BIOL 2420 Lecture Notes - Lecture 5: Vasoactive Intestinal Peptide, Enteric Nervous System, Muscular Layer
26 Apr 2018
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Unit 9 – Lecture 5
Regulation of GI Function
- motility and secretion are the primary regulated functions
o food moves too rapidly = not enough time for everything in the lumen to be digested and
absorbed
o secretion is regulated so that the appropriate digestive enzymes can break down food into
an absorbable form
o digestion depends on motility and secretion
- some nutrient absorption can be altered in response to long-term environmental changes
The Enteric Nervous System Can Act Independently
- enteric nervous system (ENS)
o first recognized more than 1 century ago
o isolated sections of intestine removed from the body created a reflex wave of peristaltic
contraction when pressure in the lumen increased
o ENS can carry out a reflex independent of control by the CNS
▪ ENS is much like the nerve networks of jellyfish and sea anemones
▪ Carried out using the brain, eyes, or a nose
o Human ENS receives stimuli and acts on them
▪ Controls motility, secretion, and growth of the digestive tract
- Anatomically and functionally the ENS shares features with the CNS
1. Intrinsic neurons
o Of the two nerve plexuses of the digestive tract – those neurons that lie completely within
the wall of the gut
o Just like interneurons are contained completely in the CNS
o Extrinsic neurons - Autonomic neurons bringing signals from the CNS to the digestive
system
2. Neurotransmitters and neuromodulators
o ENS neurons release more than 30 neurotransmitters and neuromodulators
▪ Identical to the molecules found in the brain
▪ Nonadrenergic, noncholinergic – distinguishes them from traditional autonomic
neurotransmitters norepinephrine and acetylcholine
o Best known GI neurotransmitters and neuromodulators – serotonin, vasoactive intestinal
peptide, nitric oxide
3. Glial support cells
o Glial cells of neurons in the ENS are more similar to astroglia of the brain than to
Schwann cells of PNS
4. Diffusion barrier
o Capillaries surrounding ganglia in the ENS are not very permeable and create diffusion
barrier that is similar to the BBB of cerebral blood vessels
5. Integrating center
o Reflexes initiated in the GI tract can be integrated and acted on without neural signals
leaving the ENS
o Neuron network of the ENS is its own integrating center – like the brain and spinal cord
- If we can explain how the ENS integrates simple behaviours, we could use the system as a model
for CNS function
o Studying ENS function is difficult because enteric reflexes have no discrete command
center
find more resources at oneclass.com
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o Instead, in an interesting twist
▪ GI physiologists are applying information gleaned from studies of the brain and
spinal cord to investigate ENS function
• Complex interactions between the enteric and central nervous systems,
the endocrine system, and the immune system promise to provide
scientists with questions to investigate for many years to come
Short Reflexes Integrate in the Enteric Nervous System
- Enteric nerve plexuses in the gut wall act as a little brain
o Allows local reflexes to begin, be integrated, and end completely in the GI tract
- Short reflexes - Reflexes that originate within the enteric nervous system and are integrated there
without outside input
- Submucosal plexus
o Contains sensory neurons that receive signals from the lumen of the gut
o Controls secretion by GI epithelial cells
- ENS network integrates sensory information, then initiates responses
- Myenteric plexus neurons in the muscularis externa influence motility
Long Reflexes Integrate in the CNS
- ENS
o can work in isolation
o sends sensory information to the CNS
o receives input from the CNS through autonomic neurons
- classic neural reflex begins with a stimulus transmitted along a sensory neuron to the CNS, where
the stimulus is integrated and acted on
o in the digestive system, some classic reflexes originate with sensory receptors in the GI
tract, but others originate outside the digestive system
o no matter where they originate, digestive reflexes integrated in the CNS are called long
reflexes
- long reflexes that originate outside the digestive system
o feedforward reflexes
▪ begin with stimuli (sight, smell, sound, thought of food)
▪ prepare digestive system for food that the brain is anticipating
▪ example: if you’re hungry and smell dinner cooking, your mouth waters and
stomach growls
o emotional reflexes
▪ demonstrate a link between the brain and the digestive system
▪ examples:
• traveler’s constipation
• butterflies in your stomach
• psychologically induced vomiting and diarrhea
o together these are called cephalic reflexes because they originate in the brain
o smooth muscle and glands of the GI tract are under autonomic control
▪ parasympathetic division is excitatory and enhances GI functions -> “rest and
digest”
• most parasympathetic neurons to the GI tract are found in the vagus
nerve
▪ sympathetic neurons usually inhibit GI function
GI Peptides Include Hormones, Neuropeptides, and Cytokines
- peptides secreted by cells of digestive tract can act as
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
If we can explain how the ens integrates simple behaviours, we could use the system as a model for cns function: studying ens function is difficult because enteric reflexes have no discrete command center. Short reflexes integrate in the enteric nervous system. Enteric nerve plexuses in the gut wall act as a little brain: allows local reflexes to begin, be integrated, and end completely in the gi tract. Short reflexes - reflexes that originate within the enteric nervous system and are integrated there without outside input. Submucosal plexus: contains sensory neurons that receive signals from the lumen of the gut, controls secretion by gi epithelial cells. Ens network integrates sensory information, then initiates responses. Myenteric plexus neurons in the muscularis externa influence motility. Gi peptides include hormones, neuropeptides, and cytokines peptides secreted by cells of digestive tract can act as: hormones, paracrine signals.
