Enteric Nervous System part 2
Learning Objectives:
Identify the enteric nervous system and its role in the control of gastrointestinal function
Discuss what makes the ENS unique
And what is similar to other parts of the nervous system
Discuss the elements of the enteric neural circuitry and how they can be identified
this applies to all the ENS segments
Motility Control Circuits
Intestinal Motility: After a meal, involves several processes and depends on both the enteric nervous system and intrinsic mechanisms within the smooth meal:
Segmentation: Mixing through local constrictions alternating with relaxations.
Peristalsis: Propulsion involving strong contractions that move contents towards the anus, preceded by a relaxation phase.
Retropulsion: Similar to peristalsis but moves content in the oral direction, predominantly in the duodenum and proximal colon.
Receptive Relaxation: Relaxation of smooth muscle to prepare for content arrival, significant in the stomach and intestines.
Coordinated contractions produce mixing, propulsion and waste disposal
Since activity can be blocked by tetrodotoxin, it depends on enteric neural activity
Fasting State:
Migrating Motor Complex (MMC): Occurs 4-6 hours post-meal, involved in clearing debris from the small intestine.
Still uses the same neurons, muscles and pathways but the pattern of activity is different
MMC clears debris and bacteria from small intestinal lumen
Occurs sequentially with a time delay
Repeats every 90 – 120 min
Abolished by feeding
At least 3 phases that appear to propagate from either gastric antrum or the proximal duodenum to the ileo-caecal junction
Phase 1: Quiescence for about an hour.
Phase 2: Irregular contractions build.
Phase 3: Strong, rhythmic contractions lasting 10-15 minutes.
Summary of Key Terms
Segmentation: Local constrictions alternating with relaxations.
Peristalsis: Strong contractions that propagate anally along the intestine preceded by a relaxation.
Retropulsion: Peristalsis-like contractions that propagate orally.
Receptive relaxation: Relaxation of smooth muscles to prepare a segment for arrival of content.
Phase 1 migrating motor complex: Quiescence lasting about an hour at any one site.
Phase 2 migrating motor complex: Irregular contractions.
Phase 3 migrating motor complex: Strong, rhythmic contractions lasting 10-15 minutes.
Basic Circuit of Motility Control
Simple Reflex Circuits: Studied in guinea pig small intestine, involving:
Sensory Neurons: Detect stimuli leading to excitation of local motor neurons.
Interneurons: Ascending and descending interneurons facilitate coordinated contraction and inhibition pathways, enabling effective motility control (these are immunoreactive to different compounds)
Pathway Polarization: Ascending excitation and descending inhibition allow regulation of motility propagation, compliant with the Law of the Intestine.
For the diagram below:
Intrinsic sensory neurons (ISNs) are immunoreactive for calbindin and contain acetylcholine and substance P
Excitatory motor neurons have acetylcholine and substance P.
Inhibitory motor neurons contain nitric oxide synthase (NOS).
Ascending interneurons are immunoreactive for calretinin
Descending interneurons are immunoreactive for NOS, somatostatin or 5-HT, the latter two are also cholinergic
Ascending excitation and descending inhibition:
Activate intrinsic sensory neurons at one site, simultaneous excitation of local excitatory motor neurons, local inhibitory motor neurons, ascending interneurons and descending interneurons
Excitatory motor neurons project orally, inhibitory motor neurons project anally
Pathways are polarized via motor neuron and interneuron projections
Activity follows the Law of the Intestine
But normal physiological stimuli are distributed and how does one get anal propagation of contractions from a polarized circuit
Question is: how do you get from this polarised circuit to motor constrictions that actually occur
Modelling shows that segmentation and anal propagation can come from polarized circuit
Testing the model predictions to confirm theories
Immunofluorescence studies show that descending interneurons do make synapses with ascending interneurons (and excitatory motor neurons)
Thus, circuit components exist to produce anal propagation of excitation
Model predicts that, when the stimulus is distributed along the preparation, a local disruption in the circuit will produce stationary contractions on either side of the disruption
Local disruptions in the circuit do produce increased stationary contractions in nutrient infused preparations of jejunum
What causes physiological disruptions of the circuit has not been determined
Summary:
The basic neural circuit involved in Bayliss and Starling’s law of intestine is initiated by activation of intrinsic sensory neurons (ISNs) at one site. Activation of ISNs can stimulate local interneurons and motor neurons whose projections are polarised. Excitatory motor neurons project orally to mediate contractions above the stimulation. Inhibitory motor neurons project anally to mediate relaxations below the stimulation. Motor neurons alone do not allow for propagation of the motor patterns. Interneurons extend the range of this polarisation by contacting other interneurons. Ascending interneurons project orally. At least three classes of descending interneurons have been identified and they all project anally. Some descending interneurons are connected to ascending interneurons and excitatory motor neurons and this allows anally directed constriction to follow relaxation, hence allowing propagation of motor patterns.
Enteric Sensory Transduction
How does what’s going on in the lumen change enteric circuit activity when ISN terminals do not penetrate the mucosal epithelium?
Role of Enterochromaffin Cells:
Key to sensing luminal content changes despite terminal ISN (Intrinsic Sensory Neurons) not penetrating mucosal epithelium.
Produces Serotonin (5-HT); 90% of body’s serotonin is synthesized in these cells.
Factors Altering Gut Function:
Gut function depends on the luminal content
The ongoing motor activity in any region of the gut moves the content around and responds to it to change the motor activity
Key factors altering gut behaviour:
Chemical Composition: High nutrients favour segmentation over propulsion, as they go slower to take up more nutrients
Volume: Large volumes trigger propulsive contractile patterns and receptive relaxation ahead of that activity, increasing speed but reducing segmentation thus the uptake of nutrients
Mechanical Properties: Viscosity and texture influence movement speeds; rapid for low viscosity, slow and stimulating for high viscosity. low viscosity content impacts mucosal movement and stimulation.
There must be chemoreceptive ISNs and mechanoreceptive ISNs
Summary
The ability to sense the state of the gut environment is important for the ENS to coordinate gut function. Intrinsic sensory neurons must have chemosensitive and mechanosensitive properties to detect the chemical composition, volume and the viscosity and texture of the contents. High nutrient contents favour segmentation and slow transit that assist in nutrient absorption. Large volume of contents triggers propulsion and receptive relaxation that move the contents along. High viscosity contents can deform the mucosa and slows transit.
Communication with Nervous System:
High volume signals get across the intestine as they distend the elastic elements of the intestinal wall, thereby stretching and producing mechanosensitive effects in neurons that lie within the myenteric plexus
The chemical stimuli, which is critical for the digestive process, can't cross the gut wall very easily as the mucosal epithelium forms barrier for diffusion, so it must be transported across
The mucosal epithelium contains many types of endocrine cells, which may act as sense cells
About 1% of the enterocytes are specialised endocrine (enteroendocrine, EE) cells
> 10 different types
Many have apical membrane exposed to the lumen
Basal membrane exposed to lamina propria so can interact with chemicals in the intestinal lumen, therefore polarised in the same way epithelial cells are
Derived from same stem cells as enterocytes
Significant proportion of these are enterochromaffin (EC) cells
Synthesise, store and release 5-hydroxytryptamine (serotonin, 5-HT)
>90% of body’s 5-HT synthesised in EC cells
This means it doesn't cross the blood brain barrier so it doesn't play a major role in mood changes
Circulates as a hormone, BUT does this seratonin have an active neural function within the enteric nervous system itself
Direct signalling by enteroendocrine cells:
Cells have neuropods that contact sensory neuron terminals, allowing for modulation of gut activity in response to luminal content.
Both 5-HT-EC cells and PYY-EE cells have neuropods
Neuropods contact terminals of sensory neurons (afferents)
Can get input from efferent enteric or extrinsic neurons
Neuropods have proteins involved in transmitter release
Also receptors for efferent transmitters
Summary
While mechanical stimuli can be sensed by mechanosensitive neurons via distension and stretch of the gut wall, chemical stimuli cannot be sensed by neurons quite easily because the epithelium is a barrier to diffusion and neuronal projections do not penetrate out into the lumen. Enteroendocrine cells (EECs) are specialised enterocytes that may act as sensory mediators between the gut lumen and the nervous systems. They have an apical membrane that is in direct contact with the lumen and a basal membrane that is exposed to the lamina propria where neuronal fibres are located. EECs can signal directly to the nervous system via neuropods which connect with afferent nerve fibres. EECs also have receptors that detect transmitters from efferent nerve fibres
Can 5-HT release from EC cells activate enteric circuits?
EC cells:
express taste receptors for sweet, bitter and potentially umami
mechanosensitive
Olfactory receptors
Blocking 5-HT uptake with an SSRI increases segmenting motor activity in the jejunum
Blocking 5-HT receptors in the mucosa reduces nutrient induced segmenting motor activity
Releasing 5-HT from mucosa with cholera toxin rapidly increases propulsive motor activity
Amino acids can excite local reflexes via release of 5-HT and/or ATP
Mucosal 5-HT is a sensory mediator:
Analogous with Merkel cells and taste receptor cells
But mucosal 5-HT is also a circulating hormone
Are hormones contained in EE cells sensory mediators within the enteric circuits?
Seems likely for CCK and PYY
Perhaps not for other hormones
Summary
Over 90% of the body’s 5-HT is released from enterochromaffin (EC) cells. These cells are activated by chemical and mechanical stimuli as they express taste receptors for sweet, bitter and umami and mechanoreceptors. EC cells also have neuropods that synaptically connect with afferent and efferent neurons. Blocking 5-HT receptors, 5-HT release and reuptake impacts gut motor activities. There is direct evidence that application of amino acids onto the mucosa can trigger activation of local reflexes via the release of 5-HT and/or ATP as neurotransmitters. Mucosal 5-HT is also a circulating hormone that have various roles around the body, for example in bone formation. However, circulating 5-HT cannot cross the blood-brain-barrier. Other enteroendocrine cells, such as CCK and PYY cells, also likely acts as sensory mediators in a similar way to EC cells.
Neural Control of Secretion
Neurogenic secretion across the mucosa
Secretion of water and salt is essential for digestion
Digestive enzymes are either water soluble or bound to brush border membranes of duodenal and jejunal enterocytes
Nutrients are water soluble, with exception of lipids
Both digestion of macronutrients and the turbulent flow that brings nutrient molecules to the absorptive epithelium depend on their being adequate amounts of water in the lumen
The volume of water transported across the mucosa per day is equivalent to the whole blood volume of an animal
Homeostasis depends on both secretion and absorption of water
Secretion Mechanisms:
Secretion is electrogenic (the act of movement of material across the mucosa leads to an electric current) and controlled by neurons
Cl- ions are transported into lumen from enterocytes in mucosal crypts via CFTR channels in the apical membrane
Water and Na+ ions follow passively
Process depends on water from blood stream so is enhanced by vasodilatation of mucosal and submucosal blood vessels
2 types of secretomotor neurons in submucosal plexus:
Cholinergic Neurons: Release ACh to increase Cl- gradient.
release ACh onto M3 muscarinic receptors that are located in the basolateral membrane of the epithelial cells expressing CFTR
Increase Cl- concentration gradient across the apical membrane where the CFTR is operating, increasing the responsiveness of the other class of secretomotor neuron
Non-cholinergic Neurons: Release vasoactive intestinal peptide (VIP) activating pathways to enhance secretion
activates adenylyl cyclase
Increasing Cyclic AMP which opens CFTR channels
Modulated by activity in by sympathetic nerves
Both types of secretomotor neurons are regulated by enteric circuits
Summary
Secretion occurs at the crypts of the epithelium. Cholinergic secretomotor neurons release acetylcholine that are detected by muscarinic M3 receptors located on the basolateral membrane of epithelial cells expressing CFTR. This increases Cl- concentration gradient across the apical membrane where the CFTR channel is located and also the responsiveness of these enterocytes to non-cholinergic secretomotor neurons that release vasoactive intestinal peptide (VIP). VIP activates VIP receptors located on basolateral surface of the enterocytes, leading to activation of adenylyl cyclase which increases cyclic AMP that opens the CFTR channels. Cl- are transported into the lumen via CFTR channels. Water and Na+ passively follows into the lumen, leading to secretion. Both cholinergic and non-cholinergic secretomotor neurons are regulated by enteric circuits while non-cholinergic secretomotor neurons can also be modulated by sympathetic nerve activities.
Absorption
Electroneutral processes occurring without direct neural control (constitutively active) primarily via Na+ transport.
Na+ is transported from lumen into enterocytes at tips of villi (SI) and mucosal surface of colon
Transport is via the NHE3 sodium hydrogen exchanger in the apical membrane
It drags chloride and water to the other side for the CFTR channel
Newly differentiated enterocytes express CFTR, older enterocytes express NHE3
Pathology – Diarrhea
Caused by imbalance in secretion and absorption, often exacerbated in response to infections (e.g., cholera toxin).
Occurs when secretion of water into the lumen exceeds absorption of water from the lumen (these processes occur in different sections)
Can be due to increased secretion in the duodenum and jejunum
Increased transit speed through the ileum and colon
Less absorption
Damage to mucosal epithelium leading to breakdown of mucosal barrier both to the movement of water and content
Different regions contribute different components to the overall outcome
Summary
The functionality of enterocytes changes as they migrate from the base of the crypts to the tips of the villi. Newly differentiated enterocytes at the base of the crypts express more CFTR channels. During the migration process to the tips of the villi, CFTR expression is gradually lost and the NHE3 transporter becomes more expressed. Na+ is transported by the NHE3 antiporter into the lumen in exchange for one H+ ion in the apical membrane. This drag Cl- and water in the lumen back into the body. Therefore, absorption is an electroneutral process that occurs at the tips of the villi. It is also constitutively active as it runs at the same rate at all times. Therefore, absorption is linked to transit speeds. Slow transit allows increased absorption, and this is one mechanism underlying constipation. Fast transit decreases absorption and this can lead to diarrhoea.
Cholera is the classic example
Due to Vibrio cholerae which releases a protein exotoxin – cholera toxin
Cholera toxin acts via neurons in duodenum and jejunum
By activating neural pathways that are involved in sensory transduction and the overal functionality
Effects of CT in vivo are seen in small intestine (not the large intestine)
Hypersecretion that swamps absorption in ileum and colon (without changes in motor function)
Does not have substantial effect on secretion in the colon
Effects in upper small intestine are blocked by local anaesthetics and neurotoxins
i.e activity in neural circuits is required
Acts by via a direct activation of adenylyl cyclase
Releases 5-HT from EC cells
Leads to increased propulsive contractions as well as hypersecretion
Cholera Toxin Mechanism: Activates adenylyl cyclase, leading to increased fluid secretion and propulsive contractions, thus impairing absorption.
Conclusion and Integration
Consideration of interactions between motility and secretion systems essential for understanding the gastrointestinal tract's overall contribution to maintaining homeostasis.
5-HT is not the only mediator released by CT
There are 5-HT antagonist resistant effects on both neurons and secretion
Yet to be identified
Need prolonged exposure to CT to produce hypersecretion, so plastic changes in affected neurons are relatively slow
Need to consider interactions between secretion and motility to get full grasp of contribution of GI tract to whole body homeostasis
Summary:
Diarrhoea is a characteristic feature of cholera infection that is caused by the release of cholera toxin (CT). CT modifies enteric neural activity by increasing the excitability of several classes of enteric neurons, most notably intrinsic sensory neurons (ISNs). Application of CT onto the mucosa for 90 minutes increases action potentials fired by ISNs in response to a stimulus. These ISNs form recurrent networks that produce positive feedback in the circuit. These effects are blocked by tetrodotoxin, indicating that the hypersecretion is neurogenic in nature. In some cases, the effects of CT also involve 5-HT released from enterochromaffin cells through activation of adenylyl cyclase. The increase in 5-HT leads to increased propulsive contractions that reinforces the effects of diarrhoea. 5-HT3 antagonists may block the effects of CT on ISNs in some cases, but there are 5-HT3 antagonist resistant mechanisms that are yet to be identified.
