In-Depth Notes on the Enteric Nervous System
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
The Enteric Nervous System Introduction
The ENS is the most complex nervous system outside the brain
The ENS can function independently of the brain, facilitating complex behaviours.
Contains most neurotransmitters and receptors found in the nervous system, enabling distant studies of their functions within intact circuits.
Definition and Structure of the ENS
The ENS is a network of neurons located in the wall of the gastrointestinal tract, making it the largest part of the autonomic nervous system.
Consists of more neuron cell bodies than the spinal cord
Includes axons and terminals of visceral sensory neurons, sympathetic neurons and parasympathetic neurons that innervate the GI tract
Innervates all layers of GI tract including smooth muscle, mucosal epithelium and endocrine cells
Comprising two main plexuses:
Myenteric Plexus (Auerbach's): Located between muscle layers, facilitates GI motility.
Submucosal Plexus (Meissner's): Primarily involved in regulating digestive secretions and blood flow.
Basic Properties of the ENS
Extends from the mouth to anus (over 7 meters).
Contains ganglia with clusters of neuronal bodies involved in both sensory (visceral afferents) and motor control throughout the GI tract.
The submucosal plexus starts ganglionated at the pylorus and myenteric at the upper esophageal sphincter.
Basic Structure of ENS and Wall of GI Tract:
The Enteric Nervous System (ENS) is formed within the wall of the gastrointestinal tract and is comprised of a complex network of neurons organized into distinct plexuses. Key components include:
Myenteric Plexus (Auerbach's Plexus):
Located between the circular and longitudinal muscle layers.
Primarily facilitates gastrointestinal motility by regulating the contraction and relaxation of smooth muscle layers.
The main neural supply for the gut
Has much larger ganglia (100-200 neurons)
Circular Muscle:
Forms the inner layer of muscle in the gastrointestinal wall, responsible for segmental contractions to mix and propel contents.
Longitudinal Muscle:
The outer layer of muscle that runs along the length of the GI tract, facilitating the shortening of segments for peristalsis.
Submucous Plexus (Meissner's Plexus):
Located in the submucosa, primarily regulating digestive secretions and blood flow within the GI tract.
Smaller ganglia
Muscularis Mucosae:
A thin layer of smooth muscle in the mucosal layer that aids in the local movement of the mucosa, enhancing absorption and secretion.
Mucosal Plexus:
Often considered part of the submucous plexus and involved in regulating the epithelial layer, impacting secretion and absorption. Innervates the mucosa.
Tertiary Plexus:
Represents additional network branches or connections that potentially integrate signals from the ENS to enhance functional outcomes but are less defined.
Deep Muscular Plexus and Circular Plexus:
Not as commonly referenced, but indicates a network associated with deeper muscle layers for finer control of motility functions and motor control of the smooth muscle.
Submucous Artery:
Supplies blood to the submucosal area, helping to nourish the plexuses (including the submucous plexus) and maintain intestinal health.
GANGLIA AND PLEXUS
Ganglia:
Clusters of neuronal cell bodies located outside the central nervous system.
Function as relay stations for transmitting nerve signals and integrating information from different parts of the nervous system.
In the context of the enteric nervous system, ganglia are associated with both sensory and motor functions throughout the gastrointestinal tract.
Not all plexuses are ganglionated, some are just nerve trunks that innervate different layers of the GI tract wall.
Myenteric ganglia are typically larger than submucosal ganglia.
Plexus:
A network or braid of nerve fibers, often involving complicated interconnections between nerve cells.
In the enteric nervous system, plexuses such as the myenteric and submucous plexuses coordinate various functions such as motility, secretions, and local reflexes in the GI tract.
Plexuses allow for localized control and modulation of gastrointestinal functions independent of the central nervous system.
The primary plexus consists of interganglionic connective nerve trunks that run between myenteric ganglia.
The secondary plexus is the nerve trunks that run into circular muscles.
The tertiary plexus supply the longitudinal muscles.
Submucosal plexus present in stomach, but not ganglionated.
Ganglionated submucosal plexus begins at pylorus
Ganglionated myenteric plexus begins at upper oesophageal sphincter
Several non-ganglionated plexuses
Innervate muscle, mucosa, blood vessels
Extrinsic Nerve Supply
Sympathetic Nerves: Modulate ENS function throughout the whole GI tract.
but have little direct effect on the muscle or mucosa
Parasympathetic Nerves: innervate upper and lower parts of the GI tract, affecting motility and secretion.
Come either by the vagus nerve (to innervate the oesophagus, stomach and duodenum) or the via the pelvic nerves (to innervate the colon, rectum and anus)
Extrinsic sensory neurons: Supply sensory neurons, with 90% being vagal afferents, playing crucial roles in reflex actions and sensory feedback.
Innervate the whole length of the GI tract
Extrinsic Primary Afferents
Extrinsic primary afferents that supply the GI tract includes:
Vagal afferents with cell bodies located in the nodose ganglia
Dorsal root afferents that run in splanchnic and pelvic nerves.
These afferents have distinct terminations in central processing centres because their functions are different and lead to different overall outcomes for their activity
Large supply to upper GI tract in vagus nerve
90% of vagal axons that go to the gut are primary afferents
Dorsal root ganglion cell (spinal) afferents run in splanchnic and pelvic nerves
Often thought to be nociceptive
Functions and central terminations of vagal afferents and dorsal root afferents are distinctively different
which is important since they have distinctly different overall outcomes
Functions of the ENS
Controls movement of intestinal content
Regulation of water and electrolyte transport across the mucosa:
Movement of water and salt into the body from intestinal lumen – absorption
Movement of water and salt from body into lumen – secretion
Whole body homeostasis
Contributes to control of acid secretion in stomach, mucus secretion along length and bicarbonate secretion in duodenum
Types of Enteric Neurons
Neuromuscular Contractions - Intestinal movement experiments
In vitro studies show contractions are not solely brain-dependent; can occur even when the nervous system is chemically inhibited.
Contractions only seen if there are nutrients in the lumen
Contractions blocked by antagonists of nicotinic or muscarinic acetylcholine receptors
Both receptors are required for normal functioning of the system
Contractions abolished by tetrodotoxin, which blocks nerve action potentials, but not those of smooth muscle
Contractions of intestinal smooth muscle do not depend on the brain or spinal cord
Contractions can be confined to a local region or sequentially cover long segments
Some contractions apparently move along the segment – propagate
Jejunum contains entire circuit for generation of a complex behaviour
Circuit involves two types of cholinergic neurons
Types of Enteric Neurons
Intrinsic Sensory Neurons: Responsible for detecting physical distension and chemical signals in the lumen.
Sensitive to distension
Sensitive to mucosal deformation
Sensitive to luminal nutrients
Motor Neurons: Can be excitatory (project orally) or inhibitory (project anally) affecting smooth muscle activity.
To circular muscle
To longitudinal muscle
Interneurons: Connect excitatory and inhibitory motor neurons, playing a role in coordinating muscle contractions.
Running orally, ascending
Running anally, descending
Secretomotor neurons: regulate the secretion of digestive enzymes and fluids in the gastrointestinal tract, stimulating glands to produce necessary substances for digestion
Vasodilator neurons: modulate blood flow within the GI tract by inducing vasodilation
Intestinofugal neurons: send signals from the intestine back to the central nervous system, facilitating reflexes and feedback mechanisms that help regulate gastrointestinal motility and secretion based on the state of the gut
Interplexus interneurons: provide connections and communication between different plexuses within the enteric nervous system, integrating information and coordinating responses for smooth muscle control and secretion in the GI tract.
Identifying Enteric Neurons
Functionally distinct enteric neurons are intermingled
Each ganglion may contain 20+ different neuron subtypes
Top down approach:
Define the properties specific neurons must have to produce an identified behaviour (starting point)
E.g., a neuron that has terminals in the muscle is likely to be a motor neuron
The Law of the Intestine - to explain propulsion via peristalsis
Physiological stimulus-induced contractions in the intestine occur with ascending excitation above and descending inhibition below the stimulus point.
A “physiological” stimulus applied to the intestinal wall will cause a contraction above the stimulus and a relaxation below.
Ascending excitation and descending inhibition
If a neuron sends an axon orally, then it is likely to be in an excitatory pathway, if its axon has terminals in the muscle then it may be an excitatory motor neuron
If a neuron sends an axon anally (aborally), then it is likely to be in an inhibitory pathway and if it has terminals in the muscle it may be an inhibitory motor neuron
This principle helps identify specific neuronal types and their behavioural contributions in GI motility.
The Role of Neurotransmitters
Acetylcholine (ACh) and Substance P (SP) are key excitatory neurotransmitters.
Nitric oxide plays a crucial role in mediating inhibition in smooth muscle cells.
From the video and other studies
Acetylcholine (ACh) and substance P (SP) each excite the intestinal smooth muscle
Blocking muscarinic ACh receptors reduces ascending excitation leaving a component that is blocked by antagonists of SP receptors
Specifically, the NK1 tachykinin receptor (Tacr1)
Thus, excitatory motor neurons are probably orally projecting neurons with terminals in the muscle and the ability to release ACh and SP
Thus we can probably identify these
Inhibition in the muscle appears to be mediated by nitric oxide (NO) and a purine released by the same nerve terminals
NOS synthesises NO and some NOS neurons have terminals in the muscle
Thus, some NOS neurons are probably inhibitory motor neurons
Other evidence
Studies of axonal projections using lesions and histological stains (eg immunofluorescence)
Intracellular injection of markers that diffuse along axons and dendrites (left)
Can be combined with intracellular recording and immunofluorescence
Transcriptomics
Immunofluorescence identifies neurochemical codes of 2-3 antigens for many neuron subtypes, but there are more subtypes than can be accounted for by the coding available
Isolation of individual neurons (or their nuclei and associated ribosomes) allows transcriptomes (ie expression of mRNAs) of neuron subtypes to be identified, but different results have been gathered so has limitations.
Assigning function still depends on projections and physiology of the individual neuron subtypes
Need to combine data types to identify the neurons
SUMMARY
Intestinal movements are observed in vitro when GI segments are completely isolated from an organism, indicating that the contractions do not depend on the brain or the spinal cord. As these contractions are only seen if nutrient is present, there must be intrinsic sensory components that detect nutrients in the lumen and motor components that cause the contractions. The observed contractions are either confined to a local region or sequentially cover long segments, so there must be different information processes that generates the different motor patterns. We can examine the circuit that generates these complex behaviours using pharmacological methods. The addition of tetrodotoxin, which blocks nerve action potentials, abolishes the contractile activities, therefore intestinal contractions must be neurally mediated. Nicotinic and muscarinic receptor antagonists also block the contractions. From this, we can deduce that there are
cholinergic neurons that contact neurons with nicotinic receptors and cholinergic motor neurons that activate muscarinic receptors found on smooth muscles.
Immunohistochemistry is a technique that can label for specific neurochemical markers in the ENS, allowing us to differentiate between different neurochemical subtypes that correlate with certain functional enteric neuronal classes. Each ganglion may contain more than twenty different neuronal subtypes which are intermingled. Some markers also label varicosities on nerve fibre trunks that run into the circular muscles and these may be motor neurons
According to the Law of the Intestine by Starling and Bayliss (1899), propulsion occurs by ascending excitation that leads to contractions above the stimulus and
descending inhibition that leads to relaxation below the stimulus. Therefore, a neuron that projects orally is likely to be in an excitatory pathway and a neuron that projects anally is likely to be in an inhibitory pathway. Neurons with terminals in the muscles are likely to be motor neurons while neurons with terminals within the plexus and around other ganglia are likely to be interneurons. Enteric neurons are assigned function based on their shape, projections and physiology, not by their neurochemistry or mRNA expression profiles. Neurochemical and transcriptomic methods are used to correlate with the different functional classes of enteric neurons
Identifying intrinsic sensory neurons
Properties to be expected of an Intrinsic Sensory Neuron
Start by working out the properties should be in order to perform the physiological functions we are interested in:
Axon terminals or neurites in or adjacent to sites of physiologically relevant stimulation
Need these terminals to be in or close tomucosa, because luminal nutrients initiate motor activity
Need a detection system on the neuron that allows sensory transduction
Output terminals contacting other neurons, otherwise the sensory neuron can't communicate with the pathway
Respond to physiological stimuli like nutrients (to sense nutrients, you must have neurons that respond to them)
Without an intervening synapse
Express mechanosensitive ion channels as they will respond directly to mechanical stimulation
Distension is a powerful stimulus of motor activity
But sensory modalities may not be as exclusive as in the somatic sensory system and there might be neurons that respond to both mechanical and chemical stimulation
The following are expected properties of intrinsic sensory neurons (ISNs):
ISNs should be primary responders to physiological stimuli.
ISNs should have multiple axons that span across large areas of the gut wall.
Chemosensitive properties to allow ISNs to detect luminal nutrients.
Summary
The following events are in the correct historical sequence:
Some enteric neurons were found to have axons that project to the mucosa and also supply other myenteric ganglia (Dogiel type II morphology). These are deduced to be intrinsic sensory neurons because they may be able to detect physiologically relevant stimulus and relay the signals to the rest of the circuitry.
Some enteric neurons showed prominent after-hyperpolarising potentials (AHPs) and these neurons did not seem to have synaptic inputs. These are deduced to be intrinsic sensory neurons because they are activated by physiological stimuli and not by other neurons through synaptic transmission.
Neurons that show AHPs have Dogiel type II morphology.
Most of these AH/Dogiel type II neurons are immunoreactive to a calcium binding protein called calbindin. Calbindin terminals are present in the mucosa and in the ganglia, consistent with projection patterns proposed by Dogiel.
The general consensus is that AH/Dogiel type II neurons are functionally sensory neurons. However, some scientists still believe AH/Dogiel type II neurons are not primary sensory neurons because they receive synaptic input despite the fact that there are other primary sensory neurons elsewhere in the body that also receive synaptic input. In recent years, a group of neurons, which are not AH/Dogiel type II neurons, have been identified to be mechanosensitive and receive extensive synaptic inputs. This raises the possibility that many neuronal subtypes are multifunctional and cannot be strictly classified into a single functional class.
Current Scientific Debates
There are ongoing debates regarding whether certain neuron types serve primary sensory roles due to their synaptic inputs, similar to established primary sensory neuron types across different systems.
Understanding the multifaceted roles of enteric neurons continues to evolve, as research reveals complex interactions between sensory feedback and the network of neuron circuits in the ENS.
What does this synaptic input do in relation to the sensory functionalities?
ISN axons connect to other ISNs and excite them
Implies they must exist in a recurrent excitatory feedback loop (positive feedback)
Computational modelling shows that AHPs in the neurons provide inhibition that keeps this network (positive feedback loops) stable
Intera
ction of slow EPSPs and AHPs needed for ISN networks to encode ongoing sensory information
ISNs can act as interneurons under some circumstances, as can mechanosensitive neurons
ISN network is one of the pattern generators that regulates mixing and some propulsive activity
This allows us to work out how enteric nervous circuits work