Control Of Gi Systems Function

Front 1

Describe the location, divisions, extensiveness and the unique functions of the enteric nervous system.

Back 1

*Myenteric Plexus: between longitudinal and circular muscle layers
*Submucosal Plexus: In submucosa between circular muscle layer & muscularis mucosae

Front 2

Describe the major functions controlled by the enteric nervous system and the major neurotransmitters by which these controls are mediated.

Back 2

**Controls contractile, endocrine and secretory functions of GI tract
*AcH: Gq receptor, Ca2+ messenger, contracts smooth muscle and relaxes sphincters, stimulates GI secretion
*VIP: Gs, CAMP, relaxes SM, Stim. secretion
*NO: NO receptor, cGMP, relaxes SM
*Neurotransmitters released from central nerve terminals of the IPAN are substance P
and calcitonin-gene related peptide (CGRP)
*INTERNEURONS (responsible for integration) are Serotonin and AcH (act of nicotinic receptors)

Front 3

Describe the anatomical and functional similarities and differences between intrinsic and extrinsic primary afferent neurons.

Back 3

*INTRINSIC primary afferent neurons: cell body, sensory terminals, and central terminals are entirely within the gut wall (i.e., the whole sensory neuron is within the ENS)
*EXTRINSIC primary afferent neurons: sensory terminals are in gut wall, cell body is in a ganglion outside the gut wall and central terminals are within the CNS. However, the fiber coming from the sensory terminals sometimes also branches within the gut wall
to directly synapse with ENS interneurons.

Front 4

Describe the kinds of stimuli sensed by gastrointestinal receptors.

Back 4

*Mechanoreceptors – esp. stretch
*Presence and types of nutrients -- Peptides and amino acids -- Monoglycerides and fatty acids -- Glucose (6-carbon sugars)
*pH (acidity)
*Osmolarity

Front 5

Explain direct vs. indirect stimulation of primary afferent neurons.

Back 5

**Directly: sensory mechanisms in the peripheral nerve
terminals - sense changes in their environment.
**Indirectly: sensory mechanisms in enterochromaffin
cells in mucosa which release 5-hydoxytryptamine (5-
HT, serotonin) serotonergic receptors in the
peripheral terminals of the IPAN.

Front 6

Describe the similarities and differences in enteric vs. extrinsic neural reflex control of gastrointestinal functions.

Back 6

*EXTRINSIC Reflex pathway involves CNS, esp. spinal cord and medulla -- Afferent (sensory) arm: primarily
extrinsic primary afferent neurons -- Efferent (motor) arm: Both branches of ANS
*Extrinsic Cell bodies are in ganglia outside the GI tract -- Reach the CNS via nerves that also carry PS and S efferents
*Like IPANS, VAGAL afferents can be stimulated directly and indirectly via 5-HT
*Vago-vagal reflexes most important for Extrinsic neural reflex control

Front 7

Describe the anatomical pathways by which the sympathetic and parasympathetic nervous systems control gastrointestinal functions

Back 7

*Involves both PS and S NSs
*Majority of autonomic innervation synapses with neurons of the ENS (enteric)
*Majority of autonomic control of GI function is by modulation of the activity of the ENS

Front 8

Influence of the parasympathetic nervous systems on gastrointestinal functions

Back 8

***PS: Innervates most of ENS through the vagus (X) nerves; innervates the distal portion of colon through pelvic (sacral) nerves. -- Pre-ganglionic neurons enter gut wall - synapses with neurons of the ENS – acts via release of Ach -- Post-ganglionic neurons are not distinctive – often part of the ENS -- Does not directly control GI functions - exerts control though modulating the output of the ENS
***In general, activation of the PNS: Stimulates secretion and Stimulates GI muscle contraction
***MOST IMPORTANT influence on GI motility and response to food ingestion (compared with sympathetic)

Front 9

Give the meanings of the terms: enteric reflex, vago-vagal reflex, long reflex.

Back 9

*Vago-vagal reflexes are reflexes in which both afferent and efferent limbs are contained in the vagus nerve
*Enteric reflex
*Long reflexes involve a sensory neuron sending information to the brain, which integrates the signal and then sends messages to the digestive system

Front 10

Site of secretion, stimuli for secretion, and the major action of gastrin

Back 10

SITE: G cells of stomach in pyloric glands of antrum
STIMULI: thoughts, sight, smell and presence of small peptides and AAs, distention of stomach (vagal stimulation, GRP)
MAJOR ACTION: Incr gastric H+ secretion (via histamine secretion stimulating parietal cells in oxyntic gland to secrete HCl); stimulates growth of gastric mucosa
*hormone whose secretion can be stimulated by
neural mechanisms.

Front 11

Describe how endocrine (hormonal) and paracrine control of gastrointestinal functions is different and explain why histamine and somatostatin are considered to be paracrines.

Back 11

**H is a paracrine whose secretion can be inhibited by
the paracrine action of S.
**Also, SOMATOSTATIN:
-Secreted by D cells located in several places
in the stomach and intestine – stimuli are varied and often unknown.
-General effect is to inhibit the activity of neighboring cells.
-Exerts its effect by binding to somatostatin receptors in neighboring cells

Front 12

Give site(s) of secretion, stimuli for secretion, and the major actions of histamine

Back 12

****IN STOMACH:
*SITE: Secreted by ECL cells in oxyntic glands.
*Target tissue: neighboring parietal cells --> increase HCl secretion
*Binds to histamine receptor (type: H2)
*Secretion is stimulated by gastrin and by Ach through neural reflexes
*****IN SI:
*Secreted by mast cells in lamina propria – in
response to antigenic stimulation
*Target tissue: adjacent epithelial cells --> incr Cl secretion into lumen --> incr H2O secretion
*Binds to histamine receptor (type: H2)

Front 13

Influence of the parasympathetic nervous systems on gastrointestinal functions

Back 13

***S: Post-ganglionic neuron enters the gut wall – exerts its action via release of norepinephrine that acts on α-adrenergic receptors.
***Either: synapses with neurons of the ENS … or … directly innervates effector tissue (especially arteriolar smooth muscle to control blood flow)
***In general, activation of the SNS: Inhibits secretion, Inhibits GI muscle contraction
**Mostly important in fight or flight responses

Front 14

Site of secretion, stimuli for secretion, and the major action of secretin

Back 14

SITE: S cells of Duodenum
STIMULI: H+ (acidity <4.5) and Fatty Acids in duodenum
MAJOR ACTION: Incr. pancreatic and biliary bicarb secretion to NEUTRALIZE CHYME, decr gastric H+ secretion, inhibits trophic effect of gastrin on gastric mucosa
*Major target is epithelial cells in pancreatic ducts (also hepatic ducts and duodenum)

Front 15

Site of secretion, stimuli for secretion, and the major action of cholecystokinin (CCK)

Back 15

SITE: I cells in epithelial layer of duodenum and jejunum
STIMULI: small peptides, AAs, fatty acids
MAJOR ACTION: Incr. pancreatic bicarb and enzyme secretion (via pancreatic acinar cells), stimulates contraction of gallbladder and relaxation of sphincter of iddi, stim growth of exocrine pancreas and gallbladder, inhibits gastric emptying
*Also activates vago-vagal reflexes

Front 16

Give site(s) of secretion, stimuli for secretion, and the major actions of somatostatin

Back 16

Action on pyloric gland:
*Secreted by D cells in antrum of stomach -- Stimulated by low pH of gastric contents -- Target tissue: G cells to inhibits gastrin secretion
Action on Oxyntic gland:
*Secreted by D cells in oxyntic glands of stomach -- target tissues are parietal cells to inhibit HCl secretion AND ECL cells to inhibit histamine secretion