Enteric neuroimmunophysiology and pathophysiology1,2 Jackie D. Wood  Gastroenterology  Volume 127, Issue 2, Pages 635-657 (August 2004) DOI: 10.1053/j.gastro.2004.02.017

Figure 1 The heuristic model for the ENS is the same as for the brain and spinal cord. Sensory neurons, interneurons, and motor neurons are interconnected synaptically to form the integrated microcircuits of the ENS. Similar to the central nervous system, sensory neurons, interneurons, and motor neurons are connected synaptically for flow of information from sensory neurons to interneuronal integrative networks to motor neurons to effector systems. The ENS organizes and coordinates the activity of each effector system into meaningful behavior of the whole organ. Mast cells behave similar to sensory neurons to warn the ENS of the appearance of threatening agents in the intestinal lumen. The ENS responds by running a program of intestinal behavior designed to eliminate the threat from the lumen. Defensive behavior of hypersecretion and powerful propulsive motility is accompanied by symptoms of diarrhea and cramping abdominal pain. Brain signals to the ENS via mast cells might underlie stress-evoked symptoms of diarrhea and abdominal pain. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 6 Intestinal crypts of Lieberkühn are innervated by secretomotor neurons in the submucosal plexus. Neurotransmitters (e.g., acetylcholine and vasoactive intestinal peptide) that evoke secretion are released at the junctions with the crypts when secretomotor neurons fire. Axon collaterals to blood vessels simultaneously dilate submucosal vessels to increase blood flow in support of stimulated secretion. Input from the sympathetic nervous system suppresses firing of secretomotor neurons and thereby inhibits secretion. Signal substances released from inflammatory/immune cells act at receptors on the cell body to increase excitability of secretomotor neurons and at presynaptic inhibitory receptors to suppress release of norepinephrine from sympathetic nerves and somatostatin from intrinsic neurons. Stimulation of secretomotor neurons by paracrine signals is expected to evoke secretion from mucosal crypts and can account for neurogenic secretory diarrhea. Presynaptic inhibition of norepinephrine and somatostatin release facilitates secretion by removing the braking action of sympathetic and intrinsic nerves and this also may contribute to diarrheal symptoms. A secretomotor neuron in the guinea-pig submucosal plexus was marked by injection of the marker biocytin from a microelectrode during an electrophysiologic study (inset). Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 2 Immune/inflammatory cells, in close proximity to elements of the ENS, release mediators that become paracrine signals to the ENS. (A) Mucosal biopsy specimen from an inflamed colon finds ganglia of the submucosal plexus (circles) surrounded by inflammatory cells. (B) Electron micrograph of the inflamed mucosa in A shows a plasma cell, polymorphonuclear leukocyte, and mast cell as the main constituents of the inflammatory/immune cell population surrounding the submucosal plexus. (C) Alcian blue stain of mast cells in rodent small intestinal mucosa before infection with T. spiralis. (D) Mastocytosis in rodent small intestinal mucosa after infection with T. spiralis. (E) Exposure of a ganglion in guinea-pig myenteric plexus to fluorescently labeled histamine. Histamine was bound to receptors expressed by 5 neurons in the ganglion. (F) Exposure to fluorescently labeled anti-Hu antibody labeled all neuronal cell bodies in the ganglion. (G) Digital merger of the images in E and F. Micrographs in C and D kindly provided by G. A. Castro of the University of Texas Medical Center, Houston, TX. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 3 Cellular neurophysiology of enteric AH neurons. (A) Activation of slow synaptic input by electrical stimulation of an axon in an interganglionic fiber tract in the myenteric plexus of guinea pig. The stimulus-evoked slow EPSP is characterized by slowly activating depolarization of the membrane potential and enhanced excitability reflected by high-frequency discharge of action potentials that lasted for ∼30 seconds after termination of stimulation. (B) Exposure of the AH neuron to a micropressure puff of histamine evoked a slow EPSP-like response. (C) Exposure of the AH neuron to a micropressure puff of 5-HT evoked a slow EPSP-like response. The slow EPSP-like response was preceded by a rapidly activating depolarizing response mediated by the 5-HT3-receptor subtype. (D1) Action potential in an AH neuron in its low excitability state is followed by characteristic long-lasting after-hyperpolarization. Downward deflections are electrotonic potentials evoked by repetitive intraneuronal injection of hyperpolarizing current pulses. Decreased amplitude of the electrotonic potentials during the AH reflect decreased input resistance owing to opening of Ca2+-gated K+ channels. (D2) Action potential in an AH neuron recorded with a fast time base shows characteristic shoulder on the decreasing phase of the spike. Shoulders appear on the spikes when AH neurons are in states of decreased excitability; they reflect the opening of voltage-activated Ca2+ channels. (D3) Superimposed action potentials in an AH neuron in its state of hyperexcitability do not have a shoulder on the falling phase. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 7 Gating function of the cell bodies of AH-type interneurons direct excitatory traffic in the ENS microcircuits. (A) Most AH neurons in the myenteric plexus project one of their neurites to the mucosa. 5-HT released from mucosal enterochromaffin cells acts at 5-HT3 receptors on the terminals in the mucosa to fire action potentials in the terminal. Bidirectional conduction (arrows) occurs in the mucosal projection with propagation away from the cell body in the direction of mucosa and propagation from the terminal toward the submucosal plexus and the cell body in the myenteric plexus. The level of prevailing excitability in the cell body determines how inbound information from the mucosal terminals is handled. Firing of the cell body by slow excitatory input occludes inbound spikes from the mucosal terminals. (B) Cell body of an AH myenteric neuron in an intermediate state of excitability fires a single action potential during injection of a depolarizing current pulse. (C) An overlay of histamine moves excitability of the same neuron to a hyperactive state that is reflected by repetitive spike discharge to the same depolarizing current pulse. (D) Cell body of an AH myenteric neuron in an inexcitable state. Intraneuronal injection of depolarizing current evoked no action potential discharge. (E) Model for an AH neuron shows how the somal gate for transfer of spike information across the cell body is closed when the neuron is in the inexcitable state. Slow excitatory inputs of either synaptic or paracrine origin increase the excitability of the somal membrane and open the gate for transfer of inbound spike information to other neurites distributed around the perimeter of the cell body. (F) Stimulation of a slow EPSP in the same neuron as D moves excitability to a hyperactive state that is reflected by repetitive spike discharge to the same depolarizing current pulse. (G) Inbound spikes from a neurite invading the cell body of an AH-type interneuron. The spikes arrive in bursts (arrows). Only the first spike of a burst fires the soma owing to fractionation of the inbound spikes by postspike hyperpolarization of the membrane potential. (H) One of the inbound spike bursts from G, recorded with an expanded time base. (I) The first spike of the burst in H recorded with an expanded time base shows how the invading spike from the neurite (arrow) triggers the somal spike by depolarizing the somal membrane potential to spike threshold. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 5 AH-type interneurons are important functional elements in ENS integrative microcircuits. (A) Driver circuits are positive feed-forward synaptic circuits formed by AH neurons. Neurons in the circuit make recurrent excitatory synaptic connections with one another, which results in positive feedback flow of synaptic excitation that leads to rapid build-up of firing within the population of driver neurons. Slow synaptic excitation accounts for escalation of excitation in the individual neurons and prolonged firing in the circuit, the output of which simultaneously activates pools of motor neurons. Presynaptic inhibitory receptors at the synapses in the driver circuit maintain a braking action that prevents runaway excitation in the circuit. (B) Morphology of an AH neuron revealed by intraneuronal injection from a microelectrode of the marker substance biocytin. Extensive ramification of the cell’s neurites is seen within the ganglion and in interganglionic fiber tracts (arrow) leading out of the ganglion. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 4 Histamine is a mast cell mediator that signals the ENS. (A) An overlay of histamine on the microcircuits of the ENS in the guinea-pig colon evokes repetitive cycles of mucosal secretion and contraction of the circular musculature. The full-thickness preparation of intestinal wall was placed in an Ussing chamber with a small strain gauge attached to the serosal surface. The upper trace records mucosal secretion; the lower trace records muscle contraction. (B) The blockade of presynaptic inhibitory histamine H3 receptors enhances the amplitude of histamine-evoked secretory cycles. (C) Exposure to the sensitizing antigen (β-lactoglobulin) in milk-sensitized guinea pigs evokes cyclic secretion. (D) Recurrent bursts of action potentials in an AH neuron during continuous exposure to histamine. (A) Data from Cooke et al.13 Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)

Figure 8 Immunoneural communication in the ENS of intestinal preparations from animals sensitized either to a food allergen or an infectious parasite. (A) Effects of β-lactoglobulin on an enteric neuron in the small intestine of a milk-sensitized guinea pig include slowly activating membrane depolarization associated with the discharge of multiple nerve impulses. Sharp upward deflections are action potentials (i.e., nerve impulses). Downward deflections are electrotonic potentials evoked by repetitive intraneuronal injection of constant-current hyperpolarizing pulses. Decreased amplitude of the downward deflections reflects a decrease in the input resistance of the neuron. (B) Neuron in a preparation from an animal that had not been fed milk did not respond to β-lactoglobulin. (C) Exposure to T. spiralis larval somatic antigen enhanced excitability in a myenteric AH-type interneuron from a guinea pig previously infected with the parasite. Intraneuronal injection of a depolarizing current pulse evoked 2 spikes at the onset of the pulse before application of the antigen (control). Exposure to T. spiralis antigen resulted in a dramatic increase in neuronal excitability reflected by repetitive spike discharge throughout the same depolarizing current pulse (T. spiralis). Addition of the histamine H2-receptor antagonist, cimetidine, in the presence of the antigen returned membrane excitability to the control level. (D) Antigen exposure in milk-sensitized preparation of guinea-pig intestine releases mediators that act presynaptically to suppress nicotinic fast EPSPs in the ENS. Exposure to β-lactoglobulin reduced the amplitude of the EPSP relative to control. Washing the antigen from the tissue chamber reversed the effect. (E) Antigen exposure in milk-sensitized preparations of guinea-pig intestinal submucosal plexus releases mediators that act presynaptically to suppress noradrenergic slow IPSPs in secretomotor neurons. Exposure to β-lactoglobulin reduced the amplitude of the IPSP relative to control. Washing the antigen from the tissue chamber reversed the effect. Gastroenterology 2004 127, 635-657DOI: (10.1053/j.gastro.2004.02.017)