemaker cells of the ENS

ICC in myenteric plexus generate slow waves -> smooth muscle contractions are controlled by slow waves

Slow waves propogate from oral to aboral end of the GI tract (higher frequency of depolarization of ICC at oral end)

Do not initiate contraction, only modulate.

Many different 5-HT receptor subtypes. 5-HT3 and 5-HT4 best characterized in GI function

5-HT-3 receptor: ligand gated ion channel -> fast inward current to activate IPAN

Activation leads to nerve induced contraction

GPCR: facilitate synaptic transmission in cholinergic enteric neurons

Activation indirectly augments peristaltic and secertomotor reflexes -> inc. ACh from cholinergic IPAN, interneuron, and motor neurons

Peristalsis - propulsion

Segmentation contractions - mixing

Both occur in addition to normal coordinated muscle contractions

Unidirectional propulsive motility

Propogating ring of muscle contraction at the oral side of bolus (ascending (orally) excitatory pathway

Relaxation of muscle below bolus (descending (aborally) inhibity pathways)

Mechanical/chemical stimulationn of the mucosa initiates reflex: EC responds to distension to release 5-HT and activate IPANs

IPANs release ACh onto two networks of interneurons: ascending and descending interneurons

Ascending: synaptically connected to excitatory motor neurons, release ACh and substance P onto smooth muscle cells leading to contraction

Descending: synaptically connected to inhibitory neurons, release VIP, nitric oxide, ATP onto smooth muscles leading to relaxation

Orally of the bolus: activation fo motor neurons -> contraction

Aborally of the bolus: inhibition of motor neurons -> muscle relaxation

Receiving: inhibition of circular muscles, contraction of longitudinal muscle

Propulsive: contraction of circular muscle, inhibition of longitudinal muscle

Mechanical or chemical stimuli

Stimuli from mucosa: activation of IPANs -> directly and indirectly activate secretomotor neurons -> ACh and VIP release onto effector cells

Congenital aganglionic megacolon:

- enlargement of the colon cause by bowel obstruction

- Aganglionic sections of a bowel: starts at anus and progresses upwards, reflex arc for coordination is no longer possible (contraction, sever constipation)

Surgical resection of anganglionic section of colon

Laxatives

High fiber diet

Function bowel disease

Characterized by abdominal pain, bloating, alteration of bowel habits (diarrhea (IBD-D), constipation (IBD-C) or alternating (IBD-A))

GI tract looks normal - dysfunction of 5-HT system

What are possible mechanisms of the pathology of IBS?