Alterations in network robustness upon simultaneous temperature and pH perturbations.

Nervous systems have evolved to function consistently in the face of the normal environmental fluctuations experienced by animals. The stomatogastric nervous system (STNS) of the crab, Cancer borealis, produces a motor output that has been studied for its remarkable robustness in response to single global perturbations. Changes in environments, however, are often complex and multifactorial. Therefore, we studied the robustness of the pyloric network in the stomatogastric ganglion (STG) in response to simultaneous perturbations of temperature and pH. We compared the effects of elevated temperatures on the pyloric rhythm at control, acid, or base pHs. In each pH recordings were made at 11°C, and then the temperature was increased until the rhythms became disorganized ("crashed"). Pyloric burst frequencies and phase relationships showed minor differences between pH groups until reaching close to the crash temperatures. However, the temperatures at which the rhythms were disrupted were lower in the two extreme pH conditions. This indicates that one environmental stress can make an animal less resilient to a second stressor.NEW & NOTEWORTHY Resilience to environmental fluctuations is important for all animals. It is common that animals encounter multiple stressful events at the same time, the cumulative impacts of which are largely unknown. This study examines the effects of temperature and pH on the nervous system of crabs that live in the fluctuating environments of the Northern Atlantic Ocean. The ranges of tolerance to one perturbation, temperature, are reduced under the influence of a second, pH.

Switching neuron contributions to second network activity.

Network flexibility is important for adaptable behaviors. This includes neuronal switching, where neurons alter their network participation, including changing from single- to dual-network activity. Understanding the implications of neuronal switching requires determining how a switching neuron interacts with each of its networks. Here, we tested 1) whether "home" and second networks, operating via divergent rhythm generation mechanisms, regulate a switching neuron and 2) if a switching neuron, recruited via modulation of intrinsic properties, contributes to rhythm or pattern generation in a new network. Small, well-characterized feeding-related networks (pyloric, ∼1 Hz; gastric mill, ∼0.1 Hz) and identified modulatory inputs make the isolated crab (Cancer borealis) stomatogastric nervous system (STNS) a useful model to study neuronal switching. In particular, the neuropeptide Gly1-SIFamide switches the lateral posterior gastric (LPG) neuron (2 copies) from pyloric-only to dual-frequency pyloric/gastric mill (fast/slow) activity via modulation of LPG-intrinsic properties. Using current injections to manipulate neuronal activity, we found that gastric mill, but not pyloric, network neurons regulated the intrinsically generated LPG slow bursting. Conversely, selective elimination of LPG from both networks using photoinactivation revealed that LPG regulated gastric mill neuron-firing frequencies but was not necessary for gastric mill rhythm generation or coordination. However, LPG alone was sufficient to produce a distinct pattern of network coordination. Thus, modulated intrinsic properties underlying dual-network participation may constrain which networks can regulate switching neuron activity. Furthermore, recruitment via intrinsic properties may occur in modulatory states where it is important for the switching neuron to actively contribute to network output.NEW & NOTEWORTHY We used small, well-characterized networks to investigate interactions between rhythmic networks and neurons that switch their network participation. For a neuron switching into dual-network activity, only the second network regulated its activity in that network. In addition, the switching neuron was sufficient but not necessary to coordinate second network neurons and regulated their activity levels. Thus, regulation of switching neurons may be selective, and a switching neuron is not necessarily simply a follower in additional networks.

Mfge8 is expressed by pericytes in gastric antrum submucosa from patients with obesity.

The main function of the stomach is to digest ingested food. Gastric antrum muscular contractions mix ingested food with digestive enzymes and stomach acid and propel the chyme through the pyloric sphincter at a rate in which the small intestine can process the chyme for optimal nutrient absorption. Mfge8 binding to α8ß1 integrins helps regulate gastric emptying by reducing the force of antral smooth muscle contractions. The source of Mfge8 within gastric muscles is unclear. Since Mfge8 is a secreted protein, Mfge8 could be delivered via the circulation, or be locally secreted by cells within the muscle layers. In this study, we identify a source of Mfge8 within human gastric antrum muscles using spatial transcriptomic analysis. We show that Mfge8 is expressed in subpopulations of Mef2c+ perivascular cells within the submucosa layer of the gastric antrum. Mef2c is expressed in subpopulations of NG2+ and PDGFRB+ pericytes. Mfge8 is expressed in NG2+/Mef2c+ pericytes, but not in NG2+/Mef2c-, PDGFRB+/Mef2c-, or PDGFRB+/Mef2c+ pericytes. Mfge8 is absent from CD34+ endothelial cells but is expressed in a small population of perivascular ACTA2+ cells. We also show that α8 integrin is not expressed by interstitial cells of Cajal (ICC), supporting the findings that Mfge8 attenuates gastric antrum smooth muscle contractions by binding to α8ß1 integrins on enteric smooth muscle cells. These findings suggest a novel, supplementary mechanism of regulation of gastric antrum motility by cellular regulators of capillary blood flow, in addition to the regulation of gastric antrum motility by the enteric nervous system and the SMC, ICC, and PDGFRα+ cell (SIP) syncytium.

Multiple intrinsic membrane properties are modulated in a switch from single- to dual-network activity.

Neural network flexibility includes changes in neuronal participation between networks, such as the switching of neurons between single- and dual-network activity. We previously identified a neuron that is recruited to burst in time with an additional network via modulation of its intrinsic membrane properties, instead of being recruited synaptically into the second network. However, the modulated intrinsic properties were not determined. Here, we use small networks in the Jonah crab (Cancer borealis) stomatogastric nervous system (STNS) to examine modulation of intrinsic properties underlying neuropeptide (Gly1-SIFamide)-elicited neuronal switching. The lateral posterior gastric neuron (LPG) switches from exclusive participation in the fast pyloric (∼1 Hz) network, due to electrical coupling, to dual-network activity that includes periodic escapes from the fast rhythm via intrinsically generated oscillations at the slower gastric mill network frequency (∼0.1 Hz). We isolated LPG from both networks by pharmacology and hyperpolarizing current injection. Gly1-SIFamide increased LPG intrinsic excitability and rebound from inhibition and decreased spike frequency adaptation, which can all contribute to intrinsic bursting. Using ion substitution and channel blockers, we found that a hyperpolarization-activated current, a persistent sodium current, and calcium or calcium-related current(s) appear to be primary contributors to Gly1-SIFamide-elicited LPG intrinsic bursting. However, this intrinsic bursting was more sensitive to blocking currents when LPG received rhythmic electrical coupling input from the fast network than in the isolated condition. Overall, a switch from single- to dual-network activity can involve modulation of multiple intrinsic properties, while synaptic input from a second network can shape the contributions of these properties.NEW & NOTEWORTHY Neuropeptide-elicited intrinsic bursting was recently determined to switch a neuron from single- to dual-network participation. Here we identified multiple intrinsic properties modulated in the dual-network state and candidate ion channels underlying the intrinsic bursting. Bursting at the second network frequency was more sensitive to blocking currents in the dual-network state than when neurons were synaptically isolated from their home network. Thus, synaptic input can shape the contributions of modulated intrinsic properties underlying dual-network activity.

Comprehensive characterization of antral and pyloric contractions by high resolution manometry: applied physiology in suspected gastroparesis.

Delayed gastric emptying may result from diverse pathophysiological mechanisms including antral hypomotility and pylorospasm. With increasing use of gastric peroral endoscopic myotomy and preliminary evidence of efficacy, our aim was to assess the motor functions of the distal antrum and pylorus in patients with symptoms of gastroparesis using high-resolution antropyloroduodenal manometry (HR-ADM). Sixteen patients with symptoms suggestive of gastroparesis underwent HR-ADM with 13 sensors, 1 cm apart, placed across the antropyloroduodenal (APD) junction and 2 sensors, 10 cm apart, in descending and distal duodenum. The 1-h postprandial motility was quantitated as contraction frequency/minute, average amplitude, and motility index (MI). Six healthy volunteers served as controls. In the patient group, the HR-ADM identified postprandial antral hypomotility, isolated pyloric pressure waves, and tonic elevation of baseline pressure in pylorus. Patients had significantly reduced frequency of the full-hour postprandial antral contractions/minute compared with healthy volunteers [1.52 (0.97, 1.67) vs. 2.04 (1.70, 2.67), P = 0.005], as well as reduced MI [9.65 (8.29, 10.31) vs. 11.04 (10.65, 11.63), P = 0.002]. The average contraction amplitude was numerically, but not significantly reduced [51.9 (21.9, 74.9) vs. 73.0 (59.8, 82.7), P = 0.14]. Bland-Altman plots showed similar distribution of antral contraction frequency and MI during the first and second postprandial 30-min periods for both patients and controls. High-resolution ADM can characterize a variety of postprandial antral contractile and pyloric motility dysfunctions. This technique shows promise to provide guidance for the selection of optimal treatment of patients with gastroparesis.NEW & NOTEWORTHY Current selection of different treatments for patients with gastroparesis is empiric or based on trial and error, though pyloric distensibility and diameter may predict response to pyloric interventions. High-resolution antropyloroduodenal manometry (HR-ADM) can characterize a variety of postprandial antral contractile and pyloric motility dysfunctions in patients with suspected gastroparesis. HR-ADM shows promise to provide guidance for selection and individualization of treatments such as prokinetic agents or pyloric interventions for patients with gastroparesis based on documented pathophysiology.

Luminal Chemoreceptors and Intrinsic Nerves: Key Modulators of Digestive Motor Function.

This chapter reviews data on the pathways by which luminal, mainly duodenal, chemoreceptors modulate gastro-pyloro-duodenal motor function to control emptying of nutrients into the small intestine. The vagus mediates proximal gastric relaxation caused by nutrient stimulation of duodenal/jejunal mucosal chemoreceptors. Modulation of the spatial patterning and inhibition of antral contractions during duodenal chemoreceptor activation are somewhat conflicting: both vagal control and ascending intramural nerves appear to play a role. Intraduodenal nutrients stimulate the localized pyloric contractions that prevent transpyloric flow via ascending duodenal intramural nerve pathways. Though not yet formally investigated, patterns of activation of the duodenal brake motor mechanism suggest that duodenal loop mucosal chemoreceptors signal to a brake mechanism at the most aborad region of the duodenum via descending intramural duodenal nerves.Intrinsic intramural pathways are important in the control of the first stages of digestion.

Evolution of nitric oxide regulation of gut function.

Although morphologies are diverse, the common pattern in bilaterians is for passage of food in the gut to be controlled by nerves and endodermally derived neuron-like cells. In vertebrates, nitric oxide (NO) derived from enteric nerves controls relaxation of the pyloric sphincter. Here, we show that in the larvae of sea urchins, there are endoderm-derived neuronal nitric oxide synthase (nNOS)-positive cells expressing pan-neural marker, Synaptotagmin-B (SynB), in sphincters and that NO regulates the relaxation of the pyloric sphincter. Our results indicate that NO-dependent pylorus regulation is a shared feature within the deuterostomes, and we speculate that it was a characteristic of stem deuterostomes.

Pyloric distensibility in health and disease.

Until recently, gastric motility measurements in humans were mostly limited to accommodation (using barostat or 3-dimensional imaging studies of gastric volume) and gastric emptying tests, the latter being the only one performed in routine clinical care. Accurate and easy to use techniques were lacking to assess pyloric function in health and disease. Recently, pyloric distensibility has been developed and validated to assess pyloric opening. Several studies confirmed that pyloric distensibility was decreased in gastroparesis and correlated with gastric emptying as well as gastroparesis symptoms. In addition, pyloric distensibility may predict outcome of endoscopic techniques targeting the pylorus, namely intrapyloric botulinum toxin injection and gastric per-oral pyloromyotomy. Pyloric distensibility appears therefore to be a promising and useful new tool in the workup of gastroparesis patients.

Rapid adaptation to elevated extracellular potassium in the pyloric circuit of the crab, Cancer borealis.

Elevated potassium concentration ([K+]) is often used to alter excitability in neurons and networks by shifting the potassium equilibrium potential (EK) and, consequently, the resting membrane potential. We studied the effects of increased extracellular [K+] on the well-described pyloric circuit of the crab Cancer borealis. A 2.5-fold increase in extracellular [K+] (2.5×[K+]) depolarized pyloric dilator (PD) neurons and resulted in short-term loss of their normal bursting activity. This period of silence was followed within 5-10 min by the recovery of spiking and/or bursting activity during continued superfusion of 2.5×[K+] saline. In contrast, when PD neurons were pharmacologically isolated from pyloric presynaptic inputs, they exhibited no transient loss of spiking activity in 2.5×[K+], suggesting the presence of an acute inhibitory effect mediated by circuit interactions. Action potential threshold in PD neurons hyperpolarized during an hour-long exposure to 2.5×[K+] concurrent with the recovery of spiking and/or bursting activity. Thus the initial loss of activity appears to be mediated by synaptic interactions within the network, but the secondary adaptation depends on changes in the intrinsic excitability of the pacemaker neurons. The complex sequence of events in the responses of pyloric neurons to elevated [K+] demonstrates that electrophysiological recordings are necessary to determine both the transient and longer term effects of even modest alterations of K+ concentrations on neuronal activity.NEW & NOTEWORTHY Solutions with elevated extracellular potassium are commonly used as a depolarizing stimulus. We studied the effects of high potassium concentration ([K+]) on the pyloric circuit of the crab stomatogastric ganglion. A 2.5-fold increase in extracellular [K+] caused a transient loss of activity that was not due to depolarization block, followed by a rapid increase in excitability and recovery of spiking within minutes. This suggests that changing extracellular potassium can have complex and nonstationary effects on neuronal circuits.

Graded Transmission without Action Potentials Sustains Rhythmic Activity in Some But Not All Modulators That Activate the Same Current.

Neurons in the central pattern-generating circuits in the crustacean stomatogastric ganglion (STG) release neurotransmitter both as a graded function of presynaptic membrane potential that persists in TTX and in response to action potentials. In the STG of the male crab Cancer borealis, the modulators oxotremorine, C. borealis tachykinin-related peptide Ia (CabTRP1a), red pigment concentrating hormone (RPCH), proctolin, TNRNFLRFamide, and crustacean cardioactive peptide (CCAP) produce and sustain robust pyloric rhythms by activating the same modulatory current (IMI), albeit on different subsets of pyloric network targets. The muscarinic agonist oxotremorine, and the peptides CabTRP1a and RPCH elicited rhythmic triphasic intracellular alternating fluctuations of activity in the presence of TTX. Intracellular waveforms of pyloric neurons in oxotremorine and CabTRP1a in TTX were similar to those in the intact rhythm, and phase relationships among neurons were conserved. Although cycle frequency was conserved in oxotremorine and TTX, it was altered in CabTRP1a in the presence of TTX. Both rhythms were primarily driven by the pacemaker kernel consisting of the Anterior Burster and Pyloric Dilator neurons. In contrast, in TTX the circuit remained silent in proctolin, TNRNFLRFamide, and CCAP. These experiments show that graded synaptic transmission in the absence of voltage-gated Na+ current is sufficient to sustain rhythmic motor activity in some, but not other, modulatory conditions, even when each modulator activates the same ionic current. This further demonstrates that similar rhythmic motor patterns can be produced by qualitatively different mechanisms, one that depends on the activity of voltage-gated Na+ channels, and one that can persist in their absence.SIGNIFICANCE STATEMENT The pyloric rhythm of the crab stomatogastric ganglion depends both on spike-mediated and graded synaptic transmission. We activate the pyloric rhythm with a wide variety of different neuromodulators, all of which converge on the same voltage-dependent inward current. Interestingly, when action potentials and spike-mediated transmission are blocked using TTX, we find that the muscarinic agonist oxotremorine and the neuropeptide CabTRP1a sustain rhythmic alternations and appropriate phases of activity in the absence of action potentials. In contrast, TTX blocks rhythmic activity in the presence of other modulators. This demonstrates fundamental differences in the burst-generation mechanisms in different modulators that would not be suspected on the basis of their cellular actions at the level of the targeted current.

Similarities and differences in circuit responses to applied Gly1-SIFamide and peptidergic (Gly1-SIFamide) neuron stimulation.

Microcircuit modulation by peptides is well established, but the cellular/synaptic mechanisms whereby identified neurons with identified peptide transmitters modulate microcircuits remain unknown for most systems. Here, we describe the distribution of GYRKPPFNGSIFamide (Gly1-SIFamide) immunoreactivity (Gly1-SIFamide-IR) in the stomatogastric nervous system (STNS) of the crab Cancer borealis and the Gly1-SIFamide actions on the two feeding-related circuits in the stomatogastric ganglion (STG). Gly1-SIFamide-IR localized to somata in the paired commissural ganglia (CoGs), two axons in the nerves connecting each CoG with the STG, and the CoG and STG neuropil. We identified one Gly1-SIFamide-IR projection neuron innervating the STG as the previously identified modulatory commissural neuron 5 (MCN5). Brief (~10 s) MCN5 stimulation excites some pyloric circuit neurons. We now find that bath applying Gly1-SIFamide to the isolated STG also enhanced pyloric rhythm activity and activated an imperfectly coordinated gastric mill rhythm that included unusually prolonged bursts in two circuit neurons [inferior cardiac (IC), lateral posterior gastric (LPG)]. Furthermore, longer duration (>30 s) MCN5 stimulation activated a Gly1-SIFamide-like gastric mill rhythm, including prolonged IC and LPG bursting. The prolonged LPG bursting decreased the coincidence of its activity with neurons to which it is electrically coupled. We also identified local circuit feedback onto the MCN5 axon terminals, which may contribute to some distinctions between the responses to MCN5 stimulation and Gly1-SIFamide application. Thus, MCN5 adds to the few identified projection neurons that modulate a well-defined circuit at least partly via an identified neuropeptide transmitter and provides an opportunity to study peptide regulation of electrical coupled neurons in a functional context. NEW & NOTEWORTHY Limited insight exists regarding how identified peptidergic neurons modulate microcircuits. We show that the modulatory projection neuron modulatory commissural neuron 5 (MCN5) is peptidergic, containing Gly1-SIFamide. MCN5 and Gly1-SIFamide elicit similar output from two well-defined motor circuits. Their distinct actions may result partly from circuit feedback onto the MCN5 axon terminals. Their similar actions include eliciting divergent activity patterns in normally coactive, electrically coupled neurons, providing an opportunity to examine peptide modulation of electrically coupled neurons in a functional context.

The differential contribution of pacemaker neurons to synaptic transmission in the pyloric network of the Jonah crab, Cancer borealis.

Many neurons receive synchronous input from heterogeneous presynaptic neurons with distinct properties. An instructive example is the crustacean stomatogastric pyloric circuit pacemaker group, consisting of the anterior burster (AB) and pyloric dilator (PD) neurons, which are active synchronously and exert a combined synaptic action on most pyloric follower neurons. Previous studies in lobster have indicated that AB is glutamatergic, whereas PD is cholinergic. However, although the stomatogastric system of the crab Cancer borealis has become a preferred system for exploration of cellular and synaptic basis of circuit dynamics, the pacemaker synaptic output has not been carefully analyzed in this species. We examined the synaptic properties of these neurons using a combination of single-cell mRNA analysis, electrophysiology, and pharmacology. The crab PD neuron expresses high levels of choline acetyltransferase and the vesicular acetylcholine transporter mRNAs, hallmarks of cholinergic neurons. In contrast, the AB neuron expresses neither cholinergic marker but expresses high levels of vesicular glutamate transporter mRNA, consistent with a glutamatergic phenotype. Notably, in the combined synapses to follower neurons, 70-75% of the total current was blocked by putative glutamatergic blockers, but short-term synaptic plasticity remained unchanged, and although the total pacemaker current in two follower neuron types was different, this difference did not contribute to the phasing of the follower neurons. These findings provide a guide for similar explorations of heterogeneous synaptic connections in other systems and a baseline in this system for the exploration of the differential influence of neuromodulators.NEW & NOTEWORTHY The pacemaker-driven pyloric circuit of the Jonah crab stomatogastric nervous system is a well-studied model system for exploring circuit dynamics and neuromodulation, yet the understanding of the synaptic properties of the two pacemaker neuron types is based on older analyses in other species. We use single-cell PCR and electrophysiology to explore the neurotransmitters used by the pacemaker neurons and their distinct contribution to the combined synaptic potentials.

In vivo effects of temperature on the heart and pyloric rhythms in the crab Cancer borealis.

The heart and pyloric rhythms of crustaceans have been studied separately and extensively over many years. Local and hormonal neuromodulation and sensory inputs into these central pattern generator circuits play a significant role in an animal's response to perturbations, but are usually lost or removed during in vitro studies. To examine simultaneously the in vivo motor output of the crustacean heart and pyloric rhythms, we used photoplethysmography. In the population measured (n=49), the heart rhythm frequency ranged from 0.3 to 2.3 Hz. The pyloric rhythm varied from 0.2 to 1.6 Hz. We observed a weak correlation between the frequencies of the heart and pyloric rhythms. During multiple hour-long recordings, many animals held at a controlled temperature showed strong inhibitory bouts in which the heart decreased in frequency or become quiescent and the pyloric rhythm decreased in frequency. We measured the simultaneous responses of the rhythms to temperature ramps by heating or cooling the saline bath while recording both the heart and pyloric muscle movements. Q10, critical temperature (temperature at which muscle function is compromised) and changes in frequency were calculated for each of the rhythms tested. The heart rhythm was more robust to high temperature than the pyloric rhythm.

The transient potassium outward current has different roles in modulating the pyloric and gastric mill rhythms in the stomatogastric ganglion.

The crustacean stomatogastric nervous system is a classic model for understanding the effects of modulating ionic currents and synapses at both the cell and network levels. The stomatogastric ganglion in this system contains two distinct central pattern generators: a slow gastric mill network that generates flexible rhythmic outputs (8-20 s) and is often silent, and a fast pyloric network that generates more consistent rhythmic outputs (0.5-2 s) and is always active in vitro. Different ionic conductances contribute to the properties of individual neurons and therefore to the overall dynamics of the pyloric and gastric mill networks. However, the contributions of ionic currents to different dynamics between the pyloric and gastric mill networks are not well understood. The goal of this study is to evaluate how changes in outward potassium current (I A) in the stomatogastric ganglion affect the dynamics of the pyloric and gastric mill rhythms by interfering with normal I A activity. We bath-applied the specific I A blocker 4-aminopyridine to reduce I A's effect in the stomatogastric ganglion in vitro and evaluated quantitatively the changes in both rhythms. We found that blocking I A in the stomatogastric ganglion alters the synchronization between pyloric neurons, and consistently activates the gastric mill rhythm in quiescent preparations.

Role of Ih in differentiating the dynamics of the gastric and pyloric neurons in the stomatogastric ganglion of the lobster, Homarus americanus.

The hyperpolarization-activated inward cationic current (Ih) is known to regulate the rhythmicity, excitability, and synaptic transmission in heart cells and many types of neurons across a variety of species, including some pyloric and gastric mill neurons in the stomatogastric ganglion (STG) in Cancer borealis and Panulirus interruptus However, little is known about the role of Ih in regulating the gastric mill dynamics and its contribution to the dynamical bifurcation of the gastric mill and pyloric networks. We investigated the role of Ih in the rhythmic activity and cellular excitability of both the gastric mill neurons (medial gastric, gastric mill) and pyloric neurons (pyloric dilator, lateral pyloric) in Homarus americanus Through testing the burst period between 5 and 50 mM CsCl, and elimination of postinhibitory rebound and voltage sag, we found that 30 mM CsCl can sufficiently block Ih in both the pyloric and gastric mill neurons. Our results show that Ih maintains the excitability of both the pyloric and gastric mill neurons. However, Ih regulates slow oscillations of the pyloric and gastric mill neurons differently. Specifically, blocking Ih diminishes the difference between the pyloric and gastric mill burst periods by increasing the pyloric burst period and decreasing the gastric mill burst period. Moreover, the phase-plane analysis shows that blocking Ih causes the trajectory of slow oscillations of the gastric mill neurons to change toward the pyloric sinusoidal-like trajectories. In addition to regulating the pyloric rhythm, we found that Ih is also essential for the gastric mill rhythms and differentially regulates these two dynamics.

Relationship between gastric motility and liquid mixing in the stomach.

The relationship between gastric motility and the mixing of liquid food in the stomach was investigated with a numerical analysis. Three parameters of gastric motility were considered: the propagation velocity, frequency, and terminal acceleration of peristaltic contractions. We simulated gastric flow with an anatomically realistic geometric model of the stomach, considering free surface flow and moving boundaries. When a peristaltic contraction approaches the pylorus, retropulsive flow is generated in the antrum. Flow separation then occurs behind the contraction. The extent of flow separation depends on the Reynolds number (Re), which quantifies the inertial forces due to the peristaltic contractions relative to the viscous forces of the gastric contents; no separation is observed at low Re, while an increase in reattachment length is observed at high Re. While mixing efficiency is nearly constant for low Re, it increases with Re for high Re because of flow separation. Hence, the effect of the propagation velocity, frequency, or terminal acceleration of peristaltic contractions on mixing efficiency increases with Re.

Gastric and pyloric sphincter muscle function and the developmental-dependent regulation of gastric content emptying in the rat.

Feeding intolerance is a common issue in the care of preterm neonates. The condition manifests as delayed emptying of gastric contents and represents a therapeutic challenge, since the factors accounting for its manifestations are unknown. The main goal of this study was to comparatively investigate the age-related function of rat gastric and pyloric smooth muscle and their putative regulators. We hypothesized that a reduced gastric muscle contraction potential early in life contributes to the delayed gastric emptying of the newborn. Newborn and adult rat gastric (fundus) and pyloric sphincter tissues were comparatively studied in vitro. Shortening of the tissue-specific dissociated smooth muscle cell was evaluated, and expression of the key regulatory proteins Rho-associated kinase 2 and myosin light chain kinase was determined. Gastric and pyloric smooth muscle cell shortening was significantly greater in the adult than the respective newborn counterpart. Expression of myosin light chain kinase and Rho-associated kinase 2 was developmentally regulated and increased with age. Pyloric sphincter muscle expresses a higher neuronal nitric oxide synthase and phosphorylated vasodilator-stimulated phosphoprotein content in newborn than adult tissue. Compared with later in life, the newborn rat gastropyloric muscle has a Ca(2+)-related reduced potential for contraction and the pyloric sphincter relaxation-dependent modulators are overexpressed. To the extent that these rodent data can be extrapolated to humans, the delayed gastric emptying in the newborn reflects reduced stomach muscle contraction potential, as opposed to increased pyloric sphincter tone.

Feedback control of variability in the cycle period of a central pattern generator.

We address how feedback to a bursting biological pacemaker with intrinsic variability in cycle length can affect that variability. Specifically, we examine a hybrid circuit constructed of an isolated crab anterior burster (AB)/pyloric dilator (PD) pyloric pacemaker receiving virtual feedback via dynamic clamp. This virtual feedback generates artificial synaptic input to PD with timing determined by adjustable phase response dynamics that mimic average burst intervals generated by the lateral pyloric neuron (LP) in the intact pyloric network. Using this system, we measure network period variability dependence on the feedback element's phase response dynamics and find that a constant response interval confers minimum variability. We further find that these optimal dynamics are characteristic of the biological pyloric network. Building upon our previous theoretical work mapping the firing intervals in one cycle onto the firing intervals in the next cycle, we create a theoretical map of the distribution of all firing intervals in one cycle to the distribution of firing intervals in the next cycle. We then obtain an integral equation for a stationary self-consistent distribution of the network periods of the hybrid circuit, which can be solved numerically given the uncoupled pacemaker's distribution of intrinsic periods, the nature of the network's feedback, and the phase resetting characteristics of the pacemaker. The stationary distributions obtained in this manner are strongly predictive of the experimentally observed distributions of hybrid network period. This theoretical framework can provide insight into optimal feedback schemes for minimizing variability to increase reliability or maximizing variability to increase flexibility in central pattern generators driven by pacemakers with feedback.

How can motor systems retain performance over a wide temperature range? Lessons from the crustacean stomatogastric nervous system.

Marine invertebrates, such as lobsters and crabs, deal with a widely and wildly fluctuating temperature environment. Here, we describe the effects of changing temperature on the motor patterns generated by the stomatogastric nervous system of the crab, Cancer borealis. Over a broad range of "permissive" temperatures, the pyloric rhythm increases in frequency but maintains its characteristic phase relationships. Nonetheless, at more extreme high temperatures, the normal triphasic pyloric rhythm breaks down, or "crashes". We present both experimental and computational approaches to understanding the stability of both single neurons and networks to temperature perturbations, and discuss data that shows that the "crash" temperatures themselves may be environmentally regulated. These approaches provide insight into how the nervous system can be stable to a global perturbation, such as temperature, in spite of the fact that all biological processes are temperature dependent.

Sources and range of long-term variability of rhythmic motor patterns in vivo.

The mechanisms of rhythmic motor pattern generation have been studied in detail in vitro, but the long-term stability and sources of variability in vivo are often not well described. The crab stomatogastric ganglion contains the well-characterized gastric mill (chewing) and pyloric (filtering of food) central pattern generators. In vitro, the pyloric rhythm is stereotyped with little variation, but inter-circuit interactions and neuromodulation can alter both rhythm cycle frequency and structure. The range of variation of activity in vivo is, with few exceptions, unknown. Curiously, although the pattern-generating circuits in vivo are constantly exposed to hormonal and neural modulation, the majority of published data show only the unperturbed canonical motor patterns typically observed in vitro. Using long-term extracellular recordings (N=27 animals), we identified the range and sources of variability of the pyloric and gastric mill rhythms recorded continuously over 4 days in freely behaving Jonah crabs (Cancer borealis). Although there was no evidence of innate daily rhythmicity, a 12 h light-driven cycle did manifest. The frequency of both rhythms increased modestly, albeit consistently, during the 3 h before and 3 h after the lights changed. This cycle was occluded by sensory stimulation (feeding), which significantly influenced both pyloric cycle frequency and structure. This was the only instance where the structure of the rhythm changed. In unfed animals the structure remained stable, even when the frequency varied substantially. So, although central pattern generating circuits are capable of producing many patterns, in vivo outputs typically remain stable in the absence of sensory stimulation.