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The urinary bladder epithelium comprises a specialised population of superficially placed cells called the umbrella cells. The apical membrane domain of umbrella cells has several intriguing morphological properties and is the site for various signaling activities. A key function of umbrella cells is to sense mechanical stimuli as the bladder stretches in response to filling. More specifically, the mechanotransduction of stretch into subcellular signals is brought about by the activation of Piezo1 channels that mediate calcium into the cell interior. The incoming calcium is critical to several aspects of umbrella cell signaling, including regulation of exocytosis, ATP release and downstream purinergic signaling. We report here a computational framework that models stretch-induced mechanotransduction via Piezo1 channels and the resulting calcium signaling in umbrella cells factoring in morphological details of subcellular compartment volumes. Our results show the following: (i) activation of Piezo1 conductance in response to stretch; (ii) development of varying Piezo1 mediated [Ca2+] profiles in subcellular compartments, namely, the apical sub-plasma membrane space, cytosol and mitochondria. The varying calcium amplitudes and temporal profiles in the subcellular compartments indicate highly specialised roles for stretch-mediated calcium in umbrella cells, including its potential effect on the energetics of mitochondria and the regulation of exocytosis.

The urothelium is the innermost layer of the bladder wall; it plays a pivotal role in bladder sensory transduction by responding to chemical and mechanical stimului. The urothelium also acts as a physical barrier between urine and the outer layers of the bladder wall. There is intricate sensory communication between the layers of the bladder wall and the neurons that supply the bladder, which eventually translates into the regulation of mechanical. In response to natural stimuli, urothelial cells release substances such as ATP, nitric oxide (NO), substance P, acetylcholine (ACh), and adenosine. These act on adjacent urothelial cells, myofibroblasts, and urothelial afferent neurons (UAN), controlling the contractile activity of the bladder. There is rising evidence on the importance of urothelial sensory signalling, yet a comprehensive understanding of the functioning of the urothelium-afferent neuron and the factors that govern it remain elusive to date. Until now, the biophysical studies done on UAN have been unable to provide adequate information on the ion channel composition of the neuron, which is paramount to understanding the electrical functioning of the UAN and, by extension, afferent signalling. To this end, we have attempted to model UAN to decipher the ionic mechanisms underlying the excitability of the UAN. In contrast to previous models, our model was built and validated using morphological and biophysical properties consistent with experimental findings for the UAN. The model included all the channels thus far known to be expressed in UAN, including; voltage-gated sodium and potassium channels, N, L, T, P/Q, R-type calcium channels, large-conductance calcium-dependent potassium (BK) channels, small conductance calcium-dependent (SK) channels, Hyperpolarisation activated cation (HCN) channels, transient receptor potential melastatin (TRPM8), transient receptor potential vanilloid (TRPV1) channel, calcium-activated chloride(CaCC) channels, and internal calcium dynamics. Our UAN model a) was constrained as far as possible by experimental data from the literature for the channels and the spiking activity b) was validated by reproducing the experimental responses to current-clamp and voltage-clamp protocols c) was used as a base for modelling the non-urothelial afferent neuron (NUAN). Using our models we also gained insights into the variations in ion channels between UAN and NUAN neurons.

Gap junctions provide pathways for intercellular communication between adjacent cells, allowing exchange of ions and small molecules. Based on the constituent protein subunits, gap junctions are classified into different subtypes varying in their properties such as unitary conductances, sensitivity to transjunctional voltage, and gating kinetics. Gap junctions couple cells electrically, and therefore the electrical activity originating in one cell can affect and modulate the electrical activity in adjacent cells. Action potentials can propagate through networks of such electrically coupled cells, and this spread is influenced by the nature of gap junctional coupling. Our study aims to computationally explore the effect of differences in gap junctional properties on oscillating action potentials in electrically coupled tissues. Further, we also explore variations in the biophysical environment by altering the size of the syncytium, the location of the pacemaking cell, as well as the occurrence of multiple pacemaking cells within the same syncytium. Our simulation results suggest that the frequency of oscillations is governed by the extent of coupling between cells and the gating kinetics of different gap junction subtypes. The location of pacemaking cells is found to alter the syncytial behavior, and when multiple oscillators are present, there exists an interplay between the oscillator frequency and their relative location within the syncytium. Such variations in the frequency of oscillations can have important implications for the physiological functioning of syncytial tissues.

The large conductance voltage and calcium activated potassium (BK) channels play a crucial role in regulating the excitability of detrusor smooth muscle, which lines the wall of the urinary bladder. These channels have been widely characterized in terms of their molecular structure, pharmacology and electrophysiology. They control the repolarising and hyperpolarising phases of the action potential, thereby regulating the firing frequency and contraction profiles of the smooth muscle. Several groups have reported varied profiles of BK currents and I-V curves under similar experimental conditions. However, no single computational model has been able to reconcile these apparent discrepancies. In view of the channels' physiological importance, it is imperative to understand their mechanistic underpinnings so that a realistic model can be created. This paper presents a computational model of the BK channel, based on the Hodgkin-Huxley formalism, constructed by utilising three activation processes - membrane potential, calcium inflow from voltage-gated calcium channels on the membrane and calcium released from the ryanodine receptors present on the sarcoplasmic reticulum. In our model, we attribute the discrepant profiles to the underlying cytosolic calcium received by the channel during its activation. The model enables us to make heuristic predictions regarding the nature of the sub-membrane calcium dynamics underlying the BK channel's activation. We have employed the model to reproduce various physiological characteristics of the channel and found the simulated responses to be in accordance with the experimental findings. Additionally, we have used the model to investigate the role of this channel in electrophysiological signals, such as the action potential and spontaneous transient hyperpolarisations. Furthermore, the clinical effects of BK channel openers, mallotoxin and NS19504, were simulated for the detrusor smooth muscle cells. Our findings support the proposed application of these drugs for amelioration of the condition of overactive bladder. We thus propose a physiologically realistic BK channel model which can be integrated with other biophysical mechanisms such as ion channels, pumps and exchangers to further elucidate its micro-domain interaction with the intracellular calcium environment.

Synchronization of network oscillation in spatially distant cortical areas is essential for normal brain activity. Precision in synchronization between hemispheres depends on the axonal conduction velocity, which is determined by physical parameters of the axons involved, including diameter, and extent of myelination. To compare these parameters in long-projecting excitatory and inhibitory axons in the corpus callosum, we used genetically modified mice and virus tracing to separately label CaMKIIα expressing excitatory and GABAergic inhibitory axons. Using electron microscopy analysis, we revealed that (i) the axon diameters of excitatory fibers (myelinated axons) are significantly larger than those of nonmyelinated excitatory axons; (ii) the diameters of bare axons of excitatory myelinated fibers are significantly larger than those of their inhibitory counterparts; and (iii) myelinated excitatory fibers are significantly larger than myelinated inhibitory fibers. Also, the thickness of myelin ensheathing inhibitory axons is significantly greater than for excitatory axons, with the ultrastructure of the myelin around excitatory and inhibitory fibers also differing. We generated a computational model to investigate the functional consequences of these parameter divergences. Our simulations indicate that impulses through inhibitory and excitatory myelinated fibers reach the target almost simultaneously, whereas action potentials conducted by nonmyelinated axons reach target cells with considerable delay.

NEURON is the most widely used computational modeling platform in the field of neuroscience, allowing development of single cell models as well as huge networks of cells. With increase in the sizes of networks, performance and efficiency becomes a growing concern. To investigate larger networks it is necessary to have access to appropriate computational resources. It is equally imperative, and typically more economical, to extract the maximum performance from the available resources. This can be achieved by optimizing the model implementation, along with appropriate utilization of the built-in parallelization and performance enhancement features of the NEURON simulator. In the present study, we have explored, tested and benchmarked the performance benefits gained from these approaches.

Satellite glial cells (SGCs) are glial cells found in the peripheral nervous system where they tightly envelop the somata of the primary sensory neurons such as dorsal root ganglion (DRG) neurons and nodose ganglion (NG) neurons. The somata of these neurons are generally compactly packed in their respective ganglia (DRG and NG). SGCs covering a neuron behave as an insulator of electrical activity from neighbouring neurons within the ganglion. Several studies have however shown that the somata show "cross-depolarization" (CD). Origin of CDs has been hypothesized to be chemical in nature: either from neurotransmitter release from both SGCs and somata or from elevation of extracellular potassium concentration ([K]

Urinary bladder smooth muscle (UBSM) generates spontaneous electrical activities due to stochastic nature of purinergic neurotransmitter release from the parasympathetic nerve. The stochastic nature of the purinergic neurotransmitter release was represented by a simplified ‘point-conductance’ model to mimic in vitro-like electrical activities in UBSM cell. The point-conductance was represented by the independent synaptic conductance described by the stochastic random-walk processes and injected into a single-compartment model of mouse UBSM cell. This model successfully evoked irregular spontaneous depolarizations (SDs) and spontaneous action potential (sAP) as the properties of in vitro-like electrical activities in UBSM cells. The model mimics the T- and L-type Ca2+ ion channel blocker by setting their respective conductance to zero. We also found that the point-conductance model modulates the sAP properties by adding background activity.

The vas deferens smooth muscle (VDSM) cells contract to direct and propel sperms from the epididymis to the urethra. It is well known that membrane electrical activity, particularly the action potential (AP) is an essential prerequisite for the initiation of contraction in all types of muscle cells. As the coordinated activation of a number of ion channels in the VDSM cell membrane causes AP generation, any mutation or dysfunction of any ion channel will modulate the AP generation and hence the contraction. To explore the quantitative contribution of individual active ionic current to the AP generation, a biophysically based single guinea-pig VDSM cell model is presented. The simulated ionic currents and AP show good agreement with the experimental recordings in terms of several parameters. Therefore, this electrophysiological model can be a preliminary platform to investigate the various electrical properties of VDSM cells in both normal and pathological conditions.

Satellite glial cells (SGCs) are glial cells found in the peripheral nervous system where they tightly envelop the somata of the primary sensory neurons such as dorsal root ganglion (DRG) neurons and nodose ganglion (NG) neurons. The somata of these neurons are generally compactly packed in their respective ganglia (DRG and NG). SGCs covering a neuron behave as an insulator of electrical activity from neighbouring neurons within the ganglion. Several studies have however shown that the somata show "cross-depolarization" (CD). Origin of CDs has been hypothesized to be chemical in nature: either from neurotransmitter release from both SGCs and somata or from elevation of extracellular potassium concentration ([K] o ) in the vicinity of somata. Here, we investigate the role of Kir4.1 channels on SGC and diffusion/clearance factor (β) of [K] o from the space between SGC and DRG neuron somata to the bulk extracellular space in ganglion. We show using two "Soma-SGC Units" interacting via gap junction that a combination of Kir4.1 and β could be responsible for CD between DRG neuron somata in pathological conditions.

The detrusor, a key component of the urinary bladder wall, is a densely innervated syncytial smooth muscle tissue. Random spontaneous release of neurotransmitter at neuromuscular junctions (NMJs) in the detrusor gives rise to spontaneous excitatory junction potentials (SEJPs). These sub-threshold passive signals not only offer insights into the syncytial nature of the tissue, their spatio-temporal integration is critical to the generation of spontaneous neurogenic action potentials which lead to focal contractions during the filling phase of the bladder. Given the structural complexity and the contractile nature of the tissue, electrophysiological investigations on spatio-temporal integration of SEJPs in the detrusor are technically challenging. Here we report a biophysically constrained computational model of a detrusor syncytium overlaid with spatially distributed innervation, using which we explored salient features of the integration of SEJPs in the tissue and the key factors that contribute to this integration. We validated our model against experimental data, ascertaining that observations were congruent with theoretical predictions. With the help of comparative studies, we propose that the amplitude of the spatio-temporally integrated SEJP is most sensitive to the inter-cellular coupling strength in the detrusor, while frequency of observed events depends more strongly on innervation density. An experimentally testable prediction arising from our study is that spontaneous release frequency of neurotransmitter may be implicated in the generation of detrusor overactivity. Set against histological observations, we also conjecture possible changes in the electrical activity of the detrusor during pathology involving patchy denervation. Our model thus provides a physiologically realistic, heuristic framework to investigate the spread and integration of passive potentials in an innervated syncytial tissue under normal conditions and in pathophysiology.

The current study investigates the interplay between two important regulatory cellular signals that control contractions in the muscle, namely, intracellular calcium concentration and ATP. In this work, we have attempted to study how two regulatory signals interplay to affect the contractile responses of the muscle. The action of ATP hinders the myosin and actin from binding to each other, whereas, the effect of intracellular calcium mediates their cross-linkages. Thus, the study of two opposing signals and the effect of their interplay on the filament properties as well as on the contractile responses have been studied here. The Hai and Murphy contraction model has been modified to perform the current study. The ATP dependency was introduced into this model and through our simulations, we were able to demonstrate the importance of incorporating the same. Our model was able to reproduce physiologically realistic contraction profiles and stress values that befits a smooth muscle. Further, we used our model to estimate duty ratio and sliding velocity for the filament, under normal, loaded and pathological conditions.

As in other excitable tissues, two classes of electrical signals are of fundamental importance to the functioning of smooth muscles: junction potentials, which arise from neurotransmission and represent the initiation of excitation (or in some instances inhibition) of the tissue, and spikes or action potentials, which represent the accomplishment of excitation and lead on to contractile activity. Unlike the case in skeletal muscle and in neurons, junction potentials and spikes in smooth muscle have been poorly understood in relation to the electrical properties of the tissue and in terms of their spatiotemporal spread within it. This owes principally to the experimental difficulties involved in making precise electrical recordings from smooth muscles and also to two inherent features of this class of muscle, ie, the syncytial organization of its cells and the distributed innervation they receive, which renders their biophysical analysis problematic. In this review, we outline the development of hypotheses and knowledge on junction potentials and spikes in syncytial smooth muscle, showing how our concepts have frequently undergone radical changes and how recent developments hold promise in unraveling some of the many puzzles that remain. We focus especially on computational models and signal analysis approaches. We take as illustrative examples the smooth muscles of two organs with distinct functional characteristics, the vas deferens and urinary bladder, while also touching on features of electrical functioning in the smooth muscles of other organs.

Smooth muscle that ensheathes the urinary bladder wall is known as the detrusor. The detrusor has a bundled syncytial architecture where groups of electrically coupled cells form discrete bundles. Electrical activity recorded from detrusor cells is varied. Among the electrical signals recorded from the detrusor, spontaneous excitatory junction potentials (SEJPs) are events that are sub-threshold for spike generation. SEJPs are caused by spontaneous random release of neurotransmitter molecules from varicosities of innervating nerves. SEJPs recorded from different cells of the detrusor vary in amplitude and/or kinetics, and the reasons for variability are obscure. Our hypothesis was that variety in SEJP characteristics may be attributed to the biophysical micro-environment of a cell (in a bundle), where it arises. With the help of computational models, we show how cellular environment factor, such as size of a bundle significantly alters the amplitude as well as kinetics of SEJPs. These findings are congruent with experimental observations. Our results also suggest that characteristically different SEJPs may be observed in identical detrusor cells, with the difference arising from their neighbourhood rather than the inherent nature of the cells. Consequently, SEJP characteristics might be indicative of the cellular environment of electrophysiological recording.

Smooth muscle cells from the urinary bladder display enhanced spontaneous electrical activities during the overactive bladder state. It is very well known that the electrical properties of all excitable cells are regulated by the intrinsic active ion channels. In excitable cells like the neurons, cardiac and other smooth muscle cells, the hyperpolarization-activated cation channel has been considered as a potential target to regulate cell’s excitability. The primary purpose of this research work is to develop a computational model of the hyperpolarization-activated cation channel in urinary bladder smooth muscle (UBSM) cell to analysis its modulating role in cell’s excitability. All required biophysical parameters are adapted from the published experimental studies in UBSM cell. We have successfully simulated and validated the channel model by comparing it with experimental findings in UBSM cells. We have investigated the potential role of this hyperpolarization-activated cation channel in regulating the shape of the evoked action potentials. From this quantitative analysis, we conclude that future pharmacological studies on hyperpolarization-activated cation channel can provide more insights into the underlying bladder overactivity.

The vas deferens smooth muscle (VDSM) cells contract to direct and propel sperms from the epididymis to the urethra. It is well known that membrane electrical activity, particularly the action potential (AP) is an essential prerequisite for the initiation of contraction in all types of muscle cells. As the coordinated activation of a number of ion channels in the VDSM cell membrane causes AP generation, any mutation or dysfunction of any ion channel will modulate the AP generation and hence the contraction. To explore the quantitative contribution of individual active ionic current to the AP generation, a biophysically based single guinea-pig VDSM cell model is presented. The simulated ionic currents and AP show good agreement with the experimental recordings in terms of several parameters. Therefore, this electrophysiological model can be a preliminary platform to investigate the various electrical properties of VDSM cells in both normal and pathological conditions.

Detrusor smooth muscle (DSM) instability is a common symptom of over active bladder (OAB) which effects quality of millions of lives. The enhanced spontaneous contraction due to generation of more frequent spontaneous action potentials (AP) is the fundamental cause of DSM instability. Ca2+ dependent potassium channels are abundantly distributed in DSM cells and regulate the membrane excitability by modulating AP shape and the resting membrane potential. Quantitative studies of these channels will help to predict the underlying mechanism of electrical signaling in DSM cells. Here, we present a large conductance Ca2+ dependent potassium (BK) channel based upon Hodgkin-Huxley (HH) formalism which is incorporated into a published single DSM cell model to investigate several modulating effects. This simple BK ion channel model is constructed with biophysical details from documented electrophysiological studies. As it reflects several modulating properties found in DSM cell electrophysiology, further investigation can shed light in genesis of OB.

This paper employs a computational model to study the dual gating modalities of BK channels in smooth muscles. These channels are gated by both membrane potential and intracellular calcium concentration. It has been previously reported that the sensors for these two stimuli are located at different regions of the channel. Thus, the two sensing modalities act independent of each other. Yet, they result in a concerted and synergistic opening of the channel pore. In this paper, we investigate the effects of these two gating mechanisms by computing the effective gating charges contributed by the channel's voltage and calcium sensors. Along with their independent contributions, we study and estimate the interplay and effect of these two modalities on the channel's activation. The voltage and calcium sensors appear to share the 'load' of the gating charges required to activate the channel based on the cytosolic calcium concentration and membrane potential. Thus, through our computational model, we demonstrate how the two independent sensors gate and coordinate the activation of the channel.