A computational model of urinary bladder smooth muscle syncytium

Abstract

Certain smooth muscles, such as the detrusor of the urinary bladder, exhibit a variety of spikes that differ markedly in their amplitudes and time courses. The origin of this diversity is poorly understood but is often attributed to the syncytial nature of smooth muscle and its distributed innervation. In order to help clarify such issues, we present here a three-dimensional electrical model of syncytial smooth muscle developed using the compartmental modeling technique, with special reference to the bladder detrusor. Values of model parameters were sourced or derived from experimental data. The model was validated against various modes of stimulation employed experimentally and the results were found to accord with both theoretical predictions and experimental observations. Model outputs also satisfied criteria characteristic of electrical syncytia such as correlation between the spatial spread and temporal decay of electrotonic potentials as well as positively skewed amplitude frequency histogram for sub-threshold potentials, and lead to interesting conclusions. Based on analysis of syncytia of different sizes, it was found that a size of 21-cube may be considered the critical minimum size for an electrically infinite syncytium. Set against experimental results, we conjecture the existence of electrically sub-infinite bundles in the detrusor. Moreover, the absence of coincident activity between closely spaced cells potentially implies, counterintuitively, highly efficient electrical coupling between such cells. The model thus provides a heuristic platform for the interpretation of electrical activity in syncytial tissues.

Unsupervised Classification of Spontaneous Action Potentials in Urinary Bladder

“…All the above mentioned attributes of the smooth muscle tissue systemthe small size, the non-homogeneous electrical syncytium, and the many-to-many mapping of the nerve and muscle cells make the comprehension of the smooth muscle tissue very difficult. The details of the electrical and mechanical activities of the smooth muscle tissues are still under exploration [9].…”

“…These may cause APs (called the spontaneously generated action potentials, or spontaneous action potentials, or sAPs) in the smooth muscle cells. These sAPs, if generated, could travel to the nearby smooth muscle cells through the gap junctional coupling, resulting in the contraction of the corresponding smooth muscle bundle [9], [11]. These uncoordinated contractions of smooth muscle bundles caused by the spontaneous neurotransmitter release are believed to be the way by which the bladder maintains its tone during the storage phase.…”

Classification of cells in detrusor smooth muscle syncytium based on analysis of spontaneous electrical activities

“…Also a single smooth muscle cell can receive inputs from more than one neuron. The complexity of this network is augmented when we consider the fact that the smooth muscle cells themselves are electrically coupled to each other with the help of 'gap junctions', special protein structures which allows the passage of ions and small molecules to travel from one cell to other [1], [2]. Thus the smooth muscle cells in a bundle form an electrical syncytium.…”

“…The neurotransmitters released from the varicosities depolarize the DSMC, and if a threshold is reached an action potential (AP) is generated. This AP can propagate from one DSMC to its neighboring DSMC via the gap junctions [1], [2], [3]. Apart from the neurotransmitter release, APs could also be originated from pacemaking cells present at the boundary of the DSM layer [10].…”

“…In order to explore the effect of syncytial properties on the signals generated, a thorough understanding of the syncytium is required, which is presently not available [2]. Hence we propose a reverse approach in which some aspects of the syncytium are uncovered using a detailed analysis of the intracellularly recorded (spontaneous) activities from the DSMC.…”