Exam 3 (Lecture 5) - Regulation of GI System Function 2

Question 1

Describe the structure and function of the PARASYMPATHETIC nervous system pre- and postganglionic neurons.

Answer 1

Preganglionic Myelinated Axons:
- Located inside the CNS (spinal cord)
- Long axons
- Cholinergic neurons
- Neurotransmitter = Ach
- Nicotinic receptor
- Synapse with ganglia or become integrated with fibers of the
ENS in target tissue

Postganglionic Unmyelinated Axons:
- Located outside the CNS (within the walls of target organ)
- smooth muscle and glands
- Short axons
- Cholinergic neurons
- Neurotransmitter = Ach
- Muscarinic receptor
- Synapse with ganglia of submucosal and myenteric nerve
plexuses, or interact directly with the same target tissues
- muscle, mucosal secretory and absorptive cells

Question 2

Describe the structure and function of the SYMPATHETIC nervous system pre- and postganglionic neurons.

Answer 2

Preganglionic Myelinated Axons:
- Located inside the CNS
- Short axons
- Cholinergic neurons
- Neurotransmitter = Ach
- Nicotinic receptor
- Peripheral nerves and proximal ganglia are the target tissues

Postganglionic Unmyelinated Axons:
- Located outside the CNS
- Long axons
- Adrenergic neurons
- Neurotransmitter = NE (norepinephrine)
- Alpha and Beta receptor
- Smooth muscle, cardiac muscle, and glands are target tissues

Question 3

Describe the ANS connection to the ENS.

Answer 3

Question 4

Describe how/what sensory information (afferent neurons) is relayed to the central nervous system.

Answer 4

Afferent sensory nerves of the GI tract are associated with mechanoreceptors and chemoreceptors.
- Send signal to brainstem via the vagus nerve (X)
- provides input to the CNS about changes in smooth muscle
tension and chemical conditions in the lumen
- parasympathetic input depends on sensory signals received
at the level of the brainstem

Afferent sensory axons from the gut connect with the CNS through the greater and lesser splanchnic nerves which synapse in the Celiac ganglion.
- Pass through the sympathetic trunk WITHOUT synapsing
- Convey signals arising from mechano and chemoreceptors
- Irritation to splanchnic afferent sensory axons may signal the
presence of pathological conditions
- overdistention of the gut wall
- stretching of mesenteric attachments
- inflammation
- noxious chemicals or substances in the gut lumen

Sensory axons are distributed into the serosal surfaces and mesentery of the gut, in addition to the muscular and mucosal layers.
- Convey conscious perception of pain to the CNS
- GDV (gastric dilation and volvulus) in dogs
- Equine colic
- painful stimuli from the gut causes a sympathetic motor
response, inhibiting gut motility and many glandular
secretions
- Afferent signals allow the CNS to function in a coordinated
manner with the ENS in the regulation of gut function

Question 5

Describe how EEC’s regulate GI system function.

Answer 5

EECs act as sensory cells with specialized microvilli that project into the lumen that sense the gut contents (nutrients and/or bacteria byproducts)

  - Respond to luminal stimuli by releasing hormones into the 
     lamina propria (and eventually into the bloodstream)
  - Mature intestinal endocrine cells can express several hormones 
  - Change in diet affects the release of GI hormones and alters
     the densities of GI endocrine cells
  - Interaction between ingested foodstuffs and the EECs can be 
    utilized for the clinical management of GI and metabolic 
    diseases
       - irritable bowel syndrome
       - obesity
       - type II diabetes

Afferent and Efferent nerve synaptic transmission

Endocrine signaling:
Produce hormones —> ISF —> blood capillary —> travel to site of action

Paracrine signaling:
Produce regulatory molecules that exert a local effect in the tissue
- secreted into the interstitial fluid
- travel by diffusion to nearby target cells

Great complexity underlying the roles of gut-derived hormones in both normal physiology and disease progression due to endocrine and paracrine signaling.

Question 6

Describe how the enteric immune system regulates GI system function.

Answer 6

• Monitor the antigenic environment of the gut
• Mucosal surface is exposed to numerous amounts of microorganisms and other antigens
• Majority of the immune system cells in the body reside in the gut mucosa
• Many are responsible for keeping these in check and prevent access systemically
• Cells span the entire range of immune cell types

Inflammatory mediators from immune cells:
- prostaglandins
- histamine
- cytokines
- Interact directly with the ENS and GI endocrine/paracrine cells to
modulate gut activities
- If a microbe begins to invade an area of the gut, sensitized
immune cells will secrete inflammatory mediators
- immune mediators can interact directly with cells of the ENS
(stimulate the myoenteric cells and submucosal cells) and the
GI endocrine system
- elicits a response of increased fluid secretion and motility
in the stimulated area of gut

Question 7

GI function is under highly integrated and complex control. What are the various inputs? Where does integration occur (name the 3 main intrinsic systems)? What are the various outputs? What is the overall effect?

Answer 7

Inputs:
- CNS influence
- Physical environment (lumen pressure/wall distension)
- Chemical environment (pH, osmolarity, nutrient concentration)
- Antigenic environment (microbes, other antigenic proteins)

Integration:
- Enteric Nervous System
- Enteric Endocrine Paracrine System
- Enteric Immune System

Outputs:
- GI smooth muscle motility (myenteric plexus)
- Vascular smooth muscle motility (in tunica media)
- Glandular secretion
- Absorptive cell function

Overall Effect:
- Balance of inhibitory and excitatory neurohumoral regulatory
molecules determines the activity of muscles, glands, and blood
vessels