What are the 5 GI tract functions?
- Secretion
- Digestion
- Absorption
- Motility
- Storage
How does the GI tract coordinate its five functions?
- Intrinsic Control System
- Extrinsic Control System
What is the Intrinsic Control System?
- Located in the wall of the GI tract
- Enteric nervous system, GI hormones (i.e. CCK, gastrin, etc.)
- All the neurons in the CNS, somatic nervous system, automatic nerous system have afferent, sensory, interneurons, or motor neurons, which is in the enteric nervous system.
- GI hormones present such as CCK or gastrin take part are produced in the gut in enterendocrine cells that are apart of the intrinsic control even if they act on distant tissues. (i.e. CCK acts on the hypothalamus even though it is locally produced in the gut)
What is the Extrinsic Control System?
- Located outside of the gut
- Incudes the vagus nerve for parasympathetic innervation, Splanchnic nerve for sympathetic innervation, and non-GI hormones (i.e. calcitriol)
The GI tract has to coordinate its five functions via 2 control systems, select the true statement.
a. Intrinsic control system contains vagus and splanchnic nerves
b. CCK and gastrin can act on distant tissues even though they are locally produced, therefore apart of the extrinsic control system
c. Calcitriol is a non-GI hormone apart of the intrinsic control system
d. The enteric nervous system, which has afferent, interneuorns, and motor neurons, as well as GI hormones are apart of the intrinsic control system.
The GI tract has to coordinate its five functions via 2 control systems, select the true statement.
a. EXTRINSIC control system contains vagus and splanchnic nerves
b. CCK and gastrin can act on distant tissues even though they are locally produced, therefore apart of the INTRINSIC control system
c. Calcitriol is a non-GI hormone apart of the EXTRINSIC control system
d. The enteric nervous system, which has afferent, interneuorns, and motor neurons, as well as GI hormones are apart of the intrinsic control system.
True or False. The GI tract has it’s own nervous system.
True - the enteric nervous system
Enteric Nerous System
- A component of the autonomic nervous system.
- Located within the GI wall and enables autonomic regulation of GI functions
- Plexus submucosus (Meissner’s)
- Plexus myentericus (Auerbach’s)
Plexus Submucosus (Meissner’s)
- Located between submucosa and circular musculature
- REgulates mucus secretion and food absorption
Plexus myentericus (Auerbach’s)
- Located in the muscularis externa, between longitubdinal and circular musculature
- Control of the muscular activity (tone and contractions rhythm)
Explain the arrangement of the enteric nervous system.
(fibers, smooth muscle cells)
- Smooth muscle cells are arranged in bundles of as many as 1000 parallel fibers (longitudinal or circular arranged)
- Within each bundle, muscle fibers are electrically connected with one other through gap junctions
- Each bundle of smooth muscle fibers is partially separated from the next by loose connective tissue, but the muscle bundles fust with one another at many points and work as syncytium.
Enteric neurons secrete their neurotransmitters from ________ located on often lengthy axonal collaterals or branches —> an enteric neuron affects a ________ compared to the typical CNS neuron.
a. varicosities, narrow area
b. thrombophlebitis, wider area
c. varicosities, wider area
d. thrombophlebitis, narrow area
c. varicosities, wider area
* Enteric neurons secrete their neurotransmitters from varicosities located on often lengthy axonal collaterals or branches —> an enteric neuron affects a wider area compared to the typical CNS neuron.
On the basis of function, what three cell types does the enteric nervous system have?
- Sensory nerve cells
- Interneurons
- Motor neurons
What statement is false regarding sensory nerve cells?
a. Detect changes/stimuli usually inside the lumen
b. Regulate function
c. Their axons go to other enteric nerve cells or to the CNS
d. There are 3 types of sensory cells
D –> False
* Sensory nerve cells detect changes/stimuli (usually inside the lumen) and regulate function. Their axons go to other enteric nerve cells or to the CNS
* There are 2 types of Sensory cells - mechanosensitive cells (mechanorecptors) and chemosensitive cells (chemoreceptors)
____________ cells recognize stretching by bolus/chyme of intestinal wall or volume changes (intraluminal pressure) and induce a response (like a gastric phase of gastric acid secretion).
a. Sensory nerve cells
b. Mechanosensitive cells
c. Chemosensitive cells
d. Interneurons
B - Mechanosensitive cells (mechanoreceptors) recognize stretching by bolus/chyme of intestinal wall or volume changes (intraluminal pressure) and induce a response (like a gastric phase of gastric acid secretion).
____________ cells detect the presence of nutrients in the GI lumen, chemical changes in the osmolarity, and changes in the pH and induce a response (sensibilization of enteroendocrine cells)
b. Mechanosensitive cells
c. Chemosensitive cells
d. Interneurons
C - Chemosensitive cells (chemoreceptors) detect the presense of nutrients in the GI lumen, chemical changes in teh osmolarity, and changes in the pH and induce a response (sensibilization of enteroendocrine cells)
____________ process signals coming frrom other nerve cells or from the CNS and propagate them to other neurons (motor neurons).
a. Vaso motor neurons
b. Interneurons
c. Sensory neurons
d. Secreto motor neurons
B - Interneurons process signals coming frrom other nerve cells (sensory nerve cells) or from the CNS and propagate them to other neurons (normally motor neurons).
* act as a bridge between sensory & motor neurons
True or False. Motor neurons induce change in the GI tract in response to a stimulus.
True - Motor neurons induce change in the GI tract in response to a stimulus.
* 3 types: muscle motor neurons, secreto motor neurons, and vaso motor neurons
Of the following motor neurons, which is true?
a. Muscle motor neurons regulate the rhythm, contraction, and tone.
b. Secreto motor neurons are located in both plexuses
c. Secreto motor neurons regulate the tone of blood vessels by contraction or dialation
d. Vaso motor neurons are located in plexus myentericus
TRUE - A - Muscle motor neurons regulate the rhythm, contraction, and tone and are located in plexus myentericus.
b. Secreto motor neurons are located in submucosus
c. Secreto motor neurons are involved with stretching and increase the intraluminal pressure
d. Vaso motor neurons are located in both plexuses and regulate the tone of blood vessels by contraction or dialation
How do enteric neurons communicate?
- Using neurotransmitters
- GI physio identified more than 25 NT substances that are released by the nerve endings of enteric neurons
- Some of these are “classical” or “conventional” transmitters (norepinephrine, acetylcholine - same as in ANS), whereas others are classified as “non-noadreergic, non-cholinergic” (NANC) substances
Is the enteric nervous system part of the autonomic nervous system?
- Yes, it is a kind of autonomic nervous system but it is not apart of the classical autonomic nervous system, meaning its not sympathetic and not parasympathetic but still uses norepinephrine and Ach
The specific functions of many NANC substances are not known well, what are the 4 best examples?
- Nitric oxide (NO) - inhibitory substances that reduces the motility of the GI tract
- ** VIP (vasoactive intestinal peptide)** - inhibitory substances, causes relaxation of smooth muscle spincters and possibly stimultory to secretory glands
- Substance P - excitatory substances, increases secretion and motility of the GI tract
- ATP - extracellular signal molecule acting on purinergic receptors, activates inhibitory mortor neurons inducing relaxation of the smooth muscle in the gut
- Intracellular ATP - enzymatic rxns, can be used
- Extracellular ATP - signaling molecule
What is the interaction with the CNS/ENS –> extrinsic innervation?
Why do we need the CNS/extrinsic control system?
- The CNS is like the muscle computer, it does many things but is not able to manage every thing by itself, it gets informed about some stimulus pain
- i.e. sensory neurons take it to the brain for a response. If there is stretching, very increased inctraluminal pressure, we will sense that & will induce a response in the ENS, maybe paralysis in that part of the gut.
- i.e. if there is big stretching, a high pressure in one part of the gut, one part may not move any more
How the CNS is informed on what happens in the GI tract by the outer innervation, extrinstic innervation is a very important connection. What are the 3 outer innervation comprises?
- SNS –> Norepinephrine
- PNSN –> Acetylcholine
- Visceral afferents
